Nonlinear Flight Control Systems Design for Hypersonic Vehicles: Results and Open Problems

Abstract Air-breathing hypersonic vehicles (HSVs) are regarded as one of the most promising technology for achieving reliable and cost-effective access to space. Notwithstanding the recent successes of NASA's X-43A and AFRL's X-51 experimental vehicles, the design of flight control systems for HSVs is still an open problem, due to the complexity of the dynamics and the unprecedented level of coupling between the airframe and the propulsion system. The slender geometries and light structures required for these aircraft cause significant flexible effects, whereas a strong coupling between propulsive and aerodynamic forces results from the integration of the scramjet engine into the fuselage. Because of the variability of the vehicle characteristics with flight conditions, significant uncertainties affect the vehicle models, which in turn are known to be unstable and non-minimum phase with respect to the regulated output. Finally, the presence of unavoidable constraints on the control inputs and the stringent requirements imposed by hypersonic regimes on the flight envelope render the control design an even harder endeavor, especially concerning controlling the propulsion system. In this paper, we present an account of nonlinear adaptive control techniques that have been developed, in collaboration with the US Air Force Research Laboratories, towards the design of flight control systems for HSVs. In particular, we present several steps leading to the design of a flight control system architecture comprised of a robust adaptive inner-loop controller and a self-optimizing guidance system. Finally, we briefly discuss open problems and current research directions.