During high-speed flight in the Blackbird, compression of air in the inlets generated most of the vehicle’s thrust. At Mach 2.2 the inlet produced 13 percent of the overall thrust with the engine and exhaust ejector accounting for 73 and 14 percent, respectively. At Mach 3 cruising speeds the inlet provided 54 percent of the thrust and the exhaust ejector 29 percent. At this point the turbojet continued to operate but provided only 17 percent of the total motive force. The inlet had a compression ratio of 40:1 at cruise conditions where each inlet swallowed approximately 100,000 cubic feet of air per second.
Source: Urie, David, “Case Studies in Engineering: The SR-71 Blackbird,” Course Ae107, Presented at the Graduate Aeronautical Laboratories, California Institute of Technology, Pasadena, Calif., April-May 1990.
A significant percentage of air entering the inlet bypassed the engine through ducts and traveled directly to the afterburner. At cruise Mach conditions, fuel burned more advantageously in the afterburner than in the main burner section. Hence, engineers described the powerplant as a turbo-ramjet.
Source: Matranga, Gene, and William J. Fox, “YF-12A Development and Operational Experience,” unpublished paper presented at the Supercruiser Conference, Wright-Patterson AFB, Ohio, February 17-20, 1976, p. 3. NASA Dryden Historical Reference Collection.