A description of hypersonic plane is provided by US Air Force website:
"...flight test vehicle stack is approximately 25 feet long and includes a modified solid rocket booster from an Army Tactical Missile, a connecting interstage, and the X-51A cruiser... cruiser is designed to ride its own shockwave, thus the nickname, Waverider...distinctive, shark-nosed cruiser... houses the heart of the system, an SJY61 supersonic combustion ramjet or scramjet engine built by Pratt & Whitney Rocketdyne designed to burn JP-7 jet fuel. Boeing's Phantom Works performed overall air vehicle design, assembly and testing for the X-51's various component systems."A booster allows X-51A to reach sufficient speed, where scramjet engine can fire and ride the wave. It's compared to lighting a match in a hurricane and keeping it lit!
A glimpse of intended use for X-51A is clearly stated in this space.com article, "'we could have in the future such things as hypersonic weapons that fly 600 nautical miles in 10 minutes,' said Charlie Brink, X-51A program manager with the Air Force Research Laboratory at Wright-Patterson Air Force Base in Ohio, during a June 1 teleconference. [Most destructive space weapon concepts.]" I feel safer with the knowledge that military can deliver payloads anywhere on planet in just hours! I suspect former Secretary of Defense Donald Rumsfeld, who placed programs under Air Force auspices, is smiling!