With the development of supersonic aircraft, the X-plane era ushered in a new and challenging phase of flight. Researchers found that much of the knowledge accumulated from the previous, subsonic flight era did not apply to the emerging supersonic aircraft. These turbojet-powered planes also outpaced the usefulness of the wind tunnel, previously an indispensible tool of aeronautic research.This book explores the development of the X-series research aircraft, the planes that helped bridge the gap between subsonic flight and hypersonic flight.