Hermeus’ audacious plan to build a passenger plane able to travel at Mach 5 is a longshot, but it’s won Pentagon backing.
By Jeremy Bogaisky, Forbes Staff
Earlier this month, a curved aluminum skeleton 40 feet long sat waiting in Hermeus’ cavernous Atlanta factory. It was the prototype of a drone called Quarterhorse. It will never fly. Instead, it’s scheduled for ground-testing starting in September. Hermeus CEO AJ Piplica and his cofounders believe it’s the first step on a long road to an audacious goal: building a plane capable of carrying 20 passengers at Hypersonic speed — five times faster than sound, or 3,850 miles per hour.
Imagine New York to Paris in 90 minutes. Quite an upgrade from the seven-and-a-half hours of a commercial flight today.
It’s been 20 years since the last flight of Concorde, the groundbreaking but money-losing supersonic jetliner. So far, none of a parade of startups that have tried to bring back supersonic travel have left the ground. Piplica acknowledges that Hermeus is taking on even harder technical challenges in building an airliner that can fly for long periods of time in the intense heat and weird dynamics created the farther you climb above the speed of sound. But he says that’s not the biggest problem. “The business challenges are actually the real hard ones,” Piplica, 35, tells Forbes. “You’re not just going to raise billions of dollars to develop a passenger aircraft.”
Piplica’s solution: to prove the technology largely on the Pentagon’s dime by developing smaller hypersonic drones, taking advantage of Washington’s urgency to catch up to Russia and China in fielding maneuverable hypersonic missiles.
Quarterhorse is intended to serve as a reusable testbed to subject materials and equipment to high-speed conditions. Hermeus won a $30 million contract from the U.S. Air Force that will go toward building and flying three versions of the aircraft. First flight is slated for 2024, and Piplica says he expects the total development cost to be under $100 million.
A larger second drone, Darkhorse, which Hermeus hopes to start flight-testing in 2026, is also planned to be used as a test vehicle, as well as for long-range surveillance and strike.
If it works out, by the time Hermeus gets to building Halcyon, its planned airliner, Piplica says they’ll have successively built six to ten prototypes of Quarterhorse and Darkhorse and found solutions to many of the technical unknowns of high-speed flight. After all, Piplica says, humans have only about 30 minutes of experience over Mach 4 with so-called air-breathing aircraft, which use the oxygen around them for fuel combustion instead of carrying it onboard as rockets do, leaving less room for payload like passengers. When all the testing is done, Piplica expects to have a fleet of hypersonic drones earning solid revenue performing Defense Department missions.
At that point, he says, Hermeus will “have built a strong enough financial foundation to actually invest in making the transition to Halcyon without a ridiculous amount of private capital.”
On the strength of that business plan, Hermeus has raised $119 million, with a B round completed in March 2022 at a valuation of $400 million. The company’s ambitious goals gained it a spot on this year’s Forbes’ Next Billion-Dollar Startups list of 25 venture-backed companies we think are most likely to reach a $1 billion valuation.
Weird Science
Hermeus’ goal to have airliners in service in the mid-2030s is an incredibly difficult task, experts tell Forbes.
The challenge isn’t to reach hypersonic speeds — missiles and space vehicles regularly do that. The difficulty is in building something that can sustain those speeds and stresses and be reusable, says Luca Maddalena, a hypersonics researcher at the University of Texas, Arlington.
Friction with air creates progressively more heat as an aircraft speeds up. To adapt, the SR-71 Blackbird spy plane, which set the record for the fastest air-breathing manned aircraft at Mach 3.3 in 1976, had aluminum wing sections that were corrugated to allow them to expand as the skin heated up. It leaked fuel on the runway because the tanks only sealed when the metal swelled in flight.
An aircraft goes supersonic when it exceeds the speed of sound, or Mach 1, but there’s no specific speed at which hypersonic territory begins. It’s defined by its peculiar dynamics. As speeds increase over Mach 5, heat starts to cause chemical reactions in the air around the vehicle. Oxygen and nitrogen split into individual atoms and can react with the aircraft’s skin. The aircraft is no longer flying in the air that surrounded it on the runway. “It’s a soup of different gases,” says Maddalena, “and it’s most likely different on every point of the vehicle” due to temperature and pressure variations. At higher speeds, some of the gas can turn into plasma. Predicting how an aircraft will perform requires combining chemistry and quantum mechanics with aerodynamics. Scientists still have plenty of work to do.
A key requirement to making a hypersonic airliner viable will be its durability. “It can’t come in with half the amount of life that a subsonic vehicle has today,” says Mary Jo Long-Davis, head of NASA’s Hypersonic Technology Project. “That won’t close the business model.”
