Adaptive Guidance with Trajectory Reshaping for Reusable Launch Vehicles

Abstract

Reusable launch vehicle (RLV) designs are influenced by weight and other constraints that rarely allow for significant effector redundancy. Therefore, reconfiguration of the inner-loop control system and outer-loop guidance functions can be necessary to compensate for significant control effector failures, aerodynamic uncertainties, and disturbances such as strong winds. Trajectory reshaping is also important in some situations to judiciously manage vehicle energy on-line to meet the flight objectives. This paper presents an Optimum-Path-To-Go (OPTG) algorithm, which is a general framework to perform the on-line trajectory-command generation task. The methodology was applied to Lockheed's X-33 RLV for the approach- and-landing phase of flight, and a Monte-Carlo simulation analysis was used to demonstrate the benefits of the approach. Random increments to the vehicle drag were inserted in the simulation at various downrange locations. Such variations are representative of mismodeled aerodynamics, speedbrake failure, or significant winds. The X-33 implementation of the OPTG was able to reshape the trajectories to result in a safe landing for greater than 93% of the cases. Without the OPTG capability, safe landing was accomplished for only 29% of the drag variations simulated.