The goal of this study is to establish mathematical model for computing performance and analyzing the thermodynamic parameters variation of turbine/ramjet combined cycle engine during mode transition.Firstly

the computed flow charts of performance and thermodynamic parameters of combined engine

and the criteria of determining design parameters of combined engine were given.Secondly

according to computed performance and thermodynamic parameters along flight trajectory and design limitations

such as the Mach number at ramjet burner entrance＜0.3

the reasonable mode transition periods were optimized

and geometry parameters of critical section were given.Thirdly

the effects of opening area of mode transition valve on thermodynamic parameters

especially the relations of total and static pressure

were analyzed.The quantitative opening area of mode transition valve along flight trajectory were given for satisfying the criteria of thrust smoothing.At last

the thrust and specific impulse of turbine/ramjet combined cycle engine were calculated along flight trajectory.