By the end of this month, the U.S. Air Force will begin a series of hypersonic tests that will send a scramjet into the atmosphere for about five minutes, at nearly five times the speed of sound. A scramjet is a supersonic combustion ramjet, while a ramjet is a jet engine using the engine’s forward motion to compress air.
 
If all goes as planned, this will be the first time that an aircraft will have flown at such speeds for more than a few seconds of time.

In previous attempts, the NASA  X-43 was powered-up for just 10 seconds of flight.  The X-43 was tested four times in 2004 and was hydrogen-powered.

This time around, the U.S. Air Force will be testing the X-51 Waverunner, which runs on compressed air that ignites fuel by combustion.  The X-51 is designed to be dropped from beneath a B-52 bomber.

A rocket booster will ignite and accelerate the Waverunner.  It will then run its course — from Mach 1 to Mach 6 — under its own power,  at which time the nose of the X-51 is expected to reach at least 1,480 degrees F.

The aircraft fuel will then be piped through tubes around the engine surface and will help warm the fuel to the temperature needed to ignite it as well as draw off heat to keep the engine from melting.  

According to Popular Mechanics,  the X-51 Waverunner is a global strike missile that is part of the Prompt Global Strike research project being developed by Boeing and the U.S. Air Force Research Laboratory and Defense Advanced Research Projects Agency. 

The Waverunner is said to be a warhead in the making, which will be filled with thousands of rods 12 times as destructive as a .50-caliber bullet, targeted to shower a designated area.

It is being developed for precision, speed, and range and has been designed to strike any place on the planet in an estimated 60 minutes.

The long-term goal is to design airplanes and missiles that would reach Mach 25.  The U.S. Air Force plans to conduct up to four tests of the Waverunner this year.