As it prepares the ground ahead of its safety analysis of Boeing and SpaceX commercial crew systems which will fly in 2020, NASA’s independent report into the Falcon 9 launch failure which lost Dragon CRS-7 and eight small Flock/Dove satellites on 28 June 2015 has finally been released. And it is specifically critical of a SpaceX design error – and by implication its chief designer: Elon Musk.
The vehicle exploded 2 minutes 19 seconds after launch just before first stage shutdown at an altitude of 45km. An overpressure event was detected in second stage oxygen tank less than a second before the explosion. An investigation into the cause of the Falcon 9 (v1.1) launch failure in June came to conclusion that a strut holding a helium filled composite overwrapped pressure vessel (COPV) tank down inside oxygen tank, snapped (probably near the bolt attachment point), either the helium COPV tank had struck the LOx tank dome causing it to rupture, or that it caused a helium leak which allowed helium to over-pressurise and blow up the second stage oxygen tank. The buoyancy of the helium tank actually increases with rocket acceleration.
The subsequent independent investigation by NASA agreed with the above scenarios but disagreed that it was a material defect in the part supplied by the subcontractor which was the most likely cause. NASA’s investigators argued that it was poor design in the selection of the rod/strut material (an industrial rather than an aerospace grade stainless steel was chosen without adequate analysis given the loads and cryogenic environment it was under) that was a more likely cause of its failure. The launch, spacecraft and the spacecraft’s contents were not insured.