Defense aviation company Hermeus has announced the development of Ramjet-X, its next-generation ramjet propulsion test vehicle capable of carrying test payloads to high Mach speeds.

The company said Ramjet-X, designed to launch mid-flight from a carrier aircraft, offers the key advantages of competitive high-Mach test platforms while keeping costs low enough for a wider range of mission profiles.

Pre-flight testing of Ramjet-X is already underway, with its first official prototype test flights expected in 2027.

Ramjet-X Will Launch Mid-Flight from Quarterhorse

Most conventional jet engines can’t achieve supersonic speeds without dumping raw fuel into an afterburner and literally rocketing past the sound barrier. To travel at high Mach speeds without relying on rocket-style propulsion, engineers have turned to ramjet engines.

While ramjets are effective, they rely on the high-speed influx of air to operate. This limitation means aircraft equipped with this type of propulsion must be propelled to higher speeds by another craft or booster rocket before they can engage.

In an email to The Debrief, a Hermeus spokesperson explained that current options often rely on “large and expensive” boosters that are discarded after each test. Conversely, Ramjet-X relies on the company’s reusable aircraft to reach launch velocity.

“Ramjet-X is designed to be carried by Quarterhorse, which provides the speed and altitude required for deployment,” the company told The Debrief.

The company could not disclose what speed Quarterhorse must achieve before it can launch Ramjet-X. However, the spokesperson did note that the launch craft is designed to return to home base for future missions.

When asked about Ramjet-X’s specific operating velocity once it disengages from Quarterhorse and engages its ramjet propulsion system, the company could only say the vehicle will operate at “High-Mach speeds.”

Conventional ramjets use a combination of high-velocity air and fuel to achieve extreme velocities. Hermeus said they cannot disclose specifics about Rocket-X’s fuel requirements.

Mission Profiles and Repeatable Architecture

Because of the sensitive nature of potential future missions, Hermeus was cautious about sharing specifics of any planned missions. The company also couldn’t provide technical details on what a typical mission profile might look like.

However, a Hermeus spokesperson told The Debrief, the vehicle’s architecture “allows customers and partners to integrate payloads, sensors, communications and computer systems, autonomy, material samples, propulsion technologies, and other experimental hardware and software onto Ramjet-X for testing in representative sustained high-Mach conditions.”

Hermeus also highlighted the potential cost savings of a “repeatable platform, launch architecture, and flight-test infrastructure,” designed to dramatically lower costs without sacrificing versatility or reliability.

“Instead of relying on a large dedicated expendable booster to accelerate the test vehicle into its ramjet operating regime, the launch aircraft provides much of that energy and then returns to fly again,” the company told The Debrief. “This architecture allows Hermeus to fly more by reducing the cost of expendable launch hardware.”

“High-speed flight testing is extremely expensive, and you don’t get many shots at it,” Hermeus CEO Zach Shore explained.

“Another Order of Magnitude Reduction”

Hermeus said specific unit costs are not currently available. However, they said a projected cost per minute of high-Mach test time “is in the low $100k’s per minute.” For comparison, the company highlighted the historic and current costs of high-Mach flight testing.

“Most of the below comparisons are rocket-based but will give you an idea how costly high-Mach tests are and why Ramjet-X will fundamentally change the equation,” Hermeus told The Debrief.

Older examples include NASA’s X-43, which achieved around 21 seconds of high-Mach flight, and the Air Force’s X-51, which achieved around 350 seconds. Hermeus said both multi-year efforts cost around $450 million each in today’s dollars.

Current options include rocket-powered test flights aboard Rocket Lab’s HASTE, which Hermeus said run $10-20 million each, “not including the expendable cruise or glide vehicles.” The company also pointed to Stratolaunch Talon-A missions.

“Even though fully reusable, (they) are $10-12m per flight for a few min of on-condition time,” the company told The Debrief.

Conversely, Hermeus said its current projections of less than a million per minute offer potential customers “another order of magnitude reduction in the problem that matters most.”

“Ramjet-X is being designed around simplified manufacturing, rapid production, and expendability in meaningful quantities rather than the traditional model of building a small number of highly bespoke high-speed test articles as was done in other high-speed airbreathing flight test programs like X-43 and X-51,” the company told The Debrief. 

Development of Ramjet-X is already well underway

When detailing the newly announced Ramjet-X’s benefits, Shore noted that their system gives them a way to place new systems into sustained high-Mach environments “without building an expensive one-off test program every time.”

“Bringing speed, sustained flight, and scalability all into one system lowers the barriers to gathering extremely valuable data from these conditions and accelerates programs for both us and our customers,” the Hermeus CEO explained.

When announcing Ramjet-X, Hermeus said the vehicle’s ramjet engine is already undergoing extensive testing at its Florida-based HEAT facility. Future tests will occur at the company’s California headquarters, with the first flight tests planned for 2027. 

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

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