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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The potential benefits of theoretical detonation engines have long been known from the basic thermodynamics of combustion, Perkins says, but for decades most propulsion experts had considered the challenge of controlling the engine cycle’s explosive instabilities too daunting for serious technical development. Today, however, a growing number of aerospace engineers believe the dawn of RDE rocketry is at hand.
“Rocketry has struggled with combustion instability since the very earliest days of liquid rocket engines,” Heister says. “With RDEs, we’re embracing the instability. Let’s make it unstable, but let’s try to control the way it’s unstable.”
During operation, a restless ring of fire runs at the heart of every RDE, with detonation-driven shockwaves whirling in resonance around a high-walled, torus-shaped combustor cavity at constant velocities from Mach 3 up to Mach 6. The expanding gases barber-pole around and out the cylindrical combustion chamber, delivering steady thrust. That racetracklike circulation starts when a pump sprays propellants through perforations in the floor of the annular cavity, and after ignition, the detonation waves can rocket out until the fuel tanks empty. Surprisingly, RDEs have no moving parts. Rather than high-speed fuel injectors, it is the high pressures of the supersonic shockwaves flashing around the precisely engineered circuit that automatically close and open the fuel ports just in time to instantly fuel the next fiery tidal wave that’s sweeping in fast behind.
RDE flight hardware—high-performance, fuel-efficient, compact, affordable rocket motors—will likely first soar aloft by 2030, according to NASA forecasts. In time, RDE applications could range from Mach 5 attack missiles and hypersonic aircraft for the Defense Department to second-stage launchers, deep-space transports, and lunar and Martian landers for NASA, and perhaps even to supersonic transports for the commercial airline industry.
It’s no wonder, then, that significant RDE developments are already appearing. Last year engineers from NASA, Purdue and In Space LLC conducted a series of ground tests of a full-scale RDE rocket at NASA’s Marshall Space Flight Center that, with the help of a cooling system, produced 5,800 pounds of thrust for 251 seconds, says NASA combustion devices engineer Thomas Teasley, who leads the NASA effort. For perspective, most RDE test firings last one or two seconds.
NASA is pursuing RDE rockets as powerplants for planetary landers and interplanetary spacecraft, where their high performance and compact sizes could allow design efficiencies that benefit other mission areas, Teasley says. “For a typical lander engine, we’re talking a combustor that’s a foot, foot-and-a-half long,” he explains. “With RDEs, that’s down to a couple of inches.”
Also last year GE Aerospace, one of the world’s biggest jet engine builders, sent supersonic air through a subscale lab rig that combined a Mach 2.5-class turbofan with a rotating detonation-enabled “dual-mode ramjet” thought to be capable of Mach 5 velocities. The RDE would supply the Mach 3 speeds needed to start up the ramjet in flight, velocities turbines have difficulty reaching. Joseph Vinciquerra, senior director of aerospace research at GE Aerospace, told FlightGlobal that the engine is “platform agnostic,” meaning that it could someday power missiles, aircraft or even spaceplanes bound for orbit.
Under a U.S. Department of Energy program, researchers at Purdue and Argonne National Laboratory developed a hydrogen-air RDE with innovative nozzle guide vanes for gas turbine power generation. Their plans call for retrofitting a Rolls-Royce turbine with an RDE this year.
And last fall the Defense Advanced Research Projects Agency (DARPA) selected RTX, another big aerospace group, to develop a next-gen missile called Gambit, with RTX’s Pratt & Whitney Military Engines unit developing the air-breathing RDE unit that’s to power it. “RDE is a disruptive, game-changing technology,” says Pratt & Whitney’s Military Engines president Jill Albertelli, a veteran aerospace engineer who has a team of 50 people working on the project. “The RDE has attributes that make for a pretty great missile. It has clear potential to provide high-performance, high-efficiency, long-range propulsion in a compact, cost-effective package.”
Yet despite so many fresh advances, Perkins stresses that the RDE field is still in its experimental stage: “any combustion is an unsteady process” that includes instabilities, turbulence and nonlinear behaviors, so gaining greater insight into the underlying physical and chemical forces that prevail under such extreme conditions is critical. Beyond “getting the fuel mixture just right,” many of the current and hoped-for innovations boil down to better understanding the chemical kinetics, the fluid dynamics and thermoacoustics of RDE combustors, which are said to ring like bells during firing. For instance, innovations should derive from enhanced laser diagnostics and optical techniques to map at smaller scales the convoluted flow fields that form as the submicron-thick leading edges of supersonic shockwaves shear through fuel mixtures. Making such exacting measurements of velocity, pressure, density, temperature and chemical species through fireproof quartz viewing windows in laboratory combustion chambers can provide the real-world observations needed to validate the complex computer simulations that help guide research and development.
One development that’s accelerated the critical experimental “build-and-burn” series testing and redesign that powers RDE progress in recent decades has been the arrival of more affordable ultra-high-speed cameras that can visualize events that occur at a hundredth of a microsecond, says Jiro Kasahara, the Nagoya University propulsion researcher who led the team that first operated an RDE in space. More recently NASA Marshall’s development of stronger heat-resistant metal alloys and its innovative use of laser 3D-printing systems has also helped drive progress by enabling high-performance test hardware to be built more quickly and at lower cost than by previous methods.
Pratt & Whitney, which is building on the two decades of detonation engine research it conducted in collaboration with the pioneering propulsion research groups at the Air Force Research Laboratory, is closing in on a fieldable RDE rocket motor. The latest ground tests of a prototype RDE motor “look really good,” Albertelli says. “The results are validating the model architecture that our team has developed during five years of theory and analysis of the needed speeds, flows and pressures.” The next step is to integrate the motor into a vehicle airframe and start ground testing.
Even though the theoretical concepts have been around for nearly 90 years, RDEs are still in their early days. “We really don’t know what an optimized RDE even would look like yet,” NASA’s Perkins says. “I can count the people with significant expertise in this area on 10 fingers.” Nevertheless, rising excitement is spurring RDE startup launches. Alexis Harroun, one of Steve Heister’s recent Purdue graduate students, just founded an RDE startup, Juno Propulsion, with help from an NSF small-business-incubator program. “We just hired our first employee,” she says.
Meanwhile four-year-old, Houston-based Venus Aerospace is busy testing RDE rockets that produce 4,000 pounds of thrust and more, says CTO and co-founder Andrew Duggleby. The company’s 100 employees aim to fly a Mach 5 drone powered by their RDE design to provide hypersonic-flight-testing services to defense contractors. Venus’ RDE rocket augments its burn by drawing in outside air into the combustor while adding extra fuel to the mix, creating a sort of supercharged afterburner effect that boosts speeds sufficiently to start up a ramjet stage.
In January Venus and NASA Marshall announced they had partnered on a cooled RDE rocket featuring a company-designed injector that operated for four minutes of hot-fire testing. “We’ll build a combustor design out of copper, which costs $20,000 and lasts for two seconds,” Duggleby says. “Then we iterate, iterate, iterate. And when we find a design we love, we’ll 3D-print it from NASA’s refractory alloys with integral cooling channels and test it at higher thrusts and for longer durations.”
As global interest in RDE takes off, aerospace engineering labs in the U.S., Japan, China, Europe and elsewhere comprise an emerging international R&D community, though it’s one that includes a large classified component. In any case, nearly 80 research papers on detonation propulsion were presented at January’s SciTech Forum of the American Institute of Aeronautics and Astronautics in Orlando, Fla. Who knows? Maybe rocketry’s next Goddard was there.