X-52
In a letter dated 27 September 2005, the US Air Force (HQ USAF/XPPE) officially granted the US Air Force Research Laboratory (AFRL) Propulsion Directorate’s scramjet flight test vehicle the designation X-51A. Since the introduction of the legendary X-1 in 1946, scientists have used the X-plane designations to identify experimental aircraft and rockets used to explore new aerospace technologies. As of early 2006, the X-52 designator appeared to remain un-assigned.
