Pratt & Whitney J58

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Pratt & Whitney J58.jpg
J58 engine on display at the Evergreen Aviation & Space Museum
Type Turbojet
Manufacturer Pratt & Whitney
First run 1958
Major applications Lockheed A-12
Lockheed SR-71

The Pratt & Whitney J58 (company designation JT11D-20) 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 with an afterburner.[1]

Design and development[edit]


The J58 had its origins in the bigger JT9 (J91) engine. It was a 3/4 scale JT9 with a mass flow of 300 lb/s, down from 400 lb/s and known by the company designation JT11.[2] The JT11 was initially proposed for the US Navy, hence its Navy designation J58.[2] It was also promoted for various Navy and Air Force aircraft which never materialised, for example Convair F-106, North American F-108, Convair B-58C and North American A3J.[2] The J58 was initially developed for the US Navy [3] to power the planned version (using the J58)[4] of the Martin P6M jet flying boat.[5] The P6M started out using Allison J71-A-4 engines and then switched to the Pratt & Whitney J75 as the J58 wasn't ready due to development problems. Upon cancellation of this aircraft, it was selected for the Convair Kingfish and for the Lockheed A-12, YF-12A and SR-71. Other sources link its origin to the USAF's requirement for a powerplant for the WS-110A, the future XB-70 Valkyrie.[6]

Re-design for Mach 3.2[edit]

J58 on full afterburner, showing shock diamonds

The J58 for the A-12 had to be almost completely redesigned from its original Navy proposal so that it could operate continuously at Mach 3.2. The complete engine had to endure the temperatures of sustained flight at this speed and had to be designed accordingly.

The compressor redesign, as well as addressing the need for high temperature materials such as Waspaloy in the rear stages[7] had to address the aerodynamic shortcomings inherent in any turbomachine compressor when ingesting very hot air (800 degrees Fahrenheit (427 degrees Celsius) at Mach 3.2).[8] The route chosen to keep the J58 compressor pumping was to bleed air from the compressor 4th stage through 6 external tubes to the afterburner. In addition a 2-position trailing edge flap was added to the inlet guide vanes.[9] The bleed and adjustable flap position enabled the compressor to still work efficiently despite the high temperatures delivered to it by the intake.

The afterburner received the exhaust from the turbine as well as the bleed air from the compressor. Most of the compressor bleed was required for cooling the afterburner duct and propelling nozzle and the remainder was used, together with the turbine exhaust, to burn the afterburner fuel flow.[9]

The combustor liner and flame holders were sprayed with ceramic thermal barrier coating[10] to allow sustained afterburner operation at temperatures up to 3,200 °F.[7]

Contemporary compressor solutions for Mach 3 flight[edit]

Alternative solutions to combat the adverse effects of high inlet temperature on the aerodynamic performance of the compressor were rejected by the Pratt & Whitney patentee, Robert Abernethy.[11] One of those solutions was used in a contemporary installation. The GE YJ93/XB-70 used a variable stator compressor to avoid front stage stall and rear stage choking.[12]

Another possible solution, pre-compressor cooling, was used on the MIG-25. Water/methanol was injected from a spray mast in front of the compressor to lower the intake temperature for short durations at maximum speed.[13] Pre-compressor cooling was also proposed for a Mach 3 reconnaissance Phantom[14] and the Mach 3+ F-106 RASCAL project.[15]


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 the afterburner. 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.[16] 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.[17] 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.

The complete engine installation or propulsion system[edit]

Operation of the air inlets and air flow patterns through the J58


The propulsion system consisted of the intake, engine, nacelle or secondary airflow and ejector nozzle(propelling nozzle).[18] 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%[18]


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.

Nacelle airflow and ejector nozzle[edit]

The ejector nozzle performed the reverse function of the inlet accelerating the turbine exhaust from about Mach 0.4 back up to Mach 3.[19] 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.[20] 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.


Specification of JT11D-20[edit]

Front view of a J58 turbojet as displayed at the Imperial War Museum Duxford, Cambridgeshire, UK, alongside a Lockheed SR-71A Blackbird

General characteristics

  • 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 or JP-4 or JP-5 for emergency refuelling from any tanker (Mach 1.5 limit)[21]


See also[edit]

Comparable engines
Related lists


  1. ^ Kloesel, Kurt J.; Ratnayake, Nalin A.; Clark, Casie M. "A Technology Pathway for Airbreathing, Combined-Cycle, Horizontal Space Launch Through SR-71 Based Trajectory Modeling". Dryden Flight Research Center. NASA. Retrieved 7 September 2011. 
  2. ^ a b c "The Engines of Pratt & Whitney: A Technical History" Jack Connors, ISBN 978-1-60086-711-8
  3. ^ "Factsheets: Pratt & Whitney J58 TurboJet". National Museum of the Air Force. 
  4. ^ "A Look at the Pratt & Whitney J-58JT11D-20". Copyright © 2014 Atomic Toasters. 
  5. ^ "Martin P6M Seamaster". © The Aviation History On-Line Museum. Created April 12, 1997. Updated November 2, 2013. 
  6. ^ Goodall, James and Jay Miller. "Lockheed's SR-71 'Blackbird' Family A-12, F-12, M-21, D-21, SR-71". Hinckley, England: AeroFax-Midland Publishing, 2002. ISBN 1-85780-138-5
  7. ^ a b
  8. ^ "Jet Propulsion for Aerospace Applications" Second Edition, Walter J. Hesse Nicholas V.S. Mumford Pitman Publishing Corporation 1964 Fig 14.7 "Compressor performance map showing effect of flight Mach number on operating points"
  9. ^ a b U.S. Patent 3,344,606, "Recover Bleed Air Turbojet," Robert B. Abernethy
  10. ^ "History of Thermal barrier Coatings for Gas Turbine Engines emphasising NASA's role from 1942 to 1990" Robert A. Miller, NASA TM 2009-215459
  11. ^ U.S. Patent 3,344,606, "Recover Bleed Air Turbojet", Robert B. Abernethy
  12. ^ "Jet Propulsion for Aerospace Applications- second edition" Walter J. Hesse, Nicholas V.S. Mumford,Jr. Pitman Publishing corporation p377
  13. ^ de:Mikojan-Gurewitsch MiG-25
  14. ^ "Tails Through Time" J P Santiago Wednesday,July 18, 2012 "The Mach 3 Phantom"
  15. ^
  16. ^
  17. ^
  18. ^ a b "F-12 Series Aircraft Propulsion System Performance and Development" David H Campbell J.Aircraft VolII NO 11 November 1974
  19. ^
  20. ^
  21. ^ "SR-71 Revealed"Richard H.Graham,Col USAF(Retd) ISBN 978-0-7603-0122 p46
  22. ^ a b "The Engine of Pratt & Whitney:A Technical History"Jack Connors ISBN 9781-60086-711-8 p325 J58 compressor map showing the take-off operating point

External links[edit]