I will be 57 very soon. I find that I am not aging as gracefully as I might like. I don't like the fact that I have trouble recalling people's names, specific nomenclature for tools and techniques, and generally a lot of things that used to be always easily pulled out of my memory banks that now get exceedingly slow. This worried me enough that I read several books on how aging affects the brain. I felt better after discovering that the brain compensates in other ways for this lack of speed, but it still bothers me to have a conversation constantly interrupted with pauses and a lot of "I can't think of the name right now" comments.
I have also discovered that my writing often takes on unintended comical aspects. I sometimes simply leave letters off of words as if my brain can't be bothered to complete the whole word. I regularly substitute on and in within sentences as well as this and these although these comes out "thes". My proofreading has always been terrible as I tend to read what I meant to write instead of what I actually recorded. This makes my new tendencies even more frustrating.
I have a routine I go through when I leave work to check that I have my badge to get back into work with and my cell phone in my pocket. I have twice gotten out of the car and started back into the shop to retrieve my missing cell phone WHILE I was talking to someone on the "missing" cell phone. I have even more elaborate routines while leaving home for work that involve checking and rechecking whether the stove is off or the water faucet I use to fill the dog's bowl is off. You don't even want to know what I go through when leaving on a trip. None of this used to happen when I was younger.
Quite a few years ago I figured out that I had to work out regularly to maintain any semblance of physical strength so I do that fairly routinely. Still, rigorous exercise produces levels of pain in the mornings that sometimes makes me think I should just retire permanently to the couch. Back pain, shoulder pain, ankle pain, and pain in my hands from a lifetime of manual labor and a youth of physical confrontation are constant companions. I don't enjoy these companions but I am never quite free from them either. Concentration on the task at hand and a certain sense of creative enjoyment dulls them considerably during activity, but they soon come back with a vengeance when I sit down for a while.
Some time ago, I worked with a somewhat crotchety retired guy who had come back to double dip on the system. He came in one Monday morning limping considerably. I asked him what was wrong and he whirled around pointing his finger at me accusingly, "Getting old is not for p%^^#s. You'll find that out one day." Wherever you are today Chester..... I couldn't agree more. Still.... it beats the only other alternative we have.
Sunday, October 23, 2016
Friday, October 7, 2016
A Bridge Too Far....
A bridge too far is simply an act of overreach; a step too far in advance of capability. In psychological terms it is a symptom of hubris or an over inflated sense of achievement that leads to disaster. At this point at least, it appears that Elon Musk and Space Ex are victims of their own bridge too far. Success in returning rockets and relanding them have been fantastic and extremely impressive but they seem to have led to an attitude the skips over the careful understanding of process in search of further achievement.
I will once again point out that all of the data is not in yet on what happened to cause the Falcon 9 to explode on the pad at Cape Canaveral during loading. While it appears that a COPV in the second stage LOX tank exploded and caused the destruction of the rocket itself; the investigation is ongoing.
In my last post, I went into some detail about the fragile nature of COPV's as far as mechanical external damage is concerned. I can't adequately express how important to the health of a COPV that basic handing integrity happens to be. While COPV's can and have been designed for massive overpressurization limits because of the lack of weight associated with more composite wraps, they are also extremely vulnerable to external damage that might immediately seem inconsequential, but can quickly lead to the failure of the COPV.
I have also pointed out that Space Ex didn't seem adequately concerned with handling of COPV's, both with the mounting methods they use to put them inside the LOX tanks and by the lack of understanding of some of their personnel that they are not meant to be stood on. For whatever reason neither the person standing on the tanks, nor the person distributing pictures documenting this act seemed aware that this might be a problem which of course leads to the question of how many other people in the handling chain for these tanks at Space Ex were similarly unaware.
While this is troubling, it is hardly proof that it caused either or both of the recent Falcon 9 explosions. As I pointed out in my last post, the sequence of events if that is what happened does seem to match the available data. There are a lot of other possibilities however. I will try to briefly explain some other things that could have quite possibly caused an inadvertant rupture of one of these same COPV's.
Let me go over another possible scenario that could have led to the failure of one of these COPV's well below its rated pressure. There are a couple of physics theories that have to be explained in order to understand the following sequence of events. The first one is something called the Joule-Thomsen effect. What Joule-Thomsen says is that expanding a compressed gas across an orifice predictably changes its temperature. This is largely the basic theory that led to the discovery of modern day air conditioning. Most gases at or near ambient temperature will tend to cool as they expand across an orifice. Therefore, taking a compressed gas such as Freon and expanding it across an orifice or valve will chill that gas quite dramatically. This cooling effect is exactly how a modern day air conditioner works. Compressed freon is expanded across an orifice into a cold tubing bundle. If air is forced across this bundle by means of a fan, the air itself is chilled accordingly. Air conditioners are a closed loop system. This means that the same Freon gas is later collected and forced through a compressor which is used to pressurize it again so that it can repeat this basic process again.
There is a rather odd anomaly surrounding three gases when it comes to the Joule-Thomsen effect. Hydrogen, Helium, and Neon have what is called a negative Joule-Thomsen effect under certain conditions. In other words, they can produce heat when expanded across an orifice, dependent on starting temperature of the gas and pressure. In the case of Helium space vehicle pressurization systems tend to cross over and through this barrier as a normal event. In other words, part of the time that you are transferring helium through a valve or orifice it will get colder and then it will go through a stage where it produces heat and then back to colder. It is predictable, if not easily calculated but it is something that should always be taken into effect when pressure rating helium and hydrogen systems. Higher heat inevitably leads to lower pressure ratings for containment vessels, valves, and pipelines in such systems.
The other side of this same process then has an effect. It is called adiabatic heating. Adiabatic heating is simply heat of work that is added to the Freon as it is stepped up to a higher pressure again by the compressor. This heat is usually dissipated in large coils that are exposed to circulating air which tends to cool the gas by the time it gets back to the orifice to be expanded again.
Another example of Adiabatic heating is a diesel engine. Diesel engines don't require spark plugs to light the fuel in each cylinder because they relay on the adiabatic heating process that occurs when the inlet valve is closed and the piston is driven at a high rate of speed inside the cylinder to compress the air/diesel mixture of gas in the cylinder. The adiabatic heating combined with the piston still being hot from the last stroke is high enough to light this mixture causing an explosion which forces the piston back down. This in turn causes the main crank to turn and push another piston up with the same result.
Adiabatic heating can be both large and fast, dependent on the gas being compressed and its volume. It is therefore also an ignition mechanism in fluid systems that has to be carefully considered when transferring gases at high pressures and volumes. Extreme heat weakens pressure ratings because the metals themselves lose strength and become more ductile when heated. This difference in tensile strength can be very large, depending on material, rate of heating, and purity or metallurgical control of manufacturing process.
As I discussed earlier one of the relative oddities of COPV's is that they will actually contain higher pressures at cryogen temperatures. This is because the extreme cold at tends to shrink the composite fibers more than the internal aluminum tank. This same quality reverses as a COPV is heated. The aluminum tank tends to expand to stress the carbon fibers more, putting them in a preload condition that limits the expansion they can take as the inner tank starts to try to expand with increases in internal pressure. If a COPV is slowly and uniformly heated this effect is lessened. If it is heated in a non-uniform matter it increases.
When we have used COPV's in testing Shuttle systems we routinely monitored internal temperatures in them to guard against this effect. Very minor increases in temperature inside the tank while the outside carbon fibers remain at the same temperature were deemed problematic enough that we had cutoffs that stopped the pressurization process automatically if we gained over 120 degrees F on the inside of the COPV. I know from experience that this was easily done with a very small volume pnuematic system so that we would take a very long time to charge the COPV's to operating temperature to do our tests.
These numbers were based upon a basic 50 degree F differential between the inner part of the COPV and the outer carbon fibers since we were operating at a roughly 70 degree ambient environment. It is routine practice to monitor internal temperatures on COPV's as they are pressurized so that you don't effectively make them able to withstand less pressure. From what I have been able to gather, Space Ex also monitors inlet pressure while filling the COPV's on Falcon 9. What I don't know is how they are monitoring this temperature.
Let me explain why this is also important. In the first place, the most common temperature measurements are made by utilizing a thermocouple which is simply a connection of two dissimilar metals. These dissimilar metals produce a voltage at their junction that changes with temperature. By monitoring these changes it is possible to accurately read this voltage and infer a temperature.
There are other instruments that are used to make such measurements but this is the most common and least expensive as well as being the fastest at monitoring temperature changes so I am somewhat giving Space Ex the benefit of the doubt in assuming they have an accurate measure of internal temperature on the COPV's as they fill them. When I say fast, I mean the amount of time it takes to actually reflect a change in temperature. In the data aquisition world temperatures are amongst the slowest measurements for the simple reason that even the fastest instruments for temperature measurement are rather slow. A step response of 200 milliseconds on the fastest thermocouple would be very good. In other words, dependent on instrument and data rate it is very hard to see rapid temperature changes.
Since Space Ex is filling several COPV's at the same time I am sure they are utilizing a temperature sensor on a tube instead of individual temperature sensors inside each COPV. This matters because the relatively large volume of a COPV inside of a tank filled with LOX (-297 Degrees F) will necessarily read a much lower temperature as the small area inside the tube will be more readily chilled by the LOX than the volume inside the COPV. In essence the temperature monitoring instrument they are using on the COPV's as they fill them is relatively slow to react to changes and physically masked by the physics of its mechanical location.
Let's also keep in mind that the physical environment of the COPV's is greatly exacerbating the relative weakening of its ability to hold high pressure gas. The COPV is seeing at least -297 Degrees F on its carbon fiber exterior at the same time that helium being heated by adiabatic heating and negative Joule-Thomsen effect is being introduced into its interior. We are probably talking a 400 degree F temperature differential that it is routinely seeing on Space Ex's rapid fill propellant transfer which will necessarily vastly lower its pressure rating. Depending on how high this differential actually is, it could rather easily exceed the safety margin of the COPV's pressure rating.
This is the part where we begin to get into the "Bridge too Far" analogy. Space Ex and Elon Musk have their eye on much bigger things than the resupply of Space Station that Falcon 9 is specifically tasked with performing. Musk has made no secret that his eventual goal is the colonization of Mars and they are as I write this developing heavier launch bigger vehicles to reach this goal. It is this ultimate goal that is causing them to push the envelope so hard on Falcon 9. There has been a lot of publicity about reusable rockets and a very public "competition" between Space Ex and Blue Horizon to return rockets and land them successfully in order to keep costs low on launching payloads.
This requires extra fuel and oxidizers to be loaded onto Falcon 9 in order to have the requisite power to control its landing process manuevers. To their great credit both Space Ex and Blue Horizon have managed to be very successful in recovering their rockets with such manuevers. Since this is not what Falcon 9 was originally designed or contracted to do it is has been a little problematic for Space Ex to find ways to get the extra fuel and oxidizer on board to carry out these manuevers and still deliver its payload to Space Station.
One of the ways they have managed to do this is to steadily increase the supercooled effect of the LOX they use on Falcon 9. Supercooling of LOX is not a new idea either, as NASA has used this idea to increase turbopump efficiency for quite a few years. What it involves is decreasing the temperature of LOX away from its boiling point and towards it freezing point. In other words, the colder they can get their LOX before launch, the more energy it will contain. This effectively gives them the margin they need to carry out the extra manuevers necessary to safely land the rockets on their return descent to Earth.
One of the main factors for having super cooled LOX is to load it quickly and to make sure it does not sit in the LOX tank for long before launch. The longer it sits in the tank the more it tends to stratify which leads to less efficient burns and more LOX expended on ascent. Space Ex has been constantly speeding up its rapid load process for quite some time now. This means that massive amounts of LOX and RP1 are transferred to the vehicle just 30 minutes before launch. It also means that Space Ex must launch immediately after loading so that all other checkouts and final preps must occur after loading propellants.
The latest load of propellants as a dry run test was the most rapid load they have yet attempted. This effectively means that the LOX in the tank was colder than it has ever been on a propellant load. It also means that the Helium fill which must occur after LOX is loaded to take advantage of the increased density that putting the COPV's in the LOX tank in the first place was necessitated by. In their every increasing haste to cut the time that LOX sits in the vehicle I am sure they are also accelerating the rate at which they fill their COPV's which also increases the heat load by adiabatic heating and negative Joule-Thomsen effect.
In other words, for reasons completely outside of their actual manifest task, Space Ex has neccessarily vastly increased the temperature differential their COPV's see on propellant load. Is this relationship the key to the latest loss of a Falcon 9 on the pad at Cape Canaveral? I guess time will tell if this is actually a bridge too far or just another bump in the road for Musks' frenzied path to Mars.
I will once again point out that all of the data is not in yet on what happened to cause the Falcon 9 to explode on the pad at Cape Canaveral during loading. While it appears that a COPV in the second stage LOX tank exploded and caused the destruction of the rocket itself; the investigation is ongoing.
In my last post, I went into some detail about the fragile nature of COPV's as far as mechanical external damage is concerned. I can't adequately express how important to the health of a COPV that basic handing integrity happens to be. While COPV's can and have been designed for massive overpressurization limits because of the lack of weight associated with more composite wraps, they are also extremely vulnerable to external damage that might immediately seem inconsequential, but can quickly lead to the failure of the COPV.
I have also pointed out that Space Ex didn't seem adequately concerned with handling of COPV's, both with the mounting methods they use to put them inside the LOX tanks and by the lack of understanding of some of their personnel that they are not meant to be stood on. For whatever reason neither the person standing on the tanks, nor the person distributing pictures documenting this act seemed aware that this might be a problem which of course leads to the question of how many other people in the handling chain for these tanks at Space Ex were similarly unaware.
While this is troubling, it is hardly proof that it caused either or both of the recent Falcon 9 explosions. As I pointed out in my last post, the sequence of events if that is what happened does seem to match the available data. There are a lot of other possibilities however. I will try to briefly explain some other things that could have quite possibly caused an inadvertant rupture of one of these same COPV's.
