A Crew Dragon test ended with a bang in April, as an engine meant to power ejection systems exploded a fraction of a second before ignition. SpaceX reports they now know the reason for the mishap. The accident took place on April 20, during a static fire test of the engine at Landing Zone 1 at Cape Canaveral Air Force Station in Florida.
The accident occurred at at 2:13 PM EDT, just one-tenth of a second prior to the ignition of the last thrusters. A leak of nitrogen tetroxide (NTO), the oxidizer fueling the vehicle, ignited a check valve, resulting in the blast which consumed the engine.
Just prior to the explosion, a leaking component allowed NTO to enter high pressure helium tubes during ground processing. Some of this NTO entered a helium check valve during rapid initialization of the launch escape system, resulting in a structural failure of the titanium valve. It was the failure of the valve which led to the ignition which triggered the explosion.
The Crew Dragon spacecraft includes two separate propulsion systems — 16 Draco thrusters operating at low pressure for on-orbit maneuvering, and a high-pressure SuperDraco engine, composed of eight Draco thrusters, designed to carry the crew to safety in the event of a catastrophic failure.
During static fire tests, engines are tethered to a platform, and are ignited in order to see how the systems perform. A demonstration mission to the International Space Station (ISS) in March 2019 was completed successfully. The test in April was designed to find any flaws in the system prior to an upcoming in-flight abort test.
“[T]he static fire test and anomaly provided a wealth of data. Lessons learned from the test — and others in our comprehensive test campaign — will lead to further improvements in the safety and reliability of SpaceX’s flight vehicles,” SpaceX reports.
Following the explosion, a large reddish cloud of smoke could be seen rising from the location of the test. The area was cleared of people and wind speed and direction were closely monitored. SpaceX crews worked with the US Air Force to clear the test site of debris and collect wreckage for examination. An accident investigation team was assembled, composed of officials from SpaceX, NASA, the Federal Aviation Administration (FAA) and the National Transportation Safety Board (NTSB).
Improvements to the engine include eliminating any path for propellant to enter the gaseous pressurization system. Check valves in the system have now been replaced by burst disks, which remain sealed until opened by pressure. SpaceX officials believe these changes should completely eliminate the risk of a similar accident in the future.
The site of the explosion was clean and fully functional on June 25, as SpaceX launched their Falcon Heavy rocket to space on the STP-2 mission, and landed two side boosters at landing zones one and two.
The Crew Dragon vehicle is designed to ferry space travelers to and from the International Space Station. Since the end of the Space Shuttle program in 2011, NASA has been paying $80 million per passenger to Russia for flights to the ISS. The Crew Dragon spacecraft is being developed in conjunction with NASA’s Commercial Crew Program.
Although SpaceX hoped to launch human travelers into space before the end of 2019, this accident will likely delay that mission until 2020 at the earliest, potentially putting a damper on the organization’s plans to put humans on the Moon and Mars.
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