09 December 2009

Following the Second World War, engine maker Pratt and Whitney found itself lagging behind in jet engine development as the wartime and immediate postwar demand for its piston engines was an all-time high. Nearly 50% of all engines built for US aircraft during World War II were Pratt and Whitney engines. The company's head, the legendary Frederick Rentschler (who quite literally built Pratt and Whitney into the dominant engine company it had become during World War II after leaving Wright Aeronautical in 1924), directed his head of engineering, Leonard Hobbs, to begin efforts to catch up in jet engine development.

Rentschler's goal was to catch up with the competition like General Electric, Westinghouse, and Allision by 1950 and become the dominant jet engine maker. Hobbs and his boss, however, had trouble finding an aircraft design tailored to their proposed engine designs. Gaining valuable experience by license-building the Rolls-Royce Nene turbojet for the Grumman F9F Panther as the J42, they found the USAF was in the midst of deciding between a turboprop and a turbojet for its new long range bomber that would become the Boeing B-52 Stratofortress.

Hobbs' initial design was for a larger turboprop than the Wright T35 being considered. Designated the T45, it combined the gearbox of a turboprop with the compressor/turbine core of one of their own jet engine designs. To achieve the necessary specific fuel consumption, Hobbs increased the pressure ratio to an unheard of 8:1. However, this would make the engine difficult to start and sluggish in acceleration. Hobbs' technological leap was to make the engine dual-spooled, with the front of the engine running at a slower speed than the back of the engine which ran at a faster speed. By optimizing the speeds of the different sections of the engine core, the engine would run not only more efficiently, but could generate more thrust than single spool engines of the day could ever create.

When Boeing's bomber design grew in size, the USAF decided only a turbojet could meet the needs of the design and Pratt and Whitney abandoned the T45 and the jet engine core of the turboprop became the J57 engine. With its compressor ratio increased to 12:1 (which was over double the industry standard of the day), the engine ran for the first time in January 1950 and would become the first jet engine to produce more the 10,000 lbs of thrust.

The J57 would be used in many fighter aircraft including the North American F-100 Super Sabre, Vought F8U Crusader, Douglas F4D Skyray, Convair F-102 Delta Dagger and McDonnell F-101 Voodoo. It also powered the Boeing B-52 Stratofortress and the KC-135 Stratotanker. The civilian JT3 version would be used on the Boeing 707 and Douglas DC-8 jetliners. The J57 and the JT3, would become the dominant large aircraft engine as well as fighter engine for a good part of the 1950s and 1960s, realizing Frederick Rentschler's goal set in 1945.

Source: US Naval Air Superiority: Development of Shipborne Jet Fighters 1943-1962 by Tommy H. Thomason. Specialty Press, 2007, 151-152.

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