SPS Feasability Study SD76SA0239-2

on the south end fire. From Figure 2.4-4, the total annual AV to keep inclination under 1° is 40 m/sec. Half of the AV is assumed to be provided by each group of thrusters in one three-hour burn per orbit. The total required thrust is 121.7 newtons at each end. The effects of the triaxiality of earth depend on the longitude of the satellite. In the worst case, the satellite can be maintained in the desired location with 1.83 m/sec/yr. This velocity is imparted in one three-hour burn per orbit. This burn occurs at the same orbital location as the semimajor axis burn but uses a separate set of thrusters, firing in the -Z direction, to insure sufficient control authority in all cases. The total thrust required is 11.1 newtons. Figure 2.4-2 summarizes thruster locations and requirements and Table 2.4-1 summarizes propellant requirements. No alternative to thrusters for stationkeeping were considered in this study since mass expulsion appears to be the only feasible means of imparting required velocity increments. It should be observed that stationkeeping requirements are dominated in every category by the north-south stationkeeping requirements. These can be reduced or eliminated by permitting greater orbital inclinations. However, greater inclination increases the size and the operating complexity of the ground rectenna; hence, a tradeoff is required to choose maximum allowable inclination. 2.4.3 Attitude Control For SPS class vehicles the largest torques to be counteracted by the attitude' control system are gravity gradient torques. They are so large that they effectively size the control system. In the subsequent analysis, gravity gradient torques were the only ones considered. The moments of inertia of the reference vehicle are estimated as for a total vehicle mass of 2.4 x 10^ kg. The corresponding gravity gradient torques become (in newton-meters) The Y axis is held nearly perpendicular to the orbit plane (POP) for the reference satellite concept. If the Z axis is held parallel to the sun line, the Y axis experiences a cyclic gravity gradient torque given by Equation 2.4-27 where y » wt. The Y-axis control requirements are determined by integrating the torque over one orbit to determine the total impulse requirement is given in Equation 2.4-29. For two sets of thrusters, one at each end of a moment arm, the required propellant per orbit is