Specifications:

Model: Pratt & Whitney J-58JT11D-20

Compressor: 9-stage, axial flow, single spool Turbine: two-stage axial flow

Thrust: 32,500 lbs. with afterburner

Weight: approx. 6,000 lbs.

Max. operating altitude: above 80,000 ft.

The SR-71 Blackbird is powered by two Pratt & Whitney J-58 turbo-ramjets, each developing 32,500 pounds of thrust with afterburning. The critical problems concerning supersonic flight with air breathing engines are concentrated in the air inlet area. The circular air intakes of the SR-71 contain a center body tipped with a conical spike. The spike is movable, forward for takeoff and climb to 30,000 feet after which, as speed builds up, it moves rearward, controlling the amount of air entering the engine. As it does so, Air Inlet Bypass Doors in the side of the nacelle close to establish the correct flow of air through the engine, holding the supersonic shock wave in it's critical position within the inlet. The engine itself operates at subsonic speed. At Mach 3+ the spike is three feet to the rear of it's takeoff position, slowing down the incoming airflow, establishing an area of pressure within the nacelle, which is now pushing the engine. This action is so powerful that it accounts for 58 percent of the total thrust, the engine providing only 17 percent, and the ejectors (surrounding the nacelle near the afterburner) is responsible for the remaining 25 percent. Should the shockwave be expelled from the inlet, a condition known as an "Unstart" occurs. Unstarts have been known to be so violent as to crack the pilots helmet from the severe yaw of the aircraft. If unchecked, the resulting yaw is described by SR-71 pilots as though the nose and tail are trying to swap ends. However, an automatic control system senses this problem and repositions the Spike in milliseconds, doing so with great accuracy even though air loads of up to fourteen tons are acting on the spike, dealing with the difficulty before the human brain becomes aware of the problem, and the Blackbird cruises on....faster than a rifle bullet.

A correction to the above paragraph is needed. Ken Hall, a retired Astro/Aero/Electronic engineer states: It appear from this description that FREE thrust is being generated by the pressurized air behind the inlet shocks.  Not true. A portion of the "pressurized" incoming air flow was/is piped and valved around the rotational core to the afterburner section where fuel is added and combusted with this, so called by-pass, air thus producing thrust.  An engine that functioned as you have described would be a free energy machine.

Ken Hall kenjack@hughes.net

Editors Note: Thank you for the clarification.

The first J-58s delivered to the blackbird program, all three models, had all stainless steel lines and the oil tank gold plated, the reason was for better heat dissipation. After a couple of years, and the subsequent tear down of engines, it was noted there was an abnormal amount of corrosion caused by dissimilar metal electrolysis. The gold plate was removed because the heat dissipation properties did not out weigh the cost of replacing lines as they started leaking.
Side note: When #957 crashed off the North end of the runway at Beale AFB and pictures were published, the hew and cry that came from the civilian segment about all the gold that was on the engine caused quite a commotion, even when it was explained why the gold was there. (Info courtesy Ron De Lozier)

An In-Depth Article on the J-58 Engine Lineage

by Philippe Ricco

Fri, 19 Jul 2002 08:18 Philippe Ricco Writes: 15 years ago, when I was a student and very impressed by the beautiful Blackbird, I wrote some personal notes about the Pratt & Whitney J-58. Years after, as Aeronautics Engineer, I found some more information to add to these notes. Recently, I gave the first part of these notes to one of my friends for publishing on the WEB, at the following address:

http://aerostories.free.fr/technique/J58/J58_01/page8.html

A Video on the Pratt and Whitney  J-58 and the Blackbird provided by the Discovery Channel

Page #13 of  the "SR-71 Blackbirds"