Last week, the Defense Advanced Research Projects Agency (DARPA) quietly unveiled a new high-speed missile program called Gambit. The program is meant to leverage a novel method of propulsion that could have far-reaching implications not just in terms of weapons development, but for high-speed aircraft and even in how the Navy’s warships are powered.
This propulsion system, known as a rotating detonation engine (RDE), has the potential to be lighter than existing jet engines while offering a significant boost in power output, range, and fuel efficiency.
The Gambit missile is just one of a number of programs placing a renewed focus on RDE technology, though for the most part, these systems have managed to fly under the media’s radar. That is, except for Aviation Week & Space Technology Defense Editor Steve Trimble, who has covered these recent developments at length. Trimble was kind enough to discuss that work with us as we sought to better understand just how big a deal this technology could be.
Rotating Detonation Engines may not be common in discussion today, but amid the ongoing hypersonic arms race and America’s renewed focus on deterring near-peers, this technology could help offset a number of tactical and strategic advantages presented by America’s opponents in places like Europe and the Pacific…
…And it may be closer than you think.
A new kind of propulsion system
Rotating detonation engines have been the subject of theory and speculation for decades, but have yet to cross the barrier between theory and practical application.
In theory, a rotating detonation engine promises to be much more efficient than traditional jet engines, potentially providing missile applications a serious boost in range and speed. That could also mean fielding smaller weapons capable of achieving the same speeds and ranges as today’s missiles.
In aircraft applications like jet fighters, rotating detonation engines could offer similar benefits to missiles in terms of range and speed, while potentially reducing maintenance requirements. Fighters, in particular, rely on afterburners, which effectively firehose fuel into the engine’s exhaust stream for added thrust, which, you can imagine, rapidly depletes fuel stores and reduces the fighter’s range. RDEs could potentially allow for a similar boost in thrust with a dramatically reduced fuel penalty.
But where this technology could be the most useful is in powering the Navy’s future non-nuclear surface vessels, providing increased power production, range, and speed while having a seriously beneficial impact on the Navy’s budgetary bottom line.
Harnessing the power of detonation
The concept behind rotation detonation engines dates back to the 1950s. In the United States, Arthur Nicholls, a professor emeritus of aerospace engineering at the University of Michigan, was among the first to attempt to develop a working RDE design.
In some ways, a Rotating Detonation Engine is an extension of the concept behind pulse detonation engines (PDEs), which are, in themselves, an extension of pulsejets. That might seem confusing (and maybe it is), but we’ll break it down.
Pulsejet engines work by mixing air and fuel within a combustion chamber and then igniting the mixture to fire out of a nozzle in rapid pulses, rather than under consistent combustion as you might find in other jet engines.
In pulsejet engines, as in nearly all combustion engines, igniting and burning the air/fuel mixture is called deflagration, which basically means heating a substance until it burns away rapidly, but at subsonic speeds.
A pulse detonation engine works similarly, but instead of leveraging deflagration, it uses detonation. At a fundamental level, detonation is a lot like it sounds: an explosion.
While deflagration speaks to the ignition and subsonic burning of the air/fuel mixture, detonation is supersonic. When the air and fuel are mixed in a pulse detonation engine, they’re ignited, creating deflagration like in any other combustion engine. However, within the longer exhaust tube, a powerful pressure wave compresses the unburnt fuel ahead of the ignition, heating it above ignition temperature in what is known as the deflagration-to-detonation transition (DDT). In other words, rather than burning through the fuel rapidly, it detonates, producing more thrust from the same amount of fuel; an explosion, rather than a rapid burn.
“The detonation process is a more rapid and efficient extraction of energy from your fuel from a thermodynamic standpoint when compared to deflagration,” Dee Howard endowed professor of hypersonic and aerospace engineering, Dr. Chris Combs, told Sandboxx News.
