Dynamics And Control Of Flexible Tethered Satellite In Complex Space Environment

Tethered satellites, satellites recently innovated for space probe, have become a prominent tool inscientific studies of space and outer space. Researches on theories and experimental methodologies(ground or orbital) regarding tethered satellites have been intensified in Eastern countries, as scienceand technology develop. Among them all, flexibility of the space tether, complexity of the space, andpertinent controlling mechanisms are, without doubt, the most difficult. Solving these issues hastherefore become the most urgent task at hand. This dissertation will focus the following aspects:First, the flexible tether is modeled as a series of elements with lumped masses. The number ofdegrees-of-freedom of the tethered satellite system is investigated as the discrete lumped mass pointsare transferred from inside (outside) of the satellite to the outside (inside). Flexibility, viscoelasticity,"rocket term", and dissipation effect of the space tether are taken into proper account within thismodel, as well as the friction between tethers and satellites, and its impact on Keplerian orbit. Themodel is therefore an adequate explanation of changes to the tether configuration within an operatingtethered satellite system.An improved finite difference method is then presented to calculate the dynamical response ofthe proposed model with a time-varying number of degrees-of-freedom. The core of this method is toidentify changes in the number of degrees-of-freedom of the tethered system within each iteration step.Once the value changes, the tether is re-devided into a different set of units, with properties of eachtether node added or removed, and its lumped mass, damping and stiffness matrices, and displacementand force vectors updated. The calculation continues till a final result can be obtained. Redundantcalculations in this model of node properties are removed, and thus computation efficiency can beobtained. The method can be extended to calculate the dynamics of one-dimensional continuumduring deployment/retrieval.After that, a dynamics analysis is carried out to study the impact of environment perturbations inthe space on complicated systems of tethered satellites. Simulation results have indicated thatinfluences of heating effect and debris impact on the system are significant. Impacts of J2perturbationand atmospheric drag depend on the inclination and altitude of the orbit where the tethered satellitesystem is located. As the orbital eccentricity changes, the different types of pitch motions such as theperiodic motion, quasi-periodic motion, and chaotic motion will occur in the system. Impact of solarpressure on the tethered satellite appears to be slight. Taking into account all the space perturbations discussed above, an effective approach ispresented to control the flexible tethered satellite system. Pitch altitude of the satellite system can beregulated through jet force control of satellites within the system. A PID controller subject to a set ofconstraints is designed to achieve the result. Case studies indicate that, with the proposed method,pitch or chaotic motions of the tethered satellites in arbitrary elliptic orbits can be effectivelycontrolled.Finally, the dynamics of electrodynamic tethered satellites and tether capturing involving cablesystems and related control methodologies are discussed.