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

[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?

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.

Thank you for this!

I expressed myself poorly.  The COPV pressure is rising throughout the entire filling process.  At any given interval of time, some mass of He is already in the COPV, whereas some other mass of He is added from the tank.  The latter mass increases in pressure, cooling the container.  The former mass decreases in pressure, heating the container.  So the simple-minded view would be that any net cooling/heating of the container depends on the ratio of the two masses of He, which changes over the course of the filling process.

However, upon reflection, I suspect that both cscott and you are really saying that there is no obvious way in which He could cool the container such as to freeze out SOX from LOX.

[spacekid]

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.

It's just that liquids find their way into tiny crevices. Liquids with small size (like hydrogen) are even harder to keep out.

You could encase it in an epoxy or similar but then if it expands it can crack. Using metal or similar that can bend would need to be leak proof and resistant to fatigue as it expands and contracts.

[FinalFrontier]

Well very sad to hear my initial thinking was correct. Material failure due to intrusion/delam as a result of operational errors, more likely design failure due to overstress as a result of operational mistakes.


Basically the worst case for spacex this justifies most of the criticisms recently levied against them specifically that their lower cost access approach is resulting in unintended higher risks to payloads and flight rationale.

That said it's entirely fixable, but it will be harder than just changing a few parts out. Gotta change the methodology.

[FinalFrontier]

No way ULA as a company would do it. The leadership of the company are all space geeks. And no one with a financial interest (such as LM or Boeing) would be insane enough to do it, since if it were revealed, there'd be so much worse impact than just some competition with SpaceX.

People who could do it:
Authoritarian regimes, maybe. Anti-Israeli folks, maybe. Some random disgruntled person who has whipped themselves up, maybe.

I seem to remember security personnel with big guns protecting Shuttle launches. As Jim said, this isn't new. But it also isn't likely in the least.

ULA could do it I have no doubt at all, the world of government contract competitors is very nasty. But they didn't the failure mode always suggested internal failure more over they don't need to do such a thing to compete with spacex it would be counter intuitive given they are developing new launch vehicles for this purpose.

[garidan]

I don't think this is due to SpaceX cost reduction , it's due to too fast changing Rocket and procedures

[Space Ghost 1962]

US are highly tuned for performance. For F9US clearly dry mass fraction plays a key role in the decision for submerged COPV's (and other).

For Centaur, a thin walled stainless pressure stabilized tanks also reduces same.

With any design, you have tradeoffs, which take time to tame. For Centaur it took 4 decades to tame.

F9US "scares" right now for its performance trades. Centaur did not anticipate a "pusher" escape system for a capsule, as OSP's Boeing capsule then expected a puller. So now it needs a skirt.

Which robs performance.

US issues contemporaneously.

[CyndyC]

All the back and forth about what the helium temp does under pressure and during expansion is making it difficult to understand what really happens. Combining 2 PDFs on COPV manufacturing linked below, and Jim's knowledge as references, these are the facts, plus one other, and a chart of expansion measurements characteristic of a titanium-lined COPV during testing:

Φ During pressurization, the compressed [helium] gas heats up, thus heating up the tank. External leak test using helium at 4800 psi for 30 minutes, bottom of page 8 in 1st reference.

Φ As the temperature drops in the COPV's [while sitting in the LOX and HE is expanding], more He can be loaded. Jim on 9/29/16 in this thread at 21:08Z
 
Φ COPVs are pre-expanded as part of the manufacturing process. Think pre-shrunk cotton. For COPVs this is known as "sizing" or "autofrettage", and results in residual tensile strain in the overwrap, and residual compressive strain in the liner (bottom of pg 7 in 1st ref, bottom of pg 4 in the 2nd).

1st Ref http://www.psi-pci.com/Technical_Paper_Library/AIAA2002-4349%20Astrolink%20Pres.pdf
2nd Ref https://ston.jsc.nasa.gov/collections/trs/_techrep/SP-2011-573.pdf

In the chart below, MEOP pressure at 4800psi is used for the manufacturer's external leak test.
Proof pressure at 6000psi approximates if not equals the helium source tank pressure used by SpaceX, but how far a COPV can continue to expand following autofrettage remains in question.

[Chris_Pi]

I don't think this is due to SpaceX cost reduction , it's due to too fast changing Rocket and procedures

May not even be speed of change, Just what the changes are.

Subcooled LOX and the fast fuel loading could be the issue. Oxygen inside the overwrap may never have been a problem before because nothing ever got quite cold enough to freeze it. In F9 it's very nearly frozen normally and doesn't need much of a temperature drop to get there.

The fix could be as simple as bringing up helium pressure quicker while the copv's are still warm enough that there can't be any frozen oxygen around.

[GLOUPY]

In this document:

http://www.psi-pci.com/Technical_Paper_Library/AIAA2002-4349%20Astrolink%20Pres.pdf

on page 2, it is specify that temperature should be highter than -95°C to operate the COPV's. It seems to be the limit of qualification. So i definitively do not understand how the COPV could be located inside the LOX tank...

[bluemars1]

that -95 degC number is a design requirement  spec for some other company's spacecraft being designed before  2002.

It does not represent some fundamental limit.

[CyndyC]

US are highly tuned for performance. For F9US clearly dry mass fraction plays a key role in the decision for submerged COPV's (and other).

