Propellant-free attitude control of solar sails with variable-shape mechanisms

Abstract

Solar sails can realize propellant-free attitude control if they have the ability to change their shape by moving reflective surfaces to harness the force of incident solar radiation pressure (SRP). In this paper, we formulate a stability analysis of the attitude motion of a sail spacecraft with large deformation under SRP, and propose stable attitude control methods using variable-shape mechanisms. The basic principle is that the attitude is maintained at a stable equilibrium point that suppresses the disturbance torque due to SRP and prevents disturbance angular momentum from accumulating in the reaction wheels. The equilibrium point can be shifted by moving reflective surfaces, and the attitude follows the new equilibrium point by reaction wheel control. The stability of the equilibrium points can be quickly evaluated by a stability analysis based on linearized equations of motion, and one can choose any equilibrium point by setting the reflective surfaces to a corresponding configuration. This control scheme enables the spacecraft body to be pointed towards desired directions, and even if the attitude maneuver and maintenance accumulate angular momentum in the reaction wheels, this momentum can also be unloaded without propellant by using SRP and shape variation. The dynamics formulation aims at a zero-momentum system that is generally applicable to arbitrarily shaped spacecraft, and the formulation of the stability analysis and control methods were verified by numerical simulations.