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If you watch the truly awesome video of the Jason-3 first stage landing on the ASDS Barge Just Read The Instructions, it is pretty clear that for one thing, they landed it. Then the leg gave out, gosh darn it!

However watch when it explodes, the side of the booster has just barely begun to touch the deck and it blows.

What part of the vehicle causes that explosion? The LOX tank rupturing and spilling LOX probably would not explode that fast, would it? Is it an engine getting crumpled and the TEA/TEB spilling out along with leaking LOX?

It happens like in a movie, which is never how things really happy, so what is the cause of the explosion?

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    $\begingroup$ I wondered the same thing. As an extra thought, it isn't at all clear to me what is even typical let alone in this specific situation in terms of how much the propellants react with each other, how much mixing occurs and if much unreacted propellant is dispersed. $\endgroup$ – Puffin Jan 18 '16 at 15:45
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I reviewed the video frame-by-frame. The stage falls on the collapsed leg (which was hot enough to be on fire during the landing). The leg penetrated the RP-1 tank. The tank is under pressure, so it ruptures violently and takes the oxygen tank with it. You have both gushing out under pressure (i.e. at high speed) over a hot deck and towards the even hotter engines. 5 frames later (0.2 seconds), a small fire erupts near the top of the collapsed landing leg. Then it's another 10 frames (0.4 s) before the big fireball.
So the explosion predates the flames and is purely due to the pressure in the tanks.

For the effect LOX has on a fire, see this 'don't try this at home' video.

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    $\begingroup$ That fast? It just looks like it happens too fast. Did not think of the hot leg being the ignition source. Good point. $\endgroup$ – geoffc Jan 18 '16 at 15:45
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    $\begingroup$ Thanks for that bit of inspiration, I've looked again at the video and paused it before the fireball. Is it plausible that there is rupturing of both tanks simply from the side load occurring at about the same time and independently of the combustion initiated near the bottom of the vehicle? i.e. the latter, from the hot leg as you suggest, causes RP1 to combust with ambient air in the first instance and the path of the explosion is aided by the fact that the tanks are already split wide open? $\endgroup$ – Puffin Jan 18 '16 at 15:59
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If you look and listen closely to the video, you'll hear two bangs, not one:

First, the LOX tank at the top of the rocket ruptures. Since it's pressurized, it doesn't just spill out, but quite loudly shoots liquid oxygen in all directions. But no explosion, only a small fire, likely combusting residue kerosene on the engine.

A moment later, the kerosene tank at the bottom ruptures. Now we see a fireball!

Since there was plenty of liquid oxygen around, sprayed into the air half a second before, and already a small fire to initiate the reaction, this allowed the RP-1 to explore quite rapidly.

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Liquid Oxygen has an expansion ratio of 1:861 (This means that 1 liter of liquid oxygen is equivalent to 861 liters of gaseous oxygen). Having worked with it on military aircraft, and witnessed a VERY SMALL amount being introduced to a flame source, I can tell you it very much could have just been the LOX going up. (I won't speculate if that's what it actually was)

At atmospheric pressure, LOX under pressure can convert to gaseous very quickly. If it's not under pressure, it converts much slower, but still creates a very dense fog in a matter of minutes.

Heat it up from a fire, and things get significantly more violent, in an incredibly small amount of time.

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    $\begingroup$ After re-reading my own answer here, I realize I didn't properly clarify that LOX on its own, introduced to a flame, will not go up in immense flames. A fuel is also required, to complete the fire triangle. This is addressed by radex's answer suggesting the LOX did not ignite until it came into contact with the kerosene in the vicinity of a heat source. $\endgroup$ – Rayanth Jan 20 '16 at 5:28
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Combustibles (oil, fat or grease, but also asphalt) in contact with liquid oxygen may explode on ignition or impact. See these links 1, 2, 3, 4, 5, 6, 7. This NASA document is about tests on compatibility of runways to LOX.

A very impressive cite from the NASA document: "A detonation was reported to have occured when a man walked across a gravelled surface on which liquid oxygen was spilled. Ignition was attributed to friction between gravel and the asphalt beneath."

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