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. “This design error is directly related to the Falcon 9 CRS-7 launch failure as a ‘credible’ cause,” the NASA report concluded. 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. 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. However, the NASA investigation concluded that the rod end could have also suffered manufacturing damage or been installed improperly, among other “equally credible” possibilities.
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The company no longer uses that strut in its vehicles. SpaceX publicly stated that conclusion a few weeks after the accident. 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. The two investigations differed, though, on why the COPV came loose. 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.
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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.
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“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 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 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.