On Saturday, April 20th, 2019, an explosion took place on SpaceX’s Landing Zone 1 at Cape Canaveral Air Force Station in Florida. The company was engaged in a series of static fire engine tests for their Crew Dragon‘s In-Flight Abort test vehicle. This vehicle is essential for crewed missions since it acts as a sort of ejection seat for the crew capsule in the event of an emergency.
While the initial tests of the twelve Draco thrusters on the vehicle were completed successfully, the initiation of the final test of eight SuperDraco thrusters resulted in the destruction of the vehicle. After a thorough investigation, SpaceX has concluded that the explosion was caused by a nitrogen tetroxide leak that occurred just prior to the final test.
Following the accident, and in accordance with pre-established safety protocols, the team worked with the US Air Force (USAF) to clear the test area of debris and collect and clean samples for the investigation. They monitored the local winds and other factors to make sure there was no threat to the health and safety of the public.
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SpaceX then convened an Accident Investigation Team that included officials from the NASA, and observers from the Federal Aviation Administration (FAA) and the National Transportation Safety Board (NTSB). Together, they developed a fault tree and began to systematically investigate the probable cause.
Their initial finds indicated that the anomaly occurred approximately 100 milliseconds prior to the ignition of the SuperDraco thrusters and during the pressurization of the vehicle’s propulsion systems. They also determined that a leaking component allowed nitrogen tetroxide (NTO) – a liquid oxidizer – to enter the capsule’s high-pressure helium tubes while it was still undergoing ground processing.
During the rapid initialization of the launch escape system, a slug of NTO was then driven through a helium check valve at high speed, resulting in structural failure within the check valve. To recreate the exact scenario, the accident investigation team used debris collected from the site (which identified where burning took place within the check valve) to conduct a series of tests the SpaceX rocket development facility in McGregor, Texas.
These tests bore out their initial findings and concluded that the failure of a titanium component in a high-pressure NTO environment was sufficient to cause an ignition in the check valve that led to the explosion. This type of reaction was not expected since titanium has been used in rocketry for decades by agencies all around the world.
Nevertheless, the static fire test and anomaly provided a wealth of data. In addition, the SuperDraco thrusters were recovered from the test site intact, which is a testament to their reliability. As such, SpaceX fully intends to take the lessons learned here and use them to inform future missions, as well as further improvements in the safety and reliability of its flight vehicles.
Already, SpaceX has taken measures to ensure something like this doesn’t happen again. This includes eliminating any flow path within the launch escape system to ensure that liquid propellant cannot enter the gaseous pressurization system. They’ve also taken to using burnt discs instead of check valves, which remain completely sealed until opened by high pressure – instead of allowing liquid to flow in only one direction.
SpaceX has begun testing and analyzing these mitigation methods with NASA already and indicated that they will be completed well in advance of future flights. The company has also shifted spacecraft assignments forward to remain on track for Commercial Crew Program flights.
These include the second demonstration mission (Demo-2) to the ISS, which will be flown using the Crew Dragon originally intended to fly the first operational mission (Crew-1). It’s also worth noting that SpaceX’s facility at Cape Canaveral was operational in time for the launch of a Falcon Heavy rocket, as part of Space Test Program-2 (STP-2), and the landing of its two first-stage side boosters on June 25th, 2019.
Further Reading: SpaceX