Several electric propul sion/RCS systems were evaluated, including electrostatic (ion) thrusters, magnetoplasmadynamic (MPD) and thermal arc jets, and an O2H2 chemical system in which the propellant is launched as water and electrolyzed to produce the gases. The intent was not a complete survey of all possibilities, but a range of potential options in terms of weight, performance, and power requirements, Of the systems considered, the MPD arc jet appears most attractive. Although it is in an early stage of development and performance projections are uncertain, its low weight, high specific impulse and high unit thrust make it worth further consideration. All the high-performance propulsion systems require large amounts of electrical power. It would be impractical to store sufficient electrical energy to operate these systems during eclipse. It appears feasible, however, to inhibit propulsive or RCS maneuvers during eclipse, except for short-period gravity gradient compensation. This small requirement can be satisfied by an H2O2 electrolysis system that produces and stores gases in sunlight; the system can also serve as a standby propulsion system. Development is required for the RCS. Development will also be necessary in control of very large, flexible structures, a problem that cannot be completely simulated on Earth because gravity is much greater than the forces normally acting on the structure. 5. Instrumentation, Control, and Communications A large quantity of subsystem'status data will be required for ground monitoring and control. The depth of this study has not produced sufficiently detailed definition to permit meaningful identification of instrumentation, control, and communications requirements. However, present technology should be adequate for all foreseeable needs, and the weight impact on the SPS will be insignificant. As a consequence, this subsystem has been deferred for later study. 6. Maintenance Station Because it would be impractical to design the SPS for a 30-year failure-free life, maintenance will inevitably be required periodically. The maintenance question has not been explored to any depth in this study, however, and the following comments represent a cursory consideration of the problem. To minimize the payload of the maintenance spacecraft, as many maintenance support capabilities as possible should be incorporated into the SPS. These would include a normally unmanned, habitable control station, some repair and small spares storage facilities, and servicing and local transportation vehicles. All these capabilities will be similar, if not identical, to those used during construction, so that the same development program is applicable to both.
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