Steel strut holding down a bottle of high-pressure helium snapped during ascent, though Dragon capsule would have survived if not for software glitchBy Mike Wall and SPACE.com | July 21, 2015
The disintegration of a SpaceX Falcon 9 rocket shortly after it launched on a space station resupply mission for NASA last month was most likely caused by a faulty strut inside the booster’s upper stage, company CEO and founder Elon Musk said Monday (July 20).
SpaceX investigators believe the explosion of the Falcon 9 rocket on June 28 during a launch to the International Space Station occurred because a steel strut holding down a bottle of high-pressure helium snapped during ascent. This failure allowed the bottle to shoot to the top of the booster’s upper-stage liquid-oxygen tank at high speed, causing a rapid “overpressure event” that destroyed the rocket, Musk told reporters during a teleconference. (Somewhat counterintuitively, buoyancy increases with g-forces during a launch, explaining why the bottle would travel up, Musk said.)
Every Falcon 9 launches with hundreds of such struts aboard, Musk said. Both stages of the two-stage rocket harbor many bottles that store helium at cryogenic temperatures. During flight, this helium flows to the engines, where it is warmed; the substance is then recirculated to the booster’s liquid-oxygen and fuel tanks to repressurize and structurally stabilize them, compensating for the volume of fuel and oxidizer lost during flight. [The Falcon 9 Explosion in Slow Motion (Video)]
The roughly 2-foot-long (0.6 meters) struts hold these bottles in place. Each strut is certified to withstand about 10, 000 lbs. (4, 500 kilograms) of force. Based on when the explosion happened—less than 3 minutes after liftoff—the strut in question apparently broke under a load of less than 2, 000 lbs. (900 kg), Musk said.
“It’s not something that should have ever failed at this force level, ” he said. The strut “would appear to be incorrectly made, but there was no visible way of telling that from the outside.”
SpaceX sources these struts from an outside company and will probably change suppliers now, Musk added. In addition, SpaceX plans to individually test and certify every strut that will fly, to ensure that no faulty ones make it on board, he said.
Still, he added, “right now, there doesn’t seem to be any other explanation that could make sense.”
The Falcon 9 launched from Florida’s Cape Canaveral Air Force Station on June 28 carrying SpaceX’s robotic Dragon space capsule on the company’s seventh contracted cargo mission to the space station. SpaceX holds a $1.6 billion NASA deal to make at least 12 such flights; the first six missions had all been completely successful.
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