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Pratt & Whitney J58 / JT11D




The Pratt & Whitney J58 (company designation JT11D) was a jet engine used on the Lockheed A-12, and subsequently on the YF-12 and SR-71 aircraft. The J58 was a single-spool turbojet engine with an afterburner.

The J58 was initially developed for the US Navy to power the planned version of the Martin P6M jet flying boat. Upon cancellation of this aircraft, it was selected by Convair and Lockheed for their supersonic projects. Other sources link its origin to the USAF's requirement for a powerplant for the WS-110A, the future XB-70 Valkyrie. The J-58 produced 32,000 lbf (142 kN) of thrust. It was the first engine to be able to operate on afterburner for extended periods of time, and the first engine to be flight-qualified by the United States Air Force for Mach 3. A major feature of the J58 installation were the conical spikes in the variable-geometry inlets, which automatically moved fore and aft, controlled by an Air Inlet Computer. The spike altered the flow of supersonic air, ensuring good pressure recovery and minimum distortion at the engine inlet. The conical spikes are locked in the forward position below 30,000 feet and un-locked above that altitude. Above Mach 1.6 they are retracted approximately 1⅝ inch (4 cm) per Mach 0.1, up to total of about 26 inches (66 cm).

A variety of engine starting operations were available throughout the life of the A-12, F-12 and SR-71 aircraft, including: A twin chamber mono-fuel starter, attached to the engine only for starting, an AG330 starter cart, with two Buick Wildcat V8 internal combustion engines driving a common output, spinning the J58 to 3,200 rpm before the turbojet could self-sustain.

The engine's high operating speeds and temperatures required a new jet fuel, JP-7. Its reluctance to be ignited required triethylborane (TEB) to be injected into the engine to ignite it, and to ignite the afterburner in flight; above -5 °C TEB spontaneously ignites in contact with air. Each engine carried a nitrogen-pressurized sealed tank with 600 cm3 (20.7 ounces) of TEB, sufficient for at least 16 starts, restarts, or afterburner lights; this number was one of the limiting factors of SR-71 endurance, as after each air refueling the afterburners had to be reignited. When the pilot moved the throttle from cut-off to idle position, fuel flowed into the engine, and shortly afterwards an approx. 50 cm3 (1.7 ounce) shot of TEB was injected into the combustion chamber, where it spontaneously ignited and lit the fuel with a green flash. In some conditions, however, the TEB flow was obstructed by coking deposits on the injector nozzle, hindering restart attempts. Refilling the TEB tank was a perilous task; the maintenance crew wore silver fire suits. Conversely, the JP-7 fueling was so safe that some aircraft maintenance was permitted during filling. The chemical ignition was chosen instead of a conventional igniter for reliability reasons, and to reduce mechanical complexity. The TEB tank is cooled with fuel flowing around it, and contains a disk that ruptures in case of overpressure, allowing TEB and nitrogen to discharge into the afterburner.

The fuel flowing into the engine is used as a coolant to cool the engine, hydraulic fluid, oil, TEB tank, afterburner nozzle actuator control lines, air conditioning systems, and the parts of the airframe subjected to aerodynamic heating.

The engine lubricant was a silicone-based grease. It was solid at room temperature, and was preheated prior to engine start.

Jet engines take in their air at about Mach 0.4 irrespective of their flight speed, whether it be zero or Mach 3. Efficient operation of the engine inlet requires that it delivers the air to the engine at that speed. The Lockheed Blackbird family of aircraft was designed to travel at speeds over Mach 3.2 and the slowing down of the air from this high speed to about Mach 0.4 was controlled by the intake shock cone and the shape of the downstream internal ducting(both were part of the airframe and designed by Lockheed). As air slows down, its temperature rises (to nearly 400 deg C at a flight speed of Mach 3.2) and it was this high temperature entering the compressor that prevented the J58, as originally designed, from powering the Blackbirds to Mach 3.2. This section describes how the compressor was modified for Mach 3.2 flight to handle the aerodynamic effects of high inlet temperature. The section is based on information in U.S.Patent 3,344,606 and the SR-71 Flight Manual.

