Nagoya University scientists working to bridge the supersonic-to-hypersonic gap report successful combustion in a hypersonic scramjet engine at speeds below Mach 5 using a specially designed igniter called an RD torch.

The researchers behind the achievement said their successful tests highlight the RD torch’s potential for practical use in future scramjet-powered airbreathing hypersonic engines.

Scramjets Face Challenges in Bridging the Supersonic-to-Hypersonic Gap

In their study, project leader Professor Jiro Kasahara and colleagues explain that hypersonic engines capable of propelling aircraft above Mach 5, or five times the speed of sound, “have been actively researched for the development of high-speed transport systems.” Among the most noteworthy designs developed in this pursuit is the scramjet engine.

While tests have shown they can reach these speeds, scramjets fail to maintain ignition below Mach 5. As a result, Professor Kasahara and colleagues note that “other propulsion systems are needed to accelerate a vehicle to those speeds.”

Turbo-ramjet engines have offered some support. However, these airbreathing engines top out at speeds approaching Mach 3-4. The study authors note that this discrepancy has resulted in “a gap with scramjet engines.”

To bridge this gap, the research team decided that rather than increasing the upper speed limit of the turbo ramjet, they would instead explore lowering the “operating limit of scramjet engines to Mach 3–4.”

According to the team, the primary challenge in reducing scramjet speeds below Mach 5 is the corresponding reduction in the total temperature of the airflow entering the engine. Consequently, they explain, “ignition becomes more difficult.” In fact, they concede, “autoignition is not expected” at the sub-Mach 5 speeds they were targeting. This meant the team had to find a way to improve igniter performance.

RD Torch “A Promising Candidate” for Scramjet Ignition

When selecting an ignition enhancer, the team noted that previous efforts have explored solutions ranging from spark plugs, glow plugs, and pulse detonators to lasers, plasma torch igniters, and even burned-gas (deflagration) torches. However, the authors observed that most previous studies focused on “higher Mach numbers than those targeted here.” As a result, they add, ignition performance in a scramjet engine under low-flight-Mach, low-temperature conditions “remains unclear.”

The team evaluated a cylindrical rotating detonation combustor (RDC). Unlike other electrically based solutions such as spark plugs or glow plugs, the team described the RDC as “a combustor in which detonation waves continuously propagate along the inner wall surface to burn the fuel–oxidizer mixture.”

The researchers explained that detonation waves are supersonic combustion waves initiated by shock waves inside the scramjet’s combustion chamber. Although the system has been studied as a rocket engine, to achieve their results, the team used what they called a rotating detonation torch (RD torch), comparing the design to conventional deflagration torches, which initiate ignition by “uniformly burning the fuel and oxidizer throughout the combustion chamber” and then continuously injecting the burned gas into the scramjet combustor.

“The RD torch, like conventional deflagration torches, burns fuel and oxidizer in the combustor and continuously discharges combustion gases into the cavity and is therefore also classified as an active ignition device,” the team explained, adding that “we consider the RD torch to be a promising candidate for use as an igniter in scramjet engines.”

“Ignition and Flameholding of the Main H2 Were Successfully Achieved”

Before conducting the tests, the Nagoya University team set out three primary objectives. First, they wanted to design a custom RD torch “capable of operating at a mass flow rate comparable to that of conventional deflagration torches” and use that design to “investigate the combustion conditions in the RD torch under supersonic flow.” Second, they planned to examine the detonation wave’s propagation characteristics within the RD torch, and third, they hoped to achieve ignition and flameholding under conditions equivalent to flight Mach numbers from 3 to 4.

After completing their testbed RD torch and thus the study’s first objective, the researchers conducted a series of combustion experiments. Critically, they performed these experiments in the scramjet model combustor and a vacuum chamber model.

According to their published results, the team’s RD torch, designed to bridge the supersonic to hypersonic gap, “successfully achieved continuous injection of burned gas into the cavity, with combustion nearly completed even during main H2 combustion.”