Let me go over another possible scenario that could have led to the failure of one of these COPV's well below its rated pressure. There are a couple of physics theories that have to be explained in order to understand the following sequence of events. The first one is something called the Joule-Thomsen effect. What Joule-Thomsen says is that expanding a compressed gas across an orifice predictably changes its temperature. This is largely the basic theory that led to the discovery of modern day air conditioning. Most gases at or near ambient temperature will tend to cool as they expand across an orifice. Therefore, taking a compressed gas such as Freon and expanding it across an orifice or valve will chill that gas quite dramatically. This cooling effect is exactly how a modern day air conditioner works. Compressed freon is expanded across an orifice into a cold tubing bundle. If air is forced across this bundle by means of a fan, the air itself is chilled accordingly. Air conditioners are a closed loop system. This means that the same Freon gas is later collected and forced through a compressor which is used to pressurize it again so that it can repeat this basic process again.
There is a rather odd anomaly surrounding three gases when it comes to the Joule-Thomsen effect. Hydrogen, Helium, and Neon have what is called a negative Joule-Thomsen effect under certain conditions. In other words, they can produce heat when expanded across an orifice, dependent on starting temperature of the gas and pressure. In the case of Helium space vehicle pressurization systems tend to cross over and through this barrier as a normal event. In other words, part of the time that you are transferring helium through a valve or orifice it will get colder and then it will go through a stage where it produces heat and then back to colder. It is predictable, if not easily calculated but it is something that should always be taken into effect when pressure rating helium and hydrogen systems. Higher heat inevitably leads to lower pressure ratings for containment vessels, valves, and pipelines in such systems.
The other side of this same process then has an effect. It is called adiabatic heating. Adiabatic heating is simply heat of work that is added to the Freon as it is stepped up to a higher pressure again by the compressor. This heat is usually dissipated in large coils that are exposed to circulating air which tends to cool the gas by the time it gets back to the orifice to be expanded again.
Another example of Adiabatic heating is a diesel engine. Diesel engines don't require spark plugs to light the fuel in each cylinder because they relay on the adiabatic heating process that occurs when the inlet valve is closed and the piston is driven at a high rate of speed inside the cylinder to compress the air/diesel mixture of gas in the cylinder. The adiabatic heating combined with the piston still being hot from the last stroke is high enough to light this mixture causing an explosion which forces the piston back down. This in turn causes the main crank to turn and push another piston up with the same result.
Adiabatic heating can be both large and fast, dependent on the gas being compressed and its volume. It is therefore also an ignition mechanism in fluid systems that has to be carefully considered when transferring gases at high pressures and volumes. Extreme heat weakens pressure ratings because the metals themselves lose strength and become more ductile when heated. This difference in tensile strength can be very large, depending on material, rate of heating, and purity or metallurgical control of manufacturing process.
As I discussed earlier one of the relative oddities of COPV's is that they will actually contain higher pressures at cryogen temperatures. This is because the extreme cold at tends to shrink the composite fibers more than the internal aluminum tank. This same quality reverses as a COPV is heated. The aluminum tank tends to expand to stress the carbon fibers more, putting them in a preload condition that limits the expansion they can take as the inner tank starts to try to expand with increases in internal pressure. If a COPV is slowly and uniformly heated this effect is lessened. If it is heated in a non-uniform matter it increases.
When we have used COPV's in testing Shuttle systems we routinely monitored internal temperatures in them to guard against this effect. Very minor increases in temperature inside the tank while the outside carbon fibers remain at the same temperature were deemed problematic enough that we had cutoffs that stopped the pressurization process automatically if we gained over 120 degrees F on the inside of the COPV. I know from experience that this was easily done with a very small volume pnuematic system so that we would take a very long time to charge the COPV's to operating temperature to do our tests.
These numbers were based upon a basic 50 degree F differential between the inner part of the COPV and the outer carbon fibers since we were operating at a roughly 70 degree ambient environment. It is routine practice to monitor internal temperatures on COPV's as they are pressurized so that you don't effectively make them able to withstand less pressure. From what I have been able to gather, Space Ex also monitors inlet pressure while filling the COPV's on Falcon 9. What I don't know is how they are monitoring this temperature.
Let me explain why this is also important. In the first place, the most common temperature measurements are made by utilizing a thermocouple which is simply a connection of two dissimilar metals. These dissimilar metals produce a voltage at their junction that changes with temperature. By monitoring these changes it is possible to accurately read this voltage and infer a temperature.
There are other instruments that are used to make such measurements but this is the most common and least expensive as well as being the fastest at monitoring temperature changes so I am somewhat giving Space Ex the benefit of the doubt in assuming they have an accurate measure of internal temperature on the COPV's as they fill them. When I say fast, I mean the amount of time it takes to actually reflect a change in temperature. In the data aquisition world temperatures are amongst the slowest measurements for the simple reason that even the fastest instruments for temperature measurement are rather slow. A step response of 200 milliseconds on the fastest thermocouple would be very good. In other words, dependent on instrument and data rate it is very hard to see rapid temperature changes.
Since Space Ex is filling several COPV's at the same time I am sure they are utilizing a temperature sensor on a tube instead of individual temperature sensors inside each COPV. This matters because the relatively large volume of a COPV inside of a tank filled with LOX (-297 Degrees F) will necessarily read a much lower temperature as the small area inside the tube will be more readily chilled by the LOX than the volume inside the COPV. In essence the temperature monitoring instrument they are using on the COPV's as they fill them is relatively slow to react to changes and physically masked by the physics of its mechanical location.
Let's also keep in mind that the physical environment of the COPV's is greatly exacerbating the relative weakening of its ability to hold high pressure gas. The COPV is seeing at least -297 Degrees F on its carbon fiber exterior at the same time that helium being heated by adiabatic heating and negative Joule-Thomsen effect is being introduced into its interior. We are probably talking a 400 degree F temperature differential that it is routinely seeing on Space Ex's rapid fill propellant transfer which will necessarily vastly lower its pressure rating. Depending on how high this differential actually is, it could rather easily exceed the safety margin of the COPV's pressure rating.
This is the part where we begin to get into the "Bridge too Far" analogy. Space Ex and Elon Musk have their eye on much bigger things than the resupply of Space Station that Falcon 9 is specifically tasked with performing. Musk has made no secret that his eventual goal is the colonization of Mars and they are as I write this developing heavier launch bigger vehicles to reach this goal. It is this ultimate goal that is causing them to push the envelope so hard on Falcon 9. There has been a lot of publicity about reusable rockets and a very public "competition" between Space Ex and Blue Horizon to return rockets and land them successfully in order to keep costs low on launching payloads.
This requires extra fuel and oxidizers to be loaded onto Falcon 9 in order to have the requisite power to control its landing process manuevers. To their great credit both Space Ex and Blue Horizon have managed to be very successful in recovering their rockets with such manuevers. Since this is not what Falcon 9 was originally designed or contracted to do it is has been a little problematic for Space Ex to find ways to get the extra fuel and oxidizer on board to carry out these manuevers and still deliver its payload to Space Station.
One of the ways they have managed to do this is to steadily increase the supercooled effect of the LOX they use on Falcon 9. Supercooling of LOX is not a new idea either, as NASA has used this idea to increase turbopump efficiency for quite a few years. What it involves is decreasing the temperature of LOX away from its boiling point and towards it freezing point. In other words, the colder they can get their LOX before launch, the more energy it will contain. This effectively gives them the margin they need to carry out the extra manuevers necessary to safely land the rockets on their return descent to Earth.
One of the main factors for having super cooled LOX is to load it quickly and to make sure it does not sit in the LOX tank for long before launch. The longer it sits in the tank the more it tends to stratify which leads to less efficient burns and more LOX expended on ascent. Space Ex has been constantly speeding up its rapid load process for quite some time now. This means that massive amounts of LOX and RP1 are transferred to the vehicle just 30 minutes before launch. It also means that Space Ex must launch immediately after loading so that all other checkouts and final preps must occur after loading propellants.
The latest load of propellants as a dry run test was the most rapid load they have yet attempted. This effectively means that the LOX in the tank was colder than it has ever been on a propellant load. It also means that the Helium fill which must occur after LOX is loaded to take advantage of the increased density that putting the COPV's in the LOX tank in the first place was necessitated by. In their every increasing haste to cut the time that LOX sits in the vehicle I am sure they are also accelerating the rate at which they fill their COPV's which also increases the heat load by adiabatic heating and negative Joule-Thomsen effect.
In other words, for reasons completely outside of their actual manifest task, Space Ex has neccessarily vastly increased the temperature differential their COPV's see on propellant load. Is this relationship the key to the latest loss of a Falcon 9 on the pad at Cape Canaveral? I guess time will tell if this is actually a bridge too far or just another bump in the road for Musks' frenzied path to Mars.
Thursday, October 6, 2016
The Fickle Nature of COPVs
As I have already discussed, COPV's are a vast improvement in weight vs. functionality in that quite high pressure COPV's can be manufactured that weigh a small percentage of the heavy wall tank needed to safely contain high pressure gases. When you start adding in extreme temperatures such as LOX (-297 Degrees F) then the metal tanks must also be of high purity stainless steel as well, which gets even more expensive and complicated to design and build.
One of the first groups to work with designing and building COPV's worked at Marshall Space Flight Center in Huntsville. One designer in particular spent much of his career working on developing processes to accurately design and build COPV's rated for very high pressures at very low temperatures. While COPV's have been around for a while the idea of using them inside of cryogen storage tanks was an undertaking that required a lot of research and development. What kind of fibers to use, what kind of helical winding pattern to overlay the tank in layers, what kind of glue to use and how to cure it; all of these factors and many more were relatively unknown in the beginning simply because no one had experience in building these types of tanks.
As luck would have it this designer happened to get some research and development money just about the time that our small testing lab was getting started. When he approached our group about doing failure testing on his COPV's we were able to give him some pretty low bids on doing this testing so it was a marriage of convenience for us both. He needed an inexpensive way to do failure testing on his designs and we needed funds enough to take on the relatively small amount of work this would entail at a low rate.
The first tests we did on these bottles involved hydro testing them to failure. The process involved putting them in a hydro chamber which we would then fill with water to help contain the blast wave. Then, we would fill the test bottle with water and begin pumping it up to its failure pressure. Hydro testing is much preferred to pneumo testing because water is not compressible. Therefore, when the bottle ruptures the pressure is quickly abated with a minimal shock wave as the water escapes the ruptured bottle. When you do pnuemo testing you pressurize the bottle with a gas which is very compressible. As the gas pressure increases until the tank ruptures there is quite a violent release of the stored energy in the gas as it has to completely expand back to its original state.
It is the difference between filling a balloon with air and puncturing it with a pin and filling the same balloon with water and puncturing it with a pin. The balloon filled with air will violently fail as the compressed air escapes the tiny hole and rapidly expands. The balloon filled with water will leak slightly and slowly relieve the rest of the pressure through the hole the pin made. There is no stored energy in the water because it is not compressed.
We have a heavy duty hydro chamber with blast proof glass on top so that we could film the test as well. We used a digital data recording system to record the water pressure at a high rate of speed so that we could see the exact pressure that the bottle would rupture or fail at. Unlike the water balloon analogy, we were taking these bottles to 3000-5000 PSIG before they would rupture so it was a little more violent than pricking a water balloon with a pin but still much less violent than doing the same failure with an expanding gas.
The point of the testing was to prove that the COPV designer's processes were controlled well enough so that a series of bottles manufactured with the same process would withstand the same pressure before failing. It was the first step in figuring out safety factors for the COPV's that he was building. The first tests were immediate successes. Not only were the failures all above predicted pressure, but they were very consistent as well. We tested some 20 bottles to failure at 3500 PSIG and they all failed within 100 PSIG of each other which was actually much better consistency than anyone was predicting.
What wasn't really predicted was the way in which the COPV's failed. Since we were trying to get exact data we were pumping the bottles up fairly slowly. The first bottle we ruptured exhibited some strange behavior we were not expecting. As we approached the predicted failure rate and slowed our pumping rate even more we heard a muffled popping noise followed by an immediate drop in internal pressure in the bottle. We were puzzled by this to say the least. The pressure stabilized but it had dropped some 75 PSIG immediately when we heard the popping noise. As we began pumping again we would see the pressure rise again but it would immediately fall back whenever the pumping piston retracted. Having done a lot of hydro testing over the years we suspected we were seeing some sort of tiny leak to cause the pressure to drop. Since water is not compressible even a small drop of water is leaking is enough to cause a significant and measurable drop in pressure. What didn't make sense is why the pressure stabilized again after it dropped.
We surmised that we may have found the pressure at which a thread or fitting was leaking but since the tank was under water there was no way to locate where it might be leaking. With the designers permission was decided to go ahead and see if we could overcome this leak with enough pumping pressure to cause the tank to fail. It took quite a few strokes from our pumping system and we had several more muffled pops followed by further drops in pressure but we eventually did rupture the bottle pretty violently. Once the carbon fibers gave way and the tank ruptured it looked very much like an exploded bundle of carbon wires. The aluminum tank underneath ripped violently open and frayed carbon resembling an angry porcupine angled sharply out from the breach in the tank.
It made interesting viewing on video and the data system captured the exact peak of pressure that caused the tank to rupture. As we looked at the pressure data more carefully later we could see the rises in pressure followed by the gradual drops after the popping noises started. We soon realized that the popping noises we were hearing were the individual carbon fiber strands popping in the bottom layers of the wrap. Each time a strand broke the aluminum tank swelled a little more as it was freed from the captured restriction of the composite overwrap material. We were effectively blowing the aluminum tank up like a balloon as the carbon fibers failed, adding space for more non-compressible water with each breakage. Eventually, enough carbon fibers failed so that they could no longer contain the swelling aluminum tank and the whole tank violently ruptured.
Later on when we filmed with high speed video we could actually see the tank lurch with each pop of a carbon fiber strand but by then we were well familiar with the failure mechanism of the tanks. The underlying carbon fibers break first and since there are so many layers of fibers it is quite impossible to see any change in the tank but the pressure trace sees the extra volume afforded by the resultant expansion in the form of decreased pressure.
After we completed a series of tests of water testing the designer suggested he needed to know how cryogen temperatures would affect the strength of the tanks. In other words, since we knew he could consistently predict their failure in water could he also consistenly produce the same results at cryogen temperatures? The end result of such design and testing would be to have a COPV that could be imbedded in a LOX or Liquid Hydrogen tank. The weight saving would be huge and the expense to produce such COPV's would be much less than a similar metal tank.
