The US Navy relies heavily on gas turbine engines to power both aircraft and ships, spending about $2 billion (with a “b”… or about one tenth of NASA’s entire budget last year) every year on fuel for them. Even a small reduction in fuel consumption would save millions of dollars, but future engines also need to meet increasing demands for power.
Nearly all commercial aircraft—and plenty of power plants—use gas turbines, so we’ve spent a lot of time optimizing their designs. This means that trying to continue to improve them will only eke out a few more percentage points of efficiency. In order to reach both the increased power levels and reduced fuel consumption the Navy wants—10 percent and 25 percent, respectively—we’ve got to come up with entirely new engine designs. (Just like automotive engineers, as we recently featured.)
Any new technology that the Navy develops could also be used in civilian aircraft and power plants. For a little perspective, US airlines spent over $50 billion on fuel last year.
With that in mind, a team at the Naval Research Laboratory (NRL), led by Dr. Kazhikathra Kailasanath, are developing rotating detonation engines, which should offer the higher efficiency and power output desired.
Gas turbines operate on the Brayton cycle, which consists of three steps. First, a compressor raises the pressure of the incoming air. Then, fuel is injected and mixes with the air, then burns, heating everything up. Since the system is open—rather than in a closed cylinder—this process occurs at a nearly constant pressure. Finally, this hot pressurized gas expands through a nozzle to generate thrust, either for propulsion or to push a turbine and generate electricity.
The main limitation to performance is the combustion step. Like nearly every other type of burning experienced in day-to-day life, it is a deflagration, where the flame propagates at a subsonic speed.
A detonation is a flame where the reacting gases release so much energy so quickly that the burning mixture moves faster than the speed of sound, driving a shock wave. Unlike a deflagration, a detonation significantly raises the pressure, increasing the available work—without requiring any additional mechanical components. Essentially, using a detonation allows you to reach much higher efficiencies.
While concepts for engines using detonation go all the way back to Robert Goddard and even Jules Verne, practical research beginning in the 1990s focused on the pulse detonation engine (PDE) design. As the name suggests, pulsed explosions shoot out of the nozzle for thrust, 20–100 times a second. That may sound crazy, but a number of experimental PDE engines have been developed, and the US Air Force Research Laboratory even successfully flew a plane with one.
However, there are a couple of difficulties that have prevented PDEs from reaching the high efficiencies they promised. For one thing, it’s tricky to initiate a detonation repeatedly at these rates—many times a second. (You can check out our previous report on the transition from deflagration to detonation for a more detailed explanation of those challenges.)
Rotating detonation engines, or RDEs, offer a solution to that problem. Micro-injectors squirt a pressurized mixture of fuel and air into a combustion chamber the shape of a long ring (annular cylinder). Then, this gas mixture explodes, with the explosion spinning around the circumference at supersonic speeds. The high pressure produced by the detonation forces exhaust gases down and out of the chamber, where they expand through a nozzle and produce thrust that can be used to push a turbine or aircraft.
Unlike a PDE, which requires repeatedly initiating detonations, an RDE only requires one detonation that spins around and around the chamber, continuously producing thrust.
Of course, there are a number of challenges with this design as well. The materials must be able to withstand the high pressure and temperature from the detonation. Also, the detonation occurs right near the injector inlets, where the strong pressure could actually push gases backwards.
To further complicate matters, since this is a relatively new concept, we don’t really understand (yet) the forces and heat fluxes that the combustion chamber experiences, making it difficult to optimize the design. By conducting simulations of RDEs with various fuels, researchers at NRL are trying to better understand the flow physics inside the combustion chamber. In a recent paper, simulation results showed that RDEs fueled with different hydrocarbons could reach fuel efficiencies, measured in specific impulse, of 85 to 89 percent of an ideal detonation cycle.
The US Navy isn’t the only group pursuing these types of engines. Researchers at the University of Texas at Arlington and Russia’s Lavrentyev Institute of Hydrodynamics have also explored RDEs experimentally. Hopefully, in a few years, this progress will lead to a working engine.