LQG control of an expendable launch vehicle in the atmospheric ascent phase

This paper presents the design of an LQG controller for an expendable launch vehicle during the atmospheric ascent flight. The vehicle considered has highly coupled flexible dynamics between the pitch and yaw planes. This coupling is measured at the sensor location. The vehicle short period equations are derived independently for pitch and yaw planes. Pitch-yaw combined system model is developed considering the significant flexible modes. Maximum dynamic pressure point is selected for design, which is a critical point in the case of a launch vehicle. An LQG controller is designed which includes the design of a linear quadratic regulator and a Kalman filter. The presence of Kalman filter increases the system order to twice the plant order. Hence it demands for order reduction of the LQG controller. The method used here is based on a performance index criterion where the same performance index used for designing the optimal regulator is used. The performance of the closed loop system is evaluated with both full order and reduced order controllers. The results illustrate the effectiveness of the algorithm.