Article by David Sear
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SpaceX has certainly gained worldwide attention for a series of historic milestones. It is reportedly the only private company capable of returning a spacecraft from low Earth orbit, which it first accomplished in 2010. The company made history again in 2012 when its Dragon spacecraft became the first commercial spacecraft to deliver cargo to and from the International Space Station.
Later SpaceX successfully achieved the historic first reflight of an orbital class rocket in 2017, and the company now regularly launches flight proven rockets. In 2018, SpaceX began launching Falcon Heavy, the world’s most powerful operational rocket by a factor of two.
According to the company website, Crew Dragon’s design includes two distinct propulsion systems – a low-pressure bi-propellant propulsion system with sixteen Draco thrusters for on orbit manoeuvring, and a high-pressure bi-propellant propulsion system with eight SuperDraco thrusters for use only in the event of a launch escape. After the vehicle’s successful demonstration mission to and from the International Space Station in March 2019, SpaceX performed additional tests of the vehicle’s propulsion systems to ensure functionality and detect any system-level issues prior to a planned In-Flight Abort test.
The initial tests of twelve Draco thrusters on the vehicle completed successfully, but the initiation of the final test of eight SuperDraco thrusters resulted in destruction of the vehicle. Following the anomaly, SpaceX convened an Accident Investigation Team that included officials from the National Aeronautics and Space Administration (NASA), and observers from the Federal Aviation Administration (FAA) and the National Transportation Safety Board (NTSB), and began the systematic work on a comprehensive fault tree to determine probable cause.
Initial data reviews indicated that the anomaly occurred approximately 100 milliseconds prior to ignition of Crew Dragon’s eight SuperDraco thrusters and during pressurization of the vehicle’s propulsion systems. Evidence shows that a leaking component allowed liquid oxidizer – nitrogen tetroxide (NTO) – to enter high-pressure helium tubes during ground processing. A slug of this NTO was driven through a helium check valve at high speed during rapid initialization of the launch escape system, resulting in structural failure within the check valve. The failure of the titanium component in a high-pressure NTO environment was sufficient to cause ignition of the check valve and led to an explosion.
In order to understand the exact scenario, and characterize the flammability of the check valve’s titanium internal components and NTO, as well as other material used within the system, the accident investigation team performed a series of tests at SpaceX’s rocket development facility in McGregor, Texas. Debris collected from the test site in Florida, which identified burning within the check valve, informed the tests in Texas. Additionally, the SuperDraco thrusters recovered from the test site remained intact, underscoring their reliability.
It is worth noting that the reaction between titanium and NTO at high pressure was not expected. Titanium has been used safely over many decades and on many spacecraft from all around the world. Even so, the static fire test and anomaly provided a wealth of data.
SpaceX has already initiated several actions, such as eliminating any flow path within the launch escape system for liquid propellant to enter the gaseous pressurization system. Instead of check valves, which typically allow liquid to flow in only one direction, burst disks, which seal completely until opened by high pressure, will mitigate the risk entirely. Thorough testing and analysis of these mitigations has already begun in close coordination with NASA, and will be completed well in advance of future flights.
About SpaceX
SpaceX’s Dragon spacecraft is flying numerous cargo resupply missions to the space station under a series of Commercial Resupply Services contracts. Dragon was designed from the outset to carry humans to space and will soon fly astronauts under NASA’s Commercial Crew Program.
Building on the achievements of Falcon 9 and Falcon Heavy, SpaceX is working on a next generation of fully reusable launch vehicles that will be the most powerful ever built, capable of carrying humans to Mars and other destinations in the solar system.
