7.1. 6.4 DISTURBANCE TORQUES The external torques on the SPS, listed in the approximate order of importance, result from the following sources: • Gravity gradient • Antenna rotary joint friction • Solar pressure • Microwave antenna recoil pressure • Aerodynamic • Magnetic. The gravity gradient torques are the predominant disturbances. Reaction control thrusters are used to counteract these torques. Figure 7-29 provides a plot of the peak gravity gradient torques about each of the three axes of an SPS configuration oriented in the reference attitude. The X-axis torques vary cyclically with a period of 12 h and have the peak values shown. The large Y- and Z-axis torques are secular in nature and are a result of the ±23. 5° offset from the orbital plane required seasonally to maintain the Z-body axis aligned with the solar vector. The alternate X-POP orientation greatly reduces the magnitude of the Y- and Z-torques and consequently the reaction control system propellant consumption. In this alternate orientation the magnitudes of the gravity gradient torques about Y and Z are essentially functions of the attitude control system deadband which has been assumed to be 1°. The magnitudes of the gravity gradient torques are also direct functions of the inertia differences of the three axes. From a fuel consumption standpoint, it has been found generally desirable to minimize the inertia difference (I -I ) to reduce the cyclical torques about the X-axis. X Zj 7. 1. 6. 5 PROPELLANT CONSUMPTION Reaction control system (RCS) propellant consumption estimates for both station keeping in geosynchronous orbit and attitude control for gravity gradient torques of a Z-solar oriented SPS have been calculated (Table 7-8).
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