SPS ATTITUDE CONTROL AND STATIONKEEPING - REQUIREMENTS AND TRADEOFFS R.E. Oglevie Rockwell International, Downey, California This paper summarizes the results of attitude control and stationkeeping (AC&SK) studies^'2,3 to define spacecraft and mission requirements, preferred control approaches, and feasibility issues.^ The work was partially accomplished under NASA MSFC Contract NAS8-32475. Three orbits with features attractive to SPS are shown in Figure 1. The ecliptic orbit permits direct solar viewing in a horizontal attitude, which minimizes gravity gradient disturbance torques. The 7.3° inclined orbit minimizes the north-south stationkeeping AV requirement. The geosynchronous equatorial orbit is preferred because of the large cost of the increased rectenna size associated with the two other orbits. The large size of the SPS makes appreciable changes in AC&SK requirements relative to small contemporary spacecraft. Analyses^ indicate that the solar pressure stationkeeping perturbation becomes dominant rather than the solar-lunar gravitational perturbation. Gravity gradient disturbance torques increase rapidly as a function of spacecraft size and can cause appreciable attitude control penalties without judicious choice of spacecraft reference orientation and spacecraft design parameters. Structural bending frequencies are appreciably reduced, raising concern about control system/structural dynamic interaction stability. The stationkeeping AV and RCS propellant requirements are presented in Table 1; correction of the solar pressure perturbation dominates the requirements. If uncorrected, the solar pressure perturbation will cause a ±2.5° cyclical change in longitude with a one-year period. This is unacceptable in light of the heavy use of the geosynchronous equatorial orbit projected during the SPS time frame. The stationkeeping propulsion requirements necessitate the use of high-performance propulsion (such as ion thrusters) to minimize propellant resupply expense over the SPS lifetime. Flying the SPS spacecraft in clustered constellations offers promise of minimizing their space requirements in geosynchronous orbit. Table 1. Stationkeeping A V and Propellant Requirements Figure 1. Orbit Selection Trade ^Satellite Power System Concept Definition Study. Vol. II. Rockwell International, SSD 79-0010-2-1 (March 1979). ^Oglevie, R.E., “Attitude Control of Large Solar Power Satellites,” AIAA Paper 78-1266, AIAA Guidance and Control Conference (August 1978). ^Satellite Power System Concept Definition Study. Vol. III. Rockwell International, SD 78-AP-0023-3 (April 1978). ^The assistance of D. Camillone of Rockwell in performing this work is gratefully acknowledged.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==