X-52

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. 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.

As of early 2006, the X-52 designator appeared to remain un-assigned, and subsequently it seems it was skipped to avoid confusion with the B-52, frequently used as a carrier aircraft for X-planes.

NASA's venerable B-52B launch aircraft, operated by the NASA Dryden Flight Research Center, participated in some of the most significant projects in aerospace history. At retirement in December 2004, the air launch and research aircraft held the distinction of being NASA's oldest aircraft, as well as being the oldest B-52 on flying status. At the same time, it had the lowest number of flying hours (2443.8) of any B-52 in operation, having been used exclusively in the role it continued to perform so reliably for nearly 50 years.

NASA obtained a B-52H bomber from the U.S. Air Force in 2001, intending to use the aircraft as an air-launch and testbed aircraft to support NASA, Air Force and industry flight research and advanced technology demonstration efforts at NASA's Dryden Flight Research Center, Edwards AFB, Calif. The B-52H replaced Dryden's famous B-52B "008" following that aircraft's retirement on Dec. 17, 2004. However, with no research projects requiring its capabilities on the horizon under NASA's restructured aeronautics research programs, the decision was made to return the aircraft to the Air Force.