Aviation Week and Space Technology has a long interesting article on the development of the SR-72. It succeeds the SR-71 reconnaissance aircraft built around 1959 by Kelly Johnson at the Lockheed Skunks works. The SR-72 will have a turbine engine good to Mach 3 and a ramjet engine that operates from Mach 3 to 6. The differences between the engines in the two aircraft are neatly illustrated in the SR-71 and SR-72 Engine Cycle Comparison.
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