This Day in McDonnell Voodoo History: February 9th

In 1956, the USAF approved a “standard” afterburner for the F-101B in place of the original “short” afterburner.  The short afterburners were needed to allow room to rotate the nose for takeoff and landing, but with the longer tail and extended landing gear of the F-101 it could safely accommodate a larger engine.  The afterburner nozzle of the J57-P-23 engine of the F-102 was mounted to the J57-P-13 engine to yield the J57-P-53.  Due to increased mixing of fuel and residual oxygen in the longer tail pipe, the J57-P-53 saw a 1,000-pound increase in thrust.  More advanced designs with convergent-divergent nozzles, such as the J57-P-35 promising 17,200 pounds, were being evaluated.

The original production engine for the F-101B, the Pratt & Whitney J57-P-53.  The afterburner nozzle was borrowed from the engine for its Convair F-102A stable mate, the J57-P-23.  Hot exhaust gases exiting the combustion chamber of the engine and afterburner (detailed in the lower photo) would converge toward a minimum diameter at the nozzle exit to produce a maximum amount of thrust without exceeding the temperature limits of the structure.  Thrust is the product of mass x velocity, but with the traditional convergent exhaust nozzle design, uncontrolled expansion of the exhaust gases after exiting the nozzle led to the formation of a normal shock wave at the nozzle exit, limiting its velocity to Mach 1.0.  If a means could be found to increase the exhaust velocity, thrust of the basic engine could be significantly increased.  Many engine manufacturers, including Pratt & Whitney, expended great effort towards that goal.  Success was achieved, but the learning curve needed to get there proved to be extremely steep.  

While most manufacturers concentrated on very sophisticated variable-area convergent-divergent exhaust nozzles, Pratt & Whitney finally settled on a relatively heavy design that was mechanically simple and proved very reliable.  This was incorporated into the definitive J57-P-55 engine that equipped the 16th aircraft on.  The basic nozzle design was also carried over to US Navy Vought F8U-2 Crusader aircraft in the form of the J57-P-16, J57-P-20, and J57-P-420 engines.  In its ultimate form as the J57-P-420, the J57 produced a maximum afterburning thrust of 19,600 pounds in upgraded F-8H and F-8J Crusader fighter-bombers.

Photo credit: USAF