The air inlets were the key factor that helped the J-58 engines to provide 32,000 pounds of thrust and allow the Blackbird to reach Mach 3.2.
The Lockheed SR-71 “Blackbird”, the famous high-altitude Mach 3 reconnaissance aircraft designed by Kelly Johnson at Lockheed’s Skunk Works division, is still holding, as of 2019, the record for the fastest air-breathing manned aircraft, established in 1976. The myth surrounding the Blackbird (we could say the secret of its success) is also due to the fact that no one has ever been able to replicate (at least publicly) the technology contained into its distinctive engine nacelles: the Pratt and Whitney J-58 turbojet engines and the air inlets, the key factor that made possible to reach the maximum speed of Mach 3.2.
The Pratt and Whitney J-58 engine was a dual cycle engine, the first of its kind, as it worked as a standard afterburning turbojet at subsonic and transonic speeds and then switched to a ramjet-like behavior at around Mach 2, providing more than 32,000 pounds of thrust at sea level. The engine made extensive use of high temperature nickel superalloys, especially Inconel and Waspaloy, to withstand temperatures ranging from 800° F (about 430° C) in the inlet to 3200° F (1760° C) in the afterburner duct.
The engine had a nine-stage compressor with an 8.8:1 pressure ratio. At speeds around Mach 2, six bypass tubes moved bleed air from the fourth stage of the compressor to the afterburner section, allowing the engine to act like a ramjet and to operate at a higher fuel efficiency. Even with this increase of the efficiency, at Mach 3.2, the Blackbird’s cruise speed, the engine provided only 20% of the thrust. The remaining 80% was generated by the air inlets.
The SR-71’s inlets completely surrounded each J-58 engine and included the characteristic movable conical spike used to change the inlet’s geometry. The spike was used to control the supersonic air flow and position the shockwaves generated by the air slowing down into the throat of the inlet to obtain the best performance, while preventing the supersonic flow to reach the compressor. The spike was in a full forward position during subsonic flight; above 30,000 feet and Mach 1.6 the spike started moving after into the throat to keep the shockwaves in the same optimum position. The spike moved approximatively one and 5/8 inches per 0.1 Mach, for a total maximum travel of 26 inches after into the inlet in the full retracted position.
Another important part of the nacelles are the bypass doors. On top and bottom of the engine there were the forward bypass doors, whose function was to relieve excess air pressure inside the inlet by sending some of the air flow outside. The forward bypass doors were controlled by the Air Inlet Computer (AIC), which beginning from Mach 1.4 opened them in relation to the Duct Pressure Ratio (DPR), a comparison of the pressure outside the engine and the static pressure inside the inlet throat, preventing an excessive pressure to build up in front of the compressor.
However, the slow air exiting from the forward bypass doors created a great amount of drag, so the doors were kept closed as much as possible. The doors’ movement was also related to the spike because, as it went aft during the acceleration, the inlet pressure increased, and the doors were activated to keep the pressure under control. A second set of doors were the aft bypass doors, manually controlled by the pilots to reduce drag during the acceleration. If these doors opened, the forward doors closed down and vice versa.
Another set of openings were the grills on the outside of the nacelle, which were connected to the hollow body of the spike. These openings allowed additional air to get into the inlet at low speeds, while at high speeds they were used to send outside the turbulent air of the boundary layer, which was sticking to the spike inside the inlet. Last, but not least, a group of openings in the inlet cowl directed some bleed air through shock traps to obtain a subsonic air flow between the cowl and the engine used to cool down the engine itself.
The last J-58 engines were used to burn the remaining stocks of JP-7, the special fuel created for the SR-71 to withstand the extreme conditions of the Mach 3 flight while avoiding the auto-ignition, and are now in museums. As of today, while no longer a secret, nobody has been able to replicate the technology of the SR-71 engine and inlets. Even the SR-72, the replacement for the Blackbird proposed by Skunk Works, won’t be able to use this system, using instead a turbine-based combined cycle (TBCC) that combines a turbojet and a scramjet and without the spike that characterized the SR-71’s inlets.