1277. Elimination of Aerodynamic Effects During Spin Balance of Spacecraft
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Paper
Abstract
The orbit insertion method used by RCA for its three-axis stabilized communication satellites requires that after booster separation, the spacecraft must be spinning to maintain stability prior to being located in a geostationary orbit. Maneuvering accuracies and fuel economy require that the spin axis be identical to the thrust axis of the integrated spacecraft which thus must be accurately balanced prior to launch. On the ground, appendages such as antenna reflectors cause significant aerodynamic disturbances which prevent sufficiently precise balancing unless special precautions are taken to eliminate these effects. This is not a problem in the vacuum of the space environment, since aerodynamic effects do not exist. In order to achieve precise balancing on the ground, the aerodynamic effects have been eliminated by enclosing the spacecraft within a co-rotating, smooth cylindrical shroud during spin balancing.
Two earlier RCA Satcom satellites incorporated fixed reflectors which caused relatively small aerodynamic disturbances because of their more or less symmetric antenna configuration. These satellites were spin balanced in air by averaging clockwise and counter-clockwise measurements. This technique produced a functionally satisfactory balance. Some future communication satellites will have much more complex reflectors which cause higher aerodynamic disturbance.
Test methods were developed to measure static and dynamic unbalance separately on the MRC machine at Hightstown or on the SchenckTrebel machine at Cape Canaveral. Recently, these new methods were applied to the Telesat Canadian communication satellite. It was balanced in three steps: the first two at Hightstown before and after the installation of solar panels, and the third at Cape Canaveral with the hydrazine tanks loaded and the solid rocket installed. Balance within the original specification of .020 in. C.G. offset and 50 lb-in2 product of inertia is clearly attainable on the 2000 lb spacecraft.
This technique is applicable to spin balancing all spacecraft whose irregular shapes cause aerodynamic unbalance forces.
