NASA investigation linked 2015 Falcon 9 failure to design error

Just after T+2 minutes, the NASA announcer says "Everything coming back shows the vehicle on course, on track.” The Falcon 9 then appears to explode. Credit: NASA TV

WASHINGTON — A NASA investigation into a 2015 SpaceX launch failure concluded a design flaw, rather than a manufacturing defect, likely initiated the chain of events that destroyed the vehicle.

NASA released March 12 a public summary of the report by an independent review team convened by NASA after the June 2015 accident during the launch of a Dragon cargo spacecraft bound for the International Space Station, a mission known as CRS-7. That investigation was performed by NASA’s Launch Services Program at the request of the agency’s associate administrator for human exploration and operations, Bill Gerstenmaier, in parallel with SpaceX’s own investigation into the failure.

In that failure, the vehicle suffered an “anomalous event” in the second stage liquid oxygen tank 139 seconds after liftoff, causing the vehicle to break apart. The investigation was a challenge, NASA noted in its report, because the accident happened so quickly, with no sign of “obviously degrading or trending conditions” prior to the event.

“In other words,” the report stated, “the vehicle went from flying fine to conflagration in less than a second, or ‘within a blink of an eye.’”

The NASA investigation, like the SpaceX one, tracked down the most likely cause of the failure to a composite overwrapped pressure vessel (COPV), filled with helium, that came loose in the second stage liquid oxygen tank. The COPV was able to accelerate upwards due to its buoyancy, according to the report, hitting the dome at the top of the liquid oxygen tank with “great force” and rupturing it.

The two investigations differed, though, on why the COPV came loose. SpaceX concluded the most probable cause that a bolt called a “rod end” suffered a “material defect” that caused it to break, liberating the COPV. SpaceX publicly stated that conclusion a few weeks after the accident. The company no longer uses that strut in its vehicles.

However, the NASA investigation concluded that the rod end could have also suffered manufacturing damage or been installed improperly, among other “equally credible” possibilities.

The NASA investigation also identified what it called a “design error” for the tank: the use of “industrial grade” rather than “aerospace grade” stainless steel for the rod end. That material was not properly modeled or tested for use in that application, the report noted, and did not have a 4:1 factor of safety recommended by the manufacturer.

“This design error is directly related to the Falcon 9 CRS-7 launch failure as a ‘credible’ cause,” the NASA report concluded.

The report also identified other general issues with the use of commercial parts and other vehicle design elements, such as potential for increased latency in its telemetry system. It recommended SpaceX pay more attention to the use of commercial parts. “SpaceX should apply particular emphasis to understanding manufacturer’s recommendations for using commercially sourced parts in flight critical applications,” it stated.

SpaceX argued that all potential causes, including those identified by NASA, were corrected prior to the vehicle’s return to flight.

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