XA, xA, or xa may refer to:
The United States Military Aircraft Designation System was first designed in 1919 when the US Army's Aeronautical Division became the United States Army Air Service. Before this aircraft were put into service under their manufacturers' designations.
During this period Type Designations used by the United States Army Air Service were allotted, using two or three letters, which were an abbreviation of the aircraft's purpose. Examples include GA for Ground Attack aircraft; NO for Night Observation aircraft and NBS for Night Bombardment, Short Distance aircraft.
From 1924 to 1947 the Air Service, United States Army Air Corps, United States Army Air Forces and United States Air Force used a designation system based on mission category, with each model in a category numbered sequentially. In 1947, the designation system was extensively overhauled, with several categories being dispensed with, and others renamed For instance, the Lockheed P-80 Shooting Star (Pursuit) was redesignated as F-80 (Fighter), while the A-26 medium bomber/attack aircraft was redesignated as the B-26, reusing the designation, the Martin B-26 having retired in the meantime.
The Boeing X-51 (also known as X-51 WaveRider) is an unmanned scramjet demonstration aircraft for hypersonic (Mach 6, approximately 4,000 miles per hour (6,400 km/h) at altitude) flight testing. It completed its first powered hypersonic flight on 26 May 2010. After two unsuccessful test flights, the X-51 completed a flight of over six minutes and reached speeds of over Mach 5 for 210 seconds on 1 May 2013 for the longest duration hypersonic flight.
The X-51 received the name "WaveRider" because it uses its shock waves to add compression lift. The program was a cooperative effort of the United States Air Force, DARPA, NASA, Boeing, and Pratt & Whitney Rocketdyne. The program was managed by the Aerospace Systems Directorate within the U.S. Air Force Research Laboratory (AFRL). X-51 technology will be used in the High Speed Strike Weapon (HSSW), a Mach 5+ missile planned to enter service in the mid-2020s.
In the 1990s, the Air Force Research Laboratory (AFRL) began the HyTECH program for hypersonic propulsion. Pratt & Whitney received a contract from the AFRL to develop a hydrocarbon-fueled scramjet engine which led to the development of the SJX61 engine. The SJX61 engine was originally meant for the NASA X-43C, which was eventually canceled. The engine was applied to the AFRL's Scramjet Engine Demonstrator program in late 2003. The scramjet flight test vehicle was designated X-51 on 27 September 2005.