Tether Dynamics Analysis 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 could not 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 de-orbit the resulting tethered system. This study focuses on the dynamics of the tethered system during such a de-orbit mission, and the influence thereof on mission performance. A suitable model for the tether was found in the lumped-mass model, which discretizes 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 only two nodes and three elements. The influence of four combinations of tether length, stiffness, and damping on the mission propellant requirements were investigated. This was done using a simple LQR-based relative orbit control and attitude control system, and an openloop throttle control system for the main engines based on fixed-duration-fixed-magnitude de-orbit burns. Three different thrust levels of the main engines were considered. It was found that higher thrust levels require significantly less mission propellant for the guidance and control system. Furthermore, the baseline tether model with nominal length and stiffness shows the best mission performance in terms of propellant required.