Lunar In Situ Propellants Mass Drivers 8.7 Lunar Transportation The utilization of lunar raw materials to build up a large scale solar power satellite seems to be a promising way to lower the initial LEO mass. Therefore this section gives a brief discussion of lunar transportation systems that would support this activity. Reducing the initial LEO mass that has to be transported from Earth into LEO offers the opportunity to lower the overall costs of building up a Space Solar Power Program. According to the lower gravity of the Moon compared to Earth, only about V20 of the energy is required to reach space from Moon than from Earth. In this case we assume a large lunar industrial infrastructure which is capable of producing Space Solar Power Program construction parts economically. According to different studies, the range of lunar materials of a GEO Space Solar Power Program is between 90% and 98%. This section presents a comparison of cislunar transportation systems with different propulsion systems in order to find out the most promising strategy for transporting construction materials from the lunar surface to the Space Solar Power Program construction site. The following types of propulsion are addressed: 1. Conventional chemical LO2/LH2-propulsion 2. Electric propulsion 3. Nuclear propulsion 4. Mass driver A laser propulsion system is not considered because operation requires a high power source in the order of at least 100 MW transmitted by a laser beam. Because large power sources may not be available during the Space Solar Power System construction phase we did not consider laser propulsion. 8.7.1 Conventional Chemical LO2/LH2 Propulsion The first LOX/H2 propulsion system was used in the Centaur upper stage whose development started in 1958 with the first R&D flight in 1962. Of the chemical propulsion systems, LOX/H2 -propulsion represents the propulsion system with the highest achievable specific impulse (Isp). At present an Isp of about 455 seconds is achieved by the SSME (Space Shuttle Main Engine). Currently the Advanced Space Engine (ASE) using an expander cycle is under development in the USA with the aim of increasing the Isp to about 480 seconds. A higher Isp on the order of 485 seconds is expected for the future due to higher combustion chamber pressure (more than 200 bar) and higher expansion ratios on the order of 1000 especially in vacuum operation. At present chemical propulsion provides the highest ratio of thrust to weight with a ratio up to 100 Figure 8.18. This high ratio enables very short inter orbital transfer times between LEO and LLO on the order of 3 days. With regard to the high thrust and throttling demand of lunar descent/ascent, LOX/H2-engines provide a high performance to fulfill the main mission requirements. Furthermore due to longstanding experience, LOX/H2-engines offer a relatively high reliability which is very important especially for manned missions. However, a disadvantage is the handling and storage of the cryogenic propellant for several days - especially of hydrogen - which results in high boil-off rates, compared to storable propellant such as hydrazine. Additional insulation material should solve this problem and keep the boil-off rate below 0.1 % per day. To obtain maximum performance the LO2/LH2-engine is operated at a weight mixture ratio (LO2/LH2) of about six which means that about 85% of the entire propellant is oxygen and about 15% is hydrogen. Due to the fact that oxygen is richly abundant on the Moon (the lunar regolith consist of about 45% of oxygen by weight) a lunar oxygen production facility could substitute the Earth-derived oxygen and provide lunar oxygen for the space vehicles. The initial LEO mass can be decreased in the order of up to 50% so that a cost benefit is expected due to a LEO mass saving.[Michael REICHERT, 1992],
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