Terminal control of a variable-stability slender reentry vehicle

Abstract

Various terminal control schemes are applied to a proposed slender reentry vehicle, controlled by two separately-articulating flaps. The flap deflections are summarized as symmetric and asymmetric flap deflections; the former manipulates the drag, lift-curve slope, and static margin; the latter controls the vehicle trim characteristics. The control problem is interesting because the static margin can be actively controlled from statically stable in pitch to statically unstable in pitch. Deflection limits on the flaps present a control saturation that must be considered in control system design. A baseline, angle of attack tracking linear-quadratic servo (LQ-servo) controller is detailed, including an analysis of actuator dynamics and a lead compensator. Desired time response characteristics and robustness to center of pressure uncertainty, reduced control effectiveness, and external pitch accelerations drive the selection of a symmetric deflection at specified points on the reentry trajectory. A hybrid switchinglinear controller (SLC) is developed to reduce the peak overshoot and settling time. A saturated control drives the phase plane trajectory toward a region of satisfactory linear control, where the LQ-servo controller is properly initialized and controls the phase plane trajectory to the reference command. SLC does not provide appreciable robustness gains compared to the LQ-servo controller. A model-reference adaptive controller is described. Saturation effects prevent the adaptive controller from providing additional robustness. A method to adaptively control both the symmetric and asymmetric flap deflections is proposed.