An artist's rendering of air-breathing, hypersonic X-43C, part of NASA's Hyper-X series of flight demonstrator. Now in development, the X-43C is expected to accelerate to a maximum potential speed of about 5,000 mph, and could undergo flight testing as early as the year 2008. Revolutionizing the way we gain access to space is NASA's primary goal for the Hypersonic Investment Area, managed for NASA by the Advanced Space Transportation Program at the Marshall Space Flight Center in Huntsville, Alabama. The Hypersonic Investment area, which includes leading-edge partners in industry and academia, will support future generation reusable vehicles and improved access to space. These technology demonstrators, intended for flight testing by decade's end, are expected to yield a new generation of vehicles that routinely fly about 100,000 feet above Earth's surface and reach sustained speeds in excess of March 5 (3,750 mph), the point at which "supersonic" flight becomes "hypersonic" flight. The flight demonstrators, the Hyper-X series, will be powered by air-breathing rocket or turbine-based engines, and ram/scramjets. Air-breathing engines, known as combined-cycle systems, achieve their efficiency gains over rocket systems by getting their oxygen for combustion from the atmosphere, as opposed to a rocket that must carry its oxygen. Once a hypersonic vehicle has accelerated to more than twice the speed of sound, the turbine or rockets are turned off, and the engine relies solely on oxygen in the atmosphere to burn fuel. When the vehicle has accelerated to more than 10 to 15 times the speed of sound, the engine converts to a conventional rocket-powered system to propel the craft into orbit or sustain it to suborbital flight speed. NASA's series of hypersonic flight demonstrators include three air-breathing vehicles: the X-43A, X-43B and X-43C.