In the near century since Robert Goddard, the founder of modern rocketry, fired the first liquid-fuel rocket into the sky, rocket scientists worldwide have favored liquid-fuel engines to power everything from the V-2 missile to the Saturn V moon booster to the Falcon 9 launcher. A liquid rocket motor works by pumping fuel and oxidant into a combustion chamber, where they mix and burn to create hot exhaust gases that expand out the nozzle, propelling the rocket forward.
But all that seems about to change. There’s a new liquid rocket on the launchpad, and it’s definitely not like the rockets of days past. It’s called a rotating detonation engine, or RDE, and those detonations are what make it so different. The fuel in a standard liquid rocket engine doesn’t detonate at all; instead it deflagrates—the technical term for an ignition front that spreads at subsonic speeds, as in piston engines, turbine engines and even candle flames, says Doug Perkins, a detonation propulsion scientist, who has worked at NASA’s Glenn Research Center since the 1990s. When fuel ignites in an RDE, by contrast, it doesn’t “burn” so much as it “bangs,” consumed more completely and near instantaneously via intense compression and heating by a supersonic shockwave. Simply put, rather than burning fuel as in existing powerplants, he says, an RDE explodes it to produce more thrust. Thus an RDE can capture more of the propellant’s energy to power vehicles farther, faster and with larger payloads.
“The power density—the amount of energy release we get within a certain volume—is an order of magnitude higher than today’s devices. And that’s exciting,” says Steve Heister, a Purdue University engineering professor and longtime propulsion researcher. The circa 1,200-degree Celsius combustion that occurs inside an RDE is like “hellfire,” he jests, calling it “the fastest way to eat propellant.”
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