Rupturing a COPV at 3500 PSIG in Liquid Nitrogen (-320 degrees F) turned out to be a little more problematic than anyone planned. The first need was to get the tank chilled to LN2 temperature which took quite a bit of LN2 and was a slow process involving creating a tube that the COPV would fit inside. The tube would then be slowly filled with LN2 to chill the outer part of the tank to temperature. After this was accomplished we would fill the inner part of the tank with LN2, being careful to remove all compressible gas at the same time. This was accomplished by a high point bleed that we opened until we got LN2 out as we filled the tank from the bottom.
It is important to remove all compressible gas to minimize the stored energy that will be released when the bottle ruptures. While LN2 is like water not compressible we knew it would go through a rapid phase change once the tank ruptured. At 68 degrees F LN2 expands 694-1 as it changes for a liquid to a gas. In other words one gallon of LN2 instantly increases to the volume of 694 gallons when this phase change occurs. This phase change is almost instantaneous so we knew that when the tank ruptured we could see a very violent and quick phase change shock wave.
To minimize the already considerable explosive power we were going to produce we were very careful to keep the LN2 in the bottle in a liquid state. We knew that any LN2 that flashed to gas would then be compressible, thereby increasing the explosive power we were going to release when the tank ruptured. We performed this test in the abandoned back area of the test area at Marshall. We utilized some very large steel I beams to build a barrier around the test setup knowing we could direct the shock wave upward in this manner. We also ran all our control and instrumentation wires into a blast bunker on the bottom of one of the test stands so that we would be removed from the vicinity when the rupture occurred.
We put temperature sensors on the pump feed line into the COPV and planned to keep our pumping speed low enough so that the natural heat of compression of a pumping piston would not flash the LN2 to a gas as we pushed it at increased pressures into the COPV. We set up video to capture the explosion itself but the main data we were after was the pressure at which the COPV would rupture at LN2 temperature. The designer suggested it might actually hold more pressure at cryogen temperature as the carbon wrap fibers themselves would tend to shrink and more tightly hold the inner aluminum tank in compression.
As soon as everything was set and we had cleared the surrounding area of all personnel we began our process. It took quite a while to slowly chill the COPV so that we could cover it with LN2. The real problem came after we filled the COPV with LN2 and began slowly pumping the pressure up with a cryogen pumping cart. The heat of compression would quickly overcome the boiling point of LN2 and we would begin to flash to gas on the inlet line of the COPV. Our test design review board had set a hard temperature number barrier on this line that we could not go above as it would increase the explosive power of the tank failure considerably.
After several hours of pumping we were nowhere near the pressure we thought it would take to fail the tank because we were having to stop so frequently to allow things to chill back to liquid temperature. Unfortunately, every time we stopped the return to liquid temperature would also decrease the pressure in the COPV as the density dropped. It was a losing battle and we soon knew we couldn't gain enough pressure to fail the tank.
After more study we decided to better insulate the fill lines and move the cryogen pump much closer to the test article. We moved the massive steel I beams with a crane to get everything closer and set up for another run at failing the COPV. We improved the process such that we could get closer to the pressure we were looking to fail the tank but still eventually hit a point where we could no longer gain pressure and keep everything at liquid temperatures. The next step would have been to include vacuum jacketed lines and a lot of expense that no one had funds for so after a quick phone session with the COPV designer and our test design review board everyone concluded we would let the temperature creep up as much as needed to achieve rupture pressure. The designer needed data for a conference he had coming up and since we had cleared the test area of personnel we simply bought the risk of destroying some of our test equipment when the tank ruptured.
We knew both the liquid tube the COPV was chilling in and all of our safety barriers would force the blast wave upward when the tank ruptured so we were fairly certain that we wouldn't do a lot of damage to anything besides the tube and some of the attached tubing and instrumentation lines. Once we got underway again we got back to pressure fairly quickly and then began speeding up the pumping process as we watched the temperature and the pressure in the COPV climb. We knew we were creating a compressible gas bubble in the COPV to add to the phase change explosion that was coming but everyone had agreed it was an unavoidable risk if we were to meet schedule and budget.
The tank, true to the designer's suggestion, actually ruptured at some 400 PSIG higher than the same design had failed at in water. We got a beautiful pressure trace showing the same popping and swelling scenario we had seen in water. We also got exactly two frames of video on our normal speed video showing a veritable rocket rising on a plume of cold gas out of the liquid soak tube. It took a little while to find the remains of the COPV and some of our tubing still attached to it. It was some 200 yards away in a swampy area next to the barrier fence seperating the test area from a wildlife refuge.
It was quite an impressive audible blast. The same size and design bottles that we had been more or less harmlessly popping underwater had produced a titanic blast with the phase change and added compressible gas that filled the bottom 1/4 of the tank when it ruptured. We don't really know how high it went as it went out of camera view in two frames.
We later did similar destructive testing at Liquid Helium temperatures to simulate the pressure rating of a COPV in Liquid Hydrogen. The design and control process that the designer used in making these tanks was very good as all failures were both predictable and consistent across many samples. This designer later left NASA and branched out to form his own company that now produces and sells these COPV's to space flight companies. The only company that I know of that utilizes these COPV's in cryogen tanks on vehicles is Space Ex.
Going back to the data that seemed counterintuitive on the Falcon 9 that exploded mid-flight; the confusing thing for Space Ex and many others that looked at this data was that the Helium pressure in these COPV's dropped at the same time that accelerometers picked up a popping "sound" in the area of the COPVs. It then immediately returned back to "normal" range right before the LOX tank overpressurized and exploded. As I have detailed above this is exactly the same sequence we saw when we were testing these tanks to failure in our lab. The popping noise was the inner carbon fiber strands breaking. The drop in pressure was the aluminum inner tank expanding like a balloon.
On Space Ex's Falcon 9 there are several COPV's tied together with a common tubing manifold. If one tank were to start to swell as the composite fibers break the pressure would quickly equalize between it and the other tanks creating a return to "normal" pressure reading on the whole system. However, the increasing pressure would quickly break more composite fibers until the damaged tank failed explosively. The severity of the resulting expansion would be maximized by the fact that the LOX tank itself was very full at this stage as the second stage was not in operation. This would mean there was a very small ullage or gas bubble for compression in the tank leaving only incompressible LOX which would quickly rupture the LOX tank.
Both Space Ex and the supplier of the COPV's will quickly explain that the COPV's have been thoroughly tested to much higher pressures than those they saw on flight. I will also attest to the fact that the designer of these COPV's has extensive data showing how good his design and manufacturing control processes on these bottles are. I have no doubt that these bottles will not fail under design pressure when handled properly.
Going back to my original discussion of COPV's I also explained that COPV's suffer from a couple of weaknesses as flight pressure tanks. The first is cycle limits, although this is probably not a problem in this usage. The second is the fragile nature of the exposed carbon fiber shells themselves. Each small carbon fiber is glued and interlocked in an intricate helical pattern to wrap and contain the thin aluminum tank underneath. Since the weight of the carbon fibers is minimal it is both cost effective and weight efficient to build in large overpressure ratings for COPV's. A tank that much contain 5500 PSIG can easily be manufactured to safely contain much higher pressures before failing. I have no doubt that the tanks flying on Falcon 9 are indeed rated and tested to much higher pressures at LOX temperatures than those they actually see in flight.
Musk has been especially adamant that the Carbon shells will not fail at pressure. No doubt he has seen a lot of data from the manufacturer proving this to be true. However, the fragile nature of the carbon fibers themselves require very careful handling from manufacture to test to installation to assure that they are not damaged before usage. Unfortunately, there is evidence that this has not been the case in the usage of these tanks.
Previous to flight Space Ex is in the habit of taking photographic evidence of all phases of their Falcon 9 assembly process. These pictures are known as "closeout" photos and stand as visual proof that all nuts are lockwired and all tiedown and cabling is carefully attached in the vehicle. I have not personally seen these pictures but some of the review teams I work with have. In at least one of these pictures the photographer himself is standing on the support struts that the COPV's are mounted to. No flight hardware is rated to be used as a standing platform. It is an egregiously bad practice for anyone to do so under any circumstance; either during fabrication and assembly or at any other time. The fact that someone is indeed seen standing on something as delicate as the COPVs which are inherently prone to serious and debilitating damage from relatively minor exterior mechanical force is inexcusable. The fact that people within Space Ex have seen and distributed these pictures seems to point to the fact that there is little or no understanding of the fragile nature of the COPVs.
While this is probably a little long winded it does plausibly match the actual data from the Falcon 9 that exploded in mid-flight. Could the same thing have happened to the Falcon 9 on the pad at Kennedy. Preliminary evidence suggests that it was a COPV failure in the exact same location that led to this explosion as well. There are also some necessary precautions that must be taken when initially pressurizing COPVs with gas that are not necessary with metallic tanks. I will go into that in my next post.
One of the first groups to work with designing and building COPV's worked at Marshall Space Flight Center in Huntsville. One designer in particular spent much of his career working on developing processes to accurately design and build COPV's rated for very high pressures at very low temperatures. While COPV's have been around for a while the idea of using them inside of cryogen storage tanks was an undertaking that required a lot of research and development. What kind of fibers to use, what kind of helical winding pattern to overlay the tank in layers, what kind of glue to use and how to cure it; all of these factors and many more were relatively unknown in the beginning simply because no one had experience in building these types of tanks.
As luck would have it this designer happened to get some research and development money just about the time that our small testing lab was getting started. When he approached our group about doing failure testing on his COPV's we were able to give him some pretty low bids on doing this testing so it was a marriage of convenience for us both. He needed an inexpensive way to do failure testing on his designs and we needed funds enough to take on the relatively small amount of work this would entail at a low rate.
The first tests we did on these bottles involved hydro testing them to failure. The process involved putting them in a hydro chamber which we would then fill with water to help contain the blast wave. Then, we would fill the test bottle with water and begin pumping it up to its failure pressure. Hydro testing is much preferred to pneumo testing because water is not compressible. Therefore, when the bottle ruptures the pressure is quickly abated with a minimal shock wave as the water escapes the ruptured bottle. When you do pnuemo testing you pressurize the bottle with a gas which is very compressible. As the gas pressure increases until the tank ruptures there is quite a violent release of the stored energy in the gas as it has to completely expand back to its original state.
It is the difference between filling a balloon with air and puncturing it with a pin and filling the same balloon with water and puncturing it with a pin. The balloon filled with air will violently fail as the compressed air escapes the tiny hole and rapidly expands. The balloon filled with water will leak slightly and slowly relieve the rest of the pressure through the hole the pin made. There is no stored energy in the water because it is not compressed.
We have a heavy duty hydro chamber with blast proof glass on top so that we could film the test as well. We used a digital data recording system to record the water pressure at a high rate of speed so that we could see the exact pressure that the bottle would rupture or fail at. Unlike the water balloon analogy, we were taking these bottles to 3000-5000 PSIG before they would rupture so it was a little more violent than pricking a water balloon with a pin but still much less violent than doing the same failure with an expanding gas.
The point of the testing was to prove that the COPV designer's processes were controlled well enough so that a series of bottles manufactured with the same process would withstand the same pressure before failing. It was the first step in figuring out safety factors for the COPV's that he was building. The first tests were immediate successes. Not only were the failures all above predicted pressure, but they were very consistent as well. We tested some 20 bottles to failure at 3500 PSIG and they all failed within 100 PSIG of each other which was actually much better consistency than anyone was predicting.
What wasn't really predicted was the way in which the COPV's failed. Since we were trying to get exact data we were pumping the bottles up fairly slowly. The first bottle we ruptured exhibited some strange behavior we were not expecting. As we approached the predicted failure rate and slowed our pumping rate even more we heard a muffled popping noise followed by an immediate drop in internal pressure in the bottle. We were puzzled by this to say the least. The pressure stabilized but it had dropped some 75 PSIG immediately when we heard the popping noise. As we began pumping again we would see the pressure rise again but it would immediately fall back whenever the pumping piston retracted. Having done a lot of hydro testing over the years we suspected we were seeing some sort of tiny leak to cause the pressure to drop. Since water is not compressible even a small drop of water is leaking is enough to cause a significant and measurable drop in pressure. What didn't make sense is why the pressure stabilized again after it dropped.
We surmised that we may have found the pressure at which a thread or fitting was leaking but since the tank was under water there was no way to locate where it might be leaking. With the designers permission was decided to go ahead and see if we could overcome this leak with enough pumping pressure to cause the tank to fail. It took quite a few strokes from our pumping system and we had several more muffled pops followed by further drops in pressure but we eventually did rupture the bottle pretty violently. Once the carbon fibers gave way and the tank ruptured it looked very much like an exploded bundle of carbon wires. The aluminum tank underneath ripped violently open and frayed carbon resembling an angry porcupine angled sharply out from the breach in the tank.
It made interesting viewing on video and the data system captured the exact peak of pressure that caused the tank to rupture. As we looked at the pressure data more carefully later we could see the rises in pressure followed by the gradual drops after the popping noises started. We soon realized that the popping noises we were hearing were the individual carbon fiber strands popping in the bottom layers of the wrap. Each time a strand broke the aluminum tank swelled a little more as it was freed from the captured restriction of the composite overwrap material. We were effectively blowing the aluminum tank up like a balloon as the carbon fibers failed, adding space for more non-compressible water with each breakage. Eventually, enough carbon fibers failed so that they could no longer contain the swelling aluminum tank and the whole tank violently ruptured.
Later on when we filmed with high speed video we could actually see the tank lurch with each pop of a carbon fiber strand but by then we were well familiar with the failure mechanism of the tanks. The underlying carbon fibers break first and since there are so many layers of fibers it is quite impossible to see any change in the tank but the pressure trace sees the extra volume afforded by the resultant expansion in the form of decreased pressure.
After we completed a series of tests of water testing the designer suggested he needed to know how cryogen temperatures would affect the strength of the tanks. In other words, since we knew he could consistently predict their failure in water could he also consistenly produce the same results at cryogen temperatures? The end result of such design and testing would be to have a COPV that could be imbedded in a LOX or Liquid Hydrogen tank. The weight saving would be huge and the expense to produce such COPV's would be much less than a similar metal tank.
