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Nov 01, 2016, 01:20 AM
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The Falcon 9 explosion theory


So the helium bottles on the Falcon 9 are pressurized to 5000psi. A household propane tank is 300psi at most. They're submerged in the LOX to increase the amount of helium they can store & save space. In previous rockets, they sit in the LOX bath long after filling is complete, to equalize the temperature before being pressurized.

The new Falcon 9 has to take off immediately after tanking, in order to keep the LOX supercooled far below its boiling point. The helium bottles have to be filled before the LOX filling is complete & before their temperature equalizes. The trick is getting the helium the right temperature so the interior & exterior of the tank are the same temperature even as the exterior is still cooling down. There's probably a temperature differential they have to be within. If the helium is too cold, the exterior of the tank is shrinking slower than the interior & it sheers. If it's too hot, the interior is shrinking slower than the exterior & it sheers. An easy solution would be to move the helium tanks to the bottom of the LOX tank so they have more time to cool, but this would add weight & SpaceX wants to save money by tweeking the filling procedures.

Now we know the latest explosion happened when the helium bottles were being filled & someone in LA knew immediately what happened. Still not clear how a bursting carbon fiber tank in a pure LOX environment can ignite. Perhaps it damaged something on the service tower which ignited it.
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