Guidance And Control During Direct Descent Parabolic Reentry

NASA Technical Note

Guidance and Control During Direct-descent Parabolic Reentry

"The results of studies of four reentry guidance and control techniques for the energy management of vehicles returning to the earth at escape speeds are compared in this paper. The reentry trajectories are constrained to those of direct descent, that is, where the vehicle does not leave that portion of the atmosphere where useful aerodynamic forces are available after its initial entry. The guidance techniques compared are (1) a piloted simulator study reference trajectory techniques (2) An automatic controller using reference trajectory techniques (3) A predictor system employing linear prediction (perturbation) techniques and (4) A repetitive prediction system employing rapid-time computer techniques."--P. 1.

A Study of Guidance to Reference Trajectories for Lifting Reentry at Supercircular Velocity

This report presents an analysis of the guidance of lifting vehicles onto a reference trajectory during atmosphere entry. Various reference trajectories and various input quantities to govern lift variations are considered. A guidance method is developed which uses the four-state variables of a trajectory as follows: Velocity is made the independent variable and the errors in the rate of climb, acceleration, and range relative to the reference govern the lift. A linearized form of the motion equations is used to show that this method represents a third-order control system. First- and second-order terms (rate-of-climb and acceleration inputs) are shown to determine the entry corridor depth by stabilizing the trajectory so that the vehicle does not skip out of the atmosphere or does not exceed a specified acceleration limit. The destabilizing effect that range input (the third-order control term) can have is illustrated and the results indicate that a low value of range input gain must be used at the high supercircular velocities while larger values can be used at lower velocities. The usable corridor depth and range capability with this guidance system are presented for a lifting capsule (L/D = 0.5) entering the atmosphere at a velocity 40 percent above local circular. The results show an attainable downrange increment of about 1,000 miles within a 25-mile entry corridor and 2,000 miles within a 10-mile corridor when a moderate range input gain is used only at velocities below local circular. The range increment is increased to about 3,000 miles within a 25-mile corridor and 4,000 miles within a 10-mile corridor when, in addition, a small range input gain is used above circular velocity.