A two-foot-long steel strut only an inch wide at its thickest point was to blame for last month’s midair explosion of an unmanned SpaceX Falcon 9 rocket, according to a preliminary investigation. The CEO of SpaceX, Elon Musk, said on Monday that the part, provided by an external supplier he declined to name, was “not something that should ever have failed at this force level”.

The disaster, a major setback for Musk’s ambitious space plans, also destroyed the Dragon spacecraft that the Falcon 9 was carrying. The Dragon, a partially reusable spacecraft developed by SpaceX, was due for a rendezvous at the International Space Station.

In his first public comments on the disaster, Musk said the strut “would appear to be incorrectly made but with no visible way of determining that from the outside”. The part held in place a tank of helium pressurized at about 55,000 pounds per square inch, cooled cryogenically by liquid oxygen and fed into the engine and back out again to keep the pressure equal in the rocket’s second stage.

“The strut that we believe failed was designed to handle 10,000lb of force and it failed at 2,000lb of force,” Musk said.

He also said that the Dragon could have been saved. “If the software had initiated the parachute deployment, the Dragon spacecraft would have survived,” Musk said. “We’re adding additional software so that the the spacecraft will always attempt to save itself. It’s an unfortunate thing.”



Musk said the test of SpaceX’s next version of its rocket, the Falcon Heavy, had been pushed back to the spring of 2016.

Musk cautioned that further investigation by SpaceX might reveal more over time but said that the inquiry, conducted in cooperation with the Federal Aviation Administration, Nasa and the US air force, suggested the strut had allowed the helium tank to shoot up through the stage as its buoyancy increased in proportion to the gravitational pressure of the rocket’s acceleration, ultimately releasing helium into the liquid oxygen and causing “a pressure event”.



That event took place over .893 seconds – part of the reason the findings were only preliminary. “When milliseconds matter, it’s remarkably difficult to line things up exactly right,” he said.

“It’s very difficult to test a rocket stage,” Musk said. “The only way to fully test a rocket stage would be to have an enormous centrifuge that doesn’t exist on Earth.”

The tech CEO said he believed the company had begun to rest on its laurels after several successful launches. “The company as a whole, I think, became a little bit complacent over seven years, after 20 successes in a row,” he said, “and this is an important lesson and something we’re going to take with us in the future.”

Musk said SpaceX wouldn’t be using those specific struts in the future and, when the replacements were selected, would be testing each one individually irrespective of the supplier’s certification.

Like a wedding officiant, Musk asks his employees to speak or forever hold their peace, he told reporters. “Before any flight I always send out an email saying: ‘If anyone can think of any possible reason why we should keep this flight from launching, they should call me on my cellphone or email me whether their manager agrees with it or not,’” he said. “I think sometimes it just seems like Elon being paranoid again.”



The failure, he said, wouldn’t affect the company’s submission for Nasa’s upcoming commercial cargo contract.

“Rockets are a fundamentally difficult thing,” he said.