Propellantless Attitude Control of a Nonplanar Solar Sail

Abstract

S OLAR sails enable a multitude of new mission concepts and unique non-Keplerian orbits that are either currently unattainable or prohibitively expensive. Owing to the constant force supplied by the solar radiation pressure, solar sails can be particularly beneficial inmissions requiring highΔv. The possibilities aremostly limited by the lifetime of the sail. Missions such as a Mercury sample return [1], Solar Polar Orbiter, Kuiper Belt fly-through [2], andmany others have been suggested to take advantage of solar sails’ inherent properties. Control of the spacecraft’s trajectory is accomplished by controlling the sail’s attitude relative to the sun, leading to a variety of methods that have been proposed for solar sail attitude control [3,4]. Proposed solutions include actively changing themass distribution of the sail [5], using control vanes [6], manipulating the sail’s optical properties, and using highly efficient pulse plasma thrusters [7].Most proposed solutions approximate the sail geometry as a flat plate and thus ignore significant shape-related effects [8]. A control scheme that works well while using a flat plate model may fail when consideringmoment biases and sail imperfections that are expected in the sail’s operational environment. Although the effects of a shaped sail have been presented in literature [8], they have yet to be incorporated in a design of a closed-loop attitude control system. This Note investigates propellantless attitude control of a nonplanar square solar sail with four control vanes, one at each corner of the sail, shown in Fig. 1. A proposed control strategy that incorporates passive stability and trimming the sail moments about the x and y axes has been previously presented [9]. In a sun-pointing configuration, stability about the sail’s x axis, as shown in Fig. 1, is achieved by rotating both control vanes 2 and 4 away from the sun by an identical angle. This configuration is passively stable because a positive rotation about the sail’s x axis results in vane 4 having a larger projected area toward the sun, whereas vane 2’s projected area to the sun is decreased. This combination produces a negative moment about the sail’s x axis, which forces the sail back toward the equilibrium sun-pointing orientation. A similar approach is then applied to stabilize rotation about the sail’s y axis using control vanes 1 and 3. The vane angles can be adjusted to achieve passive stability in all other orientations. To accurately control the sail’s attitude, this technique requires accurate knowledge of the on-orbit moments. This Note extends this approach in several ways and demonstrates its effectiveness in controlling solar sail attitude in the presence of the sail force model uncertainty. Disturbance torques arise primarily from a lack of an accurate prediction of the solar radiation pressure moments and forces, deviation of the flexible solar sail from the rigid-body assumption, unmodeled lifetime changing physical properties, and on-orbit faults. The controller uses an initial guess of the trim angles necessary for each desired attitude relative to the sun and adjusts this guess based on the observed sail behavior. In addition, the controller implements a standard proportionalderivative (PD) controller to approach the trimmed orientation and facilitate convergence and disturbance rejection. The control scheme developed here uses the command time history to estimate the moment bias experienced by the spacecraft and is thus much more capable in achieving robust attitude control in the presence of model uncertainty and error. The performance of the controller is determined using variations in solar radiation pressure moment and forces (scaling and bias errors), sailcraft moments of inertia, unmodeled bend and twist of the control vanes, and a z axis c.m. offset. The demonstration of a propellantless robust closed-loop attitude control system using non-flat-plate sail forces and moments creates a flyable and realizable design that enables wider adoption of solar sails as an alternative means of spacecraft propulsion.