A leaky valve in a propellant pressurization system apparently triggered a catastrophic explosion last April that destroyed a SpaceX Crew Dragon spacecraft during a planned ground test of the vehicle's emergency abort system, a company official said Monday.

Hans Koenigsmann, SpaceX vice president of build and reliability, said he's optimistic about the path forward, but additional work is needed to rule out any other issues. As such, he said, it's too soon to say when a modified Crew Dragon might be ready for a long-awaited piloted test flight to the International Space Station.

"My emphasis is really on making sure this is safe," he told reporters during an afternoon teleconference. "End of the year? I don't think it's impossible, but it's getting increasingly difficult."

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SpaceX and Boeing are both building commercial crew ships to ferry astronauts to and from the space station, ending NASA's sole reliance on Russian Soyuz spacecraft. Both crew ships feature powerful rocket motors designed to quickly push a vehicle to safety in the event of an impending booster failure.

A SpaceX Crew Dragon test article mounted on pad 40 at the Cape Canaveral Air Force Station for a 2015 test of its Super Draco abort engines. That test was successful, but during a planned abort motor test April 20 using a flight-qualified Crew Dragon a malfunction triggered a catastrophic explosion. SpaceX

The Crew Dragon in question was launched on a successful unpiloted test flight to the station in March and was being prepared for an in-flight test of its emergency abort system.

But first, SpaceX engineers mounted the capsule on a stand at the Cape Canaveral Air Force Station for a ground test of all its thrusters, 12 used for routine maneuvering in space and eight high-pressure Super Draco abort engines.

During the April 20 test, the smaller thrusters worked as expected but an instant before the planned ignition of the abort motors the vehicle suddenly exploded.

Telemetry, high-speed camera footage and analysis of recovered debris indicate the problem occurred when a small amount of a propellant known as nitrogen tetroxide, or NTO, leaked past a check valve into a helium line used to pressurize the propellant tanks. The leakage apparently occurred during pre-test processing.

To fire the Super Draco engines, the system is rapidly pressurized to 2,400 pounds per square inch.

"We believe that we had a liquid slug of the (NTO) in the pressurization system," Koenigsmann said. "When we opened the valves and pressurized the propellant system, we think that this slug was driven back into the check valve. ... That basically destroyed the check valve and caused an explosion."

The Super Draco engines can be seen firing in the 2015 on-pad abort test. SpaceX

He said no one expected that "NTO driven into a titanium component would cause such a violent reaction. We then performed tests ... with the help of NASA, and we found out when the pressure is high, the temperature is high and you drive a slug with a lot of energy into a titanium component that you can have these rather violent reactions."

Additional work is needed to rule out other less likely culprits but SpaceX is pressing ahead with plans to replace the valves in question with pressure-activated "burst discs" that have no moving parts and cannot leak.

"A burst disc is basically a device that is completely sealing left from right and only opens when you have pressure that exceeds its rating and then it opens," Koenigsmann said. "That is basically the functionality we need for the escape system to work properly in case of a vehicle abort.

"This was a ground test, it was not a flight test," he added. "We do ground tests so we can learn, and in this case we learned a lot, maybe almost more than what we wanted. It was certainly something we didn't expect and a great lesson for us."

Had the April ground test gone well, SpaceX planned to launch the same spacecraft on a Falcon 9 rocket early this summer to test the abort system in flight. That flight would have been followed by launch of two astronauts aboard a different Crew Dragon spacecraft in the July timeframe.

As it now stands, the Crew Dragon originally slated for the piloted flight will be used instead for an in-flight abort test, presumably sometime later this year and presumably preceded by another ground test of the Super Dracos. A different Crew Dragon will be moved up and prepared for the now-delayed piloted test flight.

It is not known whether either of those flights will take place this year.

Boeing, meanwhile, hopes to launch its CST-100 Starliner spacecraft on an uncrewed test flight to the space station late this summer, followed by the company's first piloted flight by end of year. But sources say the schedule is far from certain and warn delays are likely.