Approximate backstepping sliding mode control for a flexible air-breathing hypersonic vehicle

The design of a longitudinal dynamic controller for a nonlinear flexible air-breathing hypersonic vehicle (FAHV) model with aerodynamic parameter uncertainty is described in this paper. This problem is challenging because of the strong couplings between the aerodynamics, the propulsion system, and the flexibility effects. Firstly, after analyzing the model, the FAHV model is transformed into three subsystems (velocity subsystem, altitude and flight path angle subsystem, attack of angle and pitch rate subsystem), to reduce the complexity of the controller design. Secondly, for the velocity subsystem, the dynamic inversion and sliding mode control are combined to design controller. For other subsystems, they are firstly transformed into approximate strict-feedback form with uncertainties included in the formulation; then sliding mode control is incorporated into approximate backstepping control to design control inputs. The proposed controller can guarantee the property of asymptotic stability based on the Lyapunov stability analysis. Finally, numerical simulation results on FAHV model are presented to validate the performance of the designed controller; the velocity and altitude successfully track the reference trajectory.