A Passive Gravitational Attitude Control System for Satellites

It is shown how the gravity-gradient effect may be utilized to design a long-lived, earth-pointing satellite attitude control system which requires no fuel supplies, attitude sensors or active control equipment. This twobody system is provided with a magnetic hysteresis damper which effectively damps out oscillations (librations) about the local vertical. The long rods, which must be extended in space from coiled up metal tapes, provide the required large moments of inertia and possess adequate rigidity and sufficient strength to endure the rigors of the extension process. The system is compatible with the requirements of multiple satellite launchings from a single last-stage vehicle. Analysis indicates that the gravitational torques are sufficient to keep the disturbing effects of solar radiation pressure, residual magnetic dipole moments, orbit eccentricity, rod curvature, eddy currents, and meteorite impacts within tolerable limits. It is believed that the high-performance, earth-pointing system described and analyzed in this paper represents an essential step in the development of high-capacity communications satellites requiring long life. CONTENTS Page X. I N T R O D U C T I O N XI. DYNAMIC P R I N C I P L E S AND G E N E R A L D E S C R I P T I O N OF PGAC 2196 2199 2.1 Principles 2.2 Description 2.2.1 Extensible Rods 2.2.2 Damper Unit 2.2.3 Packaging and Multiple Launching 2.2.4 Weight Breakdown I I I . E L A S T I C V I B R A T I O N S V E R S U S RIGID BODY MOTION 3.1 Rigidity of Mast Rod and Influence of Distributed Mass on Natural Frequencies 2207 3.2 Natural Frequencies of Deck Assembly 2209 3.3 Torsional Oscillations of Mast 2212 4.1 Tumbling 4.2 Spinning 4.3 Umbrella Effect 2213 2199 2201 2202 2203 2205 2205 2206 I V .