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takyon writes:

Elon Musk appeared on CNBC and offered a definitive explanation for his company's recent launch explosion:

SpaceX CEO Elon Musk says that his company has finally gotten to the bottom of the September 1st Falcon 9 explosion — claiming it was the "toughest puzzle" they've ever had to solve. And now that the problem is known, he expects SpaceX to return to flight in mid-December.

Speaking on CNBC yesterday, Musk said "it basically involves liquid helium, advanced carbon fiber composites, and solid oxygen. Oxygen so cold that it actually enters solid phase." So what does that mean exactly? Musk gave some hints a little while ago during a speech he gave to the National Reconnaissance Office. According to a transcript received by Space News, he argued that the supercooled liquid oxygen that SpaceX uses as propellant actually became so cold that it turned into a solid. And that's not supposed to happen.

This solid oxygen may have had a bad reaction with another piece of hardware — one of the vehicle's liquid helium pressure vessels. Three of these vessels sit inside the upper oxygen tank that holds the supercooled liquid oxygen propellant. They're responsible for filling and pressurizing the empty space that's left when the propellant leaves the tank. The vessels are also over wrapped with a carbon fiber composite material. The solid oxygen that formed could have ignited with the carbon, causing the explosion that destroyed the rocket.

Musk called the issue one that had "never been encountered before in the history of rocketry." One of SpaceX's customers, Inmarsat, may find an alternative for one of its upcoming satellite launches. SpaceX launches could resume mid-December.

For comparison's sake, at standard pressure:

• Liquid Helium: 4 K (about −269 °C or −452.2 °F).
• Solid Oxygen: 54.36 K (−218.79 °C, −361.82 °F).
• Solid Nitrogen: 63.15 K ​(−210.00 °C, ​−346.00 °F)
• Liquid Nitrogen: 77.355 K ​(−195.795 °C, ​−320.431 °F)
• Liquid Oxygen: 90.19 K (−182.96 °C; −297.33 °F)
• Dry Ice (Solid CO2): 194.65 K (−78.5 °C; −109.3 °F).

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