Rupturing a COPV at 3500 PSIG in Liquid Nitrogen (-320 degrees F) turned out to be a little more problematic than anyone planned. The first need was to get the tank chilled to LN2 temperature which took quite a bit of LN2 and was a slow process involving creating a tube that the COPV would fit inside. The tube would then be slowly filled with LN2 to chill the outer part of the tank to temperature. After this was accomplished we would fill the inner part of the tank with LN2, being careful to remove all compressible gas at the same time. This was accomplished by a high point bleed that we opened until we got LN2 out as we filled the tank from the bottom.
It is important to remove all compressible gas to minimize the stored energy that will be released when the bottle ruptures. While LN2 is like water not compressible we knew it would go through a rapid phase change once the tank ruptured. At 68 degrees F LN2 expands 694-1 as it changes for a liquid to a gas. In other words one gallon of LN2 instantly increases to the volume of 694 gallons when this phase change occurs. This phase change is almost instantaneous so we knew that when the tank ruptured we could see a very violent and quick phase change shock wave.
To minimize the already considerable explosive power we were going to produce we were very careful to keep the LN2 in the bottle in a liquid state. We knew that any LN2 that flashed to gas would then be compressible, thereby increasing the explosive power we were going to release when the tank ruptured. We performed this test in the abandoned back area of the test area at Marshall. We utilized some very large steel I beams to build a barrier around the test setup knowing we could direct the shock wave upward in this manner. We also ran all our control and instrumentation wires into a blast bunker on the bottom of one of the test stands so that we would be removed from the vicinity when the rupture occurred.
We put temperature sensors on the pump feed line into the COPV and planned to keep our pumping speed low enough so that the natural heat of compression of a pumping piston would not flash the LN2 to a gas as we pushed it at increased pressures into the COPV. We set up video to capture the explosion itself but the main data we were after was the pressure at which the COPV would rupture at LN2 temperature. The designer suggested it might actually hold more pressure at cryogen temperature as the carbon wrap fibers themselves would tend to shrink and more tightly hold the inner aluminum tank in compression.
As soon as everything was set and we had cleared the surrounding area of all personnel we began our process. It took quite a while to slowly chill the COPV so that we could cover it with LN2. The real problem came after we filled the COPV with LN2 and began slowly pumping the pressure up with a cryogen pumping cart. The heat of compression would quickly overcome the boiling point of LN2 and we would begin to flash to gas on the inlet line of the COPV. Our test design review board had set a hard temperature number barrier on this line that we could not go above as it would increase the explosive power of the tank failure considerably.
After several hours of pumping we were nowhere near the pressure we thought it would take to fail the tank because we were having to stop so frequently to allow things to chill back to liquid temperature. Unfortunately, every time we stopped the return to liquid temperature would also decrease the pressure in the COPV as the density dropped. It was a losing battle and we soon knew we couldn't gain enough pressure to fail the tank.
After more study we decided to better insulate the fill lines and move the cryogen pump much closer to the test article. We moved the massive steel I beams with a crane to get everything closer and set up for another run at failing the COPV. We improved the process such that we could get closer to the pressure we were looking to fail the tank but still eventually hit a point where we could no longer gain pressure and keep everything at liquid temperatures. The next step would have been to include vacuum jacketed lines and a lot of expense that no one had funds for so after a quick phone session with the COPV designer and our test design review board everyone concluded we would let the temperature creep up as much as needed to achieve rupture pressure. The designer needed data for a conference he had coming up and since we had cleared the test area of personnel we simply bought the risk of destroying some of our test equipment when the tank ruptured.
We knew both the liquid tube the COPV was chilling in and all of our safety barriers would force the blast wave upward when the tank ruptured so we were fairly certain that we wouldn't do a lot of damage to anything besides the tube and some of the attached tubing and instrumentation lines. Once we got underway again we got back to pressure fairly quickly and then began speeding up the pumping process as we watched the temperature and the pressure in the COPV climb. We knew we were creating a compressible gas bubble in the COPV to add to the phase change explosion that was coming but everyone had agreed it was an unavoidable risk if we were to meet schedule and budget.
The tank, true to the designer's suggestion, actually ruptured at some 400 PSIG higher than the same design had failed at in water. We got a beautiful pressure trace showing the same popping and swelling scenario we had seen in water. We also got exactly two frames of video on our normal speed video showing a veritable rocket rising on a plume of cold gas out of the liquid soak tube. It took a little while to find the remains of the COPV and some of our tubing still attached to it. It was some 200 yards away in a swampy area next to the barrier fence seperating the test area from a wildlife refuge.
It was quite an impressive audible blast. The same size and design bottles that we had been more or less harmlessly popping underwater had produced a titanic blast with the phase change and added compressible gas that filled the bottom 1/4 of the tank when it ruptured. We don't really know how high it went as it went out of camera view in two frames.
We later did similar destructive testing at Liquid Helium temperatures to simulate the pressure rating of a COPV in Liquid Hydrogen. The design and control process that the designer used in making these tanks was very good as all failures were both predictable and consistent across many samples. This designer later left NASA and branched out to form his own company that now produces and sells these COPV's to space flight companies. The only company that I know of that utilizes these COPV's in cryogen tanks on vehicles is Space Ex.
Going back to the data that seemed counterintuitive on the Falcon 9 that exploded mid-flight; the confusing thing for Space Ex and many others that looked at this data was that the Helium pressure in these COPV's dropped at the same time that accelerometers picked up a popping "sound" in the area of the COPVs. It then immediately returned back to "normal" range right before the LOX tank overpressurized and exploded. As I have detailed above this is exactly the same sequence we saw when we were testing these tanks to failure in our lab. The popping noise was the inner carbon fiber strands breaking. The drop in pressure was the aluminum inner tank expanding like a balloon.
On Space Ex's Falcon 9 there are several COPV's tied together with a common tubing manifold. If one tank were to start to swell as the composite fibers break the pressure would quickly equalize between it and the other tanks creating a return to "normal" pressure reading on the whole system. However, the increasing pressure would quickly break more composite fibers until the damaged tank failed explosively. The severity of the resulting expansion would be maximized by the fact that the LOX tank itself was very full at this stage as the second stage was not in operation. This would mean there was a very small ullage or gas bubble for compression in the tank leaving only incompressible LOX which would quickly rupture the LOX tank.
Both Space Ex and the supplier of the COPV's will quickly explain that the COPV's have been thoroughly tested to much higher pressures than those they saw on flight. I will also attest to the fact that the designer of these COPV's has extensive data showing how good his design and manufacturing control processes on these bottles are. I have no doubt that these bottles will not fail under design pressure when handled properly.
Going back to my original discussion of COPV's I also explained that COPV's suffer from a couple of weaknesses as flight pressure tanks. The first is cycle limits, although this is probably not a problem in this usage. The second is the fragile nature of the exposed carbon fiber shells themselves. Each small carbon fiber is glued and interlocked in an intricate helical pattern to wrap and contain the thin aluminum tank underneath. Since the weight of the carbon fibers is minimal it is both cost effective and weight efficient to build in large overpressure ratings for COPV's. A tank that much contain 5500 PSIG can easily be manufactured to safely contain much higher pressures before failing. I have no doubt that the tanks flying on Falcon 9 are indeed rated and tested to much higher pressures at LOX temperatures than those they actually see in flight.
Musk has been especially adamant that the Carbon shells will not fail at pressure. No doubt he has seen a lot of data from the manufacturer proving this to be true. However, the fragile nature of the carbon fibers themselves require very careful handling from manufacture to test to installation to assure that they are not damaged before usage. Unfortunately, there is evidence that this has not been the case in the usage of these tanks.
Previous to flight Space Ex is in the habit of taking photographic evidence of all phases of their Falcon 9 assembly process. These pictures are known as "closeout" photos and stand as visual proof that all nuts are lockwired and all tiedown and cabling is carefully attached in the vehicle. I have not personally seen these pictures but some of the review teams I work with have. In at least one of these pictures the photographer himself is standing on the support struts that the COPV's are mounted to. No flight hardware is rated to be used as a standing platform. It is an egregiously bad practice for anyone to do so under any circumstance; either during fabrication and assembly or at any other time. The fact that someone is indeed seen standing on something as delicate as the COPVs which are inherently prone to serious and debilitating damage from relatively minor exterior mechanical force is inexcusable. The fact that people within Space Ex have seen and distributed these pictures seems to point to the fact that there is little or no understanding of the fragile nature of the COPVs.
While this is probably a little long winded it does plausibly match the actual data from the Falcon 9 that exploded in mid-flight. Could the same thing have happened to the Falcon 9 on the pad at Kennedy. Preliminary evidence suggests that it was a COPV failure in the exact same location that led to this explosion as well. There are also some necessary precautions that must be taken when initially pressurizing COPVs with gas that are not necessary with metallic tanks. I will go into that in my next post.
For Want of a Nail.....
There is an old parable: For want of a nail the shoe was lost;
For want of a shoe the horse was lost;
For want of a horse the battle was lost;
For the failure of battle the kingdom was lost—All for the want of a horse-shoe nail.
It seems to quite accurately describe the version of events the Space Ex has officially released to explain the loss of its Falcon 9 in mid flight. Space Ex believes that a bolt failed on a strut assembly that holds down the Ghe COPV's inside of the second stage LOX tank. The failure of one of these bolts set off a chain event that within one second resulted in the overpressurization and explosion of the second stage LOX tank and destroyed the Falcon 9.
After Space Ex was able to triangulate high speed accelerometer data and locate the original "sound" that corresponded in time with the loss of Ghe pressure in the COPV's inside the second stage LOX tank they quickly began conducting tensile strength tests on strut assemblies they had in stock. Although some of these strut assemblies failed at around 6000 pounds of force instead of the rated 10,000 pounds of the strut it still didn't explain how one could have failed at 2000 pounds of force which is the calculated load on the struts and maximum G force during ascent.
Eventually, Space Ex was able to find a bolt for one of these assemblies that failed at 2000 pounds of force. Therefore, they concluded that it was probably a bolt that holds the strut assembly to the tank that actually failed. During subsequent press conferences Elon Musk went into a lot of detail explaining how the strut assembly was not manufactured by Space Ex but was instead specified and bought based upon this manufacturers strength specifications. When pressed by reporters to name the manufacturer Musk declined and explained that it would not help the situation. Musk went on to explain that further metallurgical testing on the bolt itself showed improper grain forging which could have also led to a similar failure on the Falcon 9 that exploded. He seemed to have found the smoking gun for the failure sequence that led to the loss of his rocket.
When asked about the materials that the assembly was made of he also declined to go into details but put forward the information that it was a type of steel and was rated for a much higher failure pressure than it would have ever seen on flight. This is very curious, especially when in the same interview he went on to explain that they were considering going to an inconel material which is much harder to come by and astronomically more expensive to buy and manufacture. In the short term he suggested that they didn't actually test the strut assemblies previous to the loss of this vehicle but would certainly do so in the future. He was careful to explain that they had used the manufacturer's strength rating in lieu of actual testing but that at least one improperly forged bolt had been found in their inventory.
A "type of steel" is a red flag for anyone familiar with cryogenic atmospheres. Carbon steel materials lose all of their tensile strength when exposed to cryogenic temperatures. The immediate embrittlement that such temperatures cause in the granular structure of carbon steel is instantaneous and catostrophic. Just as an example, we experienced a catastrophic failure on a high pressure pipeline because of very short term exposure to LN2 (-320 degrees F). We had a heat exchanger controller failure that led to a small amount of LN2 trickling into the 3" 6000 PSIG pipeline previous to starting pumps to pressurize the system. The pipeline soon violently exploded at less than 200 PSIG in a location where it was in mechanical tension.
Even stainless steels with their lower proportion of carbon lose proportional amounts of tensile strength when exposed to cryogenic temperatures which is why careful control of metallurgy, forging process, and purity is required in all such systems. Different grades of stainless (depending largely on the amount of carbon to nickel composite) have different reactions to such temperatures. What has since become public knowledge is that Space Ex was buying off the shelf strut assemblies from a manufacturer that was not aware of the environment they were to be used in. Space Ex took some standard temperature de-rating tables based upon the assumed metallurgy of the struts they were buying and effectively load rated their strut system by analysis. There is nothing wrong with load rating by analysis as long as strict compliance of material pedigrees are observed but that does not appear to be what happened in this case. Since the manufacturer was not aware that these strut assemblies would be exposed to cryogen temperatures they do not track forging processes and metallurgy necessary to do such de-rating by analysis. This also explains why ultra weight conscious rocket ship designers used 10000 psi rated struts in a 2000 psi application.
As I have already mentioned in this thread, Space Ex has long struggled with configuration control of the hardware they are flying. It is a byproduct of being in a tremendous hurry to launch payloads. To be fair Space Ex is driven to this frenzy by the government that is punishing them monetarily for delays on manifests. Add to that the open competition they find themselves in to gain more launch manifests and you begin to see the extreme pressure they are under to launch vehicles. Under such pressure there is little wonder that they took the shortcut of buying off the shelf hardware and downrating it by analysis. It doesn't make it a good practice but it is understandable.
It doesn't make Musk's claim that a bolt "snuck" through the system accurate. It may have, but the real fault was in using assemblies in environments they were not made to be used in without at least explaining to the supplier that they needed to control the metallurgy and purity to make Space Ex's analysis hold up.
Unfortunately for Space Ex there is no proof that any of this is what caused the loss of the Falcon 9 mid-flight. While such a sequence of events seems to fit most of the data that Space Ex has, there are also parts of this story that do not fit the data. This is why neither NASA's independent investigation signed off on this theory, nor did any of the members of Space Ex's investigation that do not directly work for Space Ex. The broken strut assembly scenario is one of several fault tree sequences that could explain what happened. However, there is at least on major piece of data that most definitely is not explainable by this scenario.
The first actual warning that something was amiss on the Falcon launch was a minor drop in pressure on the Ghe pressure of the COPV's mounted in the second stage LOX tank. Musk's first public statements about the investigation brought out the fact that the data seemed counterintuitive. The data, which was taken at a relatively low rate of speed, showed a drop in pressure followed by a return to "normal" pressure. The transducer that monitored this pressure was mounted on a manifold assembly tied to several of the COPV's in the LOX tank. Through the years of doing this type of work I have been exposed to several instances where the data doesn't seem to match any logical sequence of events. It is frustrating to say the least. At some point it is not unusual in such a case to believe that you may have just found THE exception to the universal laws of physics. Reason always prevails but that thought can occur.
