Attitude Stabilization and Control Using Trim Tabs for Spacecraft with Sun Shields

Abstract

Solar radiation torque harnessed by large exposed surfaces such as sun shields and solar panels is of major concern for spacecraft operating at GEO and L2 orbits. The geometry and optical properties of these surfaces need to be designed to ensure attitude stability for the entire mission duration while mitigating solar radiation torque and thus momentum build-up. Using a conceptual spacecraft with a large sun shield, which contains similar characteristics of the James Webb Space Telescope, the relationships among the sun angle, sun shield geometry, surface optical properties, and vehicle center of mass are examined to define conditions that are required to provide passive attitude stability and to minimize solar radiation torque. Due primarily to optical property degradation over time, finding optimum sun shield properties that can be effective for the entire mission duration is not feasible with fixed sun shield geometry. It is shown in this paper that trim tabs can be employed effectively to augment the stability and to help maintain minimal solar radiation torque over the entire mission duration. The process of sizing the trim tabs and selecting their optical properties is presented to conduct these functions. A linearized control law for the trim tab is developed to generate commanded control torque by adjusting its deflection angles. Closed-loop attitude control examples using the conceptual spacecraft are provided to illustrate the process of employing the trim tabs to neutralize the solar radiation torque generated by the sun shield and to demonstrate the attitude stabilization capability of the trim tab control.