1166. Aerodynamic Effects on Spacecraft Dynamic Balance

Paper

Abstract

The requirements for spacecraft dynamic balance are based on vacuum conditions, but the tests are normally performed in air. Asymmetric aerodynamic forces are therefore error terms for spacecraft balance.
A test program was performed to investigate these effects. An aerodynamic model of a typical spacecraft with asymmetrical appendage was used. Measurements were made on a TRW-designed low speed balance machine. The tests environments included vacuum, and aerodynamic enclosure, and open air. The final test sequence was performed with the aerodynamic enclosure inside the vacuum chamber.
The appendage produced significant asymmetric elastic effects. A special analysis of these effects was required in order to isolate the aerodynamic effects.
The measured aerodynamic forces did not vary smoothly with spin rate and with ambient pressure. Extensive data reduction with least squares smoothing techniques was required to obtain meaningful results. The reduced data from the final test sequence is presented in terms aerodynamic moment as a function of the ambient pressure. The associated error in product of inertia is also plotted.
The aerodynamic forces were much less in the enclosure than in open air. Using an average from both clockwise and counterclockwise spin directions reduced the errors even further for the measurements in the enclosure. The averaging technique did not provide significant improvement in the open air measurements.
Six correction techniques for reducing the aerodynamic error terms were briefly discussed, as follows.
– Spin both directions and average the results
– Use an aerodynamic enclosure
– Replace the ambient air with helium
– Provide boundary layer velocity control around the periphery of a cylindrical enclosure
– Balance in a vacuum chamber
– Use an aerodynamic model to determine correction weights.