(Student Paper) Attitude Dynamics and Stability of Solar Sails During Deployment

Abstract

The attitude dynamics of a solar sail spacecraft during deployment are critically important for overall mission success. To model these dynamics, equations of motion govern ing the deployment of a solar sai l are developed and the stability of the predicted motion is evaluated . For this purpose, a n expression is derived to calculate the principal moments of inertia as the solar sail is deployed. A typical 40 m x 40 m square solar sailcraft is considered with a center of mass vs. center of pressure (cm/cp) offset of 0.25% of the sail edge length , boom deployment rate of 2.5 cm/s, and maximum incident so lar radiation pressure force density of 8.33 x10 -6 N/m 2 . The dynamic simulations show that the sailcraft exhibits an exponential decay in the spin rate about its roll axis (perpendicular to the sail membrane). This behavior is accompanied by an increase in the pointing error of the sail membrane normal vector (i.e., thrust vector). Variation s in the baseline parameters given above will alter the final pointing error. It is shown that uncertainty in the cm/cp offset produces the most severe increase in the pointing error . Adjustment of the boom deployment rate (and thus, the total time required for deployment) also significantly affects the poi nting error induced by the deployment process . It is observed that the final pointing attitude error in most cases is significantly larger than the typi cal sailcraft specification of ±1 deg misalignment of the thrust vector . The capabilities of the attitude control system and the deployment parameters ( e.g., initial spin rate, boom/sail deployment rate, and initial poi nting attitude ) must then be optimized to assure satisfactory deployment .