TABLE VI-4.- OTV CANDIDATE THRUSTER CHARACTERISTICS the trip times are measured in months. In the case of LEO assembly or partial assembly, the acceleration placed on the structure by the COTVL must be less than 0.001g, which dictates a low T/W. The transit time variation with T/W is shown in figure VI-5. Other implications of using a low thrust level, which results in an acceleration several orders of magnitude less than the local gravity, are: (1) continuous thrusting and acceptance of gravity losses is necessary rather than the three burn impulsive transfers normally practiced, (2) thrust may be horizontal to the ground for simplicity, because this is very close to the ideal velocity vector, and (3) the delta velocity required is approximately equal to the actual difference in the circular velocities of the original and new orbits. A problem associated with systems using solar energy and exerting a continuous low thrust is the occultation of the Sun by the Earth. This interrupts thrusting and may require that a system reacquire solar orientation each time the satellite emerges from the shadow. Another factor affecting the photovoltaic system is that solar cells exposed during passage through the Van Allen belts will suffer a performance degradation estimated to be as high as 30 percent for the long transit times considered. This effect is reduced because the COTV|_ does not require that all the array be exposed during orbital transfer. These problems can be avoided if a reusable chemical propulsion system is used to take the payload to a higher orbit, perhaps as high as 5000 km; then the switch to the solar-powered system is made. The COTV^'s, using payload-supplied power, can take full advantage of the more advanced thruster concepts, such as the MPD arcjet, because they do not pay the penalty of the necessarily heavy power supplies and the round-trip mission. These systems have the advantage of low propellant requirements, thus decreasing the orbital burden factor, but must pay some economic penalty because of their long transit time. Additionally, an MPD thruster/argon-propellant system operating from its own nuclear reactor appears to hold promise for low thrust, nuclear-electric payload transfer. Key factors, presently
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