A steel strut holding a bottle of helium likely gave way, causing the liquid oxygen tank in the upper-stage of a SpaceX Falcon 9 rocket to over-pressurize, triggering an explosion minutes after launch last month, company chief executive Elon Musk said Monday.
After poring over thousands of bits of engineering data, analyzing video and studying wreckage salvaged after the accident, a materials defect is the leading cause of the June 28 failure. The accident claimed a load of cargo heading to the International Space Station.
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"It's a really odd failure mode," Musk told reporters on a conference call.
Engineers tested thousands of the struts and found a few that failed far below the force they were designed -- and certified -- to withstand.
"We have been able to replicate the failure by taking a huge sample, essentially thousands of these struts, and pulling them. We found a few that failed far below their certificated level. That's what led us to think that there was one just far below its rated capability that happened to be at the wrong place at the wrong time," Musk told Discovery News.
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Over the course of 18 successful Falcon 9 flights, thousands more struts were flown, apparently without issue.
As a result of the accident, SpaceX will look for a new strut design, most likely from a new vendor, and test each one before they are installed in the rocket's tanks.
Flights are not expected to resume until September at the earliest, Musk added.
The work also will delay the debut flight of Falcon Heavy, a 27-engine version of the Falcon rocket, from late this year until next spring.
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The accident investigation remains underway, with the company casting a wide net to consider other issues and contributing factors to the June 28 accident, as well as potential concerns for future missions.
One lesson learned: Future Dragon cargo ships will have new software so that if a launch accident occurs, the capsules' parachutes will deploy. The upgrade already was planned for the passenger version of Dragon, which SpaceX is developing in partnership with NASA to fly astronauts to and from the International Space Station.
"If the software had initiated the parachute deployment then the Dragon spacecraft would have survived," Musk said. "We're now including contingency software that if something were to go wrong with the vehicle, Dragon will always attempt to save itself."