The Lockheed SR-71 was an advanced, long-range, Mach 3 strategic reconnaissance aircraft developed from the Lockheed YF-12A and A-12 aircraft by the Lockheed Skunk Works. The SR-71 was unofficially named the Blackbird; its crews often called it the Sled, or the Habu ("snake"). The SR-71 line was in service from 1964 to 1998. Clarence "Kelly" Johnson was the man behind many of the design's advanced concepts. The SR-71 was one of the first aircraft to be shaped to reduce radar cross section. However, the aircraft was not stealthy and still had a large enough radar signature to be tracked by contemporary systems. The aircraft's defense was its high speed and operating altitude; if a surface-to-air missile launch was detected, the standard evasive action was to simply accelerate. Thirteen aircraft are known to have been destroyed, all for non-combat related reasons.