Until its first retirement at the end of 1989, and then the planned reactivation of two in the mid 1990s but followed by the announcement of continuing full retirement in April 1998, the SR-71 was the world's fastest and highest-flying conventional aircraft. It was a truly extraordinary machine, designed for the strategic reconnaissance role with a mass of sensors including ASARs-1 (advanced synthetic aperture radar system) or the Itek camera that could scan to the horizon on each side of the flightpath, and two pre-programmable high-resolution cameras. The Lockheed SR-71 was originally developed as a replacement to the U-2.
The 'stealthy' airframe was designed for a crew of two (pilot and systems operator) and minimum drag, and was therefore evolved with a very slender fuselage and thin wings of delta planform blended into the fuselage by large chines that generated additional lift, prevented the pitching down of the nose at higher speeds, and provided additional volume for sensors and fuel.
The airframe was built largely of titanium and stainless steel to deal with the high temperatures created by air friction at the SR-71's Mach 3+ cruising speed at heights over 21 336 m. Power was provided by the two special continuous-bleed turbojets which at high speed provided only a small part of the motive power in the form of direct jet thrust from the nozzles (18%), the greater part of the power being provided by inlet suction (54%) and thrust from the special outlets at the rear of the multiple-flow nacelles (28%).
Nicknamed Blackbird for its special overall color scheme that helped dissipate heat and absorb enemy radar emissions, the SR-71 was developed via three YF-121-A interceptors which reached only the experimental stage, from 15 A-12 (including one trainer) Mach 3.6 reconnaissance aircraft ordered for the CIA (and, in the case of two A-12 (M)s, as launching platforms for D-21 hypersonic cruise reconnaissance drones) and first flown from Groom Lake in March 1962. The SR-71A entered service in 1966 and 30 aircraft were built, while training was carried out on a conversion type comprising one SR-71B and one similar SR-71C converted from SR-71 standard.