NASA operated two YF-12As, 60-6935 and 60-6936. 06936 crashed on 24 June 1971 and was then replaced by an SR-71, 61-7951. Because NASA was not allowed to operate the SR-71, it was given the fake designation YF-12C and spurious tail number 06937. The real 60-6937 was an A-12 (c/n 131) of the CIA that was no longer active. 61-7951 remained with NASA until 1978. The second aircraft on the photo is obviously 60-6935. It carries a cooled cylinder which was being exposed to the friction and heat of Mach 3 flight. Photo from: NASA Ames
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