“Based on the observed increases in flame height and width in the flame chemiluminescence images, ignition and flameholding of the main H2 were successfully achieved under simulated flight Mach 3–4 conditions, with the RD torch operating in detonation mode,” they write. “Notably, the mainstream flow in the scramjet model combustor remained supersonic.”

The team presents these results as “demonstrating robust torch operation,” adding that “the RD torch functioned as intended.”

When discussing the success of the RD torch in their approach, the Nagoya University team said their results “confirm successful supersonic combustion and demonstrate the RD torch’s ability to reliably ignite and sustain main H2 combustion under low-Mach scramjet conditions, highlighting its potential for practical application in future airbreathing hypersonic engines.”

The study “Experimental Study of a Cylindrical Rotating Detonation Combustor for Scramjet Ignition” was published in the Journal of Propulsion and Power.

Christopher Plain is a Science Fiction and Fantasy novelist and has spent the last six years as Associate News Editor and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him at christopher@thedebrief.org

Days after successfully flying its Quarterhorse unmanned aircraft at supersonic speeds, an industry-first defense aviation company, Hermeus, has received the largest-ever contract awarded by the Defense Innovation Unit (DIU) to demonstrate high-Mach flight combined with high-Mach payload release.

The new $ 159 million award, which increases the original contract to a total of $219 million, will allow Hermeus’ engineers to expand on previous high-Mach test flight milestones in direct partnership with the U.S. Air Force and the U.S. Navy, with the goal of providing the American warfighter with the most advanced arsenal capable of delivering munitions and payloads around the world at record-setting speeds.

“By delivering flight-ready aircraft and demonstrating payload release at speed, we will prove this technology can create a decisive military advantage on a timeline that matters,” explained Hermeus CEO and co-founder, AJ Piplica. “Our focus is on providing the Air Force and Navy with the validated data they need to transition these platforms into the future force.”

High Mach Payload Release: “A Game Changing Warfighting Capability”

When announcing the new DIU award, Hermeus executives noted that the program will include a series of “intensive flight tests.” The first is expected later this year, while the final flights are expected sometime in 2027.

Although few specifics were provided, the company said the flights will “prove the aircraft can maintain high speeds and expand its performance limits” while also demonstrating the successful carry and release of payloads at high velocity. The company also noted that all flight data gathered during the development and testing phases “will directly inform service experimentation and future acquisition decisions.”

Although the U.S. Military has several aircraft capable of exceeding the speed of sound, and several more hypersonic (exceeding five times the speed of sound) aircraft and missile prototypes, delivering payloads from high-Mach aircraft has remained a challenge. However, some military experts argue that the U.S. would benefit greatly from the capability.

When discussing the need for high-speed payload release, DIU Military Deputy Major General Joe “Solo” Kunkel said that the largest problems the military is facing in the future are “generally around time and distance,” and the associated challenges with getting to distant locations “in a timeframe that matters.”

“I care about the warfighting capability, providing advantage to our partners and us in the fight, and making sure that we can walk into every fight with swagger,” General Kunkel said, adding that mass producing the technology developed by Hermeus could move it from a test platform to a “game-changing warfighting capability.”

“I think we found a significant number of use cases where it can be used as a weapon,” the General added. 

Moving High-Mach Capability Out of the Lab

When discussing the company’s approach that convinced the DIU to offer its largest award in the organization’s history, including what they describe as “reclaiming the lost art of rapid iterative prototyping to build the fastest aircraft in the world today,” Hermeus noted that directly partnering with the USAF and USN allows their engineers to align tests with future mission needs and shorten the path from testing to deployment.

“By prioritizing relentless hardware iteration, we deliver high-speed systems at the pace of the modern battlefield,” the company’s statement explained. “We work with the Department of War to provide the high-speed capabilities our nation and its allies need to maintain a durable, asymmetric advantage.”

When discussing the significance of the upcoming tests and the overall need for high-Mach flight payload deployment, including the benefit of the performance data, the company CEO heralded the DIU’s award, noting that the financing will help them move their successful demonstrations into real-world high-Mach platforms.

“This program is about moving high-Mach capability out of the lab and into an operationally relevant environment,” Piplica said. 

Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.