SpaceX Says Faulty Valve Led to Crew Dragon Test Accident | 7/16/2019 | Jeff Foust
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WASHINGTON — SpaceX said July 15 that the explosion that destroyed a Crew Dragon spacecraft during a ground test in April was likely caused by oxidizer that leaked into the spacecraft's propulsion system and destroyed a valve, but didn't give a firm schedule for resuming test flights.

The April 20 explosion at Cape Canaveral took place during testing of the thrusters on the same Crew Dragon spacecraft that flew the uncrewed Demo-1 mission to the International Space Station in March. SpaceX was testing the thrusters, including both the smaller Draco thrusters and larger SuperDraco thrusters, ahead of an in-flight abort test of the spacecraft then planned for this summer.

SpaceX - Draco - Thrusters - Tenth - Ignition

According to SpaceX, all the Draco thrusters were tested successfully but, about a tenth of a second before the scheduled ignition of the SuperDraco thusters, the anomaly took place that destroyed the spacecraft. There were no personnel in the vicinity of the vehicle at the time of the test and thus no injuries.

SpaceX said the most likely cause of the explosion was when a "leaking component" allowed nitrogen tetroxide (NTO) oxidizer into helium tubes during processing. When the system was pressurized just before the planned ignition, a "slug" of NTO was driven at high speed into a titanium check valve, causing structural failure of the valve and igniting it, triggering the explosion.

Slug - NTO - Check - Valve - Explosion

"When you pushed the slug [of NTO] into the check valve, it basically creates an explosion," said Hans Koenigsmann, vice president of build and flight reliability at SpaceX, during a call with reporters.

He said SpaceX, with the assistance of NASA, confirmed this scenario in testing at the company’s McGregor, Texas, test site. "We found out that when the pressure is pretty high, the temperature is high, and you drive a slug with a lot of energy into a titanium component, you can...
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