Check Valve Failure Caused SpaceX Anomaly

Check Valve Failure Caused SpaceX Anomaly

SpaceX provided an update today on its investigation into the cause of the “anomaly” during a static fire test of its Crew Dragon In-Flight Abort system in April that destroyed the spacecraft.  A leak allowed liquid oxidizer to enter high-pressure helium tubes during ground processing.  Milliseconds before the eight SuperDraco engines were to be fired, a slug of the oxidizer was driven through a check valve, causing the valve to ignite and cause an explosion.  The company still has work to do and could not project a revised schedule for the In-Flight Abort test, a prerequisite for launching astronauts to the International Space Station (ISS).

SpaceX is one of two companies developing crew space transportation systems to take astronauts to and from the ISS as public-private partnerships with NASA.  Boeing is the other.  Each company must launch an uncrewed test flight and a crewed test flight in order to be certified to take NASA astronauts into space. Other tests also are required, including of the abort systems for each spacecraft — SpaceX’s Crew Dragon and Boeing’s CST-100 Starliner.  They would be used in an emergency during launch to separate the crew capsule from the rocket and return the crew safely to Earth.

Hans Koenigsmann, SpaceX Vice President of Build and Reliability (screengrab from NASA TV during a May 2019 briefing)

SpaceX successfully completed its Demo-1 uncrewed flight test to ISS in March.  On April 20, it was conducting a static fire test of the Crew Dragon’s engines at Cape Canaveral Air Force Station, FL in preparation for the In-Flight Abort test.

Crew Dragon has two sets of engines: 16 low-pressure Draco engines for orbital maneuvering, and eight high-pressure SuperDraco engines for the abort system.  After successfully testing the Draco engines, SpaceX was getting ready to fire the SuperDraco engines when the anomaly occurred.

The company stresses that the failure was not of the SuperDraco engines themselves, which survived the explosion intact.

In a statement, SpaceX said that 100 milliseconds before the engines were to ignite, a “slug” of nitrogen tetroxide (NTO) oxidizer was “driven through a helium check valve at high speed during rapid initialization of the launch escape system.”  The NTO was present due to a “leaking component” that allowed it to enter “high-pressure helium tubes during ground processing.”  The slug initiated “a 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.”

SpaceX  is replacing the check valves with burst disks in response.  Check valves allow liquid to flow in only one direction, while burst disks are completely sealed until opened by high pressure. It said that will “mitigate the risk entirely.”

Kathy Lueders, NASA Commercial Crew Program Manager (Credit: NASA)

During a teleconference today, SpaceX Vice President of Build and Reliability Hans Koenigsmann said the investigation is still ongoing.  They have completed about 80 percent of the fault tree analysis.  The Accident Investigation Team includes officials from NASA and observers from the FAA and the National Transportation Safety Board (NTSB).

NASA Commercial Crew Program (CCP) Manager Kathy Lueders also participated in today’s teleconference.  Asked whether she expects that SpaceX will be able to launch astronauts this year, she replied there is “always a chance,” but a lot of work and other tests, like parachute tests, still must be completed.

“I hope it’s this year, but we’re going to fly when it’s the right time and when we know that we’ll be flying our crew safely.” — NASA CCP Manager Kathy Lueders

Koenigsmann agreed.  “I am pretty optimistic at this point in time,” but it is “getting increasingly difficult, too.”

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