Sliding Mode Disturbance Observer-Based Control for a Reusable Launch Vehicle

Abstract

The nation's goals to replace the aging Space Shuttle fleet and pursue exploration of our solar system and beyond will require more robust, less costly launch vehicles and spacecraft. This paper presents a novel Sliding Mode Control approach, Sliding Mode Control driven by Sliding Mode Disturbance Observers with Gain Adaptation, for the reusable launch vehicle (RLV) flight control system design as a way to improve robustness to many phenomena such as modeling uncertainties and disturbances, while retaining continuity of control without using high control gains. Due to the robustness to external disturbances (including wind gusts), mission guidance trajectories and modeling uncertainties, the proposed flight control system design also can reduce cost by requiring less time in design cycle and preflight analyses. This design is applied to Terminal Area Energy Management and Approach/Landing (TAL), a flight regime that has had little research effort in recent years. The multiple-loop, multiple time-scale design features low order disturbance observers that rely only on knowledge of the bounds of the disturbance. A gain adaptation algorithm is included in the disturbance observer design that provides the least gain needed for existence of the sliding mode. High fidelity 6 DOF computer simulations of the X-33 technology demonstration sub-orbital launch vehicle for nominal and severe wind-gust tests demonstrate improved performance over a more conventional, classical control system design.