Above Mach 2.2, some of the airflows are bled from the fourth stage of the compressor and dumped into the augmentor inlet through six bleed-bypass tubes, circumventing the core of the J58 engine and transitioning the propulsive cycle from a pure turbojet to a turbo-ramjet.
The intriguing video in this post, titled The Mighty J58—The SR-71’s Secret Powerhouse, describes how the famous Blackbird’s engine works.
The J58, often known as “black magic,” pushed the SR-71, the A-12, and the YF-12 from zero to Mach 3.2.
In his NASA Technical Memorandum Predicted Performance of a Thrust-Enhanced SR-71 Aircraft with an External Payload, Timothy R. Conners outlined the three main parts of the SR-71 aircraft’s propulsion system. These include Pratt & Whitney J58 turbojet engines, airframe-mounted convergent-divergent blow-in door ejector nozzles, and axisymmetric mixed compression inlets.
The inlet spike translates longitudinally, depending on the Mach number, and controls the throat area: the spike provides an efficient and stable inlet shock structure throughout the Mach range. At the design cruise speed, most of the net propulsive force derives from flow compression pressure on the forward-facing surfaces of the spike. Besides the spike, other inlet controls include the forward and aft bypass doors, used to maintain terminal shock position and to remove excess air from the inlet; and cowl and spike bleed used to control boundary layer growth.
The SR-71 aircraft is propelled by two 34,000 lbf (151,240 N) thrust-class J58 afterburning turbojet engines, just as its predecessors, the Lockheed A-12 and YF-12A prototype interceptor.
The J58 engine was developed in the late 1950s by the Pratt & Whitney Aircraft Division of United Aircraft Corp. to meet a U.S. Navy requirement. It was designed to operate at speeds of Mach 3+ and at altitudes of more than 80,000 feet. The J58 was the first engine designed to operate for extended periods using its afterburner, and it was the first engine to be flight-qualified at Mach 3 for the U.S. Air Force.
An SR-71 set a global speed record of 2,193 mph and a world altitude record of 85,069 feet in July 1976 thanks to J58 engines.
Each engine contains a nine-stage compressor driven by a two-stage turbine. The main burner uses an eight-can combustor. The afterburner is fully modulating. The primary nozzle area is variable. Above Mach 2.2, some of the airflows are bled from the fourth stage of the compressor and dumped into the augmentor inlet through six bleed-bypass tubes, circumventing the core of the engine and transitioning the propulsive cycle from a pure turbojet to a turbo-ramjet. The engine is hydromechanically controlled and burns a special low-volatility jet fuel mixture known as JP7.
The inlet bleed and aft bypass flow mix with the engine exhaust flow just forward of the airframe-mounted ejector nozzle. Blow-in doors on the ejector nozzle remain open at low speeds and entrain additional mass flows into the exhaust stream. At high speeds, the doors close, and the airframe nozzle ejector flaps reposition to form a convergent-divergent geometry. The blow-in doors and ejector flaps are positioned by aerodynamic forces.
The engine spikes and forward bypass doors are positioned by commands from the digital automatic flight and inlet control system (DAFICS). The DAFICS provides precise control of the terminal shock position. The DAFICS has significantly improved vehicle performance and range and has virtually eliminated inlet unstart, compared to the older analog control system.