Defense Advanced Research Projects Agency developed a series of experimental platforms designated X-Planes—Chuck Yeager flew X-1 in the 1940s- the first pilot to break the sound barrier officially.


Isaac Schultz writing in Gizmodo AU, describes the X-planes, such as:
  • X-33 was a conceptual liquid hydrogen-fuelled, wedge-shaped uncrewed reusable vehicle that launched vertically and landed horizontally.
  • X43 (Hyper-X) used scramjet propulsion to reach Mach 9.6 (11,265 km per hour) for about 10 seconds.
  • Lockheed Martin X-59 QueSST is a long-nosed supersonic plane that can fly without a sonic boom.


Source Information: