Electric propulsion differs fundamentally from chemical propulsion in several ways. First, a large electrical power system is carried on-board the vehicle to provide energy to electric thrusters. This electrical energy is provided to thrusters which accelerate a propellant at very high speeds. This acceleration produces a very high ISp. Because of the high Isp, the total mass of the vehicle can be significantly reduced over chemical propulsion. This is especially true for the very high energy missions. The performance of an electric OTV is strongly dependent upon the power technology, power level and the Isp of the thrusters. For each mission type, a series of trade studies and an optimization of power level and thruster performance is therefore recommended. Secondly, the thrust levels with electric propulsion produce a low thrust-to-weight ratio: less than 10" 4 and typically 10A This requires the vehicle to thrust for a long period of time to accomplish its mission. Also because of the large power systems used, the vehicle is often very large. Because of the low thrust levels produced, the structure can be lightweight and flexible. There are several thruster technologies that are appropriate for orbital transfer vehicles. They are ion, arc jet and Magneto-Plasma-Dynamic (MPD) thrusters. Each system can use various propellants and the performance is dependent upon the propellant selection. Ion propulsion will typically use inert gas propellants, such as xenon, krypton or argon. Arc jet thruster may use hydrazine, ammonia or hydrogen. For MPD thrusters the propellants may be argon, hydrogen or even lithium for very high efficiency engines. The other important parts of an electric OTV are the structure, guidance system and other subsystems typical of chemical OTVs. Tables 8.4 & 8.5 provide the list of subsystems and the masses of a chemical and electric OTV for a lunar mission. The payload delivered to lunar orbit is 35,000 kg. The most massive part of the electric OTV is the power system [Palaszewski, 1988]. Both solar arrays and nuclear reactors may be used for powering these transfer vehicles. With a space solar power system, however, it may be advantageous to use the same power technology for the transfer vehicle as with the satellite. This would reduce the overall development cost because only one power system would need to be developed. Therefore, nuclear reactors may not likely be developed for Space Solar Power Program if only solar power were part of the system design. On the other hand, a nuclear-electric OTV is very efficient in terms of mass and is less sensitive to the radiation degradation during transfers through the Van Allen Radiation Belts. Solar arrays can be very sensitive to this radiation. Nuclear electric OTVs may also be developed for lunar transportation. Mars missions or other interplanetary missions. Thus, the selection of the power technology may be driven by many factors other than the Space Solar Power Program. Conclusions Very powerful benefits of reduced LEO mass and transportation cost reduction are possible with electric propulsion. Though the cost of development of the power system will be significant and perhaps the most expensive part of electric propulsion, the benefits in terms of long term cost reduction are undeniable. The performance of electric propulsion and its ability to reduce the mass of propulsion systems are discussed in the sections on advanced technologies for cost reduction and lunar transportation.
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