Just after T+2 minutes, the NASA announcer says "Everything coming back shows the vehicle on course, on track.” The Falcon 9 then appears to explode. Credit: NASA TV

WASHINGTON — A strut in an upper stage propellant tank that failed at a fraction of its rated strength is the leading explanation for the June 28 loss of a SpaceX Falcon 9 rocket, the company’s chief executive said July 20.

In a teleconference with reporters, Elon Musk said the preliminary conclusion of the three-week investigation was that a steel strut, designed to hold a bottle of helium in place within the upper stage’s liquid oxygen tank, snapped while the first stage was still firing. That released enough helium gas to overpressurize the tank, causing it to burst and destroying the upper stage.

“The vast majority of the people at the company have only ever seen success,” Musk said. “When you’ve only ever seen success, you don’t fear failure quite as much.”

“That’s the best explanation we can think of right now,” Musk said, emphasizing throughout the call the difficulty in identifying the cause of the failure and the preliminary nature of the conclusion. “So it’s a really hard, hard failure mode.”

Musk said that a strut failure was considered in the investigation but initially not thought likely. The strut is rated to handle up to 10,000 pounds of force, three times the calculated maximum loads, but failed at just one-fifth that rated amount. No other such struts, used hundreds of times, had experienced problems on previous launches, and tests of a small sample of the struts on the ground failed to show any problems. Photos taken of the interior of the tank during closeout of the stage also showed no signs of damage to the strut or errors in its assembly.

It was only after testing what Musk said were “thousands” of the struts did they find a few that failed at much lower forces that expected. “It was sort of a statistical thing,” he said. A closer examination of the failed test struts turned up problems with the grain structure in the steel used in the struts.

The strut, which Musk described as being about 60 centimeters long and about 2.5 centimeters thick, is provided by an outside supplier that he declined to identify. SpaceX plans to use a different strut design on future launches, including a potential switch from steel to Inconel, and will likely buy the struts from a different supplier. They will also receive more thorough individual testing.

Musk declined to give a specific schedule for resuming flight, beyond saying the next launch would be no earlier than September. “We want to turn over every piece of data” to make sure they did not overlook anything, he said. The customer for that return-to-flight mission has not been identified.

Musk did say he briefed the company’s customers last week about the preliminary results of the investigation. “They agree with our conclusions thus far,” he said. “Every one of our customers has been supportive and none of them have indicated diminished faith in SpaceX.”

“Meaningful” Losses, Falcon Heavy Delay

The failure itself, and resulting launch delays, will cause “meaningful” lost revenue for SpaceX, which Musk estimated in the hundreds of millions of dollars. He added that the first launch of the Falcon Heavy will be “deprioritized” as the company focuses on returning the Falcon 9 to flight. “We’ll probably launch in the spring of next year,” he said of the heavy-lift rocket the company has been developing for several years. That launch was previously scheduled for later this year.

The problem, though, should not affect the company’s plans to develop a crewed version of the Dragon spacecraft by 2017. “It doesn’t affect the critical path on our commercial crew timeline,” Musk said. “The critical path is really the design and validation of Dragon 2.”

Musk said that there was no evidence of problems with either the Falcon 9 first stage or the Dragon cargo spacecraft. He added the Dragon appeared to survive the vehicle’s destruction and continued to communicate until it fell below the horizon, shortly before it hit the ocean. The vehicle’s current software, though, didn’t allow it to deploy its parachutes.

SpaceX plans to upgrade the software on the cargo version of the Dragon to include contingency modes to deploy parachutes in the event of such a launch failure, something the company was already planning for the crew version of the spacecraft. “We could have saved Dragon if we had the right software,” Musk said.

He also suggested that the company’s 4,000-person workforce, most of whom have joined the company since the failure of a Falcon 1 rocket nearly seven years ago, had become “complacent” during its string of successful launches. “The vast majority of the people at the company have only ever seen success,” he said. “When you’ve only ever seen success, you don’t fear failure quite as much.”

“I think now everyone in the company appreciates just how difficult it is to get rockets to orbit successfully,” he said. “I think we’ll be stronger for it.”

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...