X-51
Artist concept of X-51A
Role	Robotic flight demonstrator
Manufacturer	Boeing
Primary user	United States Air Force

The Boeing X-51 is a scramjet demonstration aircraft for hypersonic (Mach 7, around 8,050 km/h) flight testing. The X-51 WaveRider program is a consortium of the US Air Force, DARPA, NASA, Boeing and Pratt & Whitney Rocketdyne. The program is managed by the Propulsion Directorate within the United States Air Force Research Laboratory (AFRL).^[1]
The X-51 is a descendant of earlier efforts including the Advanced Rapid Response Missile Demonstrator and the liquid hydrocarbon-fuelled scramjet engine developed under the USAF’s HyTech program. The scramjet flight test vehicle was designated X-51 on 27 September 2005.^[2] The X-51 is carried aloft by a B-52 bomber and then launched with the aid of a solid rocket booster.
DARPA had viewed X-51 as a stepping stone to Blackswift,^[3] a planned hypersonic demonstrator that was canceled in October 2008.^[4] The X-51's first flight is expected in February 2010.^[5]

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 Langley Research Center on July 27, 2006.^[6] In April 2007 an entire flight was simulated. The result was positive. Two ground test engines will be built and testing will continue.
During the flight demonstrations, a B-52 will carry the vehicle to an altitude of about 50,000 feet and then release it. Initially propelled by an ATACMS solid rocket booster, the scramjet will take over at approximately Mach 4.5, and the vehicle will accelerate to a flight speed near Mach 6.^[7]
Four test flights in 2009 were planned, however, the first captive flight of the X-51A on a B-52 was not conducted until 9 December 2009,^[8] with further captive flights in early 2010. The vehicle's first flight is scheduled for mid-February 2010 when it will be released from a B-52 at 50,000 feet over the Pacific Ocean.^[5] Boeing has proposed to the Air Force Research Laboratory to extend the test flights from four to six^[9] with flights at four to six weeks intervals, assuming there are no failures.
The purpose of the testing is to observe acceleration between Mach 4 and Mach 6 and to "show achieving hypersonic thrust isn't just luck".^[10][11]

Specifications

• Length: 26 ft in (7.9 m)
• Empty weight: 4,000 lb (1,814 kg)

• Maximum speed: Mach 7+