Less than two years after revealing ambitious plans to develop a 20-passenger hypersonic aircraft by the end of the decade, startup Hermeus has already confounded the skeptics by completing development testing of key subscale elements of its propulsion system at conditions representative of speeds up to Mach 5.
Now, with its work attracting high-speed research and study contracts from NASA and the U.S. Air Force, the growing company is starting construction of a bigger test facility to evaluate a larger flight-scale version of its turbine-based combined-cycle (TBCC) engine. Testing is expected to begin at Hermeus’ site near Atlanta later this year and is intended to pave the way for the follow-on development of a single-engine flight-test demonstrator. Last November, the company closed a $16 million funding round led by venture capital group Canaan Partners.
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“The kind of major technical milestone that we’ve set out to achieve with our Series A funding is to demonstrate this physical-scale flight engine on the ground,” Hermeus CEO AJ Piplica says. “Basically, we will complete the engine development and demonstrate that it works the way we expect it to work. The next step after that will be to go build the flight vehicle.”
Progress toward the Hermeus demonstrator comes as other high-Mach civil supersonic and hypersonic projects take shape. These include a Mach 5 airliner concept unveiled by Boeing in 2018 as well as parallel studies of a Mach-3-plus transport under study by Virgin Galactic and Rolls-Royce in association with NASA. The latest to reveal its ambitions in this high-speed arena is Aerion Supersonic. Currently developing its Mach 1.4 AS2 business jet, Aerion has set its sights on a follow-on 50-seat Mach 4-5 airliner with trans-Pacific range.
The Hermeus concept is founded on the use of a TBCC engine incorporating a ramjet—rather than a supersonic combustion ramjet, or scramjet. Initial subscale prototype tests in 2019-20 were based on a 290-lb.-thrust, off-the-shelf TJ100 turbojet supplied by Czech-based high-speed turbine maker PBS. The combined-cycle elements include an in-house-developed precooler and a ramjet. The turbojet is designed to operate from a standing start to Mach 3.3, while the ramjet operates over the transition range from approximately Mach 2.8 to above Mach 3 and then onward to power the vehicle to Mach 5.
Following sea-level static tests at the first modest Hermeus facility (AW&ST April 6-19, 2020, p. 46)—a converted shipping container—the TBCC was evaluated at Purdue University’s Zucrow Labs direct-connect site in West Lafayette, Indiana. “In that test campaign, we were able to push the turbojet up to about Mach 3.2 with our precooler,” Piplica says. “Then we configured the test article into just a pure ramjet mode and ran it up into the Mach 4-5 range.”
“We were able to show about a half of a Mach number overlap between the two systems, so we took the turbojet up to Mach 3.2, and we started the ramjet down at about Mach 2.7,” Piplica says. “That gave us good confidence in the overlap between the operability of the individual propulsion systems. Now the key question is: Can you go do the big transient maneuver of switching from one to the other and back?” he says, referencing upcoming mode-transition testing on the larger engine.
Another key challenge to be answered in upcoming integrated propulsion system tests will be the ability of the design to thermally isolate the turbojet from the heat of the ramjet during high-Mach cruise with inlet temperatures around 1,800F.
In preparation for these next tests, “we’re getting back into building hardware,” Piplica says. “We’re building up our new 5,000-ft.2 propulsion test facility at DeKalb-Peachtree Airport [in Gerogia], and that should be opening sometime in early summer. It will be getting into ground testing of that next engine shortly thereafter.”
Based on the core of a General Electric J85-21—a late-generation 3,500-lb.-dry-thrust variant with an additional compressor stage—the new TBCC engine will have 10 times the mass flow rate of the prototype. “We’re also developing a ram burner and a nozzle for the back of it as well as a bypass system, an inlet and precooler,” Piplica says. “So it’s basically the same architecture, but it’s now at a scale that we can actually put into a flight vehicle and go fly. So that’s what sized our test facility.”
Following removal of the former Northrop F-5E/F engine’s standard afterburner and integration of the TBCC bypass, precooler and other systems, the company plans to conduct sea-level static testing later this year. “Then we’ll do another direct-connect test campaign at a location that is yet to be determined,” Piplica says. “We have a couple of plans but that’ll be either toward the end of this year or early next year when we get to that.”
Meanwhile, Hermeus is also working on the design of the flight demonstrator. Although Piplica says the airframe configuration is largely driven by the engine, the vehicle design is “coming along pretty well,” he adds. “We’re working on the preliminary design of it right now, and we’ll be sharing more details about the vehicle later this year.”
