Investigators believe a leak of propellant inside Crew Dragon spacecraft’s propulsion system led to the capsule’s explosion April 20 during a ground test at Cape Canaveral, and a senior SpaceX official said Monday that delays are making it “increasingly difficult” to fly astronauts on the commercial spaceship before the end of the year.
Engineers are replacing valves inside the Crew Dragon’s launch abort propulsion system to prevent similar leaks from happening in the future, according to Hans Koenigsmann, SpaceX’s Vice President of build and flight reliability.
The explosion sent a cloud of toxic vapors into the sky over Cape Canaveral Air Force Station, but winds drove the material offshore, and no one was injured in the accident.
In a press briefing Monday, Koenigsmann described the preliminary findings of a joint SpaceX-NASA investigation board set up to determine the cause of the April 20 explosion, which occurred on a test stand at Cape Canaveral Air Force Station during a test of the Crew Dragon’s SuperDraco launch abort engines.
A leaky check valve inside the propulsion system allowed nitrogen tetroxide oxidizer, which the Crew Dragon’s eight SuperDraco rocket engines consume mixed with hydrazine fuel, to enter high-pressure helium tubes during ground processing, SpaceX said.
The helium system is used to quickly pressurize the propulsion system, allowing the SuperDraco thrusters to fire up during a launch emergency and push the Crew Dragon and its astronaut crew away from a failing rocket.
“If you have a propellant tank, and you fill that tank, and you do have a check valve, it’s conceivable that the check valve leaks backwards … and you push propellant into the pressurization system,” Koenigsmann said. “The amount might be a cup or something like that, or more than a cup, it depends on how the system is being built up. And then it’s there for a while after loading, and when you pressurize you basically open the valves really, really fast.”
The abort propulsion system needs to pressurize up to 2,400 pounds per square inch to force propellant into the SuperDraco thrusters, which SpaceX designed to nearly instantaneously ignite and power up to 16,000 pounds of thrust. The eight SuperDraco engines are clustered around the Crew Dragon capsule in four pods.
As the system pressurized April 20, roughly 100 milliseconds before the SuperDraco engines were set to ignite, “we think that this slug (of nitrogen tetroxide) was driven back into the check valve,” Koenigsmann said Monday.
“Imagine a lot of pressure driving back a slug of liquid (that) has significant force, and that basically destroyed the check valve and caused an explosion,” Koenigsmann said.
The explosion destroyed the Crew Dragon spacecraft, the same vehicle that successfully launched to the International Space Station on an unpiloted test flight March 2. The capsule returned to Earth six days later and splashed down in the Atlantic Ocean.
The April 20 test-firing was key milestone as SpaceX teams readied the same spacecraft for an in-flight abort test, in which the SuperDraco engines will be ignited to push the capsule away from a Falcon 9 rocket about a minute after liftoff.
Koenigsmann said workers recovering debris from the spacecraft found burns within the faulty check valve, and the investigation’s preliminary findings line up with video and telemetry data. The SuperDraco thrusters were recovered intact after the accident, and SpaceX officials are confident the engines themselves were not at fault in the explosion.
In a written statement, SpaceX said the failure of the check valve — made of titanium — in a high-pressure NTO (nitrogen tetroxide) environment was “sufficient to cause ignition of the check valve and led to an explosion.”
SpaceX engineers tested the check valve failure hypothesis at the company’s test site in McGregor, Texas.
“We found out that … when the pressure is high, and you drive a slug with a lot of energy into a titanium component, that you can have this rather violent reaction,” Koenigsmann said.
He added that the result was surprising. Engineers did not expect a titanium, a material commonly used for decades on space vehicles around the world, could react so explosively in such an environment.
“We still are not done with the testing,” Koenigsmann said. “We have preliminary results, basically, but we know enough … that you should actually make sure that no oxidizer can move over to the pressuring side, and then cause problems when you pressurize the system for flight.”
