Abstract: A design technique, method, and apparatus for varying the bypass ratio and modulating the flow of a gas turbine engine of the bypass type in order to achieve improved mixed mission performance. The disclosed preferred embodiments each include a gas flow control system for management of core and bypass stream pressure comprising diverter valve means downstream of the core engine to selectively mix or separate the core and bypass exhaust streams. The flow control system may also include variable geometry means for maintaining the engine inlet airflow at a matched design level at all flight velocities. Each preferred embodiment thus may be converted from a high specific thrust mixed flow cycle at supersonic velocities to a lower specific thrust separated flow turbofan system at subsonic velocities with a high degree of flow variability in each mode of operation, wherein the engine inlet airflow may be maintained at a matched design level at all engine velocities.