A Terminal Guidance Technique for Lunar Landing

Terminal guidance of a soft-landing lunar spacecraft can be achieved by perturbing a nominal gravity-turn trajectory This perturbation is imposed by offsetting the thrust vector for some period of time from the nominal attitude, anti-parallel to the instantaneous velocity vector The offset is removed when the predicted and required landing points coincide The vehicle then proceeds along a gravity-turn descent trajectory until landing Closed-form solutions obtained determine the thrust acceleration required and the predicted landingpoint coordinates For trajectories following a shallow descent, as from a parking orbit, the moon is represented by spherical geometry For trajectories that initially make large angles with the local horizon, or have small velocities, the moon is considered flat During descent an on-board computer compares the predicted and desired landing-point coordinates A pin-point guidance channel drives the down-range and cross-range components of the landing point error to zero A soft-landing control channel constrains the thrust level so as to terminate the trajectory at the lunar surface with zero velocity Separation of the two control functions has the advantage of assuring a safe landing in the event of component failui es in the pin-point guidance subsystem Simulation studies have shown this technique to be quite effective