Using Shifting Masses to Reject Aerodynamic Perturbations and to Maintain a Stable Attitude in Very Low Earth Orbit

Abstract

The aerodynamic forces are the main orbital and attitude perturbations at very low orbital altitudes ( 450 km). To minimize them, it is desirable to design spacecraft with their center-of-mass (CoM) as close as possible to the spacecraft’s center-of- pressure (CoP). Design constraints, poorly understood aerodynamics and environ- ment variability, prevent this CoP and CoM match. The use of internal shifting masses, actively changing the location of the spacecraft CoM, and thus modu- lating, in direction and in magnitude, the aerodynamic torques is proposed as a method to reject these disturbances. First, the equations of motion of a spacecraft with internal moving parts are revisited. The atmospheric environment and the aerodynamic properties of a spherically shaped spacecraft are then provided. A single-axis controller is used to analyze the disturbance rejection capability of the method with respect to several parameters (shifting mass, shifting range and alti- tude). This analysis shows that small masses and a limited shifting range suffice if the nominal CoM is relatively close to the estimated CoP. For the full three rota- tional degrees-of-freedom analysis, a quaternion feedback controller and a linear- quadratic regulator are used. Finally, a practical implementation on a 3U CubeSat using commercial-off-the-shelf components is provided, demonstrating the tech- nological feasibility of the proposed method.