The drop in pressure that Space Ex saw in their data was very short, less than a second, before the overpressurization and rupture of the LOX tank occurred. The best scenario that Space Ex was able to come up with was that the initial "sound" that high speed accelerometer data was used to triangulate to the area of the COPV's seemed to occur at the same time as the drop in pressure. If this was indeed a strut or bolt breaking then the COPV would begin to rapidly rise in the LOX tank, ripping the connecting tubing that attached it to the other COPV's as it moved. Space Ex engineers theorized that the tubing could have kinked and shut off the leaking Ghe for a few milliseconds which would explain the return towards "normal" pressure on the Ghe manifold.
I think we have reached the point described above concerning an exception to the universal laws of physics with this explanation. Or... as a friend I worked with for many years used to explain in a crude way; bull$%^t. He would usually do this quite openly in a feigned sneeze at high volume; bull$%^t as he covered his mouth. In order to understand my incredulous disbelief let me explain a little further.
A water hose can indeed kink in such a manner that you can shut off the flow of water at low pressures. I have even seen brake lines of small diameter kink such that hydraulic fluid can be restricted enough to defeat the balance on a brake system. What I have never seen and will defy anyone to produce is a 5500 PSIG Ghe line that is kinked enough to shut off the leak of Helium. In the configuration that Space Ex uses of their COPV's inside LOX tanks there are several different tanks tied to one tubing manifold. If one tank becomes detached and begins to rapidly rise the line would have to kink in two directions at once for the pressure to return to "normal." It would have to shut off leakage from the rising tank and the tank that it was tubed to at the same time. If one such kink is impossible I don't know how to describe the statistical impossibility that two simultaneous kinks would represent.
There is more....
In the constant effort to save weight Space Ex used titanium tubing to make these manifolds. Titanium is both lighter and stronger than the stainless tubing usually used in such applications so they were able to use extremely thin walled tubing for this assembly. In other words, this tubing has great strength for retaining internal pressure but almost no shear strength to resist tearing apart. If a strut assembly were to break and the COPV to start a rapid ascent in the tank it would immediately tear the tubing apart instantaneously releasing a large volume of helium into a tank with a very small ullage. In other words, the LOX tank would rupture AND there would be no rise in Ghe pressure after the initial drop in pressure.
Add to all of this the fact that all preliminary data suggests that the same type of COPV in the same second stage LOX tank just experienced a "massive breach" that destroyed a second Falcon 9 as it was filled with propellants while sitting on the launch pad and the story gets even harder to believe. Preliminary data also suggests that there was a large drop in pressure followed by a similar rise in pressure in this same Ghe system right before this vehicle exploded.
Is there a scenario that matches this sequence of events? It turns out there is and it isn't counterintuitive at all once you understand the COPV failure mechanism. More on that tomorrow......
For want of a shoe the horse was lost;
For want of a horse the battle was lost;
For the failure of battle the kingdom was lost—All for the want of a horse-shoe nail.
It seems to quite accurately describe the version of events the Space Ex has officially released to explain the loss of its Falcon 9 in mid flight. Space Ex believes that a bolt failed on a strut assembly that holds down the Ghe COPV's inside of the second stage LOX tank. The failure of one of these bolts set off a chain event that within one second resulted in the overpressurization and explosion of the second stage LOX tank and destroyed the Falcon 9.
After Space Ex was able to triangulate high speed accelerometer data and locate the original "sound" that corresponded in time with the loss of Ghe pressure in the COPV's inside the second stage LOX tank they quickly began conducting tensile strength tests on strut assemblies they had in stock. Although some of these strut assemblies failed at around 6000 pounds of force instead of the rated 10,000 pounds of the strut it still didn't explain how one could have failed at 2000 pounds of force which is the calculated load on the struts and maximum G force during ascent.
Eventually, Space Ex was able to find a bolt for one of these assemblies that failed at 2000 pounds of force. Therefore, they concluded that it was probably a bolt that holds the strut assembly to the tank that actually failed. During subsequent press conferences Elon Musk went into a lot of detail explaining how the strut assembly was not manufactured by Space Ex but was instead specified and bought based upon this manufacturers strength specifications. When pressed by reporters to name the manufacturer Musk declined and explained that it would not help the situation. Musk went on to explain that further metallurgical testing on the bolt itself showed improper grain forging which could have also led to a similar failure on the Falcon 9 that exploded. He seemed to have found the smoking gun for the failure sequence that led to the loss of his rocket.
When asked about the materials that the assembly was made of he also declined to go into details but put forward the information that it was a type of steel and was rated for a much higher failure pressure than it would have ever seen on flight. This is very curious, especially when in the same interview he went on to explain that they were considering going to an inconel material which is much harder to come by and astronomically more expensive to buy and manufacture. In the short term he suggested that they didn't actually test the strut assemblies previous to the loss of this vehicle but would certainly do so in the future. He was careful to explain that they had used the manufacturer's strength rating in lieu of actual testing but that at least one improperly forged bolt had been found in their inventory.
A "type of steel" is a red flag for anyone familiar with cryogenic atmospheres. Carbon steel materials lose all of their tensile strength when exposed to cryogenic temperatures. The immediate embrittlement that such temperatures cause in the granular structure of carbon steel is instantaneous and catostrophic. Just as an example, we experienced a catastrophic failure on a high pressure pipeline because of very short term exposure to LN2 (-320 degrees F). We had a heat exchanger controller failure that led to a small amount of LN2 trickling into the 3" 6000 PSIG pipeline previous to starting pumps to pressurize the system. The pipeline soon violently exploded at less than 200 PSIG in a location where it was in mechanical tension.
Even stainless steels with their lower proportion of carbon lose proportional amounts of tensile strength when exposed to cryogenic temperatures which is why careful control of metallurgy, forging process, and purity is required in all such systems. Different grades of stainless (depending largely on the amount of carbon to nickel composite) have different reactions to such temperatures. What has since become public knowledge is that Space Ex was buying off the shelf strut assemblies from a manufacturer that was not aware of the environment they were to be used in. Space Ex took some standard temperature de-rating tables based upon the assumed metallurgy of the struts they were buying and effectively load rated their strut system by analysis. There is nothing wrong with load rating by analysis as long as strict compliance of material pedigrees are observed but that does not appear to be what happened in this case. Since the manufacturer was not aware that these strut assemblies would be exposed to cryogen temperatures they do not track forging processes and metallurgy necessary to do such de-rating by analysis. This also explains why ultra weight conscious rocket ship designers used 10000 psi rated struts in a 2000 psi application.
As I have already mentioned in this thread, Space Ex has long struggled with configuration control of the hardware they are flying. It is a byproduct of being in a tremendous hurry to launch payloads. To be fair Space Ex is driven to this frenzy by the government that is punishing them monetarily for delays on manifests. Add to that the open competition they find themselves in to gain more launch manifests and you begin to see the extreme pressure they are under to launch vehicles. Under such pressure there is little wonder that they took the shortcut of buying off the shelf hardware and downrating it by analysis. It doesn't make it a good practice but it is understandable.
It doesn't make Musk's claim that a bolt "snuck" through the system accurate. It may have, but the real fault was in using assemblies in environments they were not made to be used in without at least explaining to the supplier that they needed to control the metallurgy and purity to make Space Ex's analysis hold up.
Unfortunately for Space Ex there is no proof that any of this is what caused the loss of the Falcon 9 mid-flight. While such a sequence of events seems to fit most of the data that Space Ex has, there are also parts of this story that do not fit the data. This is why neither NASA's independent investigation signed off on this theory, nor did any of the members of Space Ex's investigation that do not directly work for Space Ex. The broken strut assembly scenario is one of several fault tree sequences that could explain what happened. However, there is at least on major piece of data that most definitely is not explainable by this scenario.
The first actual warning that something was amiss on the Falcon launch was a minor drop in pressure on the Ghe pressure of the COPV's mounted in the second stage LOX tank. Musk's first public statements about the investigation brought out the fact that the data seemed counterintuitive. The data, which was taken at a relatively low rate of speed, showed a drop in pressure followed by a return to "normal" pressure. The transducer that monitored this pressure was mounted on a manifold assembly tied to several of the COPV's in the LOX tank. Through the years of doing this type of work I have been exposed to several instances where the data doesn't seem to match any logical sequence of events. It is frustrating to say the least. At some point it is not unusual in such a case to believe that you may have just found THE exception to the universal laws of physics. Reason always prevails but that thought can occur.
The drop in pressure that Space Ex saw in their data was very short, less than a second, before the overpressurization and rupture of the LOX tank occurred. The best scenario that Space Ex was able to come up with was that the initial "sound" that high speed accelerometer data was used to triangulate to the area of the COPV's seemed to occur at the same time as the drop in pressure. If this was indeed a strut or bolt breaking then the COPV would begin to rapidly rise in the LOX tank, ripping the connecting tubing that attached it to the other COPV's as it moved. Space Ex engineers theorized that the tubing could have kinked and shut off the leaking Ghe for a few milliseconds which would explain the return towards "normal" pressure on the Ghe manifold.
I think we have reached the point described above concerning an exception to the universal laws of physics with this explanation. Or... as a friend I worked with for many years used to explain in a crude way; bull$%^t. He would usually do this quite openly in a feigned sneeze at high volume; bull$%^t as he covered his mouth. In order to understand my incredulous disbelief let me explain a little further.
A water hose can indeed kink in such a manner that you can shut off the flow of water at low pressures. I have even seen brake lines of small diameter kink such that hydraulic fluid can be restricted enough to defeat the balance on a brake system. What I have never seen and will defy anyone to produce is a 5500 PSIG Ghe line that is kinked enough to shut off the leak of Helium. In the configuration that Space Ex uses of their COPV's inside LOX tanks there are several different tanks tied to one tubing manifold. If one tank becomes detached and begins to rapidly rise the line would have to kink in two directions at once for the pressure to return to "normal." It would have to shut off leakage from the rising tank and the tank that it was tubed to at the same time. If one such kink is impossible I don't know how to describe the statistical impossibility that two simultaneous kinks would represent.
There is more....
In the constant effort to save weight Space Ex used titanium tubing to make these manifolds. Titanium is both lighter and stronger than the stainless tubing usually used in such applications so they were able to use extremely thin walled tubing for this assembly. In other words, this tubing has great strength for retaining internal pressure but almost no shear strength to resist tearing apart. If a strut assembly were to break and the COPV to start a rapid ascent in the tank it would immediately tear the tubing apart instantaneously releasing a large volume of helium into a tank with a very small ullage. In other words, the LOX tank would rupture AND there would be no rise in Ghe pressure after the initial drop in pressure.
Add to all of this the fact that all preliminary data suggests that the same type of COPV in the same second stage LOX tank just experienced a "massive breach" that destroyed a second Falcon 9 as it was filled with propellants while sitting on the launch pad and the story gets even harder to believe. Preliminary data also suggests that there was a large drop in pressure followed by a similar rise in pressure in this same Ghe system right before this vehicle exploded.
Is there a scenario that matches this sequence of events? It turns out there is and it isn't counterintuitive at all once you understand the COPV failure mechanism. More on that tomorrow......
Confusing Data and Assumptions about Metallurgy
On September 1, 2016 a Space Ex Falcon 9 exploded on the pad at Kennedy Space Flight Center. The Falcon was in the process of loading propellants when this explosion occurred. Preliminary reports suggest that a large breach occurred in a helium tank in the upper stage which after some .9 seconds caused the rupture of the LOX tank it was contained within. The resultant fire destroyed much of the Falcon 9 and severely damaged the pad itself.
There is some careful wordsmithing going on at the moment to suggest that this accident has no relationship to the earlier Falcon 9 that exploded in flight but the truth of the matter is that the ultimate destruction of both vehicles was caused by the failure of a helium tank inside the upper stage LOX tank. Space Ex, who had earlier determined that the first loss was caused by the failure of a mounting strut that holds the tank in place during flight, immediately suggested that there is no correlation between the two failures as this strut is not under dynamic load during propellant loading. While this is true, it skips over the fact that only Space Ex believes that it fully understands what happened to the first flight. An independent NASA investigation into the same incident suggests that while the strut issue was a problem, there are several other possibilities that could have caused the same incident. In other words, while everyone agrees that a rapid overpressurization of the LOX tank caused the incident, everyone does NOT agree about what caused the failure of the helium tank that led to this overpressurization.
The first Falcon 9 that exploded in mid-flight was most definitely experiencing dynamic loads that are not present during propellant loading on the pad. Therefore, it is extremely unlikely that a strut failure occurred to cause this explosion. However, it is worth backing up a little bit here to explain how Space Ex decided that the strut failure was actually what caused the first accident.
Early data from the first incident presented some seemingly conflicting and contradictory data. Telemetry system data suggested that there was a very brief drop in Helium pressure immediately previous to the explosion. This would make sense if a tank experienced a sudden leak or breach of some kind but there was also data that suggested that the pressure immediately returned to normal before the actual explosion. Engineers from Space Ex and NASA were confused by this information to say the least.
Early on, Space Ex was concerned about the bouyancy effect of the Helium tanks within the LOX tank. Bouyancy in LOX is little different than bouyancy in water and most everyone understands that holding a balloon underwater is problematic. The same thing occurs in a COPV pressurized with helium in a LOX tank. As the G forces increase during launch the bouyancy increases. In other words, the upward pressure on the struts that hold the helium tanks in place increase as the rocket ascends rapidly. Due to the timing of the incident on the Falcon 9 it seemed that this problem occurred simultaneous with a very high G loading on the flight.
The second clue that led investigators to look at these struts was some acoustic data from microphones/accelerometers on the vehicle. The data from these intruments is taken at a high rate of speed that is inherently necessary to gather vibration data for analysis. As I have discussed in here before (see Update Rates) digital data systems take snapshots of pressure, vibration, and temperatures. These snapshots are taken at varying rates dependent on the type of data you are trying to collect. These snapshots are then arranged on a plot and a line is drawn between the points on the chart to create a graph displaying this information vs. time.
