For most aerospace engineers, an explosion in a rocket engine is a disaster. But for Kareem Ahmed, it’s the entire point. As the director of the Propulsion and Energy Research Laboratory at the University of Central Florida, Ahmed has spent the last few years developing a next generation rocket engine that uses controlled explosions to boost stuff into space. It’s called a rotating detonation engine and it promises to make rockets lighter, faster, and simpler. But before it ever heads to space, engineers and physicists need a better understanding of how the hell it works.
“The challenge has been trying to understand what’s really happening inside and to be able to predict the performance,” Ahmed says. “We want to get them to the point where they are as predictable as a traditional engine.”
Rotating detonation engines, or RDEs, sound like something out of science fiction, but the concept is about as old as the space age itself. In the late 1950s and early 60s, aerospace engineers working on rocket engines envisioned RDEs as a way to turn a problem into a solution. “Sometimes the rocket motors would get a real bad instability and you’d get an explosion,” pioneer Arthur Nicholls recalled in a University of Michigan interview shortly before his death. “Then it led to the idea—well, what if we use that?”
RDEs are fundamentally the same as all other rocket engines: A fuel and oxidizer are ignited, and as they rapidly expand they are pushed out of a nozzle at high speeds, which blasts the rocket in the opposite direction. But the devil, as always, is in the details. In conventional liquid rocket engines like the kind used by SpaceX, the fuel and oxidizer are pressurized and fed into the ignition chamber using bulky turbopumps and other complicated machinery. A rotating detonation engine doesn’t need these pressurization systems because the shockwave from the detonation provides the pressure.
In the RDE developed by Ahmed and his colleagues, hydrogen and oxygen are fed into a combustion chamber. A small tube is used to send a shockwave into the chamber, which triggers the detonation. As the pressure wave moves through the chamber, it encounters more hydrogen and oxygen being fed into the front of the engine by dozens of tiny injectors. When the detonation wave hits the fresh fuel and oxidizer, it rapidly raises the temperature and pressure of the gases. This causes them to combust and send a flame shooting out of the rocket engine.
We’re accustomed to thinking of an explosion as a one-off event—something blows up and that’s that. But a functional RDE requires sustaining the initial detonation, which is where the “rotating” part comes in. The propellant is fed to the engine through a specially-designed injection plate with dozens of small holes that act like a racetrack for the detonation wave, allowing it to rotate around the cylinder. The rotating wave feeds off new propellant and will generate new detonation waves in an endless loop until there’s no more fuel flowing into the chamber.
Earlier this month, Ahmed and a team of researchers from the University of Central Florida and the US Air Force published the test results from the first rotating detonation engine to use hydrogen and oxygen for propellant. This chemical cocktail is regularly used to propel the upper stage of a rocket on the final leg of its journey to orbit. But Ahmed says that many engineers believed this chemical mixture was too volatile to be used in a rotating detonation engine. “Hydrogen is a crazy fuel,” he says. “Most believed it wasn’t possible to detonate hydrogen and oxygen because it would tend to deflagrate like a typical rocket engine, rather than a detonation motor.”