[phkahler]

My thoughts on this. Overpressure in the LOX tank could be due to 1) failure to vent properly or 2) something heating it up.

Option 1 is based on the fact that it's constantly evaporating and needs to vent. This seems really unlikely given that it passed all test on the pad 2 minutes before. It also sounds like this was likely ruled out based on Elon's tweet about a "counter intuitive cause".

Option 2 sounds like fire or flames due to fuel leakage or something, but then realize that this is the second stage and all the action is going on 100 or more feet down in the first stage. They also have a camera on the 2nd stage engine which was shown shortly before the incident and nothing was going on in there. I wonder what the in-tank camera showed.

It seem that to get the extra energy into the tank, something must have fired up early. But if there's one thing Spacex seems to have a lot of it's data. Aside from a breech letting external air in (like the last shuttle accident), how do you get enough added energy into a tank to build pressure to the breaking point? In 2 minutes.

[JshWright]

There are plenty of other scenarios...

The IDA is the heaviest thing they have ever carried in the trunk, AFAIK. Maybe its mounting bracket failed and it impacted the top of the second stage, buckling the LOX tank. A suddenly induced crack allowed some LOX to escape (the initial 'puff'), and the suddenly reduced pressure allowed the rest of the LOX to boil off and the tank to BLEVE, causing the catastrophic failure of the second stage.

Obviously this is a completely theoretical scenario, but it's one of many I could dream up...

EDIT: While is is a fairly technical discussion, I realize I got a little heavy handed with the acronyms there...

  IDA: https://en.wikipedia.org/wiki/International_Docking_Adapter
  LOX: https://en.wikipedia.org/wiki/Liquid_oxygen
  BLEVE: https://en.wikipedia.org/wiki/Boiling_liquid_expanding_vapor_explosion

[mmedellin]

Falcon typically has a trunk-facing camera on top of the second stage, sometimes broadcasted during separation with Dragon. I wonder if CRS-7 had the camera? If the mount did fail, the footage could show that (assuming SpaceX successfully received the broadcast).

Example footage here: https://youtu.be/p7x-SumbynI?t=25m45s

[Serow225]

Interpreting Musk's tweet about 'counter-intuitive' meaning BLEVE would make sense to me.

[JshWright]

Hmm... perhaps, but that seems pretty 'intuitive' to me.

[tbabb]

Could the mass of the IDA have introduced unexpected lateral forces that caused the second stage to buckle like a soda can under the aerodynamic pressure? And perhaps simple crushing was the cause of the tank overpressure?

[emp_zealoth]

I can see why you would say BLEVE. But I'm quite curious why there wasn't a huge fireball, at any rate

[JshWright]

Oxygen by itself doesn't burn. If it was just the LOX tank that burst, there wouldn't have been anything to burn. Clearly some sort of FTS destroyed the vehicle (I don't think atmospheric forces could have taken it apart that cleanly and instantaneously), but the FTS system is probably just detcord running down the length of the propellant tanks. If the tanks were 'unzipped' quickly enough, the fuel and oxygen might not have had time to mix to a stoichiometric ratio that supported combustion.

Looking back at a gif of the incident, there was a pretty impressive amount of fire. Initially when the oxygen hit the first stage exhaust, causing the unburnt RP1 to burn (all the 'extra' flame that appears in the exhaust plume), and then there is a fair amount of fire at the front of the vehicle a second or two later (I assume the second stage RP1 tank failed at that point). Finally there is a large cloud and the rocket vanishes. That's the part that I assume is some sort of FTS, and likely blew the rocket apart with enough force that the prop didn't get a chance to mix.

[fche]

Did Jack Swigert stowaway on board to do a cryo-stir?