Attitude dynamics and thrust control for short tethered sub-satellite in deployment

Dynamic characters and control strategy for short tethered satellite system in deployment are studied in this paper. Tether elastic oscillation and sub-satellite attitude motion are considered in the dynamic model. Sub-satellite releasing method with an initial velocity and a negative acceleration is proposed for fast deployment. It is indicated that the short dimension and fast deployment make dynamic characteristics of both the tether and the sub-satellite different from those of the long-tether systems. Control strategy is designed based on these dynamic characters. Thrusters are considered to provide active control for the tether and the attitude of the sub-satellite. Variable structure control is adopted, taking into account the nonlinear dynamics of the system and the on-off output of the thrusters. Numerical simulation shows that the tethered sub-satellite can be deployed to designed direction in or out of the orbit plane with thrust control. The tension of the tether is positive during deployment. The in-plane angle of the tether is controlled within 0.1°. The out-of-plane angle passively keeps near its initial releasing value. In addition, the attitude angles of the sub-satellite are controlled or passively remain within 0.5°. This fast deployment provides flexibility for tether assisted space operations.