In June 1960 the USAF issued a specification for a long-range interdiction/strike aircraft able to penetrate Soviet air defenses at very low altitudes and very high speeds to deliver tactical nuclear weapons against crucial Soviet targets like airfields and supply depots.
Included in the specification were a low-level speed of Mach 1.2, a high-altitude speed of Mach 2.5, a combat radius of 890 mi (1,430 km), good short-field performance, and a ferry range long enough to reach Europe unrefueled.
The U.S. Navy, meanwhile, had since 1957 been searching for a long-range, high-endurance interceptor to defend its carrier groups against the new generation of Soviet jet bombers, which by then were being armed with huge anti-ship missiles with nuclear warheads.
The program was dubbed TFX (Tactical Fighter Experimental). The TFX design eventually emerged as an aircraft in the 20-ton (empty) class with a maximum take-off weight of almost 50 tons.
It had been intended to use titanium for large portions of the airframe to save weight, but this proved prohibitively expensive. The TFX was powered by two afterburning Pratt & Whitney TF30-P-100 turbofans in the 80 kN class.
The shoulder-mounted wings were attached to a pair of giant pivots, allowing it to take off, land, and loiter with a modest 16° sweep (for maximum lift and minimum landing speed), cruise at high subsonic speeds with a 35° sweep, or sweep back to a 72.5° maximum for fast supersonic dashes at more than Mach 2.
Despite its high maximum speed, its modest thrust fraction (thrust-to-weight ratio) made early versions somewhat underpowered, exacerbated by compressor stalls and other engine problems that forced a hasty redesign of the engine inlets.
Although the United Kingdom had expressed interest in the program in 1965 in preference to the home grown BAC TSR-2, the British order for the F-111 was cancelled, and the F-111's only export customer was the Royal Australian Air Force.
To replace the elderly and obsolescent Douglas EB-66, in 1972 the USAF contracted Grumman to convert some existing F-111As into electronic warfare/ECM aircraft. The Air Force had considered the Navy Grumman EA-6B Prowler, but was reluctant to adopt a Navy aircraft.
text was adapted from http://www.wikipedia.org/ )(GFDL)
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