Astrobotic’s Peregrine lunar lander failed to reach the moon due to a problem with a single valve in the propulsion system, according to a mission report released Tuesday. Company officials said in a news conference that engineers redesigned the valve and built additional redundancy into the propulsion system of the next lander, Griffin, to ensure the problem does not recur.
The report comes from a board of inquiry convened shortly after the completion of the Peregrine mission in January. That mission encountered problems just hours after launch on Jan. 8, when engineers activated the spacecraft’s propulsion system for the first time in orbit.
At that point, the fuel and oxidizer tanks should have been pressurized with helium by opening two pressure control valves (PCVs). But helium began to flow “uncontrollably” through the second valve into the oxidizer tank, Astrobotic CEO John Thornton explained during the press conference.
“This caused a significant and rapid build-up of pressure in the tank,” he said. “Unfortunately, the tank then ruptured and subsequently leaked oxidizer for the remainder of the mission.”
That PCV failed to reseal, likely due to a mechanical failure caused by “vibration-induced relaxation” between some threaded parts in the valve, said test committee chairman John Horack. Telemetry data was able to pinpoint the location and timing of the anomaly, and that data matched the autonomous PCV opening and closing sequence and the valve’s position in the propulsion system. Engineers were also able to reproduce the failure in ground tests.
Although the oxidizer leak persisted, Astrobotics’ team managed to stabilize the spacecraft, recharge its batteries, and power its payload. But the problem ultimately doomed the mission. After 10.5 days, the spacecraft returned to Earth and burned up in the atmosphere.
The 34-member review panel included 26 internal and eight external people from the company. The panel reviewed not only the data collected during the mission, but also all data from the flight qualification campaign and component tests. In the end, it concluded that the likely cause of the malfunction was the failure of this single helium PCV in the propulsion system.
The board has also created a chronology of events that led to the failure, starting as early as 2019, when Astrobotic contracted an unnamed supplier to develop the propulsion system. When that supplier began experiencing technical and supply chain issues due to the COVID-19 pandemic, Astrobotic decided in early 2022 to terminate the contract and complete the partially assembled propulsion system itself.
“By that time, we had already decided to develop Griffin’s propulsion system ourselves to achieve greater vertical integration,” said Sharad Bhaskaran, Astrobotic mission manager. “We had already developed many of the capabilities to do that propulsion integration. … This also mitigated some of the risks of the Griffin program, which is far more complex than Peregrine.”
But Astrobotic engineers encountered problems with the original supplier’s propulsion components – particularly the PCVs. In August 2022, they switched to another, unnamed PCV supplier, and those valves were installed in the lander.
A final set of propulsion system tests found leaks in one of the two PCVs – but not the one that ultimately leaked in orbit. That one performed flawlessly; the leaking one was repaired. While Bhaskaran acknowledged that the second PCV was “classified as a risk in our risk register” due to the leak in the first one during testing, engineers ultimately concluded that the failure was minor because the lander had passed final acceptance testing.
He justified not replacing the second PCV by saying that it was located much further inside the spacecraft and would have required “extensive surgical interventions” on the lander. In addition, the final inspection would have been invalidated and additional risks would have arisen from dismantling and reassembling it.
Horack confirmed that the team’s decision-making was consistently sound: “When I look at the team and look at what happened, I can really see that there were no decisions made in the run-up to the product launch where I would have said, ‘Hey, I think you should have done that differently.'”
These findings are already being incorporated into the development of the much larger Griffin lander, which is currently scheduled to launch to the Moon before the end of 2025. In addition to redesigning the valve, engineers have built a regulator into the propulsion system to control the flow of helium to the fuel and oxidizer tanks, as well as backup interlock valves for additional redundancy in case the problem with a PCV occurs again.