The patent summarizes why the J58 would not work at Mach 3.2 - "As a result of the ram air temperature rise the thrust output drops because of insufficient airflow, compressor tolerance to surge (or compressor stall) is poor, and low compressor efficiency occurs resulting in high fuel consumption. Also the compressor blades are subjected to high stress from the combination of high rotational speed and flutter from rotating stall in the front stages".

It specifies the object of the invention - "to improve the thrust generating quality of a turbojet engine with afterburner during high supersonic flight speed operation and to improve the compressor efficiency, the compressor surge margin and the compressor blade and vane fatigue problem".

It summarizes the solution - ".. by bleeding a portion of the compressor air from an intermediate compressor stage and recovering the air in the afterburner for reheating therein prior to discharge to atmosphere with the remainder of the exhaust gasses".

It was found that 20% of the engine flow needed to be bled from the fourth stage to combat the effects of high compressor inlet temperature and to restore the operation of the compressor to an acceptably high efficiency and flow capacity when operating at Mach 3.2.

The bleed air was returned to the engine to provide both cooling for the afterburner liner and more air for afterburner combustion. It passed through six external tubes from the compressor to the afterburner.

The SR-71 Flight Manual provides information on the use of the bleed ("bleed and IGV shift schedule") in terms of engine RPM and Mach number or compressor inlet temperature. It shows that this air bleed, which it calls internal, is also necessary, together with an extra bleed, which it calls external, during starting and low engine RPM.
The schedule shows that variable inlet guide vanes (IGV) were also part of the compressor design. Unlike most variable IGV, where the whole vane pivots, the J58 vanes had 2-position, part-span trailing edge flaps only.
Like the bleed flow the position of the flaps was also determined by the engine speed and compressor inlet temperature.
A schematic view of the engine is also shown on the schedule showing the engine bleed air path from the compressor to the afterburner.
The final configuration of the J58 for Mach 3.2 flight was that of a "turbojet with bleed air recovery", as stated in the patent. An alternative classification was a "turbojet with afterburner and compressor bleed bypass at high Mach", as stated in the Flight Manual. The engine designation was JT11-D20.

The propulsion system consisted of the intake, engine, nacelle or secondary airflow and ejector nozzle(propelling nozzle). The propulsive thrust distribution between these components changed with flight speed.
at Mach 2.2 inlet 13% - engine 73% - ejector 14%
at Mach 3.0+ inlet 54% - engine 17.6% - ejector 28.4%

The intake had to supply air to the engine with minimum pressure loss and distortion and at the speed dictated by the engine, namely about Mach 0.4. It had to do this at all flight conditions.

The ejector nozzle, together with the engine variable nozzle, performed the reverse function of the inlet accelerating the turbine exhaust from about Mach 0.4 back up to Mach 3. Mach 3 exhaust velocity is higher than Mach 3 flight velocity due to the much higher temperature in the exhaust. The nacelle airflow from the intake controlled the expansion of the hot engine exhaust in the ejector nozzle. This air flowed around the engine and served also to cool the hot external parts of the engine and to purge any combustible mixtures in the event of a fuel or oil leak in the nacelle.


Lockheed A-12
Lockheed M-21
Lockheed SR-71 Blackbird
Lockheed YF-12


Type: afterburning turbojet with compressor bleed bypass
Length: 17 ft 10 in (5.44 m) (an additional 6 in (15 cm) at max. temp.)
Diameter: 4 ft 9 in (1.45 m)
Dry weight: approx. 6,000 lb (2,700 kg)
Compressor: 9-stage, axial flow, single spool
Combustors: 8 can, annular
Turbine: two-stage axial flow
Fuel type: JP-7
Maximum thrust: 34,000 pounds-force (150 kN) (wet), 25,000 pounds-force (110 kN) (dry)
Overall pressure ratio: 6
Specific fuel consumption: 1.9 lb/(lbf-h) (wet), 0.9 lb/(lbf-h) (dry)
Thrust-to-weight ratio: approx. 6
Core air flow: 450 lb/s, (200 kg/s)






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