I was a USAF aircraft mechanic the first part of my career, and really wanted to work on the SR-71. And one of my instructors had done so. High cool factor! Instead, I wound up working on Aardvarks. F-111D's. The A-12 was supposed to be a fighter/attack aircraft. Unfortunately, they fly faster than the bullets and missiles available at that time, so it wasn't practical. Mach 3 is over 3000fps. The M61 Vulcan 20mm rounds had just over 1000fps muzzle velocity. The GAU-8 system's 30mm cartridge is about the same, but a much heavier projectile, and wasn't developed until much later.There is also one at the U.S. Space and Rocket Center in Huntsville, Alabama. While we drove by this place numerous times working in AL last year, we just couldn't bring ourselves to pay the admission just to get in.
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That all said, the guy that wrote this article calls Huntsville's plane an A-12 Oxcart. That I need to research to see what the differences are.
Rare Cold War Spy Plane Gets Some Love and Care in Alabama