After four months of intensive investigation, SpaceX says its Falcon 9 booster has been cleared for flight. The reusable booster was grounded after a launch pad explosion during a fueling operation last September 1. According to the company, the cause of the accident has been identified and steps have been taken to prevent its recurrence.
The September incident came as a shock to the aerospace community, with the Falcon 9 rocket carrying the AMO-6 mission suffering a launch pad mishap of a kind not seen in the United States in 62 years. Within 93 milliseconds, an anomaly in the second stage progressed to a full-on explosion that destroyed both the vehicle and the 5.5 tonne Israeli communications satellite onboard.
An investigation committee overseen by the US Federal Aviation Administration (FAA) was assembled to find the cause of the accident. Consisting of the FAA, US Air Force, NASA, the US National Transportation Safety Board (NTSB), SpaceX, and industry experts, it examined physical debris from the explosion as well as 3,000 channels of video and telemetry data. Though the incident did not occur during an actual launch, SpaceX says that the umbilicals attached to the rocket sent back a wealth of information.
The team concluded that the cause of the explosion was the failure of one of the three helium bottles set inside the second stage liquid oxygen tank. Used to pressurize the propellant tanks in flight, the bottles or Composite Overwrapped Pressure vessels (COPVs) are made of aluminum wrapped in graphite fibers as reinforcement.
According to the team’s report, recovered bottles showed voids or buckles in the fiber wrapping. Because the Falcon 9 uses supercooled liquid oxygen to improve performance, the most likely scenario is that the liquid helium, which is much colder than liquid oxygen, caused the oxygen in the voids to freeze solid into a sort of oxygen ice. This could have cause the fibers stress and generate friction, resulting in ignition of the fibers in the super-oxygenated environment and the failure of the bottle, which flooded the oxygen tank with helium making it overpressure, fail, and produce the final explosion.
SpaceX says the team found several possible causes of the helium bottle failure and fueling procedures have been altered to avoid a recurrence. These changes include using warmer helium to prevent the supercooled liquid oxygen from freezing, and using more conventional fueling procedures preferred by NASA. In the long term, SpaceX is redesigning the helium bottles to remove the voids.
The Falcon 9 is now scheduled to make its next flight on January 8 when it launches the Iridium NEXT mission from Space Launch Complex 4E (SLC-4E) at Vandenberg Air Force Base, California.