High-Torque Momentum Control for Spacecraft Docking Dynamics

The increasing need for refueling and servicing of on-orbit spacecraft motivates a fresh look at the dynamic behaviors of interconnected spacecraft. The sudden changes in attitude dynamics during connection events demand high-torque actuation, ideally through power-efficient momentum-control systems. This study examines the case of two spacecraft (i.e. a servicer and a client spacecraft) that undergo such a connection and how an array of control moment gyroscopes (CMGs) could counteract the impulsive loads imparted by the client. Momentum conservation for a perfectly inelastic collision—or docking event—establishes the requirements for the servicer’s control moment gyroscopes. The servicer’s initial conditions and relative-scaling parameters determine the angular momentum of the servicer-client composite body. Equations of motion for a system comprising the servicer, the servicer’s gyroscopes, and a client spacecraft establish first-principles design parameters for the composite spacecraft. From these equations, certain initial conditions for significantly larger-mass client spacecraft determine the upper limit for CMG array angular momentum. Conversely, clients with mass properties similar to or smaller than those of the servicer are the most feasible. These intuitive first-principles results are confirmed through simulation, in which a state-space representation models the control of the composite spacecraft’s angular momentum by commanding the gyroscopes’ gimbal rates. Simulated docking scenarios confirm the appropriateness of the example subsystem’s design, show the composite spacecraft’s attitude settling behavior, and reveal the limitations of CMG attitude control during docking operations.