The YF-12 "Blackbird" was an experimental fighter-interceptor version of the Lockheed A-12 reconnaissance aircraft but the YF-12 was never adopted by the military as an operational aircraft. It was, however, a precursor to the SR-71 Blackbird reconnaissance plane. Also, this plane had a vertical vertical fin underneath at the rear to increase stability ands ee how the tertiary doors at the rear of the engine are open?