One of the problems with digital data is that is can easily be used to draw graphs that do not resemble actual events. For instance, if you take digital data on a repeating sine wave that operates once a second you can accurately represent this sine wave if you take at least ten snapshots per second. However, if you take only two snapshots in this time period you will wind up with a graph that doesn't even resemble a sine wave. It is accurate data at that point but it completely misrepresents a sine wave. In other words, the snapshots are accurate but the resultant graph is bogus.
There are standard formulas for deciding data rates for all manner of instrumentation which I won't go into here, but it is also dependent on both the instrument you are using to gather the data and the speed of the event you are trying to capture. In the case of the Falcon 9 that exploded in flight the data gathered from the Helium pressurization system was probably rather slow in terms of trying to capture the event that actually happened. I don't know this for a certainty as I have not personally seen the data but a pressure trandsducer that is being used to monitor tank pressure there is typically no need to monitor it at a high rate of speed as the pressure is not expected to change extremely rapidly. If you knew you might want to use it to decide exactly how and when something explodes you would run it at an extremely high rate of speed but that is not what this system was designed and built to do.
Ideally, one would set all such systems up for such an eventuality but running at extremely high rates of speed on numerous channels costs money and if you aren't convinced you will ever need this high speed data you simply don't design systems to accomodate it. When we are purposefully taking COPV's to failure we would typically run our data collection pressure channels at 50,000 Hz. In other words, we would take a snapshot 50,000 times a second so that we could see exactly what the pressure was when the tank ruptured. Gathering 50,000 Hz data is not that difficult with today's systems but storing it and being able to analyze it later can be problematic and expensive.
I suspect from the information I have seen released that the pressure system Space Ex was using on the helium tanks was on the order of 10 hz. Again... I don't know this for certain but it would make sense economically and technically as they were not expecting to see rapid pressure changes in this system to begin with. It would also explain the delay between the drop in pressure and the overpressurization or loss of the vehicle. Even if they were running at 100 hz it is still not fast enough to have a lot of data as to what had actually happened in the helium system.
Space Ex was also running accelerometer/microphone data systems at several locations on the vehicle to monitor vibration during the flight. These types of instruments operate at much higher frequencies inherently because they are looking for vibration signals in the thousands of cycles per second. Using this high frequency information they look for vibrational problems that might create positive feedback loops of resonant frequencies that could damage or destroy the vehicle. This is another problem with space flight that is always a concern. You don't want to set up positive feedback loops that destroy your vehicle. Instrumentation looking for these loops is monitored and systems are throttled specifically to avoid these issues.
Using this high speed data, Space Ex determined that there were two significant incidents that occurred at different times. The first was a significant "sound" or detectable vibration and .9 seconds later the vehicle exploded. By triangulating the various signal locations Space Ex determined that the first "sound" came from the area where the helium tanks were located. This led back to their original concern about the bouyancy of the helium tanks in LOX and how it was affected by the G loading during ascent. If the original "sound" was a strut breaking, the rapid rise of the helium tank in the LOX tank could have followed. The resultant collision with a wall of the LOX tank or the top of the tank would have precipitated the instantaneous failure of the COPV, releasing 5500 PSIG of Helium into the LOX tank and overpressurizing it immediately.
Space Ex began testing mounting struts that they had in stock to see if some of them might fail at lower torque ratings than their specification. What they found was that several of them did fail at much lower ratings than their specification. Meallurgy studies found consistency problems at the granular level in these struts. Steel struts, just like steel bolts are rated for shear strength and manufactured accordingly. The second part of this problem has to do with the fact that they were being used to mount helium tanks inside a LOX tank. If a steel strut has minor inconsistencies in the granular structure but it is highly overrated for shear pressure this is not a problem. Space Ex soon came out and said that some of these struts failed at 5 times lower pressures than they were rated for. They also refused to release the name of the manufacturer of these struts but stated that from this point forward they would individually test each strut before use.
All of this sounds reasonable except for there is no mention of the more important fact that typically carbon steel struts are NEVER used in cryogenic applications. Exposing stainless steel to -297 degrees temperature changes its shear rating dramatically. Minor granular inconsistencies become major catostrophic failures under these conditions, which is why you do not use any type of steel struts in such conditions without comprehensive metallurgical pedigrees. Space Ex has been in a constant running battle with its NASA oversight groups from the beginning of its existance because of its unwillingness to do due diligence on configuration control issues such as this one. When this Falcon 9 exploded in mid-flight there were numerous parts flying on it that Space Ex could not identify as to origin or pedigree. This was not limited to struts, nuts, and bolts but went as far as valves, regulators, and all manner of complex components. In other words, they completely lost configuration control in their haste to launch vehicles on many of the systems on their vehicles. This is a much worse problem than a few struts that failed during testing. It is a problem that will have ever more serious implications in the future if it is not straightened out.
Space Ex used the data it had to locate an issue. The struts they were using to mount these tanks had basic flaws in some of them that could have caused this accident and they absolutely needed to fix this issue before continuing to launch vehicles. Unfortunately, there is no proof that this issue is what caused the loss of the Falcon 9 during flight. It makes a plausible story and it was definitely an issue that needed to be rectified. However, neither any of the investigators on Space Ex's team that did not work for Space Ex nor the independent NASA investigation team were convinced that this was THE cause of the loss of the Falcon 9 on June 29, 2015.
There is some careful wordsmithing going on at the moment to suggest that this accident has no relationship to the earlier Falcon 9 that exploded in flight but the truth of the matter is that the ultimate destruction of both vehicles was caused by the failure of a helium tank inside the upper stage LOX tank. Space Ex, who had earlier determined that the first loss was caused by the failure of a mounting strut that holds the tank in place during flight, immediately suggested that there is no correlation between the two failures as this strut is not under dynamic load during propellant loading. While this is true, it skips over the fact that only Space Ex believes that it fully understands what happened to the first flight. An independent NASA investigation into the same incident suggests that while the strut issue was a problem, there are several other possibilities that could have caused the same incident. In other words, while everyone agrees that a rapid overpressurization of the LOX tank caused the incident, everyone does NOT agree about what caused the failure of the helium tank that led to this overpressurization.
The first Falcon 9 that exploded in mid-flight was most definitely experiencing dynamic loads that are not present during propellant loading on the pad. Therefore, it is extremely unlikely that a strut failure occurred to cause this explosion. However, it is worth backing up a little bit here to explain how Space Ex decided that the strut failure was actually what caused the first accident.
Early data from the first incident presented some seemingly conflicting and contradictory data. Telemetry system data suggested that there was a very brief drop in Helium pressure immediately previous to the explosion. This would make sense if a tank experienced a sudden leak or breach of some kind but there was also data that suggested that the pressure immediately returned to normal before the actual explosion. Engineers from Space Ex and NASA were confused by this information to say the least.
Early on, Space Ex was concerned about the bouyancy effect of the Helium tanks within the LOX tank. Bouyancy in LOX is little different than bouyancy in water and most everyone understands that holding a balloon underwater is problematic. The same thing occurs in a COPV pressurized with helium in a LOX tank. As the G forces increase during launch the bouyancy increases. In other words, the upward pressure on the struts that hold the helium tanks in place increase as the rocket ascends rapidly. Due to the timing of the incident on the Falcon 9 it seemed that this problem occurred simultaneous with a very high G loading on the flight.
The second clue that led investigators to look at these struts was some acoustic data from microphones/accelerometers on the vehicle. The data from these intruments is taken at a high rate of speed that is inherently necessary to gather vibration data for analysis. As I have discussed in here before (see Update Rates) digital data systems take snapshots of pressure, vibration, and temperatures. These snapshots are taken at varying rates dependent on the type of data you are trying to collect. These snapshots are then arranged on a plot and a line is drawn between the points on the chart to create a graph displaying this information vs. time.
One of the problems with digital data is that is can easily be used to draw graphs that do not resemble actual events. For instance, if you take digital data on a repeating sine wave that operates once a second you can accurately represent this sine wave if you take at least ten snapshots per second. However, if you take only two snapshots in this time period you will wind up with a graph that doesn't even resemble a sine wave. It is accurate data at that point but it completely misrepresents a sine wave. In other words, the snapshots are accurate but the resultant graph is bogus.
There are standard formulas for deciding data rates for all manner of instrumentation which I won't go into here, but it is also dependent on both the instrument you are using to gather the data and the speed of the event you are trying to capture. In the case of the Falcon 9 that exploded in flight the data gathered from the Helium pressurization system was probably rather slow in terms of trying to capture the event that actually happened. I don't know this for a certainty as I have not personally seen the data but a pressure trandsducer that is being used to monitor tank pressure there is typically no need to monitor it at a high rate of speed as the pressure is not expected to change extremely rapidly. If you knew you might want to use it to decide exactly how and when something explodes you would run it at an extremely high rate of speed but that is not what this system was designed and built to do.
Ideally, one would set all such systems up for such an eventuality but running at extremely high rates of speed on numerous channels costs money and if you aren't convinced you will ever need this high speed data you simply don't design systems to accomodate it. When we are purposefully taking COPV's to failure we would typically run our data collection pressure channels at 50,000 Hz. In other words, we would take a snapshot 50,000 times a second so that we could see exactly what the pressure was when the tank ruptured. Gathering 50,000 Hz data is not that difficult with today's systems but storing it and being able to analyze it later can be problematic and expensive.
I suspect from the information I have seen released that the pressure system Space Ex was using on the helium tanks was on the order of 10 hz. Again... I don't know this for certain but it would make sense economically and technically as they were not expecting to see rapid pressure changes in this system to begin with. It would also explain the delay between the drop in pressure and the overpressurization or loss of the vehicle. Even if they were running at 100 hz it is still not fast enough to have a lot of data as to what had actually happened in the helium system.
Space Ex was also running accelerometer/microphone data systems at several locations on the vehicle to monitor vibration during the flight. These types of instruments operate at much higher frequencies inherently because they are looking for vibration signals in the thousands of cycles per second. Using this high frequency information they look for vibrational problems that might create positive feedback loops of resonant frequencies that could damage or destroy the vehicle. This is another problem with space flight that is always a concern. You don't want to set up positive feedback loops that destroy your vehicle. Instrumentation looking for these loops is monitored and systems are throttled specifically to avoid these issues.
Using this high speed data, Space Ex determined that there were two significant incidents that occurred at different times. The first was a significant "sound" or detectable vibration and .9 seconds later the vehicle exploded. By triangulating the various signal locations Space Ex determined that the first "sound" came from the area where the helium tanks were located. This led back to their original concern about the bouyancy of the helium tanks in LOX and how it was affected by the G loading during ascent. If the original "sound" was a strut breaking, the rapid rise of the helium tank in the LOX tank could have followed. The resultant collision with a wall of the LOX tank or the top of the tank would have precipitated the instantaneous failure of the COPV, releasing 5500 PSIG of Helium into the LOX tank and overpressurizing it immediately.
Space Ex began testing mounting struts that they had in stock to see if some of them might fail at lower torque ratings than their specification. What they found was that several of them did fail at much lower ratings than their specification. Meallurgy studies found consistency problems at the granular level in these struts. Steel struts, just like steel bolts are rated for shear strength and manufactured accordingly. The second part of this problem has to do with the fact that they were being used to mount helium tanks inside a LOX tank. If a steel strut has minor inconsistencies in the granular structure but it is highly overrated for shear pressure this is not a problem. Space Ex soon came out and said that some of these struts failed at 5 times lower pressures than they were rated for. They also refused to release the name of the manufacturer of these struts but stated that from this point forward they would individually test each strut before use.
All of this sounds reasonable except for there is no mention of the more important fact that typically carbon steel struts are NEVER used in cryogenic applications. Exposing stainless steel to -297 degrees temperature changes its shear rating dramatically. Minor granular inconsistencies become major catostrophic failures under these conditions, which is why you do not use any type of steel struts in such conditions without comprehensive metallurgical pedigrees. Space Ex has been in a constant running battle with its NASA oversight groups from the beginning of its existance because of its unwillingness to do due diligence on configuration control issues such as this one. When this Falcon 9 exploded in mid-flight there were numerous parts flying on it that Space Ex could not identify as to origin or pedigree. This was not limited to struts, nuts, and bolts but went as far as valves, regulators, and all manner of complex components. In other words, they completely lost configuration control in their haste to launch vehicles on many of the systems on their vehicles. This is a much worse problem than a few struts that failed during testing. It is a problem that will have ever more serious implications in the future if it is not straightened out.
Space Ex used the data it had to locate an issue. The struts they were using to mount these tanks had basic flaws in some of them that could have caused this accident and they absolutely needed to fix this issue before continuing to launch vehicles. Unfortunately, there is no proof that this issue is what caused the loss of the Falcon 9 during flight. It makes a plausible story and it was definitely an issue that needed to be rectified. However, neither any of the investigators on Space Ex's team that did not work for Space Ex nor the independent NASA investigation team were convinced that this was THE cause of the loss of the Falcon 9 on June 29, 2015.
Catastrophic Failure
On June 29, 2015 a Space Ex Falcon 9 lifted off from Cape Canaveral on its way for a resupply of Space Station. 2 minutes and 19 seconds into the flight it exploded spectacularly. Space Ex immediately launched an internal investigation as to the causes of the explosion. Shortly afterwards, Space Ex officials began claiming that had astronauts been on board they would have been safely jettisoned in an escape vehicle. At this same time Space Ex began showing videos of safely executed tests conducted on this jettison system at their facilities. While escape vehicles have been touted by NASA and numerous other space agencies as necessary components of a launch vehicle since the original Challenger explosion many years ago, it is by no means appropriate to think that any such vehicle can protect astronauts during an event such as happened to this Space Ex vehicle.
Escape launch vehicles require advance warning of system problems of a long enough time period so that the escape vehicle itself can be outside the deadly shock wave produced by a large explosion. In the case of this Falcon 9 explosion they had less than a second of data to warn them that there was a problem. This is not a long enough time period to activate the jettison of an escape pod and escape the shock wave. It also assumes that an anomaly such as was seen on Falcon 9, the temporary loss of Helium pressure would have initiated such a sequence. I can assure you that it would not initiate such a sequence. Less than one second of data suggesting that there is an unknown and unknowable blip in pressure would NOT be a cause to abort a mission.
