Elon Musk, CEO of SpaceX, reports that his company has discovered why SpaceX's Falcon 9 exploded at the launch pad in Cape Canaveral, Florida last September. Now that the issue has been resolved, Musk says he expects the SpaceX to return to flight in mid-December.

In an interview with CNBC, Musk reveals that the September SpaceX explosion was the "toughest puzzle" they've ever had to solve. Musk added that "it basically involves liquid helium, advanced carbon fiber composites, and solid oxygen. Oxygen so cold that it actually enters solid phase."

Part of the problem that baffled the investigators was that the explosion that destroyed the Falcon 9 rocket occurred before the ignition of the engines for a planned test. In a transcript obtained by Space News, Musk postulated that the propellant used by the SpaceX, supercooled liquid oxygen, became so cold that it turned into solid. The solid oxygen may have reacted with another piece of hardware, such as one of the vehicle's three liquid helium pressure vessels.

The pressure SapceX vessels, arranged inside the upper oxygen tank, are configured for filling and pressurizing the empty space when the propellant exits the tank. The oxygen tank holds the supercooled liquid oxygen propellant while the pressure vessels are wrapped with carbon fiber composite material. The solid oxygen that formed may have caused the carbon to ignite. Musk did not provide other details regarding how the solid oxygen affected the carbon fiber.

To prevent further disaster, SpaceX has been conducting tests at its facility in McGregor, Texas. The tests simulate the same scenario that led to last month's failure which also resulted in the loss of the satellite it was supposed to carry into space: an Israeli communication satellite called Amos-6.

In response to what the SpaceX has discovered, the company says they are now focusing on improving their techniques on helium loading. And with SpaceX still under contract with NASA to carry astronauts to the space station next year, Musk is confident that SpaceX will be back to flight by the end of the year.