When an unmanned SpaceX Falcon 9 rocket headed to the International Space Station suddenly exploded shortly after take off on June 28, it brought an end to a string of 18 successful missions conducted with the new rocket. Now, after poring over thousands of pieces of data, SpaceX engineers believe the incident was caused by a single faulty two-foot-long by one-inch-thick metal strut, in what Elon Musk referred to in a conference call with reporters as “a really odd failure mode.”


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Engineers believe just two and a half minutes after take off from Cape Canaveral, Florida, one of thousands of metal struts used in the Falcon 9 rocket broke, causing a helium tank to break free and leak within the second-stage liquid oxygen tank. This caused extra pressure within the oxygen tank that caused the rocket to break apart from its midsection.

Related: Watch SpaceX Falcon 9 rocket explode during ascent

Speaking to reporters, Musk noted that at this point the theory of the rocket’s failure is somewhat speculative, but in discussion with Discovery News, he elaborated: “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.”

Indeed, if it was a failure in the strut, it occurred at 2,000 pounds of force when it should have been able to withstand 10,000 pounds. And with that a significant lesson is learned—after SpaceX’s extraordinary number of successful flights, Musk admits: “I think to some degree, the company as a whole became maybe a little bit complacent.”

So now SpaceX is poised to develop a new strut design, most likely working with a new vendor, testing each strut before it is used in the craft. Additionally, parachutes will be used on all future Dragon capsules. While this feature was already planned for manned missions using the capsule, SpaceX has been able to determine that had the capsule had the capacity to self-abort, it would have likely survived the explosion, preserving the payload of supplies and experiments that were en route to the ISS.

Via Discovery, New York Times