Untersuchungen zum Langzeit-Rotationsverhalten von Satelliten mit Flexiblen Solarpanelen

Abstract

The number of nonfunctional satellites in important Earth orbits has increased since the beginning of space flight. This poses an increasing risk for current space craft operations, and therefore it is planned to remove these objects via robotic service satellites. The service satellites can maneuver the nonfunctional target satellites to non-critical orbits. In order to perform the necessary docking maneuver between servicer and target it is essential to have a good knowledge about the target's rotational state. It cannot safely be assumed that the target stays in a nonrotating state during its nonfunctional period. Internal and external influences can cause a spin of the satellite. For a docking maneuver it is crucial to know if the rotating motion is stable around a single axis or has tumbling characteristics. In this thesis the long period evolution of the rotational state for satellites with flexible solar panels is examined. Especially the order of magnitude for the time of the transition from a tumbling motion to a single axis spin is of interest which is resulting from internal energy dissipation within the solar panels. The calculations were performed analytically as well as numerically using the multi body simulation program "Simpack". For the latter, five models of an example satellite were created. Four of those models were generated by dynamic reduction of a Finite-Element model that was provided by the company "SpaceTech GmbH Immenstadt". The fifth model was created using only a number of rigid bodies which describe the flexible panels as accurate as possible. The models were tested and verified for different criteria concerning energy dissipation due to deflection of the solar panels. The results from the analytic approach and the numeric simulations are compared. Advantages and disadvantages of each method are described and afterwards a combined approach is presented which uses the advantages of each method. An additional topic of this thesis is the identification of rotational behavior from measurements. The goal is to predict the long term evolution of the rotational behavior by only using a few number of known points. This is relevant for example for the scenario when a service satellite is in the vicinity of a target and has to determine if it is favorable to wait until a tumbling rotation changes in to a flat spin.