Solar Sail Attitude Dynamics and Coning Control: On Developing Control Methods for Solar Sail Coning at Orbit Rate to Attain Desired Orbital Effects

摘要

In this thesis, a control method is developed for the solar sail normal vector to trace a desired circular coning trajectory at orbit rate. The coning trajectory is defined in the local vertical local horizontal (LVLH) frame and the coning occurs about an LVLH equilibrium sail attitude. Past research has shown that sail attitude equilibria exist in the LVLH frame under the influence of aerodynamic, gravity gradient and solar torques. Precession of the sail normal from these equilibria causes sail normal coning about that equilibrium attitude. If the coning happens at orbit rate, wide variety of orbital effects can be induced with minimum excitation of the sailcraft structure. This results in an inexpensive spacecraft with a longer duration mission as compared to other conventional efforts. A special case of analyzing circular cones (at orbit rate coning) revealed that new Sun-synchronous orbits were created and launch injection errors were overcome by employing the sail coning method. The control method herein minimizes the angular momentum error between the sail and desired angular momentum vectors at orbit rate. Since angular momentum is a function of sail normal, angular momentum error reduction raises hope in reducing the sail normal error between the sail normal and desired sail normal vector as well. The results show that even though the control method enables the sail angular momentum to track the desired angular momentum on the coning trajectory, the sail normal tracing can only occur about certain LVLH equilibrium points, for small cones and small initial condition angular position/velocity errors. The control method is robust for tracking the desired angular momentum at orbit rate, but not always for tracking the desired sail normal. The case where the sail normal does track the desired at orbit rate corresponds to tracing a 1° circular cone about an orbit lowering LVLH equilibrium point. Even though the control torques are on the order of 10-6 Nm (acceptable on small sailcraft) for both a spinning and non-spinning sail, a spinning sail (spun at a specific rate) requires less control torque (4 times lower than a non-spinning sail) to yield the desired orbit rate circular coning. The control torques can be applied to the sailcraft to enable orbit rate cone tracing of the sail normal and yield the desired orbital effects.