Gravity-turn descent from low circular orbit conditions

GRAVITY-TURN descent to the surface of a planetary or small solar system body has been investigated for many years and, indeed, has been used for both lunar and Mars descent vehicles. Such a descent proŽ le requires that the vehicle thrust vector is oriented opposite to the instantaneous velocity vector along the entire descent trajectory. This requirement can be achieved with knowledge of the vehicle velocity vector from an inertial measurement unit and an attitude control system that can maintain the thrust vector antiparallel to the instantaneous velocity vector.1 For pure gravity-turndescent, the steering law is, therefore, relatively easy to implement in practice, although the descent may be modiŽ ed at the terminal phase for surface hazard avoidance. An important beneŽfit of gravity-turn descent is that the landing is assured to be vertical, and the steering law is close to fuel optimum.