4A.2 Propellants
There are two most promising propellant options for lunar-LEO transport systems. The first is an oxy-hydrogen combination using lunar-derived oxygen and hydrogen imported from Earth. The second option again requires native lunar oxygen as the oxidant but combines terrestrial-imported hydrogen with silicon purified on the Moon to produce a more powerful silane rocket fuel.
(a) Lunar oxygen, terrestrial hydrogen propellant option
The relevant chemical propellant combustion reaction is:
- 2H2 + O2 → 2H2O
The molecular weight of H2 is 2 and of O2 is 32, so:
- BH = MH2/(MH2 + MO2) = 1/9
The achievable specific impulse of LOX - LH2 is about 450 sec, using heat of formation data from Weast (1978) and assuming 75% thermal efficiency. This yields an exhaust velocity of 4.41 km/sec.
(b) Lunar oxygen, Earth/lunar silane propellant option
The silane produced on the Moon is assumed here for simplicity to be entirely SiH4. The propellant chemical reaction is:
- SiH4 + 2O2 → SiO2 + 2H2O
The molecular weight of SiH4 is 32, so BH = 1/24. The achievable vacuum specific impulse is within the range 328-378 sec (Lunar and Planetary Institute, 1980). Assuming the middle of the range, Isp = 353 and C = 3.46 km/sec.
4A.3 References
Lunar and Planetary Institute: Extraterrestrial Materials Processing and Construction, Final Report NSR 09-051-001 Mod #24, Houston, Texas, 1980.
Weast, Robert C., ed.: Handbook of Chemistry and Physics, CRC Press, West Palm Beach, Florida, 1978. Fifty-ninth Edition.
