This NASA report provides a comprehensive history of the X-43A Hyper-X (Hypersonic Experiment) program. In this NASA project, a supersonic combustion ramjet (scramjet) engine was flight tested on a subscale vehicle. The X-43A Hyper-X Research Vehicle (HXRV) was launched from a B-52B mothership, then boosted to the test speed by a modified Pegasus rocket first stage, called the Hyper-X Launch Vehicle (HXLV). Once at the proper speed and altitude, the X-43A separated from the booster, stabilized itself, and then the engine test began. Although wind-tunnel scramjet engine tests had begun in the late 1950s, before the Hyper-X program there had never been an actual in-flight test of such an engine integrated with an appropriate airframe. Thus, while the scramjet had successfully operated in the artificial airflow of wind tunnels, the concept had yet to be proven in "real air." These conditions meant changes in density and temperature, as well as changes in angle of attack and sideslip of a free-flying vehicle. A wind tunnel is limited in its ability to simulate these subtle factures, which have a major impact on almost any vehicle, but especially that of a scramjet's performance. The Hyper-X project was to provide a real-world benchmark of the ground test data. The full scale X-43A engine would be operated in the wind tunnel, and then flown, and the data from its operation would then be compared with projections. If these matched, the wind tunnel data would be considered a reliable design tool for future scramjet. If there were significant differences, the reasons for these would have to be identified. Until such information was available, scramjets would lack the technological maturity to be considered for future space launch or high-speed atmospheric flight vehicles. What would eventually become the Hyper-X project had its start within the NASP. This was ironic, as NASP project managers rejected the very idea of a short-term, limited-goal effort built around a subscale vehicle. It took a long time for them to finally accept this approach. Among reasons that the NASP program was focused on development of a full-scale test aircraft was the belief of many researchers and contractors that data from a subscale scramjet could not be scaled up to a full-size engine without introducing errors. Indeed, as time passed, the contractors found that they became less confident that they could predict general performance from specific data points. There were uncertainties in many data points, and those uncertainties changed. For these reasons, all of the contractors believed that only a prototype near-full-scale vehicle with a maximum speed above Mach 20 would give them real confidence in their performance predictions for a single-stage-to-orbit design.