# Pratt & Whitney J58
The **Pratt & Whitney J58** (company designation JT11D-20) is an afterburning turbojet engine developed for continuous flight at Mach 3 and above, best known as the powerplant of the [Lockheed A-12](https://www.edgepedia.org/lockheed-a-12), YF-12A, and SR-71 Blackbird aircraft. It is a single-shaft engine with a compressor bleed bypass that shifts its operating cycle toward that of a ramjet at high speed, and it was the first engine designed to run on afterburner for extended periods and the first flight-qualified at Mach 3 for the [United States Air Force](https://www.edgepedia.org/united-states-air-force).[1](https://www.si.edu/object/pratt-whitney-j58-jt11d-20-turbojet-engine%3Anasm_A19920006000) • [2](https://www.globalsecurity.org/military/systems/aircraft/systems/j58.htm)
| Key fact | Detail |
|---|---|
| Type | Afterburning turbojet (turbo-ramjet in bypass operation), company designation JT11D-20[1](https://www.si.edu/object/pratt-whitney-j58-jt11d-20-turbojet-engine%3Anasm_A19920006000) |
| Thrust | 32,500 lbf with afterburner per NASA and Air Force Museum figures; the Smithsonian lists 133,440 N (30,000 lb)[3](https://www.nasa.gov/wp-content/uploads/2021/09/495839main_FS-030_SR-71.pdf?emrc=672af533a40c4) • [1](https://www.si.edu/object/pratt-whitney-j58-jt11d-20-turbojet-engine%3Anasm_A19920006000) |
| Compressor | Single-shaft axial compressor with 8.8:1 pressure ratio; Smithsonian describes an 8-stage unit, the National Museum of the U.S. Air Force a nine-stage one[1](https://www.si.edu/object/pratt-whitney-j58-jt11d-20-turbojet-engine%3Anasm_A19920006000) • [4](https://www.nationalmuseum.af.mil/Visit/Museum-Exhibits/Fact-Sheets/Display/Article/195665/pratt-whitney-j58-turbojet/) |
| Bleed bypass | Above about Mach 2.2, valves bleed air from the fourth compressor stage through six external ducts around the rear compressor stages, combustor, and turbine into the afterburner[1](https://www.si.edu/object/pratt-whitney-j58-jt11d-20-turbojet-engine%3Anasm_A19920006000) • [2](https://www.globalsecurity.org/military/systems/aircraft/systems/j58.htm) |
| Weight | 2,948 kg (6,500 lb) per the Smithsonian; a NASA combined-cycle study lists 6,700 lb dry weight and a 5.3:1 thrust-to-weight ratio[1](https://www.si.edu/object/pratt-whitney-j58-jt11d-20-turbojet-engine%3Anasm_A19920006000) • [5](https://ntrs.nasa.gov/api/citations/20110013567/downloads/20110013567.pdf) |
| Fuel and ignition | Low-volatility JP-7, ignited by triethylborane (TEB), which ignites spontaneously on contact with oxygen[6](https://www.enginehistory.org/Convention/2015/J58Starting/J58Starting.shtml) |
| Timeline | Development began about 1958; production began in 1964[1](https://www.si.edu/object/pratt-whitney-j58-jt11d-20-turbojet-engine%3Anasm_A19920006000) |
## Design and the bleed-bypass cycle
[Pratt & Whitney](https://www.edgepedia.org/pratt-and-whitney) designed the engine for continuous operation at high [Mach number](https://www.edgepedia.org/mach-number) with compressor inlet temperatures above 400 degrees C (750 degrees F), conditions no conventional engine materials of the time could tolerate. Engineers developed new alloys for turbine blades, new forging and heat-treatment methods for disks, and new processes to roll superalloy sheets into engine cases; even the standard ceramic engine badge cracked and flaked at J58 temperatures and had to be replaced with a stainless-steel eagle logo.[1](https://www.si.edu/object/pratt-whitney-j58-jt11d-20-turbojet-engine%3Anasm_A19920006000) • [7](https://www.prattwhitney.com/en/newsroom/news/2014/12/22/this-day-in-history-sr-71-makes-first-flight)
