LOCKHEED SR-71A second installment

J58 Turbojet

SR-71 Engine Nacelle

The National Museum of the USAF in Dayton, OH has a display of the power plant for the SR-71 Blackbird.  There were two Pratt & Whitney J58s of 32,500 lbs. thrust each with afterburner.  I remember reading about Ramjet technology in Mechanics Illustrated back in the 50's.  This is its application.  They consolidated a ramjet and a turbojet into one nacelle on each wing to save space and weight.  The turbojet was the primary power up to about 1500 mph.  Then the air began to bypass the engine compressor and turbine directly into the after burner which became the combustion chamber.

J58 Engine showing Bypasses

 It then began to function as a ramjet.

SR-71 Afterburner

 You should notice that the engine nacelle seems to point downward.  That is because the plane flew in a 2 degree nose up attitude.

SR-71 Engine Nacelle

The Wikipedia diagram of a ramjet explains the function of the cone on the front of the Nacelle.  The  airspeed begins at greater than Mach 1 before it enters the engine.  It moves over the cone and compresses moving at less than Mach 1.  Fuel is injected as it passes into the combustion chamber.  It than passes through a nozzle at Mach 1 and accelerates to greater than Mac1 in the exhaust.  The Blackbird would speed up to save fuel.  That is why its cruising speed was greater than Mach 3. 

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