Critical Dynamic Effects during the Deorbiting of a Large Satellite with a Free-Flying Robot

Abstract

This thesis attempts to show that a deorbiting of a large target satellite can be performed using a free-fying servicer satellite equipped with a robot arm. The example used for such a large target is the defective satellite Envisat. The servicer uses its robot arm and surface contact to transfer forces and torques to the target. The surface contact is assumed to be frictionless and only transfers normal forces, while the robot handles all other forces and torques. The servicer engages thrusters to propel the system and transfers the thrust force over the surface contact, while the robot arm is used to stabilize the system and prevent the masses from drifting apart or moving relative to each other. Two aspects of the system's dynamics were examined in order to show that such a mission can be performed using a relatively weak robot: The first is the one-dimensional dynamics of a mass-spring-damper system resulting from the structural elasticity in the surface contact. The second aspect is the three-dimensional dynamics of the angular velocities and accelerations resulting from an external torque. An external torque occurs when the two bodies aren't properly aligned and the thrust vector doesn't point through the system's center of mass. The resulting angular acceleration causes lateral forces between the two bodies. The dynamics were examined analytically and then veried with the help of Simpack, a multi-body simulation software. The results show that while the system is propelled by a large thrust force (1500N were assumed), the robot forces are fairly low. In a worst-case scenario the robot would have to exert up to 30N of force and 70Nm of torque at the grasping point. Additionally, several strategies can be employed to reduce this value. This demonstrates that such a deorbiting mission is generally possible, though several aspects in the design of the servicer and the robot, as well as the planning of the maneuver must be considered to ensure safe performance of the mission.