Analysis of Mode Transition Performance for a Mach 4 Over-Under TBCC Engine

The Turbine-Based Combined-Cycle Engine (TBCC) is a favored solution for powering both the future long-range high-speed vehicle and the Two-Stage-To-Orbit (TSTO) reusable launch vehicle. Mode Transition from the low-speed propulsion system to the high-speed propulsion system is one of the most critical TBCC enabling technologies. In this paper, the principles of the Mach 4 over-under TBCC engine were analyzed, and a flexible TBCC simulation model, which considered the inlet/engine mass flow matching and improved the iterative technique for engine model solving, was developed for the steady-state and transient performance analysis of all modes of operation, including mode transition. Through comparing the variations of net thrust and SFC along the flight trajectory between turbo mode and ram mode, the mode transition Mach number is selected as 3.0. Based on the basic principle of smooth thrust and air mass flow variations during mode transition, a three-phase mode transition strategy according to the turbine engine operation was proposed, and control schedules of the regulated parameters during mode transition was designed. The simulation results of mode transition dynamic performance show that the proposed strategy can basically realize the stable mode transition from turbo mode to ram mode, but there exists a short-time decrease of total thrust by about 12.5% when the turbine engine afterburner shuts down during mode transition to ensure that the total air mass flow maintains constant.