Getting back to the explosion itself, Space Ex eventually concluded that the explosion was caused by a rapid overpressurization of the second stage LOX tank. The first stage of Falcon rockets use an array of Merlin engines. The second stage uses one vacuum tuned engine of the same type with a different injector and nozzle type. Once a vehicle escapes Earth's atmosphere and gravity the power requirements for propulsion drop dramatically. One engine is sufficient to propel the vehicle at this stage but it has to be an engine that is more efficient operating in the perfect vacuum of space. NASA utilizes Hydrogen/LOX engines in this atmosphere because of the increased specific impulse energy over a RP1/LOX engine. For simplicity and the elimination of different fuel storage and handling techniques Space Ex early on opted for RP1/LOX engines for both stages of their vehicles. It is a trade off between efficiency and weight, which is at the root of all such decisions concerning space vehicles.
In order to support combustion of a powerful rocket engine you must have a very fast and efficient pumping system to provide it with the massive amounts of fuel required to produce the high thrust requirements of such a vehicle. Turbopumps have long been a source of problems in this chain. Turbopumps on LOX systems are specifically very problematic because of a couple of inherent features. A turbopump runs at extremely high rates of speed with very close tolerances. As I mentioned before, LOX systems preclude the use of lubricants because of the incompatability of LOX to hydrocarbons. Therefore, you wind up with tight tolerances at extremely cold temperatures that have to self lubricate with the LOX itself. Every rocket manufacturer has struggled with this architecture. As I will explain later, it was this same type of system that was at the heart of the Antarres explosion earlier in this same year.
One of the ways of alleviating this problem is to run the initial tank pressures at high pressures to begin with. This becomes somewhat problematic in smaller tanks as it requires a finely tuned pressurization system to balance the ullage (gas pocket) that sits on top of the liquid with the rapidly depleting liquid in the tank when the engine is firing. Falcon 9 uses gaseous helium to provide the pressurization in their LOX tank on the second stage. Compressed helium is stored in several small pressure bottles inside the LOX tank itself at 5500 PSIG.
In order to give a relative idea of what this type of pressure is capable of think of the movie Jaws where in the last scene the shark is blown up by shooting a rifle into a scuba tank inside his mouth. Scuba tanks are pressurized with breathing air to a little less than 3000 PSIG. As you might imagine such high pressures require a very thick and heavy tank wall so that the vessel itself won't rupture inadvertently from this internal force. As I have mentioned already, the crucial tradeoff in space flight is always weight vs. safety. Instead of 4-1 safety factors such as a scuba tank on earth requires, space vehicles use 1.4 - 1.5 to one as a basic safety requirement. Instead of a tank that operates at 5500 PSIG being required to be designed to withstand 22000 PSIG it is designed to withstand 7700 PSIG.
Space Ex is also doing several other things with this system to increase efficiency. The first thing they are doing is super cooling the helium tank and fill gas by storing it inside the LOX tank itself. This is not a new discovery as NASA has been doing this for quite some time on many different vehicles. By putting the pressurization tank inside the LOX itself it is possible to have a smaller pressurization tank. A tank at 5500 PSIG Helium in ambient temperature will contain much less actual usable gas than a tank containing 5500 PSIG at -279 Degrees F. The Helium molecules become denser and can be more efficiently compressed so that more gas is available to pressurize the tank.
Falcon 9's system utilizes the engine itself to heat the helium and rapidly expand it before returning it to the LOX tank ullage to pressurize the LOX system. The helium is released through chambers in the engine as it burns which rapidly expands the helium before it is returned to the LOX tank to be used as a pressurization source to force the LOX out of the tank and into the engine. As you can imagine, the control system needed to tightly control the ullage at a constant pressure utilizing this chain of events is quite complicated.
Naturally, Falcon 9's LOX system is as full as possible prior to flight. A large initial ullage would require a large tank and more weight so at launch the ullage in the LOX tank is very small. This effectively means that relatively small loss of control of this pressurization system will result in large pressure fluctuations in the LOX tank during the initial use of this system. Rapid pressure rise from uncontrolled release of the Helium in these tanks has resulted in the loss of both Falcon rockets that have exploded so far. We know it did with the first Falcon explosion and all evidence points in the direction that it did on this last one as well.
Before we go any further on this concept let me get back to the other thing that Space Ex is doing with their helium pressurization system. Remember the analogy of the scuba tank for a moment. Not only is Space Ex using a 1.4-1 safety factor on the design of the tanks to be used in but they are also using some relatively new technology to build the tanks themselves. Composite Overwrap Pressure Vessels (COPV's) have been under development for some time now. We have built and tested to failure many such vessels where I work so I am not exactly unfamiliar with the concept.
A COPV can be manufactured in several different ways. Most of them utilize a lightweight inner aluminum tank that is rated at a very low pressure. It is thin walled aluminum and extremely lightweight in comparison with an equal stainless steel tank of the same size. This thin walled aluminum tank is then wrapped with carbon fiber, usually by an automated computer controlled lathe in numerous layers of thin carbon wires that are glued in place as the wrap process takes place. The resultant tank is then tested for pressure rating. Design processes and computer programs to produce these tanks are complicated to say the least.
The end result is a pressure vessel that is vastly lighter, which makes it ideal for usage on space flight vehicles. We have tested design and process on these tanks for many years in our shop. Properly done and controlled such processes produce reliable and predictable results. We have tested series of tanks to failure at Liquid Nitrogen Temperatures (-320 degrees F) and even up to Liquid Helium Temperatures (-452 Degrees F). NASA has entertained the idea of using these tanks just as Space Ex currently does in LOX tanks. They have even considered doing the same thing in Liquid Hydrogen tanks which is why we tested at even colder temperatures.
The key to the successful production of such tanks is process control. In other words, careful control of materials and manufacturing process produces consistent results in the actual pressure these tanks will withstand. Any loss of control of either of these variables produces disastrous results which is why NASA is still reluctant to commit to using COPV's inside LOX and LH2 tanks.
There are at least two other dangers associated with COPV's and their usage. Each pressure cycle effectively stretches the composite overwrap strands to a certain extent. Therefore, COPV's are listed for usage by the number of pressure cycles they see as a matter of course. Each pressure cycle is noted and logged and after a sufficient number of cycles the tank is no longer rated for use as a pressure vessel. By itself, this is not a problem for the usage of COPV's on space vehicles as they will see a relatively small number of usages anyway.
The last part of this issue is that COPV's are extremely fragile as far as handling and usage in comparison with metal tanks. Since the thin composite strands are effectively a very large chain reaction system, any external damage to a small strand effectively makes it the weakest link in a chain and we all know the adage about the weakest link in any chain being the source of its strength. Inadvertent bumping into sharp objects or dropping of objects on COPV's can cause catastrophic failures accordingly.
Escape launch vehicles require advance warning of system problems of a long enough time period so that the escape vehicle itself can be outside the deadly shock wave produced by a large explosion. In the case of this Falcon 9 explosion they had less than a second of data to warn them that there was a problem. This is not a long enough time period to activate the jettison of an escape pod and escape the shock wave. It also assumes that an anomaly such as was seen on Falcon 9, the temporary loss of Helium pressure would have initiated such a sequence. I can assure you that it would not initiate such a sequence. Less than one second of data suggesting that there is an unknown and unknowable blip in pressure would NOT be a cause to abort a mission.
Getting back to the explosion itself, Space Ex eventually concluded that the explosion was caused by a rapid overpressurization of the second stage LOX tank. The first stage of Falcon rockets use an array of Merlin engines. The second stage uses one vacuum tuned engine of the same type with a different injector and nozzle type. Once a vehicle escapes Earth's atmosphere and gravity the power requirements for propulsion drop dramatically. One engine is sufficient to propel the vehicle at this stage but it has to be an engine that is more efficient operating in the perfect vacuum of space. NASA utilizes Hydrogen/LOX engines in this atmosphere because of the increased specific impulse energy over a RP1/LOX engine. For simplicity and the elimination of different fuel storage and handling techniques Space Ex early on opted for RP1/LOX engines for both stages of their vehicles. It is a trade off between efficiency and weight, which is at the root of all such decisions concerning space vehicles.
In order to support combustion of a powerful rocket engine you must have a very fast and efficient pumping system to provide it with the massive amounts of fuel required to produce the high thrust requirements of such a vehicle. Turbopumps have long been a source of problems in this chain. Turbopumps on LOX systems are specifically very problematic because of a couple of inherent features. A turbopump runs at extremely high rates of speed with very close tolerances. As I mentioned before, LOX systems preclude the use of lubricants because of the incompatability of LOX to hydrocarbons. Therefore, you wind up with tight tolerances at extremely cold temperatures that have to self lubricate with the LOX itself. Every rocket manufacturer has struggled with this architecture. As I will explain later, it was this same type of system that was at the heart of the Antarres explosion earlier in this same year.
One of the ways of alleviating this problem is to run the initial tank pressures at high pressures to begin with. This becomes somewhat problematic in smaller tanks as it requires a finely tuned pressurization system to balance the ullage (gas pocket) that sits on top of the liquid with the rapidly depleting liquid in the tank when the engine is firing. Falcon 9 uses gaseous helium to provide the pressurization in their LOX tank on the second stage. Compressed helium is stored in several small pressure bottles inside the LOX tank itself at 5500 PSIG.
In order to give a relative idea of what this type of pressure is capable of think of the movie Jaws where in the last scene the shark is blown up by shooting a rifle into a scuba tank inside his mouth. Scuba tanks are pressurized with breathing air to a little less than 3000 PSIG. As you might imagine such high pressures require a very thick and heavy tank wall so that the vessel itself won't rupture inadvertently from this internal force. As I have mentioned already, the crucial tradeoff in space flight is always weight vs. safety. Instead of 4-1 safety factors such as a scuba tank on earth requires, space vehicles use 1.4 - 1.5 to one as a basic safety requirement. Instead of a tank that operates at 5500 PSIG being required to be designed to withstand 22000 PSIG it is designed to withstand 7700 PSIG.
Space Ex is also doing several other things with this system to increase efficiency. The first thing they are doing is super cooling the helium tank and fill gas by storing it inside the LOX tank itself. This is not a new discovery as NASA has been doing this for quite some time on many different vehicles. By putting the pressurization tank inside the LOX itself it is possible to have a smaller pressurization tank. A tank at 5500 PSIG Helium in ambient temperature will contain much less actual usable gas than a tank containing 5500 PSIG at -279 Degrees F. The Helium molecules become denser and can be more efficiently compressed so that more gas is available to pressurize the tank.
Falcon 9's system utilizes the engine itself to heat the helium and rapidly expand it before returning it to the LOX tank ullage to pressurize the LOX system. The helium is released through chambers in the engine as it burns which rapidly expands the helium before it is returned to the LOX tank to be used as a pressurization source to force the LOX out of the tank and into the engine. As you can imagine, the control system needed to tightly control the ullage at a constant pressure utilizing this chain of events is quite complicated.
Naturally, Falcon 9's LOX system is as full as possible prior to flight. A large initial ullage would require a large tank and more weight so at launch the ullage in the LOX tank is very small. This effectively means that relatively small loss of control of this pressurization system will result in large pressure fluctuations in the LOX tank during the initial use of this system. Rapid pressure rise from uncontrolled release of the Helium in these tanks has resulted in the loss of both Falcon rockets that have exploded so far. We know it did with the first Falcon explosion and all evidence points in the direction that it did on this last one as well.
Before we go any further on this concept let me get back to the other thing that Space Ex is doing with their helium pressurization system. Remember the analogy of the scuba tank for a moment. Not only is Space Ex using a 1.4-1 safety factor on the design of the tanks to be used in but they are also using some relatively new technology to build the tanks themselves. Composite Overwrap Pressure Vessels (COPV's) have been under development for some time now. We have built and tested to failure many such vessels where I work so I am not exactly unfamiliar with the concept.
A COPV can be manufactured in several different ways. Most of them utilize a lightweight inner aluminum tank that is rated at a very low pressure. It is thin walled aluminum and extremely lightweight in comparison with an equal stainless steel tank of the same size. This thin walled aluminum tank is then wrapped with carbon fiber, usually by an automated computer controlled lathe in numerous layers of thin carbon wires that are glued in place as the wrap process takes place. The resultant tank is then tested for pressure rating. Design processes and computer programs to produce these tanks are complicated to say the least.
The end result is a pressure vessel that is vastly lighter, which makes it ideal for usage on space flight vehicles. We have tested design and process on these tanks for many years in our shop. Properly done and controlled such processes produce reliable and predictable results. We have tested series of tanks to failure at Liquid Nitrogen Temperatures (-320 degrees F) and even up to Liquid Helium Temperatures (-452 Degrees F). NASA has entertained the idea of using these tanks just as Space Ex currently does in LOX tanks. They have even considered doing the same thing in Liquid Hydrogen tanks which is why we tested at even colder temperatures.
The key to the successful production of such tanks is process control. In other words, careful control of materials and manufacturing process produces consistent results in the actual pressure these tanks will withstand. Any loss of control of either of these variables produces disastrous results which is why NASA is still reluctant to commit to using COPV's inside LOX and LH2 tanks.
There are at least two other dangers associated with COPV's and their usage. Each pressure cycle effectively stretches the composite overwrap strands to a certain extent. Therefore, COPV's are listed for usage by the number of pressure cycles they see as a matter of course. Each pressure cycle is noted and logged and after a sufficient number of cycles the tank is no longer rated for use as a pressure vessel. By itself, this is not a problem for the usage of COPV's on space vehicles as they will see a relatively small number of usages anyway.
The last part of this issue is that COPV's are extremely fragile as far as handling and usage in comparison with metal tanks. Since the thin composite strands are effectively a very large chain reaction system, any external damage to a small strand effectively makes it the weakest link in a chain and we all know the adage about the weakest link in any chain being the source of its strength. Inadvertent bumping into sharp objects or dropping of objects on COPV's can cause catastrophic failures accordingly.
Wednesday, October 5, 2016
Inherent Issues and Reasonable Response
https://www.yahoo.com/finance/news/house-republicans-just-launched-political-232200411.html
From the article....
3. Given the two recent failures of the Falcon 9, will the Air Force add more weight to mission assurance and schedule reliability vs. price in their future launch service procurements? If not please explain.
