Astrobotic has continued its policy of transparency with a report into the failings that resulted in its Peregrine lander burning up in Earth's atmosphere rather than making a soft landing on the Moon.

According to the report [PDF], "the most likely cause of Peregrine's anomaly was the failure of a singular helium pressure control valve, called PCV2, within the propulsion system."

However, how that valve came to be in the system and was "carried as a risk" merits further investigation, and the report does not disappoint. The timeline goes back to 2019, when Astrobotic contracted with another vendor for the propulsion feed system used by the lander.

It did not go well, and for various reasons, including worries about the mission schedule - more on that later - Astrobotic opted to complete the propulsion feed system in-house in early 2022.

That also did not go well. Between April and November 2022, Astrobotic began encountering problems during testing. In particular, the pressure control valves (PCVs) to control the helium pressure into the fuel and oxidizer tanks kept failing. So Astrobotic decided to pivot to another vendor for the valves. A vendor that was already providing the pyrotechnic valves.

During a final set of leak tests, one valve - PCV1 - leaked. Another - PCV2 - did not. PCV1 could be accessed and repaired. PCV2 could not as it was deep within the spacecraft, and getting to it would require dismantlement. "This would have invalidated costly, time-intensive spacecraft acceptance tests that had already been completed and could not be rescheduled without missing Peregrine's scheduled launch date."

A judgment call was made. Ideally, the same preemptive alterations made to PCV1 could have been made to PCV2. However, PCV2 passed its tests, and the risk of failure was characterized as low. After all, it had passed its acceptance tests, and launch was fast approaching.

The rest is history. The Peregrine lander arrived at Cape Canaveral for payload processing on October 31. It was launched on United Launch Alliance's Vulcan rocket, before separating successfully from the launcher to begin its journey to a planned landing on the Moon.

The initiation sequence for the propulsion system began, and the PCV1 and PCV2 valves were actuated autonomously. PCV1 opened and closed correctly. Then it was PCV2's turn. The telemetry showed the valve had closed, but the helium tank registered a fall in pressure alongside a rise in pressure in the oxidizer tanks downstream of PCV2. At least one of the tanks ruptured, and despite multiple attempts to get the PCV2 valve to reseat correctly, the leak continued.

The rest of the mission consisted of gathering whatever science could be gleaned from the lander's instruments. A landing attempt was called off, and instead the spacecraft was sent on a trajectory to burn up safely in the Earth's atmosphere.

Thanks to Astrobotic's report, lessons can be learned. There needs to be multiple dissimilar PCVs so that a single failure does not result in the loss of a mission. The design of the PCV itself has been tweaked to deal with the sealing flaw that doomed the lander. But, perhaps most importantly, "additional quality management personnel have been brought onboard to further enhance Astrobotic's mission assurance.

"While this is still a commercially minded program, Astrobotic is augmenting quality management to focus deeply on key subsystem deliveries such as propulsion components." ®

https://www.theregister.com//2024/08/30/astrobotic_peregrine_postmortem/