By Charles F. Radley; What went wrong at Mojave? In terms of the specifics of the accident I do not know more than anyone else from public sources. However, I can offer some general perspectives and insight, concerning industrial hazards.
My background includes a few years working with NASA and the USAF on range safety and payload safety involving several spacecraft. I never worked with Nitrous Oxide, and have no specific experience with that particular oxidizer. But the more I research Nitrous Oxide (N2O), the more I get the feeling that this accident might not have been due to the chemical properties.
According to all the literature I have read, including numerous MSDS’es (Materials Safety Data Sheets), N2O is chemically inert up to 300 C, and does not react with anything below that temperature.
At much higher temperatures, usually above 1200 C, N2O can explosively decompose into oxygen and nitrogen.
It is possible to make N2O decompose at lower temperature using a variety of catalysts. For example, many metals, such as those used in brazing or welding, can cause N2O to decompose at temperatures of around 650 C.
Of course it is theoretically conceivable that there might be a previously unknown catalyst which could cause some kind of N2O reaction at room temperature, but I cannot find any references to any known material today which could do that. So that would seem highly unlikely, but cannot be ruled out.
Since the accident in question occurred during a cold flow test at room temperature, it seems unlikely that a chemical reaction caused the explosion.
So what else could have happened?
Well, there are several possibilities all of which I would investigate (this list is not all inclusive).
1) The structural integrity of the pressure vessel tank
2) The thermal integrity of the cryogenic insulation layer(s)
3) The integrity of the cryogenic tank refrigeration system
4) Failure of a critical vale or plumbing feature
5) A high temperature heat source
Indeed many accidents are caused by a combination of two or more factors such as those listed above.
Let me address each in more detail.
1) The structural integrity of the pressure vessel tank
Pressure vessels in the USA must generally conform to the American Society of Mechanical Engineers (ASME) boiler code, which requires a safety factor of 4 times the maximum rated operating pressure.
Did the tank somehow experience a pressure greater than its rated pressure? It is difficult to imagine how this could happen without some kind of big temperature change. For cryogenic N2O, if the temperature exceeded the boiling point of N2O then the pressure could increase dramatically.
But even without any pressure increase, it is still possible a pressure vessel can fail. This can be for a number of reasons:
A tank wall can be weakened by several phenomena, e.g.: metal fatigue, micro-crack propagation, corrosion, mechanical damage, or design errors.
The first two (fatigue and crack growth) can happen under a combination of circumstances which involve: presence of small cracks which are not detected by visual or NDE inspection, and frequently pressurizing and depressurizing a vessel which causes cyclic varying stresses. If the stress levels exceed a certain value (depending on the material used), then micro-cracks can slowly expand in size. Over time, after multiple pressure cycles, these cracks can become large enough that a catastrophic failure occurs. This phenomenon is subtle and difficult to predict, a highly specialized process called “Fracture Mechanics” can be used to predict crack propagation patterns, but this is not commonly done with ASME type pressure vessels.
Corrosion: if a vessel becomes corroded for any reason that can compromise its structural integrity. There are various types of corrosion, including galvanic corrosion or stress corrosion. The risk of corrosion depends on the storage environment, and the material of which the tank is made.
Mechanical damage: if there were a collision, such as a vehicle crashing into the tank, or a foreign object or projectile impacting the tank, any of those could compromise integrity and result in explosion.
Other: a variety of other problems can arise. For example if there were undetected manufacturing flaws in the tank, or the material used did not conform to its specification, or there was a mistake in the stress analysis.
2) The thermal integrity of the cryogenic insulation layer(s)
To keep N2O liquid requires maintaining a temperature below -88 C. This requires a combination of insulation and refrigeration. Failure of thermal insulation can therefore be a risk. Insulation could fail by being inadvertently removed during maintenance, or by physical damage from collisions or projectiles.
3) The integrity of the cryogenic tank refrigeration system
Without refrigeration cryogens will boil and pressure will rise. If the refrigeration system fails that would be a hazardous situation. One might expect some type of warning (fire alarm?) prompting rapid evacuation of personnel if this occurred.
4) Failure of a critical vale or plumbing feature
Many pressure systems involve a variety of components, depending on what they do or how they are connected together, there can be a variety of failure scenarios which could result in explosion. For example one might expect pressure relief valves to be in place to bleed off excess pressure from the tank, e.g. to protect against of failure of the refrigeration system; but if the relief valve also failed, then an explosion could result. Seals or check valves might be used to prevent cryogenic fluids from being exposed to room temperature components, their failure could be catastrophic. For example O-ring seals can fail at low temperatures, as was seen in the Challenger accident.
5) A high temperature heat source
This is what I call the Apollo-13 scenario. Sometimes electrical components are exposed to cryogenic fluids. For example, pressure transducers, or temperature transducers, or solenoids. Normally these devices are current limited to prevent overheating, and have barriers preventing direct exposure to cryogenic fluid. But if a barrier failed (e.g. a seal) or if there were some type of electrical short circuit or surge, then the electric device can turn into a heater, and begin to boil the cryogenic fluid, causing a sudden increase in pressure. An example of this type of problem occurred on Apollo-13.
Conclusion:
Cryogenic oxidizers have numerous safety risks associated with them, some examples have been provided. It is not possible at this time to state with any certainty which (if any) of the above scenarios were involved in the particular accident which prompted this study.
We wish the accident investigators good fortune in their painstaking and difficult task of reconstructing the accident and investigating all the possibilities.
Please feel free to discuss this topic further in the forum…
Copyright 2007 The International Space Fellowship and Charles F. Radley. All rights reserved.
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