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2005 U.S. Commercial Space Transportation Developments and Concepts Enabling Technologiesof conventional aircraft structures and required athermal protection system. Because existing windtunnels are incapable of generating Mach 10 airflow,the final X-43A flight provided a one-of-a-kindopportunity for research into this flight regime. TheX-43A proved that the United States can produceair-breathing vehicles capable of sustained activelycontrolled flight at hypersonic speeds.This program was originally intended to featuretwo additional vehicles. As envisioned, the X-43B would demonstrate an engine capable of operatingin several modes. The X-43B’s combined cycleengine would function as a normal turbojet at lowaltitudes and switch to scramjet mode at high altitudesand speeds. Planned X-43B flights were tooccur sometime in 2009 after the completion ofanother Hyper-X test vehicle, the X-43C. The X-43Cwas intended to demonstrate the operation of a solidhydrocarbon-burning scramjet engine at speedsbetween Mach 5 and 7 sometime in 2008. Both vehicleswere cancelled in March 2004 because of a shiftin NASA’s strategic goals following the announcementof the President’s Vision for Space Explorationin January of that year. However, because of the successof the X-43A , the U.S. Congress added $25million to the NASA 2005 budget to continue developmentof the X-43C research vehicle. Hypersonicvehicles promise to enable future RLV systems, suchas two-stage-to-orbit systems. In addition to servingas RLV propulsion, hypersonic engines may enableproduction of hypersonic munitions for the military.Federal Aviation Administration/Office of Commercial Space Transportation 31