<p>During the late 1950s, Lockheed's Advanced Development Projects office ('Skunk Works') at Burbank, California, was involved in the design and construction of a Mach 3+ high-altitude strategic reconnaissance platform to replace the U-2 then in service with the CIA and US Air Force. With expert direction from Clarence L. ('Kelly') Johnson, the resultant A-12 was fabricated largely of titanium alloy to maintain the structural integrity of the airframe when subjected to kinetic heating. At the design speed in excess of Mach 3, some components could reach 570 degrees F (300 degrees C). Because aerodynamic drag increases exponentially with speed, the slimmest possible fuselage and thinnest delta wing went into the design, together with integral lifting chines along the forward fuselage to prevent pitch-down at increasing speed.
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<p>Power was to come from two giant turbo-ramjet engines, the 32,500-lb (144.57-kN) thrust Pratt & Whitney J58-P-1. A complex mechanical sensing system provided air data information to allow the complicated series of bleed/inlet doors to be opened or closed correctly for optimum airflow, as well as the optimum position for the huge centerbody inlet spike. A high flashpoint fuel, JP-7, was especially developed for high-altitude flight.
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<p>The first A-12 flew in great secrecy with Lou Schalk at the controls from Lockheed's test plant at Groom Dry Lake, Nevada, on 26 April 1962 and, despite several problems during the flight test phase, aircraft were soon flying clandestine missions for the CIA. Included in the initial A-12 batch were three aircraft configured as interceptors as a spin-off of the main reconnaissance program.
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<p>These YF-12As differed considerably from the A-12. Firstly, they had two cockpits, the second containing a weapon systems officer (this work incidentally being of great value to the upcoming two-seat SR-71 derivative of the A-12). Secondly, the YF-12A had its chines cut back to allow the carriage of a Hughes ASG-18 radar, which was later augmented by two infra-red sensors. Weaponry was carried in bays in the chine area, these being four Hughes XAIM-47 air-to-air missiles. Ventral fins were fitted to offset the loss of directional stability caused by the nose radome.
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<p>If the aircraft had been developed further as an interceptor, it would have done exactly what the MiG-25 and SA-2 missile did to the American B-70 bomber, namely neutralize the threat of high-altitude strategic bombing forever. It is widely believed that the YF-12As were used to test systems for the cancelled North American XF-108 Rapier interceptor.
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<p>Three YF-12As were built (60-6934, 6935, 6936), and the first public viewing came on 30 September 1964, its debut marked by the heat from its engines setting off a sprinkler system and drenching onlookers! The A-12 aircraft had been made public in February (President Johnson erroneously calling it the A-11), and when the YF-12A appeared these were widely assumed to be the same aircraft.
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<p>To further publicize the aircraft, a series of 12 world records was established by 60-6936 on 1 May 1965, including the absolute speed mark of 2,070.06 mph (3,331.35 km/h) and sustained altitude record of 80,257.84 ft (24,462.59 m), subsequently only beaten by its offspring, the SR-71A. All three YF-12As quickly passed to NASA Dryden at Edwards AFB, California, where they undertook high-altitude, high-speed trials in a program that lasted until 1979. 60-6934 was badly damaged in an accident at Edwards, and its rear half was used (together with an engineering mock-up) to create 64-17981, the SR-71C two-seat trainer. 60-6936 crashed at Edwards in 1971 but 60-6935 continued until the completion of its trials in 1979. It flew to a well earned rest in the US Air Force Museum.
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<p>The F-12B designation was to have been assigned to a service interceptor if it had been proceeded with, and the YF-12C (60-6937) was a test aircraft for NASA. In fact, it was the second SR-71A (64-17951) assigned a spurious serial (60-6937 which had earlier been applied to a CIA A-12). Stripped of military equipment, the YF-12C was better-performing than the standard SR-71As, and it too had a long and fruitful career on trials with NASA Dryden.
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<hi style=hdr1>Specification</hi>
<hi style=hdr2>YF-12A</hi>
<list style=aflst>
<item><hi format=bold>Type:</hi> single-seat research aircraft
<item><hi format=bold>Performance:</hi> maximum speed 2,189 mph (3,523 km/h) at 70,000 ft (21,336 m); initial climb rate 39,600 ft (12,070 m) per minute; service ceiling 105,000 ft (32,004 m); range 3,300 miles (5,311 km)
<item><hi format=bold>Weights:</hi> empty about 60,000 lb (27,216 kg); maximum take-off about 145,000 lb (65,772 kg)
<item><hi format=bold>Dimensions:</hi> span 55 ft 7 in (16.94 m); length 107 ft 5 in (32.74 m); height 18 ft 6 in (5.64 m); wing area 1,800 sq ft (167.22 m<su>2</su>)
<item><hi format=bold>Armament:</hi> four Hughes AIM-47 air-to-air missiles
