A Study On Dynamics And Deployment Planning Of Tethered Satellite System

Tethered satellite system consists of two or more satellites connected by long tether. It plays an important role in both space exploration and space debris deorbit. However, the dynamics and control of tethered satellite system are quite complex, due to the complexity of space environment and characteristic dynamics of attitude and orbit. A lot of investigations focus on the dynamics and control of tether deployment/retrieval, which are the most important and delicate parts of a tethered satellite system mission. Because of the effect in space, the tether is strongly susceptible to undergoing a complicated set of librations and vibrations during the deployment/retrieval phase. So it is very important to plan the deployment phase depending on the dynamics and control of tethered satellite system.According to the emerging technologies of tether deployment,the background of tethered satellite system missions is reviewed, the dynamics model and control strategies are also summarized. Based on the fundamental model of dynamics for tethered satellite system and the planning of tether deployment, the development tendency and key technologies still need to break through of tether are analyzed. The main themes and contributions of the dissertation include:Firstly, based on Lagrange principles, the attitude dynamic of tethered satellite system effected by in and out-of-plan librations was established. The two-body model takes many practical factors like the mass of tether into consideration, therefore it is broadly applicable. This model is later extended to multi-body system. The hub and spoke multi-tethered satellite system is anylasised with considerring the attitude of central body or not. The beaded formation of multi-tethered satellite system is also deduced with Kane method later.Using the dynamics model of tethered satellite system with concerning the deformation and tension of the tether, the libration and tension variation of the tether also with the attitude of main-satellite are researched during tether deploying. Combining the model with the gravity gradient torque and tether tension, the uniform speed deployment, uniform acceleration deployment and power index deployment are analysed. The equilibrium state and the length of the tether tension could be neglected in the new model are calculated.Secondly, on the basis of the dynamics model of two-body tethered satellite system, changes of orbital elements are analysed by considering the attitude of the electro-dynamic tether system. Based on the attitude dynamics model and and the International Earth’s Magnetic model, the Lorentz forces and torques are calculated. Taking the Gauss perturbation equation into considering and the numerical models for Electro-Dynamic Tether deorbit are established. The libration and non-libration deorbit mission were simulated respectively.Finally,the deployment strategies for “Hub-and-spoke” configured tethered satellite formation are investigated. For systems without considering the attitude of central body,two deployment strategies which are uniform speed and power index are analysed and simulated. The efficiency and libration of tether are also discussed during deployment phase. For systems with considering the attitude and dimension of central body, the deployment strategies are developed for the cases of with and without gravity gradient compensation. To prevent the libration motion of tether after the deployment respectively, a smooth transition phase is added to the mission and the simulation results indicate that by using the proposed deployment strategies, the configuration can be maintained effectively and steadily.