The promise of traveling from New York to London in less than 60 minutes, or Los Angeles to Tokyo in three hours, has been around since 1949, when an unmanned two-stage Bumper rocket blasted through the sky at 5,150 mph. