The Boeing X-51 "Waverider" is an unmanned technology demonstrator designed to further develop the concept of shockwave-assisted, high-speed flight by way of an internal scramjet engine. The product was born from the 1990s-era Scramjet Engine Demonstrator program in which the SJX61 hydrocarbon-fueled engine was originally developed for the X-43, a NASA-sponsored unmanned hypersonic technology demonstrator (the "Hyper-X" program). While the X-43 eventually fell to naught, the research was later rolled into the Boeing X-51. Following a steady stream of ground tests beginning in 2006, the X-51 prototype recorded its first flight on May 26th, 2010 to which the test vehicle reached speeds of 4,000 miles per hour (Mach 6) when launched from a Boeing B-51 Stratofortress mothership. As of this writing (2013), the X-51 has been produced in four development examples.
A scramjet engine ("Supersonic Combustion RAMJET") - a concept initially developed in the technology evolution of the 1950s - makes use of incoming air (via an air intake) through extremely high-speed flight, the air being slowed prior to combustion . An inlet body manages the supersonic compression to which fuel is then injected during the combustion process and a nozzle manages the outgoing supersonic exhaust, therefore producing the required thrust. A scramjet, therefore, is related to a "ramjet" in its general approach though, in a ramjet engine, combustion occurs in subsonic airflow as oppose to supersonic. A scramjet can provide speeds rated in Mach values - even as high as Mach 12 - and is a future alternative to the conventionally-powered turbofan engines in wide-scale use today. One of the key limitations of modern scramjet technology is the lack of low-altitude, low-powered capabilities which require the airframe to be airlifted though use of a "host" / "mother" ship for its launching. From there, the scramjet must also be first brought to speed by way of a rocket booster.
The X-51's scramjet powerplant makes use of the same fuel as in the Lockheed SR-71 Blackbird, designated in the USAF inventory as "JP-7". The X-51 is hauled into the air by the host ship (B-52) and then air-launched. The X-51 initially utilizes the power of a modified Lockheed Martin MGM-140 ATacMS (Army Tactical Missile System) solid rocket booster to help clear the host ship and reach initial speeds (approximately Mach 4.5). Once the booster system is spent, the assembly is jettisoned, allowing the Pratt & Whitney Rocketdyne SJY61 scramjet to take over in helping the airframe reach speeds nearing Mach 6. The aerodynamic principles involved in the X-51 fuselage design then react to the naturally-occurring shockwaves to add required lift (hence the nickname of "Waverider"). The fuselage consists of a well-sloped nose cone assembly, streamlined nearly-rectangular fuselage and short stubby wings about the design. An air intake is identified under the fuselage with the scramjet exhausting through the rear of the design in a conventional fashion. In essence, the X-51's configuration is similar to that of a streamlined cruise missile ala the "Tomahawk". Despite its unmanned design, the X-51's project data may very well be applied to all manner of future aircraft still to come - such as a scramjet-powered civilian airliner.
The X-51 sports a running length of 25 feet with an empty weight of 4,000lbs. Its presented operational range is 460 miles with a service ceiling of 70,000 feet.
As of 2013, the X-51 is still in active testing.