<hi style=afpcap>One of the most closely guarded secrets of the twentieth century was the A-12 and its derivatives. The USAF had a requirement for an advanced interceptor which Lockheed responded with a version of the A-12 which was allocated the designation YF-12A. The aircraft was fitted with the Hughes AN/ASG-18 radar, which was originally intended for the cancelled North American XF-108 Rapier interceptor, while armament consisited of four AIM-47A missiles. However funds were not made available for a production run, with the three aircraft, together with a fourth which was a hybrid constructed from a damaged SR-71, were transfered to NASA at Edwards AFB for high speed, high altitude research. </hi>