Piplica freely admits there’s no way to predict a lifespan or maintenance schedule for Halcyon. “The data doesn’t exist,” he says. Hermeus aims to get the information through Quarterhorse and Darkhorse, and it may have a shot due to the U.S. government’s current zeal to develop hypersonic weapons.
Weapon Of Choice
Congress gave the Pentagon $5.8 billion in fiscal 2023 for roughly 70 hypersonic programs, up from less than half a billion dollars in 2016. But progress has been slow, in part due to an insufficient number of wind tunnels able to simulate hypersonic conditions and the slow pace of flight testing. Many programs are only managing a couple of flight trials a year.
That’s one reason why Hermeus may be onto something in aiming to first build drones that can be used to flight-test components.
Others are also pursuing the opportunity. Stratolaunch, previously bankrolled by the late Microsoft billionaire Paul Allen, is developing a rocket-powered autonomous hypersonic test vehicle that will be air-launched from its Roc aircraft, the world’s largest plane. This spring the Defense Innovation Unit awarded contracts to the Australian company Hypersonix and California’s Fenix Space and Rocket Lab to develop hypersonic testbeds.
Piplica and his cofounders — CTO Glenn Case, COO Skyler Shuford and Chief Product Officer Mike Smayda — all have backgrounds in the space industry and worked together at Generation Orbit, which was developing an air-launched liquid rocket for hypersonic flight testing. They think they can hold down costs by mimicking space startups in building a series of aircraft that progressively work through the problems of hypersonic flight.
For example, Quarterhorse will only fly over Mach 3 for a few minutes, so Hermeus won’t have to solve the heat management problems of a longer-flying hypersonic drone, let alone those required to keep passengers alive. Quarterhorse will test the air-breathing propulsion system, which is designed to be relatively affordable.
Both Quarterhorse and Darkhorse will be powered from takeoff by an off-the-shelf fighter-jet engine that will propel the aircraft to near Mach 3, fast enough to ignite a ramjet, which has no moving parts and depends on forward motion to compress the air needed to burn fuel efficiently. Ramjets have been in development for decades and are a relatively mature technology, Piplica says.
Last year, the company demonstrated that the propulsion system, called Chimera, could make the transition to a ramjet in a high-speed wind tunnel.
Darkhorse is planned to be roughly 50% longer than Quarterhorse and capable of a range at Mach 5 of over 1,000 miles. With advances in radar threatening to nullify the stealth properties of U.S. aircraft like the F-35 fighter, Piplica argues that Darkhorse’s speed will give the Air Force the ability to continue to operate in the airspace of adversaries like China that have advanced air defenses.
Other hypersonic passenger plane startups are aiming to go even faster than Hermeus, with more exotic technology.
Houston-based Venus Aerospace is working to perfect a rotating detonation rocket engine, which it plans to power a 12-passenger plane that can fly at Mach 9 at the edge of space. Investors including the venture capital arm of Airbus have given it $48 million, according to PitchBook.
Swiss startup Destinus is planning a hydrogen-fueled plane designed to fly at Mach 15. It has $39 million in funding, per PitchBook.
Cost Of Speed
The higher the Mach number, the more the exotic materials required — and the higher the maintenance costs, says NASA’s Long-Davis. Also rising with speed are fuel costs and likely ticket prices, according to a handful of studies NASA has funded over the past four years on the economics of high-speed air transportation. They’re heavy on assumptions, but the studies suggest the biggest market potential is for aircraft that cruise in the relatively sedate range of Mach 2 to Mach 3, with 20 to 30 passengers.
A notional 20-passenger Mach 5 airplane with 4,000 miles of range could have ticket prices topping $10,000, according to a study by SpaceWorks.
Piplica says the speed range Chimera will be optimized for — Mach 3 to 5 — gives Hermeus flexibility. Even if Mach 1 to 2 has more potential for passenger service, Piplica argues that supersonic aircraft startups face the disadvantage of having less appeal to the military and no prospects for selling intermediate products to finance their way to the end goal, as Hermeus hopes to do with its planned drones.
But the Pentagon is famous for experimenting with new technologies and then changing direction. After the steep increase in spending on hypersonic weapons development in the Trump Administration, critics are questioning whether they’ll be worth it. They could be a third more expensive than ballistic missiles of the same range, and less survivable, the Congressional Budget Office concluded in a January report.
It’s proving hard enough to build a maneuverable hypersonic vehicle on a one-way mission to explode, so talking about hypersonic transportation at this point is a “Ray Bradbury story,” says Richard Aboulafia of AeroDynamic Advisory, referring to the science-fiction author. “The idea that a small startup could move the needle on a century-long path is a little strange.”
Maddalena says it’s good that startups like Hermeus are attracting attention and capital with exciting aircraft designs, but away from the spotlight, other companies and academics are quietly working on building-block components that will enable hypersonic flight. “It’s not that the success of our hypersonic program depends exclusively on a few startups.”
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