First off.... let me start out by saying that the investigation is still underway and no one knows for certain what happened to cause the Falcon rocket to explode on the pad during fueling operations on the pad at Cape Canaveral as of yet.
However, what we do know is that it did explode. Preliminary investigation reports indicate that it was a massive rupture of a Ghe vessel in the second stage LOX tank on the vehicle. Space Ex has been quick to point out that this is NOT the same issue that caused the spectacular mid air explosion on the first Falcon loss less than a year ago. At best, this is a partial truth. At worst, we may soon find that it has an identical cause in the very near future.
Having been personally involved in similar accident investigations in the past, I can say that the whole process of building a fault tree of possible causes and carefully and painstakingly running every possible cause to ground is a slow and painstaking process; especially when much of the evidence is no longer existing due to the inherent dangers that come with using a perfect oxidizer. LOX is Liquid Oxygen and it is the perfect oxidizer of which I am speaking.
LOX is necessary on rocket engines flying out of the earth's atmosphere for a couple of reasons, the main one being that all combustion requires oxygen to support it and once a vehicle is it out of the earth's atmosphere it cannot burn the oxygen in the surrounding atmosphere because there is none. Therefore, there must by an oxidizer on board and LOX is the most efficient oxidizer known to man. Since rocket engines are all about efficiency; ie.... weight to lift capacity on very narrow margins of failure.... LOX is a necessary evil when it comes to launching vehicles out of the earth's atmosphere.
As I have discussed before, the dangers inherent in using LOX are numerous and manifold in nature. It is a cryogen, operating at -297 degrees fahrenheit in it's liquid or condensed version. Exposed to anything above this temperature it will begin to boil with rapid explosive expansion on the order of 861-1 as it changes from a liquid to a gas. LOX is intolerant of hydrocarbons; such that even small traces of almost all known lubricants can cause instantaneous ignition on contact with LOX under almost any pressure at all.
Add just these two inherent conditions alone and you begin to see the difficulty in working with LOX. Since materials naturally tend to contract and shrink as temperatures drop close tolerances on rotating parts such as pumps needed to move LOX become problematic to lubricate effectively. Add in pressures often in the thousands of PSIG necessary to feed and sustain a rocket engine and the problems become infinitely greater.
LOX itself is inflammable. However, being the perfect oxidizer it strongly supports combustion on any type of fuel. It is such a strong supporter of combustion that it makes readily combustible fuels out of things that are normally imflammable. This includes but is not limited to the containment vessels used to store LOX. High grade 316 Stainless steel itself burns readily in such an oxygen rich environment.
As any boy scout can tell you, the basic fire "triangle" necessary to support combustion requires three things. Fuel, Oxidizer, and ignition source. In any LOX system you have two of the three present at all times. The perfect oxidizer (LOX, often at high pressure in our application) and abundant fuel (the storage vessel AND the vehicle structure in our case). All that is between a LOX system and utter catastrophe on a space vehicle is an ignition source.
Anyone who wants to see what a LOX fire looks like can easily look up both recent Falcon infernos, the Antarres accident last year, or any number of other accidents involving space vehicles that NASA has lost in the past. Much of the evidence of what caused the accident is consumed in the inferno that results from the introduction of an ignition source. However, the charred remains plus existing recorded data can tell the story of what happened if considered in enough detail. We have been quite successful in detailing such causes in the past and I expect we will this time as well.
As the letter in the above article points out, we do have some basic problems in both this investigation and the previous investigation of the Falcon vehicle losses. The first and most glaring problem is that both accident investigations were ran by and controlled by the same private entity that created the vehicle. I would also point out that this same entity, necessarily in an ultracompetitive environment, has a vested interest in coming to a rapid conclusion so that it can get back to the business of making money by launching payloads. This inherent conflict of interest should preclude the possibility that Space Ex heads up its own investigation that will determine when they get back into profitable operation.
Unfortunately, this basic common sense idea seems to be overridden in the present situation just as it was in the last investigation into the loss of a Space Ex vehicle. I would also point out that the something similar just happened in the investigation of the Antarres rocket belonging to Orbital Sciences the exploded shortly after liftoff in Wallops, Virginia last year. Having been personally involved in that investigation, I can assure you that this inherent conflict of interest not only hindered the investigation that NASA's oversight group performed; but eventually precluded the possibility of completely understanding the root cause of the loss of that particular vehicle (more on that a little later).
From the article....
3. Given the two recent failures of the Falcon 9, will the Air Force add more weight to mission assurance and schedule reliability vs. price in their future launch service procurements? If not please explain.
First off.... let me start out by saying that the investigation is still underway and no one knows for certain what happened to cause the Falcon rocket to explode on the pad during fueling operations on the pad at Cape Canaveral as of yet.
However, what we do know is that it did explode. Preliminary investigation reports indicate that it was a massive rupture of a Ghe vessel in the second stage LOX tank on the vehicle. Space Ex has been quick to point out that this is NOT the same issue that caused the spectacular mid air explosion on the first Falcon loss less than a year ago. At best, this is a partial truth. At worst, we may soon find that it has an identical cause in the very near future.
Having been personally involved in similar accident investigations in the past, I can say that the whole process of building a fault tree of possible causes and carefully and painstakingly running every possible cause to ground is a slow and painstaking process; especially when much of the evidence is no longer existing due to the inherent dangers that come with using a perfect oxidizer. LOX is Liquid Oxygen and it is the perfect oxidizer of which I am speaking.
LOX is necessary on rocket engines flying out of the earth's atmosphere for a couple of reasons, the main one being that all combustion requires oxygen to support it and once a vehicle is it out of the earth's atmosphere it cannot burn the oxygen in the surrounding atmosphere because there is none. Therefore, there must by an oxidizer on board and LOX is the most efficient oxidizer known to man. Since rocket engines are all about efficiency; ie.... weight to lift capacity on very narrow margins of failure.... LOX is a necessary evil when it comes to launching vehicles out of the earth's atmosphere.
As I have discussed before, the dangers inherent in using LOX are numerous and manifold in nature. It is a cryogen, operating at -297 degrees fahrenheit in it's liquid or condensed version. Exposed to anything above this temperature it will begin to boil with rapid explosive expansion on the order of 861-1 as it changes from a liquid to a gas. LOX is intolerant of hydrocarbons; such that even small traces of almost all known lubricants can cause instantaneous ignition on contact with LOX under almost any pressure at all.
Add just these two inherent conditions alone and you begin to see the difficulty in working with LOX. Since materials naturally tend to contract and shrink as temperatures drop close tolerances on rotating parts such as pumps needed to move LOX become problematic to lubricate effectively. Add in pressures often in the thousands of PSIG necessary to feed and sustain a rocket engine and the problems become infinitely greater.
LOX itself is inflammable. However, being the perfect oxidizer it strongly supports combustion on any type of fuel. It is such a strong supporter of combustion that it makes readily combustible fuels out of things that are normally imflammable. This includes but is not limited to the containment vessels used to store LOX. High grade 316 Stainless steel itself burns readily in such an oxygen rich environment.
As any boy scout can tell you, the basic fire "triangle" necessary to support combustion requires three things. Fuel, Oxidizer, and ignition source. In any LOX system you have two of the three present at all times. The perfect oxidizer (LOX, often at high pressure in our application) and abundant fuel (the storage vessel AND the vehicle structure in our case). All that is between a LOX system and utter catastrophe on a space vehicle is an ignition source.
Anyone who wants to see what a LOX fire looks like can easily look up both recent Falcon infernos, the Antarres accident last year, or any number of other accidents involving space vehicles that NASA has lost in the past. Much of the evidence of what caused the accident is consumed in the inferno that results from the introduction of an ignition source. However, the charred remains plus existing recorded data can tell the story of what happened if considered in enough detail. We have been quite successful in detailing such causes in the past and I expect we will this time as well.
As the letter in the above article points out, we do have some basic problems in both this investigation and the previous investigation of the Falcon vehicle losses. The first and most glaring problem is that both accident investigations were ran by and controlled by the same private entity that created the vehicle. I would also point out that this same entity, necessarily in an ultracompetitive environment, has a vested interest in coming to a rapid conclusion so that it can get back to the business of making money by launching payloads. This inherent conflict of interest should preclude the possibility that Space Ex heads up its own investigation that will determine when they get back into profitable operation.
Unfortunately, this basic common sense idea seems to be overridden in the present situation just as it was in the last investigation into the loss of a Space Ex vehicle. I would also point out that the something similar just happened in the investigation of the Antarres rocket belonging to Orbital Sciences the exploded shortly after liftoff in Wallops, Virginia last year. Having been personally involved in that investigation, I can assure you that this inherent conflict of interest not only hindered the investigation that NASA's oversight group performed; but eventually precluded the possibility of completely understanding the root cause of the loss of that particular vehicle (more on that a little later).
Commercial Space Problems
I am a little disgusted at the moment. The recent explosion at Cape Canaveral is the last in a string of accidents on commercial space ventures that were completely avoidable. It is way too premature for anyone to know what caused this one yet but recent experience has taught me that the cause will once again be found to be schedule haste combined with the reckless and deadly nature of commercial space.
I have written about NASA's accidents and how they were all caused by this same problem before. Schedule pressure is dangerous in the space flight industry. It has to be offset by the understanding that technical concerns ALWAYS overrides schedule pressure. This is literally impossible in commercial space because there is nothing that counterbalances schedule pressure in commercial space. Combine this with a total lack of practical experience that prevails at most commercial space companies and you get what we have now; a continuing string of disasters.
In the first place space flight travel is hard. It takes extremely powerful engines to lift cargo out of earth's orbit. These engines need oxidizers such as liquid oxygen to burn at the rates needed and liquid oxygen is an extremely unforgiving substance to deal with. Because of the energy involved and the close ratio between energy available and load to be lifted space flight vehicles operate on the bare margins of safety to begin with. The standard pressure to strength ratio for mechanical facilities on earth is 4-1. In other words if a tank is designed to withstand 100 pounds of pressure per inch it is designed to withstand 400 pounds of pressure per inch. On a space vehicle, this same tank is designed to withstand 150 pounds of pressure per inch, or 1.5-1.
Add in the extreme temperature changes involved in using a cryogen like Liquid Oxygen (-297 Degrees F) and one can begin to understand the difficulties involved. Each component is designed on the ragged edge of strength to weight ratio to maximize the effective cargo that such a vehicle can carry. Why not just go to 4-1 safety factors you might ask? Well, if we did that we wouldn't have the energy to get out of earth's orbit.
NASA has a long record of dealing with these margins yet they have also experienced many different failures of their own in its own history. Besides the two shuttle disasters that everyone is familiar with there were a lot of other accidents in testing and design phases at different NASA centers across the nation. It is an inherently dangerous business that requires inherently stringent testing and design characteristics. I think everyone understands this. Unfortunately, the degree of stringency is where the argument comes in.
There is a huge disagreement on this at the moment within the commercial space industry. NASA is charged with oversight on commercial space entities and I can tell you from direct experience NASA is losing the argument at the moment. Commerical space entities have a lot of Congressional support from local districts where these ventures are benefitting the local economy with good paying jobs. Hence the continuing level of confidence and support from Congress espousing the full confidence in these ventures. I can assure you that this level of confidence is NOT being expressed by NASA employees charged with actually doing the oversight.
In the next few posts I hope to go over some details about why these problems are occurring and what the particular problems are. The basic struggle is between the technical experts at NASA who have experienced these problems in the past and a young, energetic group working private space who believe NASA is hopelessly slow and restrictive by its very nature. Neither group is completely wrong in their assumptions about the other. However, the reason that NASA is slow and restrictive is that they have already learned some of the lessons that commercial space is struggling with now.
I have written about NASA's accidents and how they were all caused by this same problem before. Schedule pressure is dangerous in the space flight industry. It has to be offset by the understanding that technical concerns ALWAYS overrides schedule pressure. This is literally impossible in commercial space because there is nothing that counterbalances schedule pressure in commercial space. Combine this with a total lack of practical experience that prevails at most commercial space companies and you get what we have now; a continuing string of disasters.
In the first place space flight travel is hard. It takes extremely powerful engines to lift cargo out of earth's orbit. These engines need oxidizers such as liquid oxygen to burn at the rates needed and liquid oxygen is an extremely unforgiving substance to deal with. Because of the energy involved and the close ratio between energy available and load to be lifted space flight vehicles operate on the bare margins of safety to begin with. The standard pressure to strength ratio for mechanical facilities on earth is 4-1. In other words if a tank is designed to withstand 100 pounds of pressure per inch it is designed to withstand 400 pounds of pressure per inch. On a space vehicle, this same tank is designed to withstand 150 pounds of pressure per inch, or 1.5-1.
Add in the extreme temperature changes involved in using a cryogen like Liquid Oxygen (-297 Degrees F) and one can begin to understand the difficulties involved. Each component is designed on the ragged edge of strength to weight ratio to maximize the effective cargo that such a vehicle can carry. Why not just go to 4-1 safety factors you might ask? Well, if we did that we wouldn't have the energy to get out of earth's orbit.
NASA has a long record of dealing with these margins yet they have also experienced many different failures of their own in its own history. Besides the two shuttle disasters that everyone is familiar with there were a lot of other accidents in testing and design phases at different NASA centers across the nation. It is an inherently dangerous business that requires inherently stringent testing and design characteristics. I think everyone understands this. Unfortunately, the degree of stringency is where the argument comes in.
There is a huge disagreement on this at the moment within the commercial space industry. NASA is charged with oversight on commercial space entities and I can tell you from direct experience NASA is losing the argument at the moment. Commerical space entities have a lot of Congressional support from local districts where these ventures are benefitting the local economy with good paying jobs. Hence the continuing level of confidence and support from Congress espousing the full confidence in these ventures. I can assure you that this level of confidence is NOT being expressed by NASA employees charged with actually doing the oversight.
In the next few posts I hope to go over some details about why these problems are occurring and what the particular problems are. The basic struggle is between the technical experts at NASA who have experienced these problems in the past and a young, energetic group working private space who believe NASA is hopelessly slow and restrictive by its very nature. Neither group is completely wrong in their assumptions about the other. However, the reason that NASA is slow and restrictive is that they have already learned some of the lessons that commercial space is struggling with now.
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