The defining feature is the compressor bleed bypass. At lower speeds, including up to about Mach 2.2, all air flows through the engine core and the J58 operates as a conventional turbojet.[2](https://www.globalsecurity.org/military/systems/aircraft/systems/j58.htm) Above roughly Mach 2.2, bypass valves open and bleed air from the fourth compressor stage; six ducts route it around the rear compressor stages, combustor, and turbine, and it re-enters the turbine exhaust around the front of the afterburner, where it adds thrust and provides cooling. The main fuel control scheduled the transition to bypass operation as a function of compressor inlet temperature and engine speed.[1](https://www.si.edu/object/pratt-whitney-j58-jt11d-20-turbojet-engine%3Anasm_A19920006000) • [2](https://www.globalsecurity.org/military/systems/aircraft/systems/j58.htm) • [7](https://www.prattwhitney.com/en/newsroom/news/2014/12/22/this-day-in-history-sr-71-makes-first-flight) Pratt & Whitney credited this bleed bypass cycle with boosting thrust significantly with no significant increase in fuel burn.[7](https://www.prattwhitney.com/en/newsroom/news/2014/12/22/this-day-in-history-sr-71-makes-first-flight)
At cruise the engine itself contributed only a minority of the propulsive force. NASA's SR-71 fact sheet states that less than 20 percent of the total thrust used to fly at Mach 3 came from the engine itself, with the balance produced by the inlet and the movable conical spike at the front of each nacelle; air entering the inlets bypassed the engines and went directly to the afterburners and ejector nozzles, acting as ramjets.[3](https://www.nasa.gov/wp-content/uploads/2021/09/495839main_FS-030_SR-71.pdf?emrc=672af533a40c4) A NASA history of the Blackbird's development gives the breakdown by speed: at Mach 2.2 the inlet produced 13 percent of overall thrust, the engine 73 percent, and the exhaust ejector 14 percent; at Mach 3 cruise the inlet provided 54 percent, the ejector 29 percent, and the turbojet only 17 percent. Each inlet swallowed approximately 100,000 cubic feet of air per second at a compression ratio of 40:1 at cruise conditions.[8](https://ntrs.nasa.gov/api/citations/20090007797/downloads/20090007797.pdf?pubDate=20260501) Lockheed designer Kelly Johnson joked that Pratt & Whitney's engine functioned merely as an air pump for the nacelle at Mach 3 and above.[9](https://www.nasa.gov/wp-content/uploads/2015/01/601333main_NASAsContributionsToAeronauticsVolume2-ebook.pdf)
## Development history
The [Central Intelligence Agency](https://www.edgepedia.org/central-intelligence-agency) awarded Pratt & Whitney a contract for J-58 development in September 1959, as part of the OXCART program that produced the A-12.[10](https://www.cia.gov/readingroom/docs/DOC_0001474972.pdf) The engine had been begun around 1958, originally to meet a U.S. Navy requirement, before being taken up for the Mach 3 reconnaissance aircraft.[4](https://www.nationalmuseum.af.mil/Visit/Museum-Exhibits/Fact-Sheets/Display/Article/195665/pratt-whitney-j58-turbojet/)
Development proved harder than expected. In March 1961 Pratt & Whitney found unexpectedly great trouble bringing the J-58 up to OXCART requirements, and in September 1961 it informed Lockheed of continuing difficulties with weight, delivery, and performance. Because the engine would not be ready, the first A-12 flights used the older J-75 engine, which could power the aircraft safely to 50,000 feet and Mach 1.6.[11](https://www.cia.gov/resources/csi/static/The-Oxcart-Story.pdf) The first A-12 flight with one J58 came on 5 October 1962, and with two J58s on 9 March 1963; the aircraft first reached Mach 3.0 on 20 July 1963.[12](https://www.cia.gov/readingroom/docs/DOC_0001458639.pdf) To simulate the stress of maximum power output at Mach 3.2 and 97,000 feet, the engine was tested in the exhaust stream of a J75; by January 1963 Pratt & Whitney had delivered ten J58 engines to the test site.[13](https://www.cia.gov/readingroom/docs/DOC_0000645397.pdf) Early J58 deliveries gave poor performance, and in spring 1963 the test program was plagued by foreign-object ingestions.[14](https://www.cia.gov/readingroom/document/cia-rdp89b00980r000500010001-8)
