Descent-phase control of a reusable launch vehicle

This paper presents the results of a design study involving the application of a combined LQR/dynamic inversion/μ-synthesis control strategy to the descent phase of a reusable launch vehicle (RLV). A nominal desired plant response is formed by deriving a set of linear quadratic regulator gains for a plant model representing an intermediate point along the descent trajectory. A dynamic inversion controller, based on the linearized plant dynamics, is employed to drive the plant response at off-nominal conditions towards the regulated dynamics at the nominal point. The equalization provided by the inner-loop controller makes it possible to use a fixed outer-loop controller, derived using μ-synthesis, to provide robust performance throughout the descent trajectory. The use of body-axis rotational accelerations as generalized controls within the inner-loop provides a means for blending the reaction control system (RCS) with the aerodynamic control effectors during the low dynamic pressure portion of the descent trajectory. Results of a six-degree-of-freedom, nonlinear simulation demonstrate the effectiveness of the control strategy.