Peregrine Mission One was launched on the maiden flight of the Vulcan Centaur rocket on January 8, 2024. The goal was to land on the Moon, carrying payloads from NASA and commercial partners. Unfortunately, the spacecraft never touched down on our satellite due to a propellant leak. Now, Astrobotic reveals the cause of the failure.
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Trouble begins
The spacecraft separated from the Centaur V upper stage 50 minutes after lift-off. By this time Peregrine had been put on a trans-lunar trajectory by the launch vehicle. The spacecraft then began activating its own propulsion system. As part of this sequence, pressure control valves (PCV) were used to inject pressurized helium gas into the fuel and oxidizer tanks. When the required pressure was reached, these valves closed. However, even after the valve to the oxidizer tanks (PCV2) was supposed to have closed, pressure in these tanks kept increasing. Then, 92 minutes after separation, the spacecraft’s attitude suffered a sudden uncontrolled change.
Flight controllers theorized that the overpressurization caused one of the oxidizer tanks had ruptured. The ensuing leak would have exerted torque on the spacecraft, thus changing its attitude. The propulsion operator meanwhile made three attempts to cycle and close PCV2. However, as indicated by dropping pressure in the helium tank, this did not stop the leak, suggesting that the valve was leaking internally. Shortly before the communication window closed, the GNC team managed to correct the spacecraft’s attitude. This ensured the solar panels were pointed towards the Sun to keep the battery charged.
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Calling off the landing
Due to the oxidizer leak, it became unlikely for Peregrine to carry out its intended mission. Moreover, the anomalous conditions in the oxidizer tank would alter the fuel-to-oxidizer ratio. This would increase the combustion temperature in the engine, putting it at risk of heat damage. Thus, the lunar landing was called off, and the mission’s focus was shifted to making the most out of the coasting phase. Experiments were run, returning publishable data, and many components demonstrated successful operation.
Meanwhile, onboard cameras gave credit to the tank rupture hypothesis by showing that the thermal insulation blankets had inflated. The affected area was near the suspected rupture point. Astrobotic worked with the engine vendor to develop a pulsed firing strategy for the main engine. This allowed them to perform small correction burns, overcoming the overheating issues.
Ranging data showed the leak had altered Peregrine’s trajectory to the point of putting it on a collision course with the Earth. After consulting with NASA, Astrobotic determined the safest course of action was a controlled reentry of the spacecraft. The main engine performed four sequences of pulses, ensuring Peregrine wouldn’t reenter over a populated area. Peregrine burned up over the South Pacific on January 18, 2024, 10 days after launch.
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The causes of the failure
Astrobotic assembled a team of 34 experts, chaired by Dr. John Horack, Professor and Neil Armstrong Chair, Ohio State University. The team conclued the most likely cause of failure was PCV2 losing sealing capability as a result of vibrations during launch. Such vibrations would have caused a relaxation of threaded components, and thus the leak. Tests were conducted on a similar PCV. Astrobotic’s team subjected it to vibration and pressurization, and observed a leak similar in rate to that encountered during the mission. Disassembling the valve revealed that a threaded joint had loosened and an O-ring had eroded.
The PCVs themselves had a troubled history. The propulsion system underwent many changes during the years. Troubles arose with valve leaks and vendors having supply-chain issues, partly caused by COVID-19. Ultimately valves from a new supplier were installed. During testing PCV1, the PCV for the fuel tanks, encountered leaks, while PCV2 did not. PCV1 was easily accessible and was quickly repaired, passing all acceptance tests. PCV2 did not receive the same fixes because it was located deep within the spacecraft, and a lenghty fix would have created the risk of missing the launch opportunity. Coupled with the risk of damaging other components during disassembly and the succesful acceptance tests PCV2 had passed, Astrobotic decided not to modify the valve.
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The future
Astrobotic is a addressing this issue by redesigning PCVs for future landers. The company is also implementing further redundancy, more quality controls, and is enhancing cooperation with external suppliers. In particular, future landers will use multiple different PCVs to ensure no single failure will lead to the loss of the entire mission.
Astrobotic’s next scheduled mission is Griffin Mission One. A larger vehicle, it will carry payloads from the company itself, NASA, and ESA. The launch is currently scheduled for late 2025 aboard a SpaceX Falcon Heavy rocket. The lander was supposed to also carry NASA’s VIPER rover, but the vehicle has been shelved due to budget overruns.
