SpaceX Falcon 9 - AMOS-6 - (Pad Failure) - DISCUSSION THREAD (2)

[Lar]

I am not hoping to see a video.  I would not want to see such a thing become generally known.  It will give too many wackos ideas that I do not want to be spread around.

Security through Obscurity never works but I get what you mean...

Word to the wise, although things have taken a surreal turn here in some ways, let's not get carried away. Things are going OK so far for the most part.

[CyndyC]

The idea is the that helium tank liner is colder than the freezing point of oxygen (perhaps since the tank was being loaded with very cold helium).  The LOX soaks through the overwrap, hits the cold metal liner, and freezes into a crystal(s).  Then pressurizing the tank continues, and compresses the LOX crystal against the overwrap as the tank liner expands due to increased pressure.  If the LOX was liquid, then it can squeeze out the way it came in.  But the crystal is solid and can't get out, so it gets compressed until it explodes.

Thank you for the very thorough and clear description, but why is everyone supposed to think this wouldn't be a routine occurrence?

[gospacex]

I'm surprised to hear that LOX is so difficult to protect against seeping into composites. I would think a relatively thin metal foil sheath on the outside should do it.

[LouScheffer]

The idea is the that helium tank liner is colder than the freezing point of oxygen (perhaps since the tank was being loaded with very cold helium).  The LOX soaks through the overwrap, hits the cold metal liner, and freezes into a crystal(s).  Then pressurizing the tank continues, and compresses the LOX crystal against the overwrap as the tank liner expands due to increased pressure.  If the LOX was liquid, then it can squeeze out the way it came in.  But the crystal is solid and can't get out, so it gets compressed until it explodes.

Thank you for the very thorough and clear description, but why is everyone supposed to think this wouldn't be a routine occurrence?

(a) Sub-cooled LOX is much closer to freezing, and not everyone puts their tanks in the LOX.  Perhaps this explains why there was no industry experience in avoiding this problem.
(b) Since SpaceX was looking at longer hold times, maybe they were using extra-cold LOX, or extra-cold helium, etc.
(c) Changing the loading schedule for LOX and helium could aggravate this problem.  This particular problem requires a cold tank, then cold LOX, then more pressurization.  Again, SpaceX was known to be fiddling with the loading schedule.
(d) Maybe it creates a dangerous condition, but does not always explode.  Maybe it only explodes one time in hundred, and they were lucky up until now.

[vulture4]

It is surprising the helium would be loaded at less than 54K. Maybe it expanded and cooled as it entered the tank from the manifold? Was there a temperature sensor in the COPV?  The failure mode should be demonstrated in a test rig before drawing conclusions.

Moreover, cryogen permiation into a composite structure can lead to delamination when the liquid boils and expands, i.e. when the tanks are drained after a launch scrub. There is no protection over the outer surface of the composite and unless a composite system that is truly impervious to LOX can be demonstrated, intrusion is likely.

On the Saturn I believe the kerosene tank was pressurized with turbopump exhaust gasses, why wouldn't that be possible here? The LOX tank could be autopressurized with heated gaseous oxygen.

[Proponent]

I believe loading the helium tank is, to a good approximation, a throttled expansion (Joule-Thomson process).  The fact that the helium tank expands a bit as the pressure rises means that some work is being done, so it's not strictly a throttled expansion, but I'm pretty sure the work done is negligible.  That means, then, that enthalpy is conserved.  For helium, the Joule-Thomson coefficient (the partial derivative of temperature with respect to pressure at constant enthalpy) is in the neighborhood of -0.5 K/MPa for temperatures around 55 K and pressures of 40-100 MPa (tank pressure is 5500 psi, or about 40 MPa).  The negative Joule-Thomson coefficient indicates that the helium will warm up as pressure drops.

Re the S-I stage, the lox tanks were pressurized in-flight with gaseous oxygen generated by circulating lox through heat exchange on the engines.  Helium may have been used before lift-off.  The RP-1 tanks were pressurized with nitrogen: see pages 85ff of the attachment to this post.  The S-IB apparently switched to helium (see p. 2 of the attached document).

A drawback to pressurizing the Falcon 9, particularly its upper stage, with gaseous lox or turbine exhaust might be the mass of the residual gases:  it can be surprisingly high.  Obviously, some mass would be saved by eliminating the high-pressure helium tank, but given the lengths to which SpaceX goes to lighten it, I would not be surprised if a lox tank load of cold lox vapor or turbine exhaust is were massive than a tank load of helium gas together with the high-pressure vessel needed to contain it initially.  Control of lox pressurization when using gox as a pressurant might be tricky, given that the lox is sub-cooled.

[guckyfan]

It is surprising the helium would be loaded at less than 54K. Maybe it expanded and cooled as it entered the tank from the manifold? Was there a temperature sensor in the COPV?  The failure mode should be demonstrated in a test rig before drawing conclusions.

My understanding is they did reproduce it at McGregor. They did not only hire a sniper to shoot at a stage mockup.

[Kaputnik]

I'm surprised to hear that LOX is so difficult to protect against seeping into composites. I would think a relatively thin metal foil sheath on the outside should do it.

Oh goody, more layers...

[aameise9]

... For helium, the Joule-Thomson coefficient (the partial derivative of temperature with respect to pressure at constant enthalpy) is in the neighborhood of -0.5 K/MPa for temperatures around 55 K and pressures of 40-100 MPa (tank pressure is 5500 psi, or about 40 MPa).  The negative Joule-Thomson coefficient indicates that the helium will warm up as pressure drops.

I'm trying to follow the implications.  Does this create two phases of COPV filling?  Phase I: contents warm up container, as He pressure drops from tank level.  Phase II: contents cool down container, as He pressure rises to launch level.

