The Pratt & Whitney J-58 turbo-ramjet engine was developed in the late 1950's to push an airframe beyond Mach 3 and above 80,000 feet. Also known by the alphanumeric soup "JT11D-20A," it was designed and manufactured by Pratt & Whitney Aircraft Division of United Aircraft Corporation. Design of the engine's thermodynamic cycle was begun in 1956, and it was flying by April 1962. It was made to suit a Navy requirement for altitude, thrust, and extended periods in afterburner.

This engine has only been used on the SR-71, and appropriate prototypes and derivatives such as the A-11, A-12, YF-12A, M-21, SR-71B and SR-71C.

A Pratt & Whitney J-58 turbo-ramjet engine will exert upwards of 30,000 pounds of thrust --32,500 with afterburning. At cruising speed, the engine operates as a ramjet, with the distinctive conical inlet seen on the SR-71 forcing supersonic air through the inlet, where it is mixed with fuel and ignited. A ramjet has no moving parts, and so is far simpler mechanically than a conventional jet engine. The aerodynamics of ramjet propulsion are, however, far trickier. Ramjets can only operate at supersonic speeds, and the faster the airplane moves the better the ramjet works. For take off and accelerating to Mach speeds the J-58 uses its afterburner. Once the ramjet is ignited, however, it is very fast. The world speed record for an air-breathing engine is held by two J-58s (mounted to the angular wings of an Air Force SR-71 Blackbird, as they were) at some 2,200 mph.

These engines burn only a highly refined jet fuel called JP-7. The mixture was created solely for the Blackbird, and is so stable you can extinguish a match in it. The fuel will only burn in the J-58, and requires chemical injection of tetra-ethyl borane to ignite.



An extraordinary picture of a J-58 being ground tested:
http://www.mobiledyne.com/pub/j58_1498x769.jpg