SpaceX appeared all set to breeze past another milestone as part of its Crew Dragon testing program in April, after the spacecraft successfully completed its maiden journey to and from the International Space Station in March. But an unexpected explosion during engine testing on Earth threw up some questions about both the timeline and dependability of its vehicles, which it is now offering some answers to following joint investigations with NASA.

The test-firing of the Crew Dragon capsule was what is known as a static fire test, where the spacecraft is tethered to the ground and the engines are fired at full throttle. The April 20 test was designed to put the Crew Dragon's In-flight Abort systems, which are designed to blast the capsule clear of the rocket and keep astronauts safe if something goes awry on the launch pad, through their paces.

This system is made up of eight Draco thrusters, which are separate from the thrusters that help the spacecraft maneuver in space, and as was earlier suspected, were the cause of the April 20 explosion. SpaceX announced today that the explosion occurred around 100 milliseconds before the eight thrusters fired up as a result of a liquid oxidizer, which helps ignite the rocket fuel, leaking into the vehicle's high-pressure helium tubes.

This "slug" of liquid oxidizer passed through a titanium helium check valve at very high speed, causing structural failure of the component, its ignition and ultimately the destruction of the vehicle. SpaceX says it has already moved to address the issue by replacing the check valves with burst disks, which it says seal completely until opened by high pressure and "mitigate the risk entirely."

How all of this affects SpaceX and NASA's plans for the Crew Dragon remain unclear. The spacecraft is part of NASA's Commercial Crew Program, which aims to see human spaceflight depart from US soil for the first time since the Space Shuttle program was discontinued in 2011 (US astronauts currently travel to space courtesy of Russia, at considerable cost).

As of November last year, SpaceX was targeting June 2019 for its first manned flights of the Crew Dragon capsule, though it will need to carry out unmanned Abort Tests, where the craft is blown clear of the Falcon 9 rocket, before that eventuates. Meanwhile Boeing, the other contractor aboard NASA's Commercial Crew Program, is targeting September for unmanned flights of its Starliner spacecraft.

Source: SpaceX