J58 engine was originally developed by Pratt & Whitney for the US Navy’s Martin P6M jet flying boat capable of dash speeds of up to Mach 3, a project that was cancelled after several production aircraft were built. After the Navy abandoned the Martin P6M project, Lockheed and Pratt & Whitney worked together to redesign the enginefor A-12 use.
The J58 was the first engine designed to operate using its afterburner for extended periods of time, and it was the first engine to be flight-qualified at Mach 3 for the Air Force. It was designed to operate at Mach 3+ for extended periods of time, at altitudes above 80,000 ft.
The J58 is housed in large nacelles on each wing. The large “spikes” or cones, at the front of each engine nacelle, slid in and out to slow airflow through the engines from Mach 3 to Mach 0.5 when the aircraft was to fly subsonic. Efficient operation of the engine inlet requires that it delivers the air to the engine at around 0.4 Mach, regardless of what speed the aircraft if flying at. It essentially becomes a ramjet with a majority of the thrust developing from the airflow through a combination of variable geometry supersonic inlets, bypass doors, and ejector flaps.
The J58 engine was made of special metals that withstood temperatures above 600 degrees, which was the requirement for the titanium skin of the A-12 and SR-71 aircraft. The oil tanks and fuel lines were coated with a very thin layer of gold plating, to reduce temperatures.
The engine had to use special low volatility JP-7 fuel, due to the high-temperature environment in which the engine operates, which would detonate normal jet fuel. The fuel flowing into the engine was used as a coolant to cool the engine, hydraulic fluid, oil, triethylborane tank (required to be injected into the engine to ignite it), afterburner nozzle actuator control lines, air conditioning systems, and the parts of the airframe subjected to aerodynamic heating.
Two engines burned 700 pounds of fuel per minute during its cruise speed of 30 miles per minute.
Characteristics and Performance:
Type: Bypass turbojet (turbo-ramjet)
Weight: 6,000 lb (est.)
Length: 17 ft. 10 in. (expands up to 3 in. at max alt and temp)
Diameter: 4 ft. 9 in. (expands up to 3 in at max alt and temp)
Combustion Chambers: 8-can annular
Fuel Nozzles: 48 (6 per chamber)
Photos courtesy of cia.gov