The demonstrator is targeted to achieve Mach 5 with a reusable air-breathing vehicle. “Those are two things that have never been done before,” Piplica says. “We really want to be very ruthless in our requirements, such that we don’t worry too much about cruise range and things like that. It’s really about flight-testing the engine, and that’s it.”
Flight testing with the uncrewed vehicle will be risky, but the approach should reap rewards, Piplica says. “We can go through envelope expansion because we’ve got a way to get the vehicle back,” he explains. “You can fly a hypersonic vehicle with blue lines and red lines [operating limits] on the propulsion system and abort, recycle and land. So you’ll learn, iterate and go fly again the next day. That’s a very different approach to the development of these types of propulsion systems and one that has never been tried before.”
Hermeus plans to outline the target year for flight tests when it unveils the demonstrator design, but Piplica says the vehicle is “still a few years away.”
The company’s plan is to develop multiple demonstrators in a way similar to SpaceX’s Starship program. “We’ve set ourselves up this way to be able to push some of that risk to flight test,” Piplica tells Aviation Week. “If we can build a vehicle for less than the cost of going into a wind tunnel, then if we put a hole in the desert, we’re not losing that much.
“We have to be hardware-rich,” he continues. “I’m very jealous of what Elon [Musk] gets to do at Boca Chica [SpaceX’s Texas test site]. I don’t think we’ll be quite that aggressive, but the process will be a lot closer to that than your traditional aerospace development program.”
In the midst of Hermeus’ development work on the aforementioned propulsion system and management of its expansion through the COVID-19 pandemic, the company is also engaged with the U.S. Air Force’s AFWerx innovation arm on a study involving the Presidential and Executive Airlift Directorate. The work focuses on integrating the startup’s hypersonic design with Air Force requirements for high-speed mobility—potentially including the eventual development of an Air Force One variant capable of a high-Mach cruise speed. “We’ve completed our modeling and simulation updates, and we’re now working through a set of trade studies for some kind of specific requirements for their aircraft mission systems, functionality, cabin layouts and so on,” Piplica says. “We are very excited to see AFWerx potentially pursuing a supersonic-hypersonic program.”
The company partially attributes its recent success in gaining traction with the Air Force to the pandemic. “We were able to get virtual face time with a lot of senior leadership and folks who would otherwise have been a lot more difficult to get a hold of,” Piplica says. “So we’ve been able to drive a much faster rate of understanding of some of the defense and national security use cases that we have in our road map.”
Hermeus is also starting high-speed flight study work with NASA as part of the agency’s Hypersonic Technology Project. The collaboration was outlined in a Space Act Agreement signed in March. The two partners will work in tandem on the development of aircraft concepts of operation, including analysis of high-Mach thrust performance, thermal management, integrated power generation and cabin systems.
In particular, the NASA study will feed into the design of the final multiengined airliner project. Getting to that end stage “relies on iterative hardware development,” Piplica says. “There’s all these technical risks that go into developing a hypersonic passenger aircraft, whether it’s propulsion, aerodynamics, thermal control, environmental control, or stability and control. If we tried to bite off all those risks in one chunk, that’s untenable. So we’ve taken all of those technical challenges and broken them up into a set of vehicles that we will build over the next decade.”
The test vehicles will be progressively larger in scale and more capable in terms of payload and range. At the same time, Hermeus plans to establish close links with civil regulators. “The type of vehicles that we’re going to be building are very new, so there’s going to be a lot of learning on both sides,” Piplica says. “For us to be successful as an OEM, we’ve got to have a very strong, mutually trusting relationship with the FAA. They need to understand our business processes, our engineering processes, our safety policies, our safety culture, as well as the technology.”
“Of course, we’ve got to work our way up to the point where we can actually certify an aircraft,” Piplica says. “The first aircraft we build is not intended to be certified, and I doubt the second aircraft will be either. So having those kinds of full life-cycle iterations along the way is critical to this concept being able to get over that certification hump. That hump lies in the way of frankly every commercial air vehicle that’s on the road map today. Whether it’s an [electric vertical-takeoff-and-landing aircraft], hybrid, hydrogen or supersonic aircraft—we all have the same challenges ahead of us. The key is: how do you approach it from a technical perspective with the regulators, and then how do you bridge the financial gap from here to there?”
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