Astrobotic's Peregrine rover failed to reach the Moon due to a single valve failure, prompting a redesign of the valve and the entire propulsion system on the larger Griffin lander.
On August 27, Astrobotic released the report of the Accident Investigation Board, which examined the January flight of Peregrine Mission One. That mission suffered a fuel leak a few hours after launch, preventing the spacecraft from attempting a lunar landing. Instead, the spacecraft flew through lunar space for 10 days before reentering the atmosphere over the South Pacific Ocean.
The investigation concluded that the leak was most likely caused by a malfunction of the pressure control valve (PCV), which allowed an uncontrolled flow of pressurized helium into the spacecraft's oxidizer tank, rupturing it. The valve, designated as PCV2, worked fine during pre-launch tests, but failed after launch.
The valve has lost its ability to seal due to "vibration-induced loosening" in the threaded components, which caused a mechanical failure in the valve, said John Horak, an Ohio State University professor who chaired the review panel, at a briefing on the report.
«If you shake it a lot, you can get some changes in the mechanical configuration that will prevent the valve from sealing. It's almost no different than when your sink starts to drip»,— he said.
As part of the investigation, engineers took a spare PCV and subjected it to shocks and vibrations similar to those experienced by the valve on the spacecraft. "Then we put helium on it, and after a fairly small number of cycles, the valve leaked. You could hear it coming out from about four feet away,&rdash; he said. The report states that the leak rates in these tests were similar to those observed on the spacecraft.
The valve leak occurred after several twists and turns in the development of the Peregrine powerplant. In 2019, Astrobotic initially decided to outsource the development of the powerplant's power system to a supplier, but that company faced supply chain issues after the pandemic, forcing Astrobotic to outsource the work in-house in early 2022.
Astrobotic then experienced problems with the original PCVs in this system and in August 2022 the company decided to switch suppliers. While the oxidizer valve, PCV2, from this new supplier passed acceptance tests, the same one on the fuel tank pressurization line, PCV1, was leaking. Astrobotic repaired the PCV1 and found it to be working fine. The company then conducted a series of environmental tests of the spacecraft, including vibration and acoustics.
"We still risked PCV2" after these pre-launch tests, said Sharad Bhaskaran, director of Peregrine Mission One at Astrobotic, due to PCV1 repairs. The company decided not to perform a preventive maintenance on PCV2 because this valve was not leaking and because its location in the spacecraft was much more difficult to access.
"To access it for repair or replacement would require major spacecraft surgery,— he said, canceling the environmental tests that had just been completed in the process. "That, along with the risk of some damage if we disintegrated and reassembled the spacecraft, led us to conclude that it was better to move on to the next phase of the program rather than replace PCV2.
Horak said the commission did not blame the company for such a decision. "I don’t see any decisions made in the process leading up to launch where I would say, "Hey, I think you should have done it differently,", — he said. "These decisions were quite correct. I think the team made very good decisions.
Lessons for Griffin
Astrobotic is incorporating Peregrine's technical and other changes into its larger Griffin lander, which is scheduled to launch by the end of 2025. Steve Clark, Astrobotic's vice president of landers and spacecraft, said the company is working with a valve supplier, which the company declined to name, to redesign the valve.
The Griffin powerplant will also have a regulator to control the flow of helium used to pressurize the tank, as well as back-up gate valves in case the redesigned PCVs fail. "If we saw the same failure mechanism on the pressure control valves, the shutoff valve would also be a way to control the flow to the oxidizer and fuel tanks,— he said.
Astrobotic also includes other corrective and preventative actions stemming from the Peregrine mission. In addition to the valve problem, the Peregrine experienced 24 anomalies during the flight, eight of which were considered “critical” but were rectified. These Bhaskaran problems included problems with the flight and guidance software, navigation and control systems, and a problem with the NASA Deep Space Network used to communicate with Peregrine.
«These were resolved in the mode real-time flight control team», — he said. "All these lessons learned from them were also brought into GM1". GM1 — is the company designation for Griffin Mission One.
That mission was to deliver NASA's Volatiles Investigating Polar Exploration Rover (VIPER) to the moon's south polar region, but in July NASA announced the cancellation of VIPER due to cost and schedule overruns, despite the rover being assembled and undergoing environmental CONTROL. testing NASA is retaining the Commercial Lunar Payload Servicing (CLPS) order awarded to Astrobotic for this mission, with plans to replace VIPER with another payload or ballast.
John Thornton, Astrobotic's executive director, said during the call that on GM1 has several smaller payloads still flying, including a small CubeRover developed by the company, as well as by NASA, ESA and an unknown customer.
"We have had dozens of conversations with people who want to fly on the Griffin. Some of them are more advanced than others», – he said. "Now that we have the additional cargo capacity, we are in talks with several parties.
He and other company executives said they are optimistic about the prospects for a successful landing of the Griffin, even though the Peregrine is not was even able to try to land. The Peregrine mission allowed the company to gain flight experience in many of the subsystems that will be used on the Griffin. The mission also gave the company personnel experience in operating a spacecraft and dealing with anomalies.
Thornton added that the CLPS program's approach to working with new landers at a cost much lower than traditional government missions requires more risk-taking. "We are trying to accomplish a mission at a cost that has never been possible before, and so we have decisions about what to focus on and how quickly we can get a launch,— he said.
"I think we got really, really close" with Peregrine, he said. "I am very confident that Griffin will strike the right balance and we will be able to pull off this landing.