Seen as a possible way of making space access more affordable, air-breathing hypersonic vehicles have been studied sporadically over the last five decades as the technical advances necessary for their development were missing. New technology has brought a renewed interest in the research area, as demonstrated by the successful flights of NASA's X-43A in April and November of 2004. Air-breathing hypersonic vehicles are characterized by their unique design, incorporating a supersonic combustion ramjet engine located beneath the fuselage. This esoteric configuration results in strong coupling between the thrust and pitch dynamics of the vehicle, which in combination with flexible effects and static instability make the vehicle a challenging application for control.
For a particular model of the longitudinal dynamics of an air-breathing hyper¬sonic vehicle, a nonlinear high fidelity simulation is developed. Tables are generated which provide a static mapping of the states/inputs to the aero-dynamic forces for the purpose of creating bounds to be used in future nonlinear control development. The tables also allow the calculation of the aerodynamics forces and moments to be replaced by a table look-up with multidimensional interpolation.
A linear controller is designed using a linearized model derived numerically from the nonlinear simulation about a specific trim condition. The controller is designed to track given reference commands in velocity, angle-of-attack, and altitude using a LQR technique. The gains are tuned to achieve the performance objectives without violating the input constraints. The controller is then tested on the nonlinear simulation and functions adeptly.
Anti-windup control is added to allow the input constraints to be considered in the formulation of the linear controller. In doing so the gains of the linear controller can be increased without manually keeping the inputs within their constraints through tuning of the weighting matrices. This allows both the speed of the closed loop response and that of the reference trajectories to be increased, resulting in better performance.