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The SR-71 aircraft, built by Lockheed, is a long-range, two-place, twin-engine airplane capable of cruising at speeds up to Mach 3.2 and altitudes over 85,000 ft (26,000 m). The aircraft is characterized by its black paint scheme; long, slender body; large delta wing; and prominent, spiked engine nacelles located midway out on each wing. The propulsion system of the SR-71 aircraft has three primary components. These components are axisymmetric mixed compression inlets, Pratt & Whitney J58 turbojet engines, and airframe-mounted, convergent-divergent blow-in door ejector nozzles.

The J58 engine was developed in the late 1950s by Pratt and Whitney Aircraft Division of United Aircraft Corporation to meet a U.S. Navy requirement. It was designed to operate for extended speeds of Mach 3.0+ and at altitudes of more than 80,000 ft. 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 Air Force. The J58 was only used on the Lockheed YF-12 interceptor and its descendents, the A-12 and SR-71.

The inlet spike translates longitudinally, depending on Mach number, and controls the throat area. The spike provides 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 bleeds, used to control boundary layer growth.

The SR-71 aircraft is powered by two 34,000 lbf (151,240 N) thrust-class J58 afterburning turbojet 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 airflow is 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. At Mach 3.2 cruise the inlet system itself actually provided 80 percent of the thrust and the engine only 20 percent, making the J58 in reality a turbo-ramjet engine. 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 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 flow 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 hock position. The DAFICS has significantly improved vehicle performance and range and has virtually eliminated inlet unstart, compared to the older analog control system.

A structurally modified SR-71 aircraft can carry external payloads weighing up to 20,000 lbm (9072 kg). This large weight limit permits flexibility in the configuration of a research package. However, within this weight limit, it is easy to design an external payload package whose additional drag exceeds the excess thrust capability of an SR-71 aircraft using unmodified J58 engines. To provide supplemental SR-71 acceleration, methods have been developed that could increase the thrust of the J58 turbojet engines. These methods include temperature and speed increases and augmentor nitrous oxide injection. The thrust-enhanced engines would allow the SR-71 aircraft to carry higher drag research platforms than it could without enhancement.

At maximum output the fuel flow rate in the J58 is about 8,000 gallons per hour and the exhaust-gas temperature is around 3,400 degrees. The J58 required the use of a special AG330 engine starter cart to spool the engines up to the proper rotational speed for starting. The cart was powered by two unmuffled Buick Wildcat V-8 racing car engines which delivered a combined 600 horsepower through a common gear box to the starter drive shaft of the aircraft engines. The J58s had to be spun up to about 3,200 RPM for starting.

At the speeds the SR-71 operated, surface temperatures were extremely high due to aerodynamic heating: 800 degrees at the nose, 1,200 degrees on the engine cowlings, 620 degrees on the cockpit windshield. Because of the operating altitudes, speeds, and temperatures, Lockheed designers were forced to work at the cutting edge of existing aerospace technology, and well beyond in many cases. Many features and systems simply had to be invented as they were needed, since conventional technology was inadequate to the task. New oils, hydraulic fluids, sealants, and insulations were created to cope with the ultra-high temperatures the craft would encounter. A new type of aviation fuel, JP-7, was invented that would not "cook off" at high operating temperatures, having such a low volatility and high flash point that it required the use of triethylborane as a chemical ignitor in order for combustion to take place. The fuel itself was rendered inert by the infusion of nitrogen and then circulated around various components within the airframe as a coolant before being routed into the J58 engines for burning.

The B-58C was proposed as a lower cost alternative to the North American XB-70 or as a medium bomber to fill the gap between the XB-70 and the XF-108 Rapier Mach 3 fighter (proposal). The B-58C, or BJ-58, was proposed as a enlarged version of the B-58A to be powered by Pratt & Whitney J58 turbojet engines. The 32,500 thrust J58 was the same engine used on the Lockheed SR-71. Design studies were conducted with two and four engine designs. As enemy defenses against high speed, high altitude penetration bombers improved, the value of the B-58C diminished and the program was canceled in early 1961.

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