Topics:Private spaceflight, mission status, human spaceflight, International Space Station
The demise of an International Space Station-bound Falcon 9 rocket last month was likely caused by a broken liquid helium bottle strut, according to SpaceX CEO Elon Musk. Speaking to reporters by phone, Musk said the mishap, which occurred 139 seconds into the flight of CRS-7, flooded the upper stage liquid oxygen tank with excess helium. The tank overpressurized, ruptured, and the rocket disintegrated shortly thereafter.
"I do have to point out that this is an initial assessment, " Musk said. "Further investigation may reveal more over time."
High-pressure liquid helium bottles are stored inside the first and second stage liquid oxygen tanks. The helium, which is presurized to 5, 500 pounds per square inch, is used to replace spent propellant as the rocket ascends. This gives the tanks structural stability while providing the compression necessary to power the engines.
Inside the upper stage liquid oxygen tank, a steel strut about two feet long and an inch wide at its thickest point appears to have snapped, Musk said. "One of those struts appears to have failed, and as a result was unable to hold the helium bottle down. So the helium bottle was shot to the top of the tank at high speed."
The strut was not made in-house by SpaceX, but Musk declined to name the supplier. Falcon components are tested on the ground, culminating with a hold-down engine firing a few days before launch. But no simulation can completely mimic the extreme forces of a ride to orbit, said Musk. When Falcon disintegrated, the rocket was experiencing about 3.2 times the force of gravity.
SpaceX CRS-7 liftoff video (NASA TV)
In launch videos, the gumdrop-shaped Dragon spacecraft can be seen tumbling away from Falcon through the cloud of rapidly venting liquid oxygen. Dragon continued to communicate with ground controllers until it fell behind the horizon, impacting in the Atlantic Ocean. The cargo carft was carrying 1.8 metric tons of ISS supplies, along with a docking adapter in its unpressurized trunk designed to allow future commercial crew vehicles to dock at the station.
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