[ILLUSTRATION OMITTED] The North American X-15 hypersonic research aircraft was among the most successful of all the X-series. A cooperative research program among the U.S. Navy, Air Force and the National Aeronautics and Space Administration (NASA), the X-15 exceeded all expectations as it expanded the known flight envelope. When the program ended after 199 flights, the U.S. was well along the path towards operational space vehicles and human spaceflight.
