The spectacular June 28 failure of a SpaceX Falcon 9 rocket apparently was triggered by a support strut inside the booster's second stage liquid oxygen tank that suddenly broke free, releasing a buoyant high-pressure helium bottle that caused the rocket to break apart from the inside out, company founder Elon Musk told reporters Monday.
Providing initial results of an internal failure investigation, Musk said the strut in question, one of several aboard the rocket that was purchased from an unidentified vendor, apparently failed well below its design specification, allowing the high-pressure helium bottle it was holding in place to twist away.
Helium, contained in several composite overwrap bottles at 5, 500 pounds per square inch, is used to pressurize the first and second stage propellant tanks to maintain the rocket's structural integrity as liquid oxygen and kerosene fuel is consumed during the climb to space.
The bottles are immersed in the liquid oxygen tanks of the first and second stages to cool the gas before it is routed to the engines, warmed up and sent back to the tanks to maintain the proper pressure.
During the June 28 launch, the Falcon 9's first stage operated normally to boost the rocket out of the dense lower atmosphere. But near the end of its planned burn, about two minutes and 39 seconds into flight, the second stage appeared to rupture, spilling its load of propellant in a thick cloud of white vapor.
The first stage continued firing, but moments later the rocket broke up in a shower of debris. The Dragon cargo capsule it was carrying, loaded with more than 4, 000 pounds of equipment and supplies bound for the International Space Station, apparently broke away from disintegrating rocket intact.
Musk said engineers were able to receive telemetry from the spacecraft until it disappeared over the horizon on the way to a catastrophic ocean impact. He said a software change will be implemented on future space station resupply flights to enable the Dragon's parachutes, normally used to lower the craft to a gentle ocean landing at the end of a mission, to deploy in the event of a launch failure.
"That's an unfortunate thing, " Musk said. "We could have saved Dragon if we had the right software" on board.
It was the first failure in 19 flights of a SpaceX Falcon 9 and the second loss in a row for the space station program following the April 28 failure of a Russian Soyuz rocket carrying a Progress supply ship. The Russian cargo craft reached orbit, but it suffered severe damage moments before, or during, release from the booster's upper stage and crashed back to Earth on May 8.
Lost in the SpaceX mishap, along with supplies and equipment for the station crew, was the first of two docking mechanisms needed to enable commercial crew capsules being built by Boeing and SpaceX to dock with the lab complex starting in 2017.
Musk said the failure and subsequent launch delays likely will cost the company several hundred million dollars in lost revenue. But he said he's hopeful flights can resume as early as September, although it's not yet clear how the manifest will shake out or what customer will get the next launch slot.
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