It’s always the valves —
“Through this process, we will continue to learn things that will help us fly safer.”
On Monday, officials from SpaceX and NASA provided an update on the investigation of an anomaly thatoccurred in April, which destroyed a Crew Dragon spacecraft. Generally, they were upbeat with their assessment: “I’m pretty optimistic right now, because we have a good path forward,” said Hans Koenigsmann, SpaceX’s vice president of mission assurance.
After nearly three months of work—which has included the collection of debris from the ground-based incident, assessing large volumes of data, and a series of tests at SpaceX’s rocket development facility in McGregor, Tex.—the company is about 80% complete with its analysis, Koenigsmann said. He characterized the findings discussed Monday as “preliminary.”
The accident occurred during tests of the Crew Dragon’s thruster systems in Florida. The capsule has “Draco” thrusters used to maneuver in space as well as powerful “SuperDracos.” They would fire in the event of an emergency with the rocket to pull the crew safely away during a launch. Specifically, the April 20 anomaly occurred during the activation phase of the SuperDraco thruster system, when it is is pressurized and valves are opened and closed.
Approximately 100 milliseconds prior to ignition of Crew Dragon’s eight SuperDraco thrusters, a leaking component allowed about one cup of liquid oxidizer—nitrogen tetroxide—or NTO—into the wrong fuel tank plumbing.
“A slug of this NTO 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 company saidin a statement. “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.”
Koenigsmann said the company is already taking steps to prevent such a problem from occurring again. This includes the use of “burst disks” instead of check valves to eliminate the possibility of any liquid propellant flowing into the gaseous pressurization system.
The hardware components needed to mitigate this problem are relatively straightforward, he said. What may take more time is the process of checking other fuel systems in Crew Dragon for similar vulnerabilities, as well as characterizing the basic physics of the NTO and titanium that ignited in this situation. “The hardware is probably the smaller part,” he said.
New schedule uncertain
After flying a virtually flawlessuncrewed Dragon test mission to the International Space Station in February, SpaceX has been working toward a crewed flight of the vehicle (by NASA astronauts Doug Hurley and Bob Behnken) later this year. That now appears unlikely, but Koenigsmann was optimistic that this issue could be worked in parallel to several other challenges between Dragon and that flight, such as qualification of the spacecraft’s parachutes.
NASA’s chief of the commercial crew program, which is paying for and managing development of Crew Dragon and Boeing’s Starliner vehicles to ferry astronauts to and from the International Space Station, expressed satisfaction with the pace and quality of SpaceX’s investigation into the mishap.
“In a lot of ways, this was a gift for us,” said Kathy Lueders, the NASA manager. “It was a test on the ground, we had a lot of instrumentation on the vehicle, we had high-speed cameras, [and] we were able to get the hardware and the data. Through this process, we will continue to learn things that will help us fly safer.”
The company also confirmed its plan to use Crew Dragon spacecraft originally assigned to SpaceX’s first crewed mission to the International Space Station for an In-Flight Abort test—which will test the SuperDraco thrusters during an ascent to space. The spacecraft originally assigned to the first operational mission will now launch Hurley and Behnken on their demonstration flight.
Sources told Ars that SpaceX will probably do well to complete the In-Flight Abort test in 2019, with a crewed flight occurring early in 2020, provided all goes well during the abort test. On this schedule, the competition between SpaceX and Boeing to launch the first people into orbit from US soil since 2011’s retirement of the space shuttle will be very close.