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Why was the U-2 so different from the SR-71? - Aviation Stack Exchange
As for why U2 has outlived the SR-71, the main reason is the operating costs- the operating costs of SR-71 is quoted to be anywhere from $ 85,000 to $ 200,000 per hour, while the U-2 costs much less than that (incidentally, U-2s operating cost is less than that of its proposed replacement, the RQ-4 Global Hawk ). From the same source as above: