Closed loop reentry guidance law of a space plane: Application to Hermes☆

Abstract A closed loop guidance law has been synthesized for the whole reentry trajectory (from orbit to a landing runway) of a space plane. The principle was to observe position and speed deviations of an optimal reference trajectory in order to ensure an optimal rendezvous in position and speed with the final point. The application of Pontryagin's principle allowed to yield analytically, from the instantaneous deviations in position and speed, the variation of the aerodynamic acceleration, the lower in module, necessary to obtain the rendezvous. From this three-dimensional vector of acceleration, the attack angle and bank angle were computed using the projections of this vector along the ground speed axis and along: the vertical axis, in order to keep the altitude vs speed profile of the trajectory for controlling the flux constraint, in the first part of trajectory; the horizontal axis orthogonal to the ground speed, in order to preserve the ground track of the trajectory, when the heat flux has been lowered; both of the previous axes, below Mach 2.5, when the air-brake control became available. This new guidance law has been checked from the point of view of sensitivity to thermal phenomena and position sensitivity to Mach 2 for several causes of perturbations: variation of mass balance, lift-to-drag ratio, initial conditions, atmosphere. The results have been compared with the corresponding ones obtained with the U.S. Shuttle guidance law applied to Hermes.