If so, oxygen ice could form only during phase II, correct?

[Jim]

1.  On the Saturn I believe the kerosene tank was pressurized with turbopump exhaust gasses,
2.  why wouldn't that be possible here? The LOX tank could be autopressurized with heated gaseous oxygen.

1.  no, it was nitrogen
2.  That doesn't work for restartable stages.  Second stage needs outside source of pressurization. 

[rsdavis9]

2.  That doesn't work for restartable stages.  Second stage needs outside source of pressurization.

So how is ITS going to restart.

[Jim]

2.  That doesn't work for restartable stages.  Second stage needs outside source of pressurization.

So how is ITS going to restart.

See IVF

[cscott]

... For helium, the Joule-Thomson coefficient (the partial derivative of temperature with respect to pressure at constant enthalpy) is in the neighborhood of -0.5 K/MPa for temperatures around 55 K and pressures of 40-100 MPa (tank pressure is 5500 psi, or about 40 MPa).  The negative Joule-Thomson coefficient indicates that the helium will warm up as pressure drops.

I'm trying to follow the implications.  Does this create two phases of COPV filling?  Phase I: contents warm up container, as He pressure drops from tank level.  Phase II: contents cool down container, as He pressure rises to launch level.

If so, oxygen ice could form only during phase II, correct?

The feed pressure will always be higher than the tank pressure, or else the flow wouldn't be into the tank.  So there's no "phase two" as far as I understand it.

I think the "unexpected acoustic heat engine" theory makes the most sense to me so far: the helium is heating up its tank, and the LOX being filled has to be above the melting point or it wouldn't flow into the tank.  So all temperatures are warm enough and there's "no way" they could get solid oxygen crystals... until they quite unexpectedly did, as a side effect of a particular loading process exciting a resonance.

[Fred Bonyea]

Oxygen plus carbon plus pressure = boom.

But this would just be a spontaneous thing?  It's sitting there, then boom?

Yes, it's spontaneous. Oxidizer and fuel will autoignite once they reach a critical pressure - the exact pressure depends on the type of fuel and oxidizer, and the temperature. This is the operating principal of the diesel engine, which does not have or need an ignition source to start combustion of the fuel-air mixture.

This is not quite correct. In a diesel engine, the piston and/or fuel injector compress the gas into a smaller volume; which increases the temperature of the gas mixture until the critical temperature for ignition is reached.
(However, this is complicated by the fact that the ignition temperature for many fuel/oxidizers does lower slightly at higher pressures.)

The way I am reading this, is that the proposed mechanism is liquid LOX wicking into microvoids and then freezing into solid plugs. When the tank internal to the plugs expands, it quickly increases the pressure of the O2 trapped inside the frozen plugs to the ignition temperature. In an O2 saturated high pressure environment, this needlepoint at critical temperature is all it would take.

It is an interesting theory, it does explain why Elon was vexed initially because there was no obvious source of ignition. It will be interesting to see if it can be duplicated, because it may not be easy to construct a micro-crack environment that will cause sufficient pressure/temperature rise for ignition.

The reason it is not safe to freeze Nitroglycerin is that the frozen crystals create stress points that can create frictional heat - especially as the explosive is thawed. This seems like a more likely solid O2 failure mode: frictional heat as crystals of O2 grind against each other during compression.  Again, a very difficult scenario to duplicate: 99+% of the time you freeze and thaw nitrated oils, nothing happens. 

[GLOUPY]

Can someone explain to me how the oxygen becomes solid? The oxygen tank is filled with oxygen at 66k. The COPV are filled with helium at ambiant temperature  which is cooled to the temperature of oxygen by conduction to the wall. I really do not or might come extra cooling that would solidify oxygen.

[Johnnyhinbos]

2.  That doesn't work for restartable stages.  Second stage needs outside source of pressurization.

So how is ITS going to restart.

See IVF

I once suggested the the MCT would utilize an internal combustion engine for power generation and pressurization (as a backup system), which sparked a lively conversation.

An internal combustion engine is a big part of the ULA IVF system. Jim, what are your thoughts on utilizing such an engine for the ITS?

EDIT: Apologies, this should be moved to the ITS discussion thread.

[king1999]

2.  That doesn't work for restartable stages.  Second stage needs outside source of pressurization.

So how is ITS going to restart.

Sparkplug, just like you gasoline car.

[Jim]

2.  That doesn't work for restartable stages.  Second stage needs outside source of pressurization.

So how is ITS going to restart.

See IVF

I once suggested the the MCT would utilize an internal combustion engine for power generation and pressurization (as a backup system), which sparked a lively conversation.

An internal combustion engine is a big part of the ULA IVF system. Jim, what are your thoughts on utilizing such an engine for the ITS?

It is a great way of using boil off

[mclumber1]

2.  That doesn't work for restartable stages.  Second stage needs outside source of pressurization.

So how is ITS going to restart.

Sparkplug, just like you gasoline car.

I think it was already known that the Raptors will utilize spark ignition.  That makes sense.  But how will they spin up the turbo pumps?  AFAIK, the Merlins use high pressure helium to spin up the turbo pump.

[jpo234]

Can someone explain to me how the oxygen becomes solid? The oxygen tank is filled with oxygen at 66k. The COPV are filled with helium at ambiant temperature  which is cooled to the temperature of oxygen by conduction to the wall. I really do not or might come extra cooling that would solidify oxygen.

The assumption is, that the helium gets compressed somewhere in the GSE and then cryogenic cooled. So cryogenic He under high pressure gets into the partially filled COPV, expands and those adiabatically cooles below the freezing temperature of LOX.