For Centaur, a thin walled stainless pressure stabilized tanks also reduces same.

With any design, you have tradeoffs, which take time to tame. For Centaur it took 4 decades to tame.

F9US "scares" right now for its performance trades. Centaur did not anticipate a "pusher" escape system for a capsule, as OSP's Boeing capsule then expected a puller. So now it needs a skirt.

Which robs performance.

US issues contemporaneously.

What is US referring to in this context? Not United States I assume.

[bluemars1]

Upper Stage

[inonepiece]

Having read through the posts, I don't think anybody has yet clearly and explicitly stated in any detail what the theory or theories is/are here.  There seem to be two being discussed here in the context of Elon's reported comments at the NRO: 1. direct ignition, and 2. indirect ignition via COPV compromise.

Also, some people seem to be making comments without stating, as context, which of those two theories they are discussing.

For theory 1: Is there data explaining how solid oxygen compressed to high pressure in contact with carbon fibre composite might directly cause ignition at these low temperatures?  Even at hundreds of atmospheres oxygen is at < 100K when solid, right?  Does ignition really take place at those temperatures?  Is the idea that there would be local heating, before that causes melting of the oxygen?

For theory 2: In the case of the theory that solid oxygen caused compressive damage to fibres and subsequent COPV failure, how would a burst COPV cause ignition?  Frictional heating?  Again, data?

[dorkmo]

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.

It's just that liquids find their way into tiny crevices. Liquids with small size (like hydrogen) are even harder to keep out.

You could encase it in an epoxy or similar but then if it expands it can crack. Using metal or similar that can bend would need to be leak proof and resistant to fatigue as it expands and contracts.

instead of foil could they plate it?

lead? gold?

[Mike_1179]

Having read through the posts, I don't think anybody has yet clearly and explicitly stated in any detail what the theory or theories is/are here.

1. Thermo-acoustic resonance when filling the COPV while there is a temperature gradient in the He system. Couple this to a mechanical resonance and the He system (some part of it, not necessarily a COPV) fails.

2. Compression of oxygen inside the polyurethane matrix of the composite as pressure inside the COPV increases.

Is that about right so far?

[Kabloona]

Having read through the posts, I don't think anybody has yet clearly and explicitly stated in any detail what the theory or theories is/are here.

1. Thermo-acoustic resonance when filling the COPV while there is a temperature gradient in the He system. Couple this to a mechanical resonance and the He system (some part of it, not necessarily a COPV) fails.

2. Compression of oxygen inside the polyurethane matrix of the composite as pressure inside the COPV increases.

Is that about right so far?

#1 was being batted around here based on a second-hand report of "weird harmonics" observed during tanking, but Elon's most recent statements at NRO didn't mention any such theory. He only said their leading theory was LOX solidifying inside the COPV overwrap.

http://forum.nasaspaceflight.com/index.php?topic=41252.msg1598513#msg1598513

Inonepiece's question about theories seems to be, assuming Elon's "solidified LOX compressed inside overwrap" theory is correct, what was the failure mechanism, ie did the compression of solidified LOX against the COPV material cause ignition, or was there some other burst/ ignition mechanism caused by the solidified LOX.

[Stan-1967]

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.

It's just that liquids find their way into tiny crevices. Liquids with small size (like hydrogen) are even harder to keep out.

You could encase it in an epoxy or similar but then if it expands it can crack. Using metal or similar that can bend would need to be leak proof and resistant to fatigue as it expands and contracts.

instead of foil could they plate it?

lead? gold?

Electroplated foils have problems with being porous, and at large scale, they are not highly engineered in regards to microstructure important to diffusion of gases or liquids like LOX.   At small scale, i.e the metallurgical grain boundaries, they are sites for discontinuities, voids, and impurities.   They are no magic bullet. 

If you are talking of plating a Titanium liner, that is even more difficult.  Titanium generally can't be electroplated well.  Aluminum is much easier to plate.  Plated films will also set up a big CTE mismatch between the substrate and the foil.  At the extreme cryogenic temperatures, the deposit would likely blister and peel away.  Part of the plating process is to remove the native surface oxide from the Al or Ti, so if it was on the outer part of the metal liner, exposed to LOX, you would have a very reactive metal, with no oxide, in contact with LOX.  BAD!

[Steven Pietrobon]

I wonder, do they pre-cool the helium first to very cold temperatures (i.e. below freezing point of oxygen) in order to allow them to fill much faster?

In past technical broadcasts you can hear SpaceX calling our "cryohelium loading" during the count. I don't know what temperature the cryohelium is at though. If the cryohelium was below the LOX freezing point, that might be a contributing factor to oxygen crystals forming.

For theory 2: In the case of the theory that solid oxygen caused compressive damage to fibres and subsequent COPV failure, how would a burst COPV cause ignition?  Frictional heating?  Again, data?

I would imagine that the kinetic energy of the explosion would be sufficient to cause ignition.

[docmordrid]

Wonder if the reasonance could produce sonoluminescence? When those bubbles collapse temps can reach 5,000°K+. More intense if a noble gas is present.

[Rocket Science]

In my quest for "what's the same, whats different" in S1 and S2: Are not the He tanks submerged in LOX? Are the temperatures involved not the same" Are not the He tanks identical for S1 and S2? Are fill rates not the same?