Control synthesis and parabolic flight experiment for tethered active debris removal mission

In the next future, the necessity of Active Debris Removal (ADR) missions will dramatically increase to ensure the continuity of the mankind space exploitation. Each uncontrolled man-made space object is defined as space debris, and thousands of dangerous space debris float into space. One of the proposed ADR mission configuration, and one of the most appreciated by the European Space Agency (ESA), is a mission in which a space tug exploits flexible systems, as net and tether, to capture a debris, pull it to decrease its energy, and bring it back into the Earth atmosphere. Guidance and control design of space tugs is a big challenge. Once the capture occurs, a passive orbiting target and the active chaser are connected through a flexible link. Then, a manoeuvre is performed by the chaser that excites the stack dynamics. The chaser has to robustly and reliably perform operations, while damping dangerous vibrations of flexible elements and connections, avoiding instability, collisions and tether entanglement. The bounceback effect is one of the most critical instabilities that flexible systems undergo during towing operations. It means that, whenever the thrusting phase is over, the tether slackens and the residual tension accelerates the two objects towards each other, increasing the collision risk. The control recovery is then difficult and not always possible. Within this new research branch this thesis falls. It is focused on synthesis, implementation and experimental validation of a suitable control law able to control the mated space objects dynamics and mitigate the negative effect of flexible systems exploitation. The theoretical simulation results are compared to the experimental data obtained by the SatLeash experiment, a microgravity experiment tested into 65th ESA Parabolic Flight Campaign (PFC), as part of the ESA education programme Fly Your Thesis! 2016. The implementation and integration of the experiment is presented, focusing on the controller network and control law implementation. After presenting the mathematical analysis of the control strategy, the experimental results are presented. Moreover, their implications on the control strategy validation to mitigate the bounce-back effect are deeply discussed. The presented results will be part of the validation process of a multibody toolbox, developed at Politecnico di Milano by Department of Aerospace Science and Technology (DAER), as instrument to Guidance, Navigation, and Control (GN&C) design for future ADR mission.