Figure 8.21. At present the ratio of thrust to weight can reach up to 30, thus the time for inter-orbital transfers can be in the order of less than 30 days depending on payload size and thrust. The first investigations concerning thermal nuclear propulsion which were carried out in the USA was the NERVA program which was canceled in 1973. This kind of propulsion has stimulated the interest from many Nations in the last twenty years. It might be the next non chemical propulsion system to be developed and to be used for large scale applications within the next thirty years. 8.7.4 Mass Driver The purpose of the mass drivers is to accelerate payloads of materials to a high velocity by the transformation of electrical energy (using an electromagnetic field) to mechanical energy of motion. In our application the mass driver would be placed on the lunar surface, and it would accelerate the payloads to the escape velocity of the Moon (2340 m/s). These payloads would be collected at a point in space ( Lagrangian point of the system Earth-Moon) to serve as the material depot for space manufacturing or to be sent to the Earth. Several Mass Drivers have been built until today. Mass Driver III has achieved over 1800 gravity acceleration. The length of the lunar machine to obtain the Moon escape velocity requires 160 meters. The mass that had been carried was about 500 grams. Many studies are conducted today on advanced Mass Drivers. Summary As shown in Figure 8.21, electric propulsion can reduce the initial LEO mass which is required to transport materials for building up Space Solar Power Program. This would have enhanced benefit to Space Solar Power Program (with the exception of H2-arc jet propulsion) as compared to conventional chemical or nuclear thermal propulsion. The reason for this is the higher specific impulse which can be achieved by electrical propulsion. However electric propulsion provide a low thrust which will cause relatively long inter-orbital transfer times. So electric propulsion seems only appropriate for cargo missions. For manned missions high thrust chemical propulsions seems to be the most promising solution because of short inter-orbital trip times of about 3 days. The use of lunar oxygen for chemical space vehicles can reduce their initial LEO mass requirement on the order of up to 50 percent. If the safety and reliability of thermal nuclear propulsion can be enhanced, medium thrust nuclear propulsion could compete with chemical propulsion. 8.8 Scheduling The scheduling of space transportation activities is based on the assumption that the first three demonstrations (Demo 0, 1 and 2) will only require existing or presently under development launchers. The only milestone for these three demonstrations will therefore be the choice of a specific vehicle. Demo 3 and future demos, on the contrary, will need tailored developments in the field of Space Transportation (personnel, priority cargo, bulk, etc.). Therefore, a set of technology studies and the design of launch vehicles will have to take place before the launch and assembly of the demonstration spacecrafts.
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