8000 seconds with input powers of 69 and 150 kW respectively. The MPD thruster performance was taken from the projections of Fig. 1 to be 57% efficient at 5000 sec with an input power of 6 MW. The results of the calculations for a 2 x 106 kg payload and a 150 MW power supply are presented in Fig. 2. The results show that the MPD propulsion system gives a shorter trip time with the same power and payload when compared to the ion thruster propulsion system at either value of specific impulse. More important than even the trip time benefit, may be the advantage a MPD propulsion system provides in system simplicity. Due to the large amount of input power handled per thruster, a MPD propulsion system needs far fewer thrusters than an ion thruster propulsion system. Therefore the propulsion system will be much simpler and less costly. Another interesting COTV concept using MPD thrusters is the use of a remote power supply located on the Earth, at GEO, or somewhere in between to transmit power to the COTV in a microwave transmission. For an initial evaluation of this concept, (see Fig. 3), three transmitters were assumed to be in orbit at GEO equally spaced around the Earth. The transmitter longitudes correspond approximately to those of southern Japan, West Germany or southern France, and the western U.S. These locations may be quite practical if the SPS program becomes an international venture. The power supply for the transmitters could be a prototype, a partially completed, or a complete SPS. This concept assumes that the MPD-COTV is equipped with a microwave rectenna to convert the power to D.C. and that the vehicle receives power from only one transmitter at a time. The transmission frequency was chosen to be 22.125 GHz because at this frequency no power will reach the ground due to atmospheric absorption. The areas of the transmitter and rectenna were each assumed to be 1 km2 and the transmitter and rectenna efficiencies are 30% and 50% respectively. Using these assumptions the LEO to GEO trip time for the MPD-COTV (including a 28.5° plane change) was calculated by integrating the equations of motion which included the dependence of the power transmission efficiency on the rectenna and transmitter separation distance. The results are presented in Fig. 4 where, for a payload of 10$ kg and a transmitted power of 100 MW, the trip time is 105 days and the initial vehicle mass is 1.39 x 10^ kg. Even with only this preliminary evaluation, this concept appears promising in terms of trip time and payload in addition to its elimination of the costly solar array and the need for subsequent annealing of the array after each trip. These calculations assumed 3 transmitters, but the concept is still feasible with only one transmitter, but the trip times will be longer. The single transmitter could be a SPS demonstration article that could be retrofitted with the high frequency transmitter.
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