RDRE

The RDRE, or Rotating Detonation Rocket Engine, is a type of combustion engine that uses detonation, as opposed to traditional engines using deflagration. In detonation, the flames of combustion travel supersonically, while flames from deflagration, which is akin to a campfire, travel subsonically. A RDRE injects fuel and an oxidizer (air, liquid oxygen, dinitrogen tetroxide, etc.) in a rotating pattern, as shown by the red arrows above. This system allows detonation to happen continously. All of these elements combine to provide an RDRE with a 20% efficiency increase when compared to a traditional deflagration engine.

Liquid Rocket Engine

While RDREs are more efficient, they have one flaw: they have never propelled an aircraft independently. Instead, propulsion has traditionally been sourced from deflagration engines(with liquid rocket engines being the most popular form of these today) and ramjets. Liquid rocket engines function by combusting two liquids, a fuel and an oxidizer, with a subsonic flame. Many modern rockets use these engines as they are reliable and well-understood. The Bell X-1 seen below used a liquid rocket engine to be the first supersonic manned aircraft. Ramjets are air breathing engines, meaning that instead of sourcing their oxidizer from an onboard tank, they take in air. The problem with ramjets is that they are notorious for losing efficiency at speeds below Mach 0.5. Along with this, ramjets must be designed for a specific altitude and velocity due to the fact that they “breathe” air.