SpaceX and NASA officials said Monday investigators have determined a leaking valve is the what caused the Crew Dragon capsule to explode during a ground test in April.
SpaceX which is in process of certifying its spacecraft to fly NASA astronauts as part of the commercial crew program. On April 20, the company was test-firing the Crew Dragon SuperDraco thrusters at Cape Canaveral Air Force Station when the capsule exploded sending red-tinted smoke across the Cape.
Prior to the testing mishap, the same Crew Dragon capsule successfully launched from Kennedy Space Center on a Falcon 9 rocket, docked at the space station’s new international docking adapter and splashed down in the Atlantic Ocean after a six-day stay.
The Crew Dragon capsule was being prepared for a launch abort test from the Cape when the capsule was destroyed during the ground test. During which SpaceX will launch a Falcon 9 with the spacecraft then cause a problem to trigger the capsule to be safely jettisoned away from the rocket.
The launch abort was the final test scheduled before for a launch with NASA Astronauts Bob Behnken and Doug Hurley, which was slated for this summer.
The SuperDraco thrusters are what the spacecraft would use to shuttle the astronauts away from the rocket to safety in case of an emergency during launch.
After the explosion, SpaceX created an accident investigation team that included officials from NASA, the Federal Aviation Administration and the National Transporation Safety Board to determine the cause.
Early on in the investigation it was determined the problem occurred approximately 100 milliseconds prior to ignition of Crew Dragon’s eight SuperDraco thrusters and during pressurization of the vehicle’s propulsion systems.
The investigation team attempted to recreate the scenario during multiple tests at its facilities in McGregor, Texas.
In a statement SpaceX explained what caused the valve to fail:
"Evidence shows that a leaking component allowed liquid oxidizer – nitrogen tetroxide – to enter high-pressure helium tubes during ground processing. A slug of this (nitrogen tetroxide) was driven through a helium check valve at high speed during rapid initialization of the launch escape system, resulting in 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."
"The reaction between titanium and nitrogen tetroxide at high pressure was not expected," SpaceX said in a statement, because "titanium has been used safely over many decades and on many spacecraft from all around the world."
In a call with reporters Monday, SpaceX vice president of build and flight reliability Hans Koenigsmann said the company is replacing about four check valves with burst disks on the astronaut capsule to remedy the problem.
Check valves, typically allow liquid to flow in only one direction while burst disks seal completely until opened by high pressure, according to SpaceX.
"SpaceX also eliminated any flow path within the launch escape system for liquid propellant to enter the gaseous pressurization system," a statement from the company said.
Koenigsmann said the static fire test and anomaly provided a wealth of data and will lead to further improvements in the safety and reliability of SpaceX’s flight vehicles.
"We learned a very valuable lesson on something going forward," he said.
"These changes, along with thorough testing and analysis will be completed "well in advance of future flights," according to SpaceX.
NASA’s Commercial Crew Program Manager Kathy Lueders said NASA still has not determined what this means for when SpaceX can begin launching astronauts.
"What’s really critical is you always learn things when you're testing, I know everyone would like us to be able to say 'This is when we are launching,'" Lueders said, adding, "This will help us fly safer."
Both Doug Hurley and Bob Behnken, the astronauts who will fly on the first crewed flight of Crew Dragon, have been in the loop during the investion, Lueders said.
"They really appreciated SpaceX’s openness -- SpaceX’s inclusion of NASA-- on the investigation team, and the continued interchange that we’ve had over the last few months," Lueders said.