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.”
RD torch and cavity region of the scramjet model combustor. Image Credit: Nagoya University, Kasahara et al.
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.”
Flame chemiluminescence images captured across the entire scramjet model combustor in test S-13. Image Credit: Nagoya University, Kasahara et al.
“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