Nonlinear control of non-minimum phase hypersonic vehicle models

Longitudinal rigid-body models of air-breathing hypersonic vehicle dynamics are characterized by exponentially unstable zero-dynamics when longitudinal velocity and flight-path angle (FPA) are selected as regulated output. To enable application of stable dynamic inversion methods (and their adaptive counterparts), previous studies have considered the addition of a canard control surface to eliminate the occurrence of the unstable zero; however, the addition of a canard may negatively impact the design of the thermal protection system. In this paper, we present a methodology for robust nonlinear control of the rigid-body longitudinal hypersonic vehicle dynamics which employs only the elevator as aerodynamic control surface. The method reposes upon a nonlinear transformation of the equations-of-motion into the interconnection of systems in so-called feedback and feed-forward forms that allows the combination of high-gain and low-amplitude feedback, achieved through the use of saturated functions. Simulation results using the flexible vehicle model are presented to illustrate the effectiveness of the method.