Why doesn't carbon fiber overwrapping in LOX catch fire? (watch this video first)

Question

Sir Martyn Poliakoff, also CBE FRS and his team produce a fascinating series of Periodic Videos.

In this one they team up with Smarter Every Day's Destin Sandlin to do high speed video of a piece of coal dropped into liquid oxygen.

The coal spontaneously combusts on contact.

Please watch the video first, then explain why immersing carbon fiber over-wrapped helium tanks into tanks of liquid oxygen in the presence of additional sources of energy (e.g. expansion, contraction, friction, vibration) is not likely to catch fire, and is likely to be safe and reliable for human-rated spaceflight.

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edit: I'll also reproduce part of the block quote from this answer where the idea of putting a combustible material in direct contact with LOX is roundly balked-at:

Finally, for completeness, page 146 of Clark's book contains a rather disturbing mention of LOX + liquid methane as a proposed monopropellant:

"If Tannenbaum's mixtures were bad, that proposed at a monopropellant conference in October 1957 by an optimist from Air Products, Inc., was enough to raise the hair on the head of anybody in the propellant business. He suggested that a mixture of liquid oxygen and liquid methane would be an extra high-energy monopropellant, and had even worked out the phase diagrams of the system.* How he avoided suicide (the first rule in handling liquid oxygen is that you never, never let it come in contact with a potential fuel) is an interesting question, particularly as JPL later demonstrated that you could make the mixture detonate merely by shining a bright light on it. Nevertheless, ten years later I read an article seriously proposing an oxygen-methane monopropellant! Apparently junior engineers are allergic to the history of their own business."

I think we can see why this one never took off.

John D. Clark's famous book Ignition! (1972, online)

my bold

Related: Final conclusion/description of the cause of the SpaceX Sept. 1, 2016 anomaly.

“When you think about it boys”, he said brokenly, “that’s what holds us together more than anything else, except maybe gravity. We few, we happy few, we band of brothers - joined in the serious business of keeping our food, shelter, clothing and loved ones from combining with oxygen.”

--Kurt Vonnegut, God Bless You, Mr. Rosewater (1965)

Answer 1

Liquid oxygen mixed with carbon powder has been used as an explosive for mining, see 1, 2. But there has been an explosion in a helium purifier 3.

The use for mining required safe explosives, the rate of spontaneus self-ignitions should be very, very low. The charges should explode only when triggered by a detonator, but not when handling them or by static discharges. They were used for decades. The accident rate was lower than with conventional explosives. The charges were explosive only when soaked with liquid oxygen. If unused, after some hours when the oxygen was evaporated, the carbon powder was not explosive anymore.

In the experiment video shown, the piece of charcoal did not ignite on contact with LOX, it was ignited before and still glowing. Throwing it into a container filled with gaseous oxygen would intensify the reaction too. Compare with these two videos 4 and 5 showing hot and burning charcoal thrown into LOX.

The surface of carbon powder is greater than that of carbon fibers, therefore the risk of self ignition should be smaller. But hydrocarbons are known to be a risk in contact with pure oxygen. The binder used with the carbon fibers contains hydrogen and carbon too. A contamination with other hydrocarbons may be possible if the procedure of cleaning it before contact with oxygen was not followed strictly.

Answer 2

In all cases I know of, fiber reinforced tanks consist of a thin metal inner tank called a "liner" wrapped with the supporting composite material. Hence, the oxidizer and fiber are never in contact. This is also important because the composite is porous and would not hold it's contents for long.

One source: (sorry, I don't have time to type the relevant passage right now)

https://books.google.co.jp/books?id=5LBx4EmBix8C&lpg=PA309&ots=yNiJNo8XLb&hl=de&pg=PA309#v=onepage&

Answer 3

As he says the charcoal in the experiment is very hot (plus charcoal has a lot of carbon dust all over it, which is really flammable stuff), even then the charcoal doesn't burn until after it bounces out of the liquid oxygen, having vaporised some oxygen. To summarise we have hot carbon dust in an almost pure oxygen atmosphere.

By contrast COPV's use carbon fibres which are bonded (and therefore far more stable) structures of carbon (their structures are actually somewhat similar to graphite which is used on some rocket nozzles). They are also in the middle of a tank full of LOX which is at -183C or lower. Furthermore oxygen isn't very flammable in it's liquid state, only when it's gaseous.

Edit: Above I said that oxygen isn't very flammable in it's liquid state, this is somewhat incorrect as oxygen is an oxidiser and therefore isn't technically "flammable" under any conditions. What I was intending to communicate it's ability to oxidise is greatly reduced in it's liquid state and therefore significantly more energy must be added to make it combust. This can actually be seen in rocket engines as LOX/RP1 combinations require a pyrophoric (hypergolic with oxygen) TEA/TEB (Tri-ethyl-borane, Tri-ethyl-aluminium) mixture to ignite and HydroLOX engines required either the aforementioned TEA/TEB mixture or spark plug ignitors (I don't know if that's their technical name but it's essentially what they are). In both cases this shows that a significant amount of activation energy must be provided.