Aerodynamic Three-Axis Attitude Stabilization of a Spacecraft by Center-of-Mass Shifting

This paper proposes a spacecraft attitude control technique based on the use of center-of-mass shifting. In particular, the position vector of the spacecraft's center of pressure with respect to the center of mass is modified by shifting masses, which results in a change of the aerodynamic torque vector within the plane perpendicular to the aerodynamic drag. This results in an underactuated control system. To achieve full three-axis stabilization, additional actuators (either a reaction wheel or a set of magnetic torquers) are considered. An adaptive nonlinear attitude regulation control law was designed in order to obtain an ideal control torque based on the Lyapunov method and its stability was proven by LaSalle's invariance principle. The control torque was then allocated to steer three shifting masses and either a reaction wheel or three magnetic torquers. Numerical simulations are reported, confirming the analytic results. The proposed method decreases the residual oscillation error typically associated with magnetic controlled attitude in the presence of residual aerodynamic torque. Therefore, it might contribute to achieve higher pointing accuracy of small spacecraft in low Earth orbit.