Modeling and analysis for integrated airframe/propulsion control of vehicles during mode transition of over-under Turbine-Based-Combined-Cycle engines

Mode transition control is a critical issue of Turbine-Based-Combined Cycle (TBCC) engines when the primary thrust provider changes from gas turbine engines to ramjets/scramjets. Improper control laws might incur unexpected propulsion performance and even lead to mission abortion. Existed control laws are designed from the perspective of the combined engine itself without aerodynamic/propulsive couplings considered. However, like scramjet-powered aircraft, those with TBCC engines propelled also have intricate couplings between the engine and the airframe, especially during TBCC mode transition. In this paper, a nonlinear longitudinal TBCC-powered dynamic vehicle model is derived from first principles with extra aerodynamic effects and adjustable surfaces involved when a typical TBCC engine is integrated into the airframe. Then, a control-oriented model with six control inputs is obtained by approximately expressing aerodynamic coefficients through curve-fitting methods. Trim results indicate that the total thrust required to maintain a steady flight increases at different steady stages during mode transition of the TBCC engine. One feasible control law is designed with dynamics of the integrated system considered for a typical mode transition process to reveal a way that control inputs should be regulated to maintain a steady flight during mode transition of the TBCC engine.