WASHINGTON — SpaceX Chief Executive Elon Musk said Nov. 4 he expects the Falcon 9 rocket to return to flight in the middle of December after overcoming a problem he claimed was unprecedented in the history of spaceflight.

Musk, briefly discussing the status of SpaceX during a half-hour interview on the cable news network CNBC Nov. 4, said that investigators had determined what caused the Sept. 1 pad explosion that destroyed a Falcon 9 and its satellite payload during fueling for a static-fire test.

“I think we’ve gotten to the bottom of the problem,” he said. “It was a really surprising problem. It’s never been encountered before in the history of rocketry.”

Musk, confirming earlier discussion about the investigation, said the failure involved liquid helium being loaded into bottles made of carbon composite materials within the liquid oxygen tank in the rocket’s upper stage. This created solid oxygen, which Musk previously said could have ignited with the carbon composite materials. However, he did not go into that level of detail in his CNBC comments.

“It’s never happened before in history, so that’s why it took us a while to sort it out,” Musk said, adding that SpaceX has been working with NASA, the U.S. Federal Aviation Administration and commercial customers on the accident investigation. “This was the toughest puzzle to solve that we’ve ever had to solve.”

Musk, though, suggested that the puzzle is now solved and that launches can resume in December. “It looks like we’re going to be back to launching around mid-December,” he said. He did not disclose what payload would fly on that return-to-flight mission, or from where the launch would take place.

SpaceX’s last public statement about the accident investigation, published on its web site Oct. 28, said that the company had narrowed its focus to one of three helium bottles inside the liquid oxygen tank that burst, noting it was able to replicate the tank failure with helium loading conditions. “The root cause of the breach has not yet been confirmed,” the company said at the time.

Musk’s statement echoes comments made Nov. 3 by Inmarsat Chief Executive Rupert Pearce, whose company has three satellites awaiting launches on SpaceX’s Falcon 9 and Falcon Heavy vehicles. “We believe they now have found a root cause that is fixable quite easily and quite quickly,” he said in a conference call with investors. “So they should be able to return to flight in December.”

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...