Fig. 3. The trajectory of a vehicle travelling around the Earth in an initially circular path and then subjected to a continuous accelerating force that is tangential to its orbital path is a spiral path that eventually carries it out of the capture by the Earth's gravitational field. It is assumed that intermittent applications of force, distributed approximately uniformly in the angular coordinate, will approximate the same trajectory. payload, cost of transporting that propellant mass from Earth to LEO, and energy consumed by the ion engines per kilogram of payload. The transit times in Table 2 assume that the rectenna and ion engines will be operated at full power level up to geosynchronous orbit. However, a scenario in which the rectenna and ion engines are operated at full power level up to an altitude of 14,600 km and at a power level inversely proportional to the square of the radius after that altitude results in significantly lower first costs associated with the transmitter. Under these assumptions, the transit times will be increased from those given in Table 2 by about 27% More detailed data for the case of a payload mass fraction of 58.2%, as presented in the second row of Table 2 but modified by the 27% larger transit time, is shown in Fig. 4. Figure 4 shows the altitude of the vehicle as a function of days of elapsed time from the start of the LEO to GEO transfer. Note that the return trip of the vehicle, assumed to have no payload fraction and just enough propellant for the return trip, is three times as fast as the outbound trip. The relatively slow increase in altitude at the start of the interorbital journey is caused by the relatively low duty cycle (Fig. 2) and the large amount of work required to pull away from the Earth’s gravitational field, which is inversely proportional to the square of the radius of the orbit. On the other hand, the microwave power transmission system selected for this scenario operates at very high efficiency at low altitudes, and its consumption of 60-cycle energy from an Earth source is very low at low altitudes. Figure 5 shows the accumulative consumption of 60-cycle energy as a function of time, including that for the return journey. The overall efficiency resulting from the scenario selected for the microwave power transmission system is poor. From Table 2 it is seen that the ion engines consume
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