TABLE 1 INTERORBITAL VEHICLE — ASSUMPTIONS AND SPECIFICATIONS argon as an acceptable propellant, convert d.c. power to beam power at 80% efficiency, and to also have a mass of 10,000 kilograms. An additional 10,000 kilograms are allowed for the structure of the vehicle, the empty propellant tanks, and any power conditioning that is needed, to produce a vehicle whose total mass without payload and propellant is 30,000 kilograms. The total propulsive force exerted by the ion engines is 200 N, and the acceleration of the empty vehicle is 0.0067 m/sec2. To establish transit times, it is also necessary to examine the duty cycle or the fraction of the elapsed time that the microwave beam engages the rectenna on the interorbital vehicle. Figure 2 gives the duty cycle as a function of orbital altitude of the vehicle for a microwave beam that is able to scan 50° from the vertical in either the east or west direction along the equator. It should be noted that the rectenna is assumed to always remain parallel to the surface of the Earth and that, if this position is maintained, no force is required to contend with gravitational gradient effects. Further, the efficiency of the rectenna is relatively independent of the angle with which the microwave beam meets the rectenna. This is of concern at low altitudes only because at high altitudes the angle of incidence is nearly at 90° throughout the engagement period. It should also be noted that Fig. 2 shows the duty cycle to suddenly change from about 25% to 100% when the vehicle reaches geosynchronous altitude. This, of course, does not occur and is the result of the mathematical model which does not adequately deal with the duty cycle as the vehicle approaches geosynchronous orbit but which is adequate for most of the interorbital transfer. Finally, an assumption is made about the trajectory that the vehicle will follow when it is intermittently accelerated. The assumption is made that the trajectory will be essentially a spiral, very similar to that of a vehicle which is continuously accelerated by a force parallel to the Earth's surface, as shown in Fig. 3 (7). This assumption is justified on the basis that the angular periods at which the intermittent accel-
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