For most outer space engineers, a rocket engine explosion is a disaster. But Karim Ahmed is everything. As director of the University of Central Florida’s Energy and Energy Research Laboratory, Ahmed has spent the last few years developing a next-generation rocket engine that uses controlled explosions to boost things in space. It’s called a rotary-detonation engine and promises to make missiles lighter, faster and simpler. But before heading into space at all, engineers and physicists need a better understanding of how the hell it works.
“The challenge is trying to figure out what’s really going on inside and be able to predict performance,”
Rotating detonation engines or RDEs sound like something out of science fiction, but the concept is almost as old as the space age itself. In the late 1950s and early 1960s, space engineers working on rocket engines envisaged RDE as a way to turn the problem into a solution. “Sometimes rocket motors will get real bad instability and you get a blast,” recalled pioneer Arthur Nichols in an interview with the University of Michigan shortly before his death. “Then that led to the idea – well, what if we used it?”
RDEs are the same as all other rocket engines: The fuel and oxidizer are ignited and, with rapid expansion, they are pushed out of the high-speed nozzle, which blows the rocket in the opposite direction. But the devil, as always, is in the details. In conventional liquid rocket engines like the type used by SpaceX, the fuel and oxidizer are pressurized and fed into the ignition chamber using bulky turbo pumps and other sophisticated machines. The rotary detonation motor does not need these pressure systems because the detonation shock wave provides the pressure.
In 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 that triggers the detonation. As the pressure wave moves through the chamber, it encounters more hydrogen and oxygen supplied to the front of the engine by dozens of small injectors. When the detonation wave hits the fresh fuel and the oxidizer, it rapidly raises the temperature and pressure of the gases. This causes them to burn and send flames firing from the rocket engine.
We are used to thinking of an explosion as a one-off event – something blows up and that’s it. But functional RDE requires maintaining the original detonation of where the “rotating” part comes from. The drive is fed to the engine via a specially designed injection plate with dozens of small holes that act as a racing track for the detonation wave, allowing it to rotate around the cylinder. The spinning wave is fed with new fuel and will generate new detonation waves in a continuous loop until no more fuel is flowing into the chamber.
Earlier this month, Ahmed and a team of researchers at the University of Central Florida and the United States Air Force released the test results of the first rotary detonation engine to use hydrogen and oxygen for fuel. This chemical cocktail is regularly used to propel the upper stage of the rocket on the last leg of its journey into orbit. But Ahmed says many engineers thought the chemical mix was too volatile to use with a rotary detonation engine. “Hydrogen is a crazy fuel,” he says. “Most believe it is not possible to blast hydrogen and oxygen, as this would tend to deflate as a typical rocket engine rather than a detonation engine.”