NASA's Altair lunar lander ascent engine is facing the challenges of not having enough propellant tank pressure for an emergency start if the astronauts decide to abort a landing, but too much pressure long before the end of the planned 210 day lunar base missions.
The engine, expected to be a development of the Apollo programme's Pratt & Whitney Rocketdyne RS-18, uses liquid methane (LCH4) and liquid oxygen that is fed at about 22.4bar (325lb/in2).
Using computational fluid dynamics (CFD) NASA's cryogenic fluid management project's storage analysis simulated propellant tank conditions with pressures starting at 6.9bar and 13.8bar, for a 210-day outpost mission.
But before the mission's end, the 6.9bar liquid oxygen tank venting started at 182 days with the subsequent propellant loss, and at 13.8bar it was 103 days. If venting does not occur, pressure increases can damage a tank. Pressure increases over time as propellant fluid becomes vapour due to heat absorption.
Heat sources include the lunar soil, the lander itself and the Sun. The LCH4 performed better with the 6.9bar tank reaching a pressure of 13.8bar at 210 days, but for 13.8bar it had to vent at 203 days. These figures were extrapolated from a simulation of two days of lunar surface storage that required two weeks of computation.
The need to retain propellant pressures at 13.8bar or below undermines the Altair's landing abort capability as these storage levels are to low for the 22.4bar the ascent engine requires.
Cryogenic fluid management project team member and NASA Glenn Research Center researcher Jeffrey Moder said at the American Institute of Aeronautics and Astronautics' Joint Propulsion Conference in Hartford, Connecticut that "the ascent engine must start 0.5s after the pilot-astronaut pushes the ignition button [for landing abort]. I don't know how they can pressurise in time. They will need extra helium for the [22.4bar] needed for ascent."
At the conference, NASA Johnson Space Center Altair ascent engine test researcher John Melcher declined to comment on the RS-18's propellant pressure needs. But he did say that last year cryogenic propellant igniters had been tested, while Moder said "reliable ignition" was a key issue.
A tank pressure study by Moder's team did not include transport from low-Earth orbit to the lunar surface. Future work will include verifying the CFD simulations and varying the simulation parameters such as non-uniform tank wall heat flux and launch fill level.
NASA is also considering hypergolic propellants for the ascent engine. Its minimal functional Altair design, which is not a point of departure vehicle, has a monomethyl hydrazine, nitrogen tetroxide ascent engine. That design is part of an industry evaluation exercise that NASA awarded contracts for in March.