Cryogenics vs. Hypergolics for the Ascent Stage

We need a delta-v from the ascent engines of about 6,136 ft/s.

LH2/LOX Cryogenic Propellants

Engine	RL-10A-3-3A
Weight	305 lb
Vacuum Thrust	16,500 lb
Expansion Ratio	61:1
Oxidizer/Fuel Ratio	5:1
Isp	444.4 sec
Propellant Mass	856.7 lb (142.8 lb LH2, 713.9 lb LOX)
Propellant Volume	32.33 ft3 LH2, 10.03 ft3 LOX

N2O4 / MMH Hypergolic Propellants

Engine	Kaiser Marquardt R-40A
Weight	22.5 lb
Vacuum Thrust	870 lb
Propellants	MMH / N2O4
Expansion Ratio	20:1
Oxidizer/Fuel Ratio	1.6:1
Isp	281 sec (range from 281 to 306 sec)
Propellant Mass	1553.6 lb (597.5 lb MMH, 956.1 lb N2O4)
Propellant Volume	10.87 ft3 MMH, 10.56 ft3 N2O4

These propellants might actually freeze (freezing point is 261K for N2O4, 220.7K for MMH), but a tiny heating coil would be a trivial addition. Other than that, they are much easier to store than the LH2/LOX propellant, and their tankage will occupy a smaller volume.

Further Work Needed:

• Detail hypergolic engine
  Should work with cryogenics as well, for use in the descent stage
• Calculate cryogenics boiloff, total fuel required
• Calculate tankage masses
• Determine storage requirements
  Such as insulation, heating, leak control, etc.
• Determine approximate risk for both systems
• Determine overall cost for both systems
  Such as cost of materials, cost of getting the extra mass to the lunar surface, etc.
• Combine cost and risk; decide on ascent stage fuel