credit: NASA / caption: this is the LDAC-2 ascent stage design, LDAC-3 will finish in March
It looks like the NASA Altair Lunar Lander's ascent stage could use liquid methane (LCH4) instead of the baselined hypergolics as the space agency has just published a synposis that says: "NASA system trade studies have shown that LCH4 ascent stage propellant tank fluid venting can be eliminated on the lunar surface for the 210 day mission..."
But can the pressurant last for that seven month mission? That's another issue but in a synopsis for a forthcoming request for propoals, which will kick off the procurement process to design, fabricate and install a low heat leak, tank-applied Multi-layer Insulation (MLI) system, it says that 60 layers will be needed for the LCH4 fuel
That and apparently loading at the Kennedy Space Center (KSC) launch pad the LCH4 tanks with "densified LCH4 at 165 oR" and starting with a tank ullege of approximately 15% will do the job; so now you know!
Here is a description of the densification process care of a NASA technical paper from 2005
Each densification production process uses a high-efficiency, subatmospheric boiling bath heat exchanger to cool the working fluid. A near triple-point hydrogen boiling bath is used to condition and subcool hydrogen to 27 R (15 K), and a nitrogen boiling bath is used to cool the liquid oxygen to 120 R (66.7 K). Multistage centrifugal compressors operating at cryogenic inlet conditions maintain the heat exchanger bath vapor pressure below 1 atm. The LO2 propellant densification unit shown in the photograph has a 30 lb/sec capacity, whereas the LH2 unit was designed to process 8 lb/sec of propellant.
To make sure this 60-layer MLI solution works NASA will test a "flight-representative, spherical LCH4 ascent stage propellant tank" as part of its Cryogenic Fluid Management (CFM) Project
An "inter-center team" from Ames Research Center, Glenn Research Center (GRC), KSC and Marshall Space Flight Center will carry this out at a GRC facilty that will be used to conduct experiments in "a simulated lunar thermal environment"
But there may still be problems with a LCH4c ascent stage. I wrote last year about pressurisation issues relating to landing aborts and excess pressure at the end of a 210-day mission. It would seem NASA is now more confident of the 210-day stay but what of the landing abort?
NASA has already test fired a LCH4 engine that is either a candidate for Altair's ascent stage or will contribute to its propulsion system's development
The slide below, which is from NASA's December 2007 briefing to the US National Research Council about work done under the space agency's Propulsion and Cryogenics Advanced Development programme shows the benefits of using methane over hypergolics
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