Elon Musk said Friday that SpaceX could resume launching rockets as soon as mid-December, and that engineers had established the cause of an explosion in September.

“I think we’ve gotten to the bottom of the problem,” Musk said. “Really surprising problem that’s never been encountered before in the history of rocketry.”

He described investigating the cause of the explosions that destroyed the Falcon 9 rocket as the toughest puzzle his SpaceX company has ever had to solve. The explosions occurred before engine ignition, during a test for a launch scheduled 2 days later. A 200 million-dollar satellite was also destroyed in the blast. The incident has delayed NASA’s International Space Station operations, as SpaceX was one of only two companies bringing cargo to the space station. The next SpaceX launch, which was planned for this month, is now postponed indefinitely.

According to Musk, the explosions were caused by a problem during fueling. While liquid oxygen flowed into a tank on the rocket’s second stage, the propellant froze solid, which started a chain reaction that caused the series of explosions.

The news follows a statement from Space X that investigations would focus one of three helium containers in the oxygen tank. While liquid oxygen is consumed during launch, helium is heated and released, in order to maintain pressure within the tank. SpaceX said it had replicated the rupture of helium containers, which are made of carbon fiber composite materials.

“It basically involves a combination of liquid helium, advanced carbon fiber composites and solid oxygen, oxygen so cold that it actually enters solid phase,” Musk explained, giving no other information about how the carbon fiber composites had been affected by the solid oxygen.

SpaceX has been using an upgraded Falcon 9 rocket since December of last year, a design that uses liquid oxygen that has been supercooled to -340 degrees Fahrenheit, which is 40 degrees cooler than what is commonly used for rocket propulsion. SpaceX says that the cooler temperatures increases the oxygen’s density and improves engine thrust.

Helium in a liquid state would be as cold as -452 degrees, which seems to have provided the additional cooling to turn a portion of the oxygen solid.

Observers have questioned the company’s procedures for launching their rockets, with the Wall Street Journal reporting earlier this week that they could pose hazard for astronauts in future launches. When using supercooled fuel, the rocket is launched soon after fueling occurs. SpaceX has proposed plans to have astronauts board the rocket and strap in before fueling, instead of afterwards.

Retired Lt. Gen. Thomas P. Stafford of the US Air Force said these plans were “contrary to booster safety criteria that has been in place for over 50 years, both in this country and internationally.”

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