To provide a near term technology prototype aircraft, a Mach 6 airbreathing configuration was selected. This vehicle, a turboramjet powered aircraft, provides capability for technology demonstration of advanced airbreathing propulsion systems, as well as a broad spectrum of research applicable to the defined potential operational systems. The vehicle is designed for steady state cruise at Mach 6 for five (5) minutes, operates in a conventional ground takeoff mode, and is manned. It employs the existing Pratt and Whitney J58 JP fueled turbojet engine together with a LH2 fueled wrap around ramjet modification. This will provide early research on a near term turboramjet engine. The engine selected is the P&W STRJ11A-27, which employs an existing, currently in use, JP fueled, J58 turbojet engine modified to include a wraparound hydrogen fueled annular ramjet.
Engine Selection - In selecting the engine for this vehicle, two sequential tasks were involved. First, a choice was made between using an advanced technology TRJ engine, or an engine which would incorporate an existing turbojet core and require developing only the ramjet subsystem (near-term technology). Phase I results indicated that the selection of an advanced TRJ engine would necessitate large engine development costs and thereby increase the cost of the overall program significantly.
At the same time, both Phase I and Phase II results point to the research value to be gained by flight experience with advanced airbreathing systems. To reduce the engine development costs and preclude pacing the aircraft development to that of an advanced engine, the near-term technology approach was selected. Having made this decision, the second task involved selecting the turbojet core to be employed from existing turbojet engines. Five candidate turbojet engines were considered: J58, F-100, GE4/J5P, J93, and J97. The selection was based on thrust-to-weight ratio, thrust, availability, and maximum Mach number capability. Three engines--the GE4/J5P, J93, and J97--were eliminated. In choosing between the J58 and F-100, a comparison was made of the thrust-to-weight characteristics of the two engines across the applicable speed range. The J58 was selected as the core turbojet for the basic Mach 6 based on the combination of a higher T/W at high supersonic speeds and a higher maximum Mach number capability. Engine modifications include using a fixed geometry convergent nozzle and closure doors to seal off the turbojet at flight speeds above Mach 3.5. The regeneratively cooled ramjet burns LH2 fuel and uses a variable geometry convergent-divergent nozzle. The ramjet size as defined in the engine specification was used for the propulsion system of the Mach 6 vehicle, and resulted in satisfactory performance without need for scaling.