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The X-15 rocket-powered spaceplane was arguably the most important -- and famous -- X-plane of the Cold War Era. Prior to the X-15 program's abrupt cancellation in 1968, the Air Force was drafting ambitious plans for the aircraft, including a "next generation" X-15-D that would test the limits of hypersonic flight (Mach 5+) using an exotic new hydrogen-fueled scramjet engine. Unfortunately, the X-15-D never made it past the drawing board. Had the program been allowed to continue, the X-15-D would have no doubt added further luster to the X-15's sterling legacy.