The USAF's SR-71A two-seat strategic-reconnaissance aircraft originates from the remarkable Lockheed A-11, detail design of which began in 1959. Almost certainly intended to follow into service the Lockheed U-2, the A-11 derived from the design team led by C. L. 'Kelly' Johnson. Four A-11 were ordered, the first being flown on 26 April 1962.Three were later modified into YF-12A interceptors, entering service for evaluation in 1964. They were capable of speeds in excess of Mach 3 and of sustained supersonic flight at heights of up to 24,385m. Consequently construction was largely of titanium to maintain structural integrity, for as a result of kinetic heating, localized skin temperatures of up to about 427°C could be reached. To retard as much as possible the effects of such heating, these aircraft were finished in a high-heat-emissive black paint, leading to the name Blackbird. The fourth A-11 (ordered on the original contract) was subsequently redesignated YF-12C. From it was developed the SR-71A reconnaissance aircraft, the first of which flew on 22 December 1964. The readily recognizable configuration of this aircraft results from extensive wind-tunnel testing to evolve a minimum-drag fuselage providing maximum speed while keeping kinetic heating to the minimum; and to maintain the best possible handling characteristics at supersonic, take-off (about 370km/h) and landing (about 278km/h) speeds.
Power plant comprises two 144.6kN Pratt & Whitney turbojets. The 36,287kg of special fuel for these engines - which is contained within upper-fuselage and inner-wing tanks - acts as a heat sink for the entire aircraft, fuel temperature being raised to 320°C before being injected into the engines. Highly complex air intakes with computer-controlled fail-safe systems are essential to ensure that smooth airflow to the engines is maintained over the enormous forward speed range of 0-3,200km/h, at the upper limit of which the engines are virtually operating as turbo-ramjets. SR-71A began to enter USAF service in January 1966 and it is believed that as many as 31 may have been built. They have the capability to survey an area of 155,400km2 within an hour and in 1976 established a closed-circuit speed record of 3,367.221km/h; a world absolute speed record of 3,529.56km/h; and a sustained-altitude record of 25,929.031m.