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How The SR-71 J58 Engines Work

by John M. White |

SR-71 J58 Engine On Full AfterburnerThe SR-71 J58 engines have been able to push the "Blackbird" to both the absolute altitude and absolute speed records which have remained in place since 1976. The SR-71 J58 engines are single-spool turbojet engines with an afterburner. It has a unique bleed from the compressor to the afterburner which gives it increased thrust at high speeds. SR71 J58 Infographic

Origins

This engine was originally intended to be used to power the Martin P6M jet flying boat, but when that project was cancelled it was selected for the Convair Kingfish, Loockheed A-12, YF-12A and SR-71 aircraft. The J58 for the A-12 had to be redesigned from its original configuration to allow the aircraft to operate at Mach 3.2 continuously. The temperatures of sustained flight are so high that at this speed the engine compressor needed to be redesigned and the engine required high temperature materials for the rear stages of the J58.

Absolute Altitude Record

This incredible aircraft has set a number of world records for aircraft, including the absolute altitude record for aircraft. On July 28th, 1976 SR-71 s/n 61-7962 broke the world record for sustained flight at the absolute altitude record of 85,069 feet. At this altitude the sky is black and the curvature of the earth can be seen. Another issue is the life support system for an aircraft flying this high. Standard oxygen masks only provide enough oxygen to 43,000 feet; above that altitude the crew must wear specialized protective pressure suits made by the David Clark Company (who also manufacture headsets for general aviation pilots).

Absolute Speed Record

Again this great airplane holds the absolute speed record which remains to this day. On July 28th, 1976 SR-71 s/n 61-7962 broke the absolue speed record of 1,905.81 knots (Mach 3.3), although SR-71 pilot Brian Shul claims in his book "The Untouchables" that he flew in excess of Mach 3.5 on April 15th, 1986 over Libya to avoid a missile. This "Need For Speed" is why the development of the Pratt & Whitney J58 engine has been developed. It took some time to figure out the fuel which finally settled on JP-7. However, starting the engine with JP-7 fuel was difficult, so to start the engines triethylborane is injected to produce high enough temperatures to light off the JP-7. This also required the use of special KC-135Q tankers for aerial refueling because the SR-71 can not take off with a full load of fuel which would place too much stress on the brakes and tires during takeoff plus ensuring the aircraft could still takeoff should one engine fail.

The Mighty J58 Video

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