Four months after a Falcon 9 rocket exploded on a Florida launch pad, SpaceX said Monday that the cause was a failed composite overwrapped pressure vessel used to store liquid helium.
In the final update to its investigation, the space company said one of three such tanks in the rocket's larger, second-stage liquid oxygen tank failed, probably after a buildup of liquid oxygen between the vessel's aluminum inner liner and its carbon overwrap ignited.
The company said its accident investigation team — comprising officials from SpaceX, NASA, the National Transportation Safety Board, the Air Force, the Federal Aviation Administration and industry experts — found "buckles," or valleys, in the vessels' inner liners.
Super-chilled liquid oxygen can gather in buckles, and when pressurized, the oxygen can get trapped. Then, "breaking fibers or friction can ignite the oxygen in the overwrap," causing an explosion, SpaceX said.
The company also said 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."
To fix the problem in the short term, SpaceX said it will change the configuration of the composite overwrapped pressure vessels "to allow warmer temperature helium to be loaded." It will also change helium loading operations to a "prior flight proven configuration" that is based on procedures that were used in hundreds of successful launches.
SpaceX plans to eventually change the design of the vessels to prevent buckles, which the company said will allow for faster loading.
The company plans its next launch Jan. 8, pending FAA approval. It would launch 10 satellites for Iridium Communications from Vandenberg Air Force Base in California.