Modeling And Dynamic Analysis Of Electrodynamic Tether Deorbiting System

The electrodynamic tether (EDT) is a new technology in recent years, and it has many potential applications in spent or dysfuctional satellites deorbiting, auxiliary orbit maneuver, and spacecrafts protecting. Due to strongly nonlinear dynamics of electrodynamic tether system and complicated multi-fields coupling problems, the dynamic models in existence do not meet the requoments of the electrodynamic tether system design, deorbiting characteristic analysis and tether deployment control. To solve such problems, this dissertation investigates the deorbiting time forecast and the dynamic modeling of EDT in deployment and stationkeeping phase. The main contents of this dissertation are as follows:The deorbiting time is an important indicator to evaluate the effect of EDT deorbiting system. In order to precise deorbiting time forecast, the flexible cable model of EDT system is established. The flexible cable model is used to discuss effects of flexible deformation on electromotive force and Lorentz force. Theoretical analysis and numerical simulation both indicate that the flexible cable model is the same essence with the rigid bar model, and the flexible cable model improves the veracity of deorbiting time estimate. Based on the flexible cable model, a six-orbit-parameters iterative cumulative algorithm is present to forecast the deorbiting time, in which the variation of magnetic field strength and space environmental perturbation is taken into account. By the flexible cable model, the influence of tether system parameters and orbit parameters to deorbiting time is researched, to discuss the design of electrodynamic tether system and proper satellite orbits suit for EDT deorbiting.The dynamics behavior of EDT system in stationkeeping phase is very complex , resultingly the effection of a series of perturbations. The study on the dynamics character of EDT system in stationkeeping phase, which presents a continuous flexible tether model to capture the gravitational, inertial, tensile forces and electrodynamic drag in the tether, and uses Newton's laws to derive the dynamic equations of electrodynamic tether system in stationkeeping phase. The simulation results show that the motion of electrodynamic tether in stationkeeping phase is similar to the vibration of cable with two immovable pinned-supports, in which the mid-span has the largest amplitude of vibration. According to this conclusion, the main satellite and the endmass at both ends of tether can be treated as immovable pinned-supports when the tether vibration and tension of electrodynamic tether system in impermanency is investigated. Following the above research, 2D and 3D simplified vibration models of EDT system are presented to study the relationship between vertical and longitudinal vibration, and the relationship between in-plane and out-of-plane vibration. The research results indicate that the nonlinear dynamics of EDT is associated with the tether structure parameterλ. The vertical and longitudinal response of the tether can be excited by each other, but they cannot excite out-of-plane response. The out-of-plane response can cause vertical and longitudinal response in plane. For solving the problem of little damping of EDT system, a damper is presented to add to EDT system. The effects of damping to tether dynamic characteristic is investigated. The simulation results show that the amplitudes are weakened and the vibrations are restrained when a damper is added into EDT system.For solving the problems of dynamics and control of EDT deployment, a continuous system differential equation model is presented to investigated dynamics of deployment of EDT system.. The dynamic model of space tether deployment provides a way to study motion laws of drum releasing tether, tether vibration and end-mass moving during the tether deployment. According to dynamic characteristics of deployment, uniform deployment and expontial deployment are adopted to control the deployment course. The deployment united control of tether tension/durm torque/thruster force is designed by pole-assignment method. The simulation results show that the deployment united control produces a very fast and stable process for the deployment of EDT system.