Tether Dynamics Analysis and Guidance & Control Design for Active Space Debris Removal

Abstract

Recent years have seen a steep increase in research being performed towards active space debris removal: space debris has proven to be a very real threat to operational spacecraft, and studies indicate that the frequency of collisions will only increase if nothing is done to remove large pieces of debris. In particular, ESA has done studies towards removing Envisat from orbit, after communications were lost and were unable to be reestablished. To this end, a scenario was proposed in which a robotic chaser satellite would use a tether to interface with Envisat, either using a net or a harpoon, and proceed to deorbit the resulting tethered system. This research tackles two main challenges relating to this scenario. First, a suitable model for the tether was developed by discretizing the tether into a number of point masses and massless Kelvin-Voigt elements. The influence of the number of nodes was investigated, showing that increasing the number of nodes used does not significantly increase the fidelity of the solution. Therefore, it was chosen to model the tether with two nodes and three elements. Additionally, multiple combinations of tether length, stiffness, and damping were investigated. Second, a preliminary design for a guidance and control system was developed. This system uses multiple fixed-duration burns as the main deorbit strategy. The guidance system controls the relative state of the chaser with respect to the target during and after these main engine burns: during burns, a hold point is established at the equilibrium length of the tether, and during coasting phases the tether is kept slightly in tension to reduce target and tether motion. At the transition between thrusting and coasting phases, the thrust is gradually throttled down to further decrease these motions. The control system is designed to keep the chaser level with the local horizon at all times. During the design process, the performance of guidance and control systems based on linear quadratic regulators was used as a baseline for the same systems based on sliding-mode control. Furthermore, three different thrust levels for the deorbit burn were examined. It was found that the system based on sliding-mode control offered considerable performance improvements over the LQR-based system: total propellant consumption was reduced by an average of 48\%, while adhering to the same tolerances. Furthermore, it was determined that high thrust levels are desirable for both reducing propellant consumption as well as reducing target rotation. In terms of safety, collision can best be avoided by using lower thrust levels or longer tethers. While this seems to conflict with minimizing required propellant, minimal target rotation and minimal propellant consumption can still be achieved by using long tethers with high stiffness. Finally, it was found that while sensitive to small changes in the initial conditions, the precision of the terminal point is still high enough to allow the target to be deorbited in the South Pacific Ocean. This result was independent of tether model, although higher thrust levels do increase the precision further.