Directory of U.S. Military Rockets and Missiles
Appendix 4: Undesignated Vehicles
Copyright © 2006-2009 Andreas Parsch

Boeing X-51

In early 2003, the Air Force Research Laboratory (AFRL) established a program called EFSEFD (Endothermically Fueled Scramjet Engine Flight Demonstrator), a name since changed to SED-WR (Scramjet Engine Demonstrator - WaveRider). This program effectively continued earlier studies made under the waverider part of DARPA's ARRMD (Advanced Rapid Response Missile Demonstrator) program. In January 2004, AFRL selected a team of Boeing (airframe) and Pratt & Whitney (engine) to build the SED-WR flight test vehicle. In September 2005, this vehicle was officially designated as X-51A.

Image: Pratt & Whitney
X-51A (design concept)

The X-51A will be an air-launched expendable missile powered by a hydrocarbon-fueled scramjet engine. This engine has been in development by P&W since at least 2000 under the AFRL's HySET (Hypersonic Scramjet Engine Technology) program, a part of the USAF's overall HyTech (Hypersonic Technology) effort. A prototype, the GDE-1 (Ground Demonstration Engine 1), has already been ground-tested between September 2002 and June 2003 at simulated air speeds between Mach 4.5 and 6.5. Originally the AFRL had planned to fly the P&W engine on NASA's X-43C vehicle, but that program was cancelled in March 2004. It's possible that the SED-WR program was started early to have a follow-on project up and running in the case of a (possibly anticipated) cancellation of the X-43C.

The X-51A will be launched by a B-52 at about 10700 m (35000 ft), and accelerated to the Mach 4.5 scramjet ignition speed by the solid-propellant rocket motor of a surplus MGM-140 ATACMS missile. Estimated target speed for the X-51A is between Mach 6 and 7. The first flight is scheduled for October 2009, to be followed by three more in 2010.


No detailed design data for the X-51A has been published so far.

Main Sources

[1] Stanley W. Kandebo: "Landmark Tests Boost Scramjet's Future", Aviation Week & Space Technology, 26 March 2001
[2] Stanley W. Kandebo: "Researchers plan civil and military scramjet flight tests", Aviation Week & Space Technology, 4 June 2003
[3] "Scramjet Demonstrator Get X-Plane Designation", AFRL Propulsion Directorate Monthly Accomplishment Report, September 2005
[4] Pratt & Whitney Website
[5] Stephen Trimble: "X-51A flight may lead to B version", article in "Flight International", 31 March 2009

Back to Directory of U.S. Military Rockets and Missiles, Appendix 4

Last Updated: 24 June 2009