The big player in the aerospace field since the 1940s has been the chemical rocket engine. These powerhouses are currently the only way to get beyond the Earth's atmosphere and it's a job they do very well, indeed. Liquid-fueled rockets, sometimes aided by their solid counterparts, put the first satellites into orbit and the first astronauts on the Moon. They sent robotic probes to each of the planets with a few moons, asteroids, and comets thrown in for good measure.

On the down side, these rockets of either fuel also sit in silos and on submarines around the world with suborbital nuclear weapons that still threaten mass destruction to this day, as well as powering smaller weapon systems down to personal grenade launchers and even pistols with rocket-propelled shells.

Unfortunately, rockets made a lot of progress in a short period of time, very early in their history. Though there have been many refinements over the years, chemical rockets have been operating very close to their theoretical limits since the first Nazi V-2 left the Earth's atmosphere in 1942.

As a result, DARPA, NASA, and others have been looking at new, more efficient engines for both space travel and to propel the next generation of hypersonic missiles. RDREs are one especially promising alternative by exploiting a different principle that seems a bit paradoxical on the surface.

Essentially, an RDRE works by turning an explosion into a controlled detonation wave that is capable of sustaining itself without the need for moving parts. Where a rocket has a combustion chamber in which a fuel and an oxidizer are injected, an RDRE has two coaxial cylinders with a gap between them. Into this gap, the fuel/oxidizer mixture is introduced and ignited. If this is done right, they form a closely coupled reaction and shock wave. This wave speeds around inside the gap at supersonic velocity that generates more heat and pressure.