to determine optimum mixture ratios or specific impulse for best economic performance (higher I does not necessarily give better economic performance [9]). Hydrogen is an obvious fuel candidate, since it has the highest specific impulse of the fuels listed. However, hydrogen has associated problems with storage and leakage that may outweigh its advantages of high performance. For long term storage on Mars extensive refrigeration equipment may be required. Due to the unreliability of predicting the location of water on the Martian surface, hydrogen would most likely have to be imported from Earth. The advantages of methane are also clear. It has a relatively high performance, and it is a high O/F ratio fuel, improving the utilization of hydrogen that must be brought to Mars. Methane has fewer storage problems than hydrogen since its normal boiling point is slightly above that of liquid oxygen, and it can be used as a refrigerant. Methane could be imported from Earth, or manufactured entirely on Mars using local water and the atmosphere. It might also be manufactured using Earth derived hydrogen, which would be brought for this purpose. With hydrogen and an available CO2 source, methane can be produced stoichiometrically with a chemical efficiency approaching 100% using the Sabatier reaction. Finally, carbon monoxide is included as a potential fuel candidate since it can be a by-product of oxygen production from CO2. Although its performance as a rocket fuel is relatively low compared to hydrogen and methane on a mass basis, performance on an energy basis and plant mass and simplicity basis is relatively good. Storage is relatively easy and it requires fewer plant components to produce. The mass requirements for the three propellants considered are given in Table 3. These are the calculated masses necessary to return a 750 kg sample return vehicle or a 4200 kg manned return vehicle from the surface of Mars to LEO. For each of the three return trip propellant combinations, the total mass that must be delivered to the Martian surface was calculated, along with the amount that is stoichiometrically produced. For hydrogen and methane, this includes a certain amount of terrestrial hydrogen which must be imported for each mission if local water is not available. The mass at Mars for the non-ISMU case, minus the mass at Mars for the ISMU case, leads to a maximum allowable amortized plant mass at Mars. This is a
RkJQdWJsaXNoZXIy MTU5NjU0Mg==