## Fuel, lubricants, and starting
The extreme operating environment required special fuel and oil.[1](https://www.si.edu/object/pratt-whitney-j58-jt11d-20-turbojet-engine%3Anasm_A19920006000) The fuel, JP-7, was formulated for low volatility and high thermal stability, since on the [Blackbird](https://www.edgepedia.org/blackbird) the fuel also served as coolant and lubricant; a lit match dropped into it would be extinguished. Because of its high flash point, JP-7 had to be ignited with triethylborane during engine start.[6](https://www.enginehistory.org/Convention/2015/J58Starting/J58Starting.shtml) • [15](https://www.nasa.gov/wp-content/uploads/2023/04/sp-4108.pdf) The onboard TEB supply was 600 cc per engine, enough for a ground start and up to 16 afterburner ignition sequences.[6](https://www.enginehistory.org/Convention/2015/J58Starting/J58Starting.shtml)
Starting was itself unusual. The early AG330 start cart coupled two 401-cubic-inch Buick Wildcat V-8 engines, delivering 600 to 650 horsepower, to spin the J58 to a self-sustaining 3,200 rpm; later carts used 454-cubic-inch Chevrolet LS-7 engines, and the ultimate arrangement was a hangar-based Garrett AirResearch compressed-air system driving a turbine on the starter dog.[6](https://www.enginehistory.org/Convention/2015/J58Starting/J58Starting.shtml)
In continuous afterburner during cruise, each engine consumed over 11,000 pounds of fuel per hour.[16](https://www.nasa.gov/wp-content/uploads/2021/04/88796main_YF-12.pdf?emrc=e17503)
## NASA research and engine control legacy
From 1969 to 1979 NASA and the Air Force flew two YF-12s in a joint research program on high-speed flight. NASA's Lewis Research Center analyzed a 32,500-pound-thrust J58 in its Propulsion Systems Laboratory, beginning in the summer of 1973 when a full-scale J58 became the first hardware tested in the laboratory's new second altitude chamber. Technicians had to protect instrumentation from the engine's 1,000-degree-[Fahrenheit](https://www.edgepedia.org/fahrenheit) surface temperatures. The tests also measured the engine's emissions for studies of potential supersonic transports.[17](https://www.nasa.gov/aeronautics/nasa-center-boosted-yf-12-supersonic-engine-research/)
The program produced a lasting control-technology legacy. Pratt & Whitney commercially developed the Digital Electronic Engine Control (DEEC) based on its J58 experience during the YF-12 flight research program; the fuel-cooled, engine-mounted digital controller was later flight-tested on an F100 engine in a NASA F-15, integrating engine functions to improve performance and extend engine life.[18](https://www.nasa.gov/wp-content/uploads/2015/01/601288main_NASAsContributionsToAeronauticsVolume1-ebook.pdf) A digital cooperative control system flown on the YF-12C in the late 1970s integrated inlet control, autothrottle, air data, and navigation, and the concept was later implemented on the SR-71 fleet.[19](https://ntrs.nasa.gov/api/citations/19920018344/downloads/19920018344.pdf)
## Variants and ratings
Published thrust figures differ with configuration and source. The NASA SR-71 fact sheet and the National Museum of the U.S. Air Force both give 32,500 pounds of thrust with afterburning per engine,[3](https://www.nasa.gov/wp-content/uploads/2021/09/495839main_FS-030_SR-71.pdf?emrc=672af533a40c4) • [4](https://www.nationalmuseum.af.mil/Visit/Museum-Exhibits/Fact-Sheets/Display/Article/195665/pratt-whitney-j58-turbojet/) while the Smithsonian's specimen record lists 133,440 N (30,000 lb).