Astrobotic’s Peregrine rover failed to reach the Moon due to a broken valve, which led to the valve and entire propulsion system being redesigned on the larger Griffin lander.

On August 27, Astrobotic released the Accident Investigation Board report examining the January flight of Peregrine Mission One. That mission suffered a fuel leak a few hours after launch, preventing the spacecraft from attempting to land on the moon. Instead, the spacecraft flew over the lunar terrain for 10 days before re-entering the atmosphere over the South Pacific Ocean.

The investigation concluded that the leak was most likely caused by a failure of the pressure control valve (PCV) that allowed an uncontrolled flow of pressurized helium to enter the spacecraft’s oxidizer tank and rupture it. The valve, labeled PCV2, worked well during pre-launch tests but failed after launch.

The valve lost its ability to seal due to “vibration-induced loosening” of threaded components, causing a mechanical failure in the valve, John Horak, an Ohio State University professor who chaired the review panel, said in a briefing on the report.

“If you shake it too much, you can get some changes in the mechanical configuration that will prevent the valve from sealing. It’s almost no different than your sink starting to drip,” he said.

As part of the study, 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 very small number of cycles, the valve leaked. You could hear it coming from about four meters away,” he said. The leak rates in these tests were similar to those observed on the spacecraft, the report said.

The valve leak occurred after several twists and turns in the development of the Peregrine engine. In 2019, Astrobotic initially decided to outsource development of the plant’s power system to a supplier, but that company encountered supply chain issues following the pandemic, forcing Astrobotic to outsource the work in-house in early 2022.

Astrobotic later experienced issues with the original PCVs in this system, and the company decided to switch suppliers in August 2022. While this new supplier’s oxidizer valve, PCV2, passed acceptance tests, the same one in 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 on the spacecraft, including vibration and acoustics.

“We were still running the risk of PCV2 after these pre-launch tests,” said Sharad Bhaskaran, Peregrine Mission One manager at Astrobotic, due to the PCV1 repair. The company decided not to perform preventive maintenance on PCV2 because that valve is not leaking and its location on the spacecraft is much more difficult to access.

“Accessing it for repair or replacement would require major surgery on the spacecraft,” he said, canceling environmental tests that had just been completed in the process. “This led us to conclude that, aside from the risk of some damage if we were to disassemble and reassemble the spacecraft, it was better to move on to the next phase of the program rather than replace PCV2.”

Horak said the commission did not fault the company for that decision. “I don’t see any decisions leading up to the launch where I said, ‘Hey, I think you should have done this differently,'” he said. “Those decisions were pretty accurate. I think the team made some really good decisions.”

Lessons for Griffin

Astrobotic is incorporating Peregrine’s technical and other changes into its larger Griffin lander, scheduled to launch in late 2025. Steve Clark, Astrobotic’s vice president of landers and spacecraft, said the company is working with a valve supplier, whose name he declined to name, to redesign the valve.

The Griffin plant will also have a regulator to control the flow of helium used to pressurize the tank, as well as backup valves in case the redesigned PCVs fail. “If we were to see the same failure mechanism in 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 preventive activities resulting from the Peregrine mission. In addition to the valve problem, Peregrine experienced 24 anomalies during flight, eight of which were rated “critical” but resolved. These Bhaskaran issues included issues with the flight and guidance software, navigation and control systems, and an issue with the NASA Deep Space Network used to communicate with Peregrine.

“These were resolved in real time by the air traffic control team,” he said. “All the lessons learned from them were also passed on to GM1.” GM1 is the corporate name for Griffin Mission One.

This mission was to deliver NASA’s Volatile Matter Investigation Polar Exploration Rover (VIPER) to the south polar region of the moon, but in July NASA announced that VIPER had been canceled due to cost and schedule overruns, despite the rover being assembled and passing environmental testing. NASA retains the Commercial Lunar Payload Servicing (CLPS) order awarded to Astrobotic for this mission, with plans to replace VIPER with another payload or ballast.

Astrobotic’s chief executive John Thornton said during the call that GM1, a small CubeRover developed by the company, as well as several smaller payloads are still flying, including those from NASA, ESA and an undisclosed customer.

“We’ve had dozens of conversations with people who want to fly the Griffin. Some are more advanced than others,” he said. “Now that we have the additional cargo capacity, we’re in discussions with several parties.”

He and other company executives said they were optimistic about Griffin’s chances of a successful landing, even though Peregrine failed in its landing attempt. The Peregrine mission allowed the company to gain flight experience with many of the subsystems that will be used on Griffin. The mission also gave the company’s personnel experience operating spacecraft and dealing with anomalies.

Thornton added that the CLPS program’s approach to working with new launch companies at much lower prices than traditional government missions requires taking more risks. “We’re trying to do a mission at a cost that hasn’t been possible before, and so we have decisions to make about what we’re going to focus on and how quickly we can launch,” he said.

“I think we’re getting very, very close” with Peregrine, he said. “I’m pretty confident Griffin will strike the right balance and we’ll be able to pull off this landing.”