Sliding mode disturbance observer-based control for a reusable launch vehicle

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 a reusable launch vehicle 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. Because of the robustness to external disturbances 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. The multiple-loop flight controller 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. Six degree-of-freedom computer simulations of the X-33 technology demonstration suborbital reusable launch vehicle for nominal and severe wind-gust tests demonstrate improved performance over a more conventional, classical control system design.