The RCR was found to provide potential solutions in areas of efficiency, weight, and temperature; however, large-scale tests on the RCR must be made to verify analytical predictions. 6. Failure Mode Analysis of MPTS Requires Study — The effects of MPTS antenna failure has not been evaluated in detail. Modes of failure and design solutions to minimize blackouts and radiation hazards from distorted beams must be initiated. 7. Maintenance of ''Hot" Conversion Devices Requires Further Study — The maintenance procedure of hot tubes requires a detailed study to determine methods by which the RCR can be maintained without major shutdowns. The hot conversion devices may be extremely difficult to handle and special design considerations in the MPTS array must be allowed for this discipline. 2.4 ATTITUDE CONTROL/STATIONKEEPING 2.4.1 Reference Control System The reference vehicle was analyzed to determine its requirement for attitude control and stationkeeping. A reference control system consisting of high performance electric thrusters was chosen to provide a common basis for comparison with other SPS studies. The thruster^ of Figure 2.4-1 was selected as the reference thruster. Its high thrust reduces the total quantity of engines required, leading to a simpler overall system. It uses argon as the propellant, which exists in significant quantities and is relatively non-polluting. Thrusters are grouped in two modules at each end of the vehicle as shown in Figure 2.4-2. Each module includes propellant storage facilities and power conditioning units to service its thrusters. The thrusters are oriented to provide simultaneously the necessary forces and torques for attitude control and stationkeeping. Needed attitude control torques are supplied by continuously throttling the thrusters. Minimum thruster counts are determined by the maximum force that must be produced. Sufficient force is provided to keep the vehicle oriented within ±1° of its nominal orientation in all three axes. Stationkeeping maneuvers are accomplished in three-hour burns at maximum thrust. No throttling is required for these thrusters. Elimination of perturbations in semi-major axis is required only during periods of eclipsing. A maximum of one three-hour burn per orbit is required. North-south stationkeeping requires one three-hour burn off the north end and one three-hour burn off the south end of the vehicle per orbit. East-west stationkeeping to eliminate the effects of gravity perturbations requires a maximum of one three- hour burn per orbit. Solar pressure induced eccentricity is not controlled.