[1](https://www.si.edu/object/pratt-whitney-j58-jt11d-20-turbojet-engine%3Anasm_A19920006000) A NASA paper on the Blackbird's development gives a maximum afterburner rating of 34,000 pounds at sea-level, standard-day conditions for the engines as fitted,[8](https://ntrs.nasa.gov/api/citations/20090007797/downloads/20090007797.pdf?pubDate=20260501) and a NASA combined-cycle study lists the J58/JT11D-20A at 32,000 lbf afterburning thrust at standard conditions with a 6,700 lb dry weight.[5](https://ntrs.nasa.gov/api/citations/20110013567/downloads/20110013567.pdf) For the NASA test-bed SR-71s, modifications to the J58 engines provided increased thrust, with about 5 percent thrust enhancement obtained on both engines by increasing turbine exhaust gas temperature and rotor speed.[20](https://ntrs.nasa.gov/api/citations/20020057965/downloads/20020057965.pdf)
## References
1. [Pratt & Whitney J58 (JT11D-20) Turbojet Engine, Smithsonian National Air and Space Museum](https://www.si.edu/object/pratt-whitney-j58-jt11d-20-turbojet-engine%3Anasm_A19920006000)
2. [J58, GlobalSecurity.org](https://www.globalsecurity.org/military/systems/aircraft/systems/j58.htm)
3. [SR-71 Fact Sheet, NASA Dryden Flight Research Center (FS-2008-6-030)](https://www.nasa.gov/wp-content/uploads/2021/09/495839main_FS-030_SR-71.pdf?emrc=672af533a40c4)
4. [Pratt & Whitney J58 Turbojet, National Museum of the United States Air Force](https://www.nationalmuseum.af.mil/Visit/Museum-Exhibits/Fact-Sheets/Display/Article/195665/pratt-whitney-j58-turbojet/)
5. [Combined Cycle Propulsion Work at NASA Dryden Flight Research Center, NASA NTRS](https://ntrs.nasa.gov/api/citations/20110013567/downloads/20110013567.pdf)
6. [J58 Start Cart, Engine History (2015 Convention)](https://www.enginehistory.org/Convention/2015/J58Starting/J58Starting.shtml)
7. [This Day in History: SR-71 Makes First Flight, Pratt & Whitney Newsroom](https://www.prattwhitney.com/en/newsroom/news/2014/12/22/this-day-in-history-sr-71-makes-first-flight)
8. [Design and Development of the Blackbird: Challenges and Lessons Learned, NASA NTRS](https://ntrs.nasa.gov/api/citations/20090007797/downloads/20090007797.pdf?pubDate=20260501)
9. [NASA's Contributions to Aeronautics, Volume 2, NASA](https://www.nasa.gov/wp-content/uploads/2015/01/601333main_NASAsContributionsToAeronauticsVolume2-ebook.pdf)
10. [OXCART background document, CIA FOIA Reading Room](https://www.cia.gov/readingroom/docs/DOC_0001474972.pdf)
11. [The Oxcart Story, CIA Center for the Study of Intelligence](https://www.cia.gov/resources/csi/static/The-Oxcart-Story.pdf)
12. [OXCART important dates document, CIA FOIA Reading Room](https://www.cia.gov/readingroom/docs/DOC_0001458639.pdf)
13. [OXCART development history document, CIA FOIA Reading Room](https://www.cia.gov/readingroom/docs/DOC_0000645397.pdf)
14. [The Directorate for Science and Technology 1962-1970, Volume Two, CIA FOIA](https://www.cia.gov/readingroom/document/cia-rdp89b00980r000500010001-8)
15. [The Smell of Kerosene: A Pilot's View of NASA's Flight Research, NASA SP-4108](https://www.nasa.gov/wp-content/uploads/2023/04/sp-4108.pdf)
16. [Mach 3+ NASA/USAF YF-12 Flight Research, 1969-1979, NASA](https://www.nasa.gov/wp-content/uploads/2021/04/88796main_YF-12.pdf?emrc=e17503)
17. [NASA Center Boosted YF-12 Supersonic Engine Research, NASA](https://www.nasa.gov/aeronautics/nasa-center-boosted-yf-12-supersonic-engine-research/)
18. [NASA's Contributions to Aeronautics, Volume 1, NASA](https://www.nasa.gov/wp-content/uploads/2015/01/601288main_NASAsContributionsToAeronauticsVolume1-ebook.pdf)
19. [Integrated control research on the YF-12C, NASA NTRS](https://ntrs.nasa.gov/api/citations/19920018344/downloads/19920018344.pdf)
20. [LASRE/SR-71 test program introduction, NASA NTRS](https://ntrs.nasa.gov/api/citations/20020057965/downloads/20020057965.pdf)
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*Topic: Encyclopedia › Technology and the built world › Engineering and manufacturing › Mechanical engineering › Turbines and jet propulsion*
*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license