Generator mass required for 14 GW output from the thermal engine is a major concern. Even with a specific mass of 0. 453 kg/kW, the mass would be 6. 3 x 106 kg. However, this specific mass appears to be an achievable goal for the SPS. The projection is based on Atomic Energy Commission studies [17] for liquid metal cooled machines, on work underway by Wright-Patterson Air Force Base on machines for laser and radar applications, and on large commercial generators. Specific weights below 0. 9 kg/kV A were defined through point designs in an Atomic Energy Commission funded study, while Wright-Patterson had demonstrated short term operation of 0. 045 to 0.113 kg/kV A machines [16]. Commercial water-cooled generators are available with specific weights of 0. 385 kg/kV A at 1500 MW ratings. The efficiency chain for the system postulated for the SPS is shown in Figure 7-37. Estimates of component masses are included in the figure. In comparison to the solar array distribution system, power loss was approximately the same but system mass was doubled for the the rmionic-Brayton system. A 400 kV, 400 Hz transmission system was studied for comparison. Step-up transformers were added at the power sources to increase the voltage level to 400 kV for transmission. The slip-ring assembly used for the baseline de system was replaced by a rotary transform er/rectifier circuit to provide 20 kV de for the transmitter tubes. Power line mass and loss were reduced considerably from the 20 kV de system, but the advantage was lost through added requirements for transformers and rectifiers. As a result, the system power loss for the two systems was equal but the mass of the 400 kV ac system exceeded the mass of the 20 kV de system by 9 x 106 kg. 7. 2. 5 MAINTENANCE AND REPAIR Collision damage and component failure will be the sources of maintenance requirements for the thermal conversion system just as it is for the photovoltaic system. Since the total area is approximately one-fourth that of the photovoltaic system, the collision rates should be proportionally smaller. The consequences of impacts on the concentrators are more significant, however, in view of the maze of plumbing, valves, radiators, etc. Fluid loss could become significant even if individual reserves are provided for each concentrator cell. Plumbing component failures will probably be more serious than amplitron failures. Automatic installation of plumbing spares will be required to maintain operation. Automatic plumbing maintenance will demand innovations and a designed maintainability philosophy. Automatic maintenance of the antenna systems is as described in subsection 7.1. 7.2.
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