SpaceX is replacing four check valves in the Crew Dragon’s abort system with burst disks, which seal off the flow path between the propellant tanks and the plumbing for the abort system’s gaseous pressurization system.
“A burst disk is basically a decide that is completely sealing left from right, and only opens when you have pressure that exceeds its rating, and then it opens and works,” Koenigsmann said. “That is basically the functionality that we need for the escape system for it to work properly in the case of a vehicle abort.”
Koenigsmann said the hardware changes inside the propulsion system will be “relatively small.”
“The burst disk we have now is definitely the safer approach overall going forward,” Koenigsmann said. “We didn’t really expect this to be a problem prior to that (accident), but that’s what you learn when you test. You find out some components might be better off exchanged with other components.”
The check valves are designed with a spring to open and close as needed.
“The problem is that sometimes the spring is a little bit sticky,” Koenigsmann said. “The valve has moving parts, and so that’s why things sometimes, especially at low pressure, are not quite sealing as well as they’re supposed to in check valves.”
On rockets and spacecraft, burst disks are designed for a single use. The burst disks block the pathway between the propellant and pressurization systems until they rupture during the engine startup sequence.
SpaceX originally intended the Crew Dragon spacecraft to use its SuperDraco thrusters for propulsive helicopter-like landings on the ground, assuming they were not needed for a launch abort –. The company nixed those plans entirely in 2017, electing to use parachutes for the capsule to splash down in the ocean.
Koenigsmann said the investigation into the accident is still ongoing. He estimated the SpaceX-led inquiry, which includes NASA participation, is about 80 percent complete.
SpaceX wants to make sure the failure mode in the nitrogen tetroxide pressurization system to not cross over to other parts of the Crew Dragon spacecraft, such as the fuel system that feeds hydrazine to the SuperDraco thrusters.
“Through this process, we will continue to learn things that will help us fly safer,” said Kathy Lueders, NASA’s commercial crew program manager.
“When you have something like a test anomaly like this one — as big as this one — then you look at other systems,” Koenigsmann said. “You make sure you don’t have vulnerabilities in … other systems, on the fuel side, and so on.
“We must characterize the basic physics of this how does this happen? How do NTO and titanium ignite, and what does it mean for flammability,” he said. “So, in general, we still have work ahead of us.”
Schedules for commercial crew test flights remain murky
In a conference call with reporters Monday, Lueders declined to offer a schedule Monday for SpaceX’s next two Crew Dragon test flights.
After completing the unpiloted test flight to the space station in March, known as Demo-1, SpaceX was gearing up for the in-flight abort test when the capsule exploded at Cape Canaveral. SpaceX was building a second space-worthy Crew Dragon spacecraft for the first test flight with astronauts to the International Space Station, known as Demo-2.
At the time of the April 20 explosion, SpaceX and NASA aimed to conduct the in-flight abort test in July, followed by the Demo-2 mission with astronauts as soon as September.
Lueders was non-committal when asked about the chances of astronauts flying to space on a Crew Dragon this year.
“Obviously, there’s always a chance that we’re going to fly crew on a SpaceX vehicle this year,” she said. “But I think right now we’re playing very close attention to the work thats being done coming out of the anomaly investigation, along with … getting through parachute testing and other key tests, along with conducting our in-flight abort test. All those things need to occur before we’re going to be confident that these systems will safely fly our crews.
“We’re going to fly when we feel like the certification work’s been done to be able to safely fly our crews,” Lueders said. “I hope it’s this year, but we’re going to fly when it’s the right time, and when we know that we’ll be flying our crew safely.”
Koenigsmann acknowledged the delays are making it less likely SpaceX can launch astronauts for the first time before the end of the year.
“I want to say I’m pretty optimistic at this point in time because we have a good path forward, but like I said, we’re still not quite done. We could find other things, so there’s always the unknown between now and then.
“End of the year? I don’t think it’s impossible, but it’s getting increasingly difficult, too,” Koenigsmann said.
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