Boeing X-51

The Boeing X-51 (also known as X-51 WaveRider) is an unmanned scramjet demonstration aircraft for hypersonic (Mach 6, approximately 4000 mph 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 300 seconds and reached speeds of over Mach 5 on 1 May 2013 for the longest duration hypersonic flight.

The X-51 is named "WaveRider" because it uses its shock waves to add lift. The program is run as a cooperative effort of the United States Air Force, DARPA, NASA, Boeing, and Pratt & Whitney Rocketdyne. The program is managed by the Aerospace Systems Directorate within the United States 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.

Design and development
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.



In flight demonstrations, the X-51 is carried by a B-52 to an altitude of about 50000 ft and then released over the Pacific Ocean. The X-51 is initially propelled by an MGM-140 ATACMS solid rocket booster to approximately Mach 4.5. The booster is then jettisoned and the vehicle's Pratt & Whitney Rocketdyne SJY61 scramjet accelerates it to a top flight speed near Mach 6. The X-51 uses JP-7 fuel for the SJY61 scramjet, carrying some 270 lb on board.

DARPA once viewed X-51 as a stepping stone to Blackswift, a planned hypersonic demonstrator which was canceled in October 2008. In May 2013, the U.S. Air Force plans have X-51 technology applied to the High Speed Strike Weapon (HSSW), a missile of similar in size to the X-51. The HSSW could fly in 2020 and enter service in the mid-2020s. It is envisioned to have a range of 500-600 nmi, fly at Mach 5-6, and fit on an F-35 or in the internal bay of a B-2 bomber.

Testing


Ground tests of the X-51A began in late 2006. A preliminary version of the X-51, the "Ground Demonstrator Engine No. 2", completed wind tunnel tests at the NASA Langley Research Center on 27 July 2006. Testing continued there until a simulated X-51 flight at Mach 5 was successfully completed on 30 April 2007. The testing is intended to observe acceleration between Mach 4 and Mach 6 and to demonstrate that hypersonic thrust "isn't just luck". Four test flights were initially planned for 2009, but the first captive flight of the X-51A on a B-52 was not conducted until 9 December 2009, with further captive flights in early 2010.

Powered flight tests
The first powered flight of the X-51 was planned for 25 May 2010, but the presence of a cargo ship traveling through a portion of the Naval Air Station Point Mugu Sea Range caused a 24 hour postponement. The X-51 completed its first powered flight successfully on 26 May 2010. It reached a speed of Mach 5, an altitude of 70000 ft and flew for over 200 seconds; it did not meet the planned 300 second flight duration, however. The test had the longest hypersonic flight time of 140 seconds while under its scramjet power. The X-43 had the previous longest flight burn time of 12 seconds,  while setting a new speed record of Mach 9.8 (12,144 km/h, 7,546 mph).

Three more test flights were planned and will use the same flight trajectory. Boeing proposed to the Air Force Research Laboratory (AFRL) that two test flights be added to increase the total to six, with flights taking place at four to six week intervals, provided there are no failures.

The second test flight was initially scheduled for 24 March 2011, but was not conducted due to unfavorable test conditions. The flight took place on 13 June 2011. However, the flight over the Pacific Ocean ended early due to an inlet unstart event after being boosted to Mach 5 speed. The flight data from the test was being investigated. A B-52 released the X-51 at an approximate altitude of 50,000 feet. The X-51’s scramjet engine lit on ethylene, but did not properly transition to JP-7 fuel operation.

The third test flight took place on 14 August 2012. The X-51 was to make a 300 second (5 minutes) experimental flight at speeds of Mach 5, more than 3,600 mph. After separating from its rocket booster, the craft lost control and crashed into the Pacific. The Air Force Research Laboratory (AFRL) determined the problem was the X-51's upper right aerodynamic fin unlocked during flight and became uncontrollable; all four fins are needed for aerodynamic control. The aircraft lost control before the scramjet engine could ignite.

On 1 May 2013, the X-51 performed its first fully successful flight test on its fourth test flight. The X-51 and booster detached from a B-52H and was powered to 4.8 Mach by a booster rocket. It then separated cleanly from the booster and ignited its own engine. The test aircraft then accelerated to 5.1 Mach and flew for 240 seconds until running out of fuel and plunging into the Pacific Ocean off Point Mugu; this test was the longest air-breathing hypersonic flight. Researchers collected flight telemetry data for 370 seconds. The test signified the completion of the program. The Air Force Research Laboratory believes the successful flight will serve as research for practical applications of hypersonic flight, such as a missile, reconnaissance, transport, and air-breathing first stage for a space system.