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Tuesday, November 21, 2017

SpaceX conclude accident investigations, targets return to flight on Sunday

Just four months after rocket loss, SpaceX ready to begin launching a packed manifest.
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Fours month after a fueling accident led to the loss of a Falcon 9 rocket and its satellite payload, SpaceX said Monday morning that it has concluded an investigation into the incident and submitted its findings to the Federal Aviation Administration. The company also announced a target date of January 8th for a return to flight.

The SpaceX investigation, in concert with the FAA, US Air Force, NASA, and the National Transportation Safety Board, concluded that one of three composite overwrapped pressure vessels, or COPVs, inside the rocket's second stage liquid oxygen tank failed. "Specifically, the investigation team concluded the failure was likely due to the accumulation of oxygen between the COPV liner and overwrap in a void or a buckle in the liner, leading to ignition and the subsequent failure of the COPV," the company stated in an update.

COPVs are used in rocketry to contain high pressure fluids and offer a substantial weight savings over all-metal tank designs. In a general sense, a composite simply means a matrix of carbon fibers contained within a resin, which is then wrapped over a pressure barrier. The Falcon 9 rocket uses COPVs that consist of a carbon wrap over an aluminum liner to store cold helium, which in turn is used to maintain tank pressure.

"The recovered COPVs showed buckles in their liners," SpaceX said Monday in its update. "Although buckles were not shown to burst a COPV on their own, investigators concluded that super chilled liquid oxygen can pool in these buckles under the overwrap. When pressurized, oxygen pooled in this buckle can become trapped; in turn, breaking fibers or friction can ignite the oxygen in the overwrap, causing the COPV to fail. In addition, investigators determined that the loading temperature of the helium was cold enough to create solid oxygen, which exacerbates the possibility of oxygen becoming trapped as well as the likelihood of friction ignition."

The investigation identified several "credible causes" for this failure, all of which can be avoided in the short term by changing the COPV configuration to allow for the loading of warmer helium, and returning helium loading procedures to a "prior flight proven configuration." Presumably this means prior to December 2015, when the company began using supercooled liquid oxygen and kerosene fuels to increase the performance of its rocket, known as the Falcon 9 Full Thrust vehicle. Since the accident did not involve the rocket fuels themselves, Ars understands that the new procedures will not substantially affect the performance gains of the full thrust Falcon 9 for upcoming launches.

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