Microwave beam pointing is achievable with an antenna pointing accuracy of 0.05 deg and electronic beam steering for precise vernier pointing. Solar collector pointing accuracy requirements are a function of collector concentration ratio and are on the order of 0.5° for CR=2. These accuracies are achievable with existing technology and current studies indicate they can be met without active figure (structural shape) control. Simple active figure control in the microwave antenna may prove to be useful in simplifying the structural design and assembly tolerances. Attitude control techniques considered for the SPS include: spin and gravity gradient stabilization, solar pressure vanes, large erectable momentum wheels, quasi-inertial free drift modes, and various reaction control thruster types (Figures 2-4). The spin, gravity gradient, and solar pressure vane stabilized approaches were all found to be inferior to the selected baseline because of their larger mass and complexity penalties. The large erectable momentum wheels (Figure 3) and the quasi-inertial free-drift attitude mode are useful in eliminating propellant consumption due to cyclical disturbance torques. The propellant requirements for various RCS thruster types (Figure 4) indicate that high-performance propulsion (such as argon ion thrusters) is required to avoid the high propellant resupply costs of contemporary chemical propulsion systems for a 30-year spacecraft lifetime. The attractiveness of RCS thrusters for attitude control is enhanced by combining attitude control and stationkeeping requirements, and satisfying them jointly with the same propulsion systemsJ The approach is illustrated in Figure 5. Thruster groupings are at each corner of the spacecraft and nominally thrust continuously toward the sun to correct solar pressure orbit perturbations. Other stationkeeping perturbations are considerably smaller and are corrected by gimbaling the thrusters through small angles. Similarly, the attitude control torques are obtained by a combination of differential throttling and gimbaling. The system is capable of simultaneously providing stationkeeping forces and attitude control torques about all three axes. Since the required gimbal angles are small, these functions are satisfied with a propellant quantity that is only slightly greater than that required to correct the solar pressure stationkeeping perturbation. Gimbaled thrusters are preferable to body-fixed thrusters because of a significant reduction in the number of thrusters and propellant required. During earth eclipse periods only attitude control torques are provided. This control approach minimizes the system requirements for attitude control and is selected for the baseline reference configuration (Figure 5). Nominally 36 thrusters are required; however, 64 are provided to accommodate for failures and servicing. The mass of the overall AC&SK
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