LOGAN, Utah — A NASA lunar cubesat mission failed to go into orbit around the moon earlier this year when debris blocked propellant lines for the spacecraft’s thrusters.
NASA’s Jet Propulsion Laboratory declared an end to the Lunar Flashlight mission May 12, five months after its launch as a secondary payload on a Falcon 9 that carried ispace’s HAKUTO-R M1 lunar lander. That decision came after months of efforts to troubleshoot problems with the spacecraft’s thrusters, which used a green propellant called ASCENT, that prevented it from performing maneuvers needed to go into orbit around the moon.
In an Aug. 8 presentation at the 37th Annual Small Satellite Conference here, Celeste Smith and Nathan Cheek of JPL said that problems with the thrusters became clear shortly after launch. Only one of the four thrusters, designated thruster four, was performing normally, and troubleshooting efforts failed to restore full thrust on the other three.
Engineers developed a technique to perform trajectory correction maneuvers with a single thruster, rotating the spacecraft to prevent momentum buildup. That approach initially worked, but on the ninth such maneuver the thrust dropped to zero. “Thruster four was basically considered dead then, and we knew a lunar orbit was no longer possible,” Cheek said.
Spacecraft controllers then attempted an alternative trajectory using thruster three, which was producing 25% of its rated thrust, to enable a series of lunar flybys rather than going into lunar orbit. That thruster, though, soon failed as well.
Cheek said the mission then considered “high-risk options” to restore the thrusters, such as reversing the propellant pump to dislodge any debris blocking propellant lines. They also increased the pressure in another thruster to its rated limit, which initially worked. “During a repeat test, something went horribly wrong,” with a big drop in temperature and pressure that he said was likely caused by a ruptured propellant line.
Engineers had long suspected that foreign object debris somewhere in the propulsion system caused the thruster problems. An investigation after the primary mission was over excluded alternative explanations.
Smith said a likely source of the debris is titanium particles. Part of the propulsion system was additively manufactured, and sintered particles on the interior of propellant lines could have come loose from vibrations before and during launch, as well as pressurization cycles of the system.
Cleaning of the lines and additional filters could have prevented the problem, but she noted the project had limited resources. The propulsion system “was designed, integrated and tested under extreme schedule pressure,” she said, because the mission was originally planned to launch as a secondary payload on the Space Launch System for the Artemis 1 mission.
Despite the failure of Lunar Flashlight to enter orbit around the moon and carry out a science mission to look for water ice in permanently shadowed lunar craters, NASA considers other aspects of the mission successful. That includes demonstrations of the laser reflectometer science instrument as well as a new flight computer and upgraded radio.
“Our Lunar Flashlight spacecraft unfortunately did not make it to the moon,” said Chris Baker, program executive for small spacecraft technology programs in NASA’s Space Technology Mission Directorate, during a NASA town hall meeting at the conference Aug. 7. “But we learned a lot from that mission.”
