Control of thrusting effects in tethered spacecraft formations

Recently proposed mission concepts involving harpoons or nets to capture and de-orbit debris represent an actual case study for tethered formations, i.e. sys- tems where bodies are connected by a flexible link. These systems present a complex behavior, as flexibility characteristics combine with orbital dynamics. The focus of the paper is on the dynamic behavior of the tethered system in the final phase of the de-orbiting mission, when a powerful apogee motor is used to change the debris orbit. The thrust action introduces significant issues, as elastic waves propagate along the tether, and the relevant oscillations couple to the or- bital dynamics. Input shaping techniques are proposed to limit or cancel these oscillations. However, the performance of these techniques drops when non- ideal scenarios are considered. In particular, an initially slack tether is a serious issue that must be solved if acceptably low oscillations of the tether are to be ob- tained. Two strategies are proposed and discussed in this paper: a controlled maneuver for adjusting the relative distance between chaser spacecraft and de- bris, and a retrieval mechanism for changing the tether length