The detonations still occur in pulses, like in a pulsejet, but a pulse detonation engine is capable of propelling a vehicle to higher speeds, believed to be around Mach 5. Because detonation releases more energy than deflagration, detonation engines are more efficient â€” producing more thrust with less fuel, allowing for lighter loads and greater ranges.
This detonation shockwave travels significantly faster than the deflagration wave leveraged by today’s jet engines, as Trimble explained: up to 2,000 meters per second (4,475 miles per hour) compared to 10 meters per second from deflagration.
In May of 2008, the Air Force Research Laboratory made history by building the world’s first crewed pulse detonation-powered aircraft, using a Scaled Composites home-build plane called the Long-EZ. The unusual franken-plane managed a speed higher than 120 miles per hour during its test flight, with test pilot Pete Siebold at the stick, and reached altitudes of between 60 and 100 feet.
“This is a potential game-changer in terms of fuel efficiency,” The AFRL’s Propulsion Directorate’s Fred Schauer said of the PDE powering Long-EZ.
“For comparison, if we had operated this same engine with conventional combustion we would have made less than a third of the thrust for the same fuel burn. In comparison to traditional engines, fuel savings of 5 to 20 percent could be expected.”
The Air Force assessed at the time that improvements to their PDE engine could eventually propel aircraft to speeds beyond Mach 4, and higher if combined with other advanced propulsion systems like scramjets. A rotation detonation engine could be even more effective, but many within the academic and engineering communities questioned whether such an engine could ever actually be built. That is, until recently.
The Rotation Detonation Engine emerges
A rotating detonation engine takes the PDE concept to the next level. Rather than having the detonation wave travel out the back of the aircraft as propulsion, it travels around a circular channel within the engine itself.
Fuel and oxidizers are added to the channel through small holes, which are then struck and ignited by the rapidly circling detonation wave. The result is an engine that produces continuous thrust, rather than thrust in pulses, while still offering the improved efficiency of a detonation engine. Many rotation detonation engines have more than one detonation wave circling the chamber at the same time.
As Trimble explains, RDEs see pressure increase during detonation, whereas traditional jet engines see a total pressure loss during combustion. As a result, RDEs offer greater efficiency. In fact, rotating detonation engines are even more efficient than pulse detonation engines, which need the combustion chamber to be purged and refilled for each pulse.
“In theory, RDE is a bit like the leap from turbojets to turbofans in the 1960s, but for high-supersonic vehicles. It should give you a big jump in specific impulse (aka fuel efficiency), and if you can figure out how to package it in a way that doesnâ€™t make things significantly heavier or less aerodynamic, you should be able to get a nice range boost out of it,” Trimble explained.
In 2020, a team out of the University of Central Florida, working with the Rotating Detonation Rocket Engine Program at the Air Force Research Laboratory, successfully built and tested the world’s first working RDE that continued firing until its fuel was cut off, effectively proving the concept was possible. The three-inch copper test rig developed by the team successfully produced 200 pounds of thrust in laboratory conditions.
Since then, several other programs have followed suit, with noted engine manufacturer Pratt and Whitney among those leading the charge.
A new generation of high-speed, long-range weapons
On July 18, DARPA released a Special Notice about their new Gambit Missile program, announcing a “Proposer’s Day” for firms to get more information about the effort and its aims. Within the notice, DARPA included a description of the program and its objectives, as well as their anticipated timeline, from inception to flight test.
“The objective of the Gambit program is to develop and demonstrate a novel Rotating Detonation Engine (RDE) propulsion system that enables a mass-producible, low-cost, high-supersonic, long-range weapon for air-to-ground strike in an anti-access/area denial (A2AD) environment.”
The program will be conducted in two 18-month phases. The first will entail competitors completing their preliminary designs with some limited testing, while the second would finalize designs and culminate in full-scale flight tests of an RDE system.
While the release offers scant details on the overarching goals of Gambit, some of the language within the announcement point toward specific challenges America’s defense apparatus currently finds itself facing. The reference to Gambit’s use in an “anti-access/area denial (A2AD) environment” could pertain to anywhere American forces are squaring off against a near-peer adversary. But there’s one such environment that has been the focus of multiple Defense efforts in recent years: the 1,000-mile-plus area denial bubble extending from Chinese shores thanks to a growing array of anti-ship weapon systems.
America’s carrier-based fighters, the F-35C and F/A-18 Super Hornet, each have a combat radius of fewer than 650 miles, which would mean having to sail carriers into harm’s way to launch combat sorties without longer-range munitions.
The United States obviously has the capability of fielding air-launched missiles with significant range, but it isn’t as simple as mounting a massive rocket under the Super Hornet’s wing, as Trimble points out. The size of these weapons matters a great deal, which is why RDE engines, with their improved efficiency and smaller mass, could be a game changer.
“That is probably most useful for the US Navy, which needs to find a way to equip fighters with long-range, high-speed (Mach 4-6) cruise missiles that are small enough to squeeze onto an aircraft carrierâ€™s weapons elevators and land back on the carrier under a fighterâ€™s wing without slamming into the deck.”
According to the Air Force Research Lab, RDE technology could make high-speed weapons much more affordable, which is of particular import following a recent Defense Department analysis that indicated the hypersonic (Mach 5+) weapons in development for the Air Force may cost as much as $106 million each.
And, according to a list of efforts supported by the Pentagon’s High-Performance Computing Modernization program in 2022, the Air Force Research Lab has begun development on at least three RDE weapons or demonstrators.
One aims to field a liquid-fueled rotating detonation scramjet that will power an air-to-surface missile that can be carried internally by 5th-generation fighters. Another will leverage solid fuel for an air-to-air missile, and a third effort aims to develop a vehicle for free jet testing on the ground.
RDE technology could eventually also lead to smaller weapons that offer the same range and speed as today’s missiles, allowing stealth aircraft like the F-35 to carry more munitions inside their internal weapons bays. Likewise, missiles of the same size as today’s could fly further faster, which has far-reaching benefits in both air-to-air and air-to-surface operations.
Rotation Detonation Engines could power fighters and even the Navy’s warships
Among the weapon-oriented programs being developed by the Air Force Research Laboratory is another RDE enterprise that could offer America’s fighters a big boost in range and speed: one that could be used in place of a fighter’s afterburner.
An afterburner effectively combines the remaining oxygen leaving the jet engine with more fuel by spraying fuel directly into the outflow of the exhaust. Needless to say, this method of increasing thrust takes a heavy toll on the aircraft’s fuel stores, forcing pilots to choose between speed and range or loiter time.
A rotation detonation engine afterburner could provide an increase in thrust while leveraging the design’s inherent efficiency, providing the same gains for less fuel expended.
In the longer term, air-breathing RDEs could even find their way into the fuselage of an aircraft as the primary means of propulsion. But not all of the potential applications for RDEs are in the sky. One of the most promising may actually be out at sea.
While the Navy’s aircraft carriers and submarines are famously nuclear-powered, the rest of the fleet still runs on good old-fashioned F-76 marine diesel fuel â€” an estimated 86 million barrels of it in 2016 alone. So it may come as little surprise that the Navy has been very interested in this approach to high-efficiency propulsion. In fact, the Navy filed its own patent for a “rotary detonation engine” as far back as 1982.
According to the Navy in 2012, rotation detonation engines could increase a warship’s thrust by 10% and reduce fuel consumption by 25%, giving them more speed and range for the fuel expended. In 2012, that kind of improvement was projected to result in savings of $300 to $400 million per year, which equates to $387 million to $516 million when adjusted to today’s dollars.
The truth is, Rotation Detonation Engines could help fighters fly further, missiles fly faster, ships sail longer, and even rocket launches become cheaper. There aren’t many places in America’s defense apparatus this forward-reaching tech couldn’t benefit.
And while for many years the question surrounding RDEs was always if, increasingly, it now appears to be when.