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Since SpaceX has isolated the AMOS-6 Falcon 9 pad explosion to the upper stage LOX tank during fueling, it seems useful to consider in general terms what was happening at the time and what materials and equipment were around. Rocket explosions usually involve firing engines and/or structural failures in flight, and I have little sense how this explosion was possible.

The current SpaceX statement is thus:

At approximately 9:07 am ET, during a standard pre-launch static fire test for the AMOS-6 mission, there was an anomaly at SpaceX's Cape Canaveral Space Launch Complex 40 resulting in loss of the vehicle.

The anomaly originated around the upper stage oxygen tank and occurred during propellant loading of the vehicle. Per standard operating procedure, all personnel were clear of the pad there there were no injuries.

I have read that LOX makes most anything extremely explosive and it takes very little to trigger that. How little does it take?

Would a spark or some other trigger have been necessary, and if so, what sources of such a thing were nearby? Would it have been easy for some source of ignition to have ended up there from the environment?

Would a leak of the equipment have been necessary for this to occur?

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    $\begingroup$ Any kind of organic substance can cause an explosion if it comes in contact with LOX. wiki.nasa.gov/oxygen-fire-incidents/wiki/home/… $\endgroup$ Commented Sep 2, 2016 at 22:31
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    $\begingroup$ @OrganicMarble Interesting. It implies that an impact was involved in that case. The units aren't intuitive to me, so i checked using this calculator. An impact of 40 to 50 lb/ft is like an object weighing 6 kg (13.2 lbs) being dropped 1 m (3.25 ft). $\endgroup$
    – kim holder
    Commented Sep 2, 2016 at 22:46
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    $\begingroup$ Also "1- 3 c. Oxygen systems shall be kept clean because organic compound contamination, such as hydrocarbon oil, can ignite easily and provide a kindling chain to ignite surrounding materials. Contamination can also consist of particles that could ignite or cause ignition when impacting other parts of the system" from hq.nasa.gov/office/codeq/doctree/canceled/1740151.pdf $\endgroup$ Commented Sep 2, 2016 at 22:55
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    $\begingroup$ Also this (GOX, not LOX) tested.com/science/space/… $\endgroup$ Commented Sep 2, 2016 at 23:00
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    $\begingroup$ There's some interesting (armchair?) slo-mo analysis and commentary here: youtube.com/watch?v=Ye0EOENUw0c At about 3:45-4:00+ the commentator pinpoints the second stage fueling interface as the source of the initial explosion. Unfortunately, the source video is only 60fps and a LOT happens in the 1/60th second between the last frame where nothing looks out of the ordinary and the next frame where flame is first visible. $\endgroup$
    – Anthony X
    Commented Sep 3, 2016 at 2:10

3 Answers 3

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Just to mention one facet of this multifaceted problem, any electrical field potential that exists (i.e. items that aren't on a common bias electrically) can and usually will at some point create an arc (spark) when it discharges. Coupling and de-coupling of interfaces, feedlines, etc especially would be a danger if their electrical fields are uneven. Also, aside from dynamic mechanical actions just stated, static arc production is always possible with higher potential field imbalances, and would be theoretically more prominent. And as Organic Marble brought forth, with high oxygen concentrations, it doesn't take much more for dust or greasy fingerprints to become a type of fuel in itself, conveying the reactive production of (fire) (flame).

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Liquid oxygen contains contains 4000 times more oxygen than normal air (by volume). A lot of materials that are almost not flameable in normal air may burn in liquid oxygen. Even pure oxygen at a pressure of 200 bar is a dangerous thing. Even stainless steel tubes did burn due to a steep pressure rise of the oxygen inside. Even an asphalt floor may be dangerous if liquid oxygen is spilt.

A paper about Resonance Tube Hazards in Oxygen Systems https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19820013541.pdf

NASA SAFETY STANDARD FOR OXYGEN AND OXYGEN SYSTEMS https://www.hq.nasa.gov/office/codeq/doctree/canceled/1740151.pdf

A NASA technical memorandum Test of LOX Compatibility for Asphalt and Concrete Runway Materials.

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  • $\begingroup$ This is an interesting answer! Is it possible to find one or two links to support it - the stainless steel tube burning perhaps? I've no doubt what you are saying is true, I'd just like to read more about it. $\endgroup$
    – uhoh
    Commented Oct 21, 2016 at 0:51
  • $\begingroup$ I think you mean "non-flammable". "Inflammable" means it burns in air. $\endgroup$
    – zeta-band
    Commented Apr 1, 2019 at 19:26
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    $\begingroup$ @zeta-band: "Inflammable" can mean either "flammable" or "non-flammable". (I agree that this is confusing.) $\endgroup$
    – Vikki
    Commented Apr 28, 2021 at 22:47
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Organics make better or worse fuels, but all are at least marginally combustible; LOX makes anything worse. The issue at such high oxygen concentrations is that even tiny amounts of organic contamination result in flammable mixtures. In any industrial area, the air contains organic pollutants; the relevant question is how much, how bad, in what context and to what consequences. In a place like Florida, the air contains plant volatiles , from leaves shedding, i. e., isoprenes, pollen, ethylene, the light components of sap and resin, etc. For most purposes, we don’t care… except for KABOOM purposes.

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  • $\begingroup$ A tank to be filled with LOX should be flushed with clean dry nitrogen or helium to remove organic contamination of the air. After cleaning the pressure inside the tank should be slightly higher than ambient pressure to prevent any contamination. $\endgroup$
    – Uwe
    Commented Jun 4, 2023 at 17:17

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