• Define an attitude control system for the SSPS and investigate analytically the elastic coupling between the control system and the spacecraft's structural modes. • Evaluate the performance of the baseline SSPS and perform a parametric sizing study to determine the influence of structural flexibility upon system performance. Results of those studies are presented in References 29 and 30. Establishment of Flight Loading Conditions.— The SSPS nominal orientation in synchronous orbit is defined in Reference 31. External forces acting on the satellite will cause deviations in the nominal orbit which, unless corrected by the spacecraft attitude control system, will cause pointing errors in the solar collectors and antenna. Pointing errors of the antenna can be corrected by a combined mechanical and electronic system and do not require any corrections of the SSPS main structure. For the purposes of this study, allowable deviation angles in the solar collector pointing accuracy were limited to ±1.0 degree about all axes. Mass expulsion actuators are used to control angular deviations and orbital drift. The external forces acting on the SSPS have four sources: aerodynamic, magnetic, solar pressure, and gravity gradient (Figure 11). The external forces associated with these conditions are given in Reference 31. At synchronous altitude, the aerodynamic forces are negligible. The magnetic forces acting on the SSPS due to the interaction of the magnetic fields of the Earth and the SSPS are shown to be small. The total solar pressure force acting on the satellite is 224 newtons (50 lbs) with small resultant torques. The largest external torques acting on the satellite are caused by gravity-gradient effects. Values of these torques are a direct function of the angular offset from the nominal orientation. Values for the gravity-gradient torques are given in Reference 31, and the control actuator forces required to correct for these torques are given in Reference 30. The passage of electrical currents along the bus/structure elements generates forces in these members. The magnitude of these electromagnetic forces is given in Reference 32. These forces are internal and are self-balancing. A study was conducted to determine the force created by the radiation of electromagnetic energy from the SSPS antenna (in effect this is a “recoil” from the MW beam). The study showed that the total force normal to the antenna was 1.8 kg (4 lbs). As the Earth rotates about the Sun in the ecliptic plane, the Earth casts a circular shadow of approximately its own diameter. Every satellite in equatorial synchronous orbit crosses this shadow daily during a 45-day period, twice per year at the time of the vernal and autumnal equinoxes. The time duration spent by the SSPS in this eclipse varies from near zero to a maximum of about 72 minutes at the equinox date. As the satellite enters the eclipse, it experiences a temperature drop of about 270°K. The thermal transient is very rapid at the beginning of this period, and a satisfactory method of assessing the loading and dynamic effects of rapid dimensional fluctuations, induced by an eclipse interval, remains to be analyzed. The overall structural response of the SSPS during this period and after reentry into the sunlight represents a significant future study area involving the dynamic effect on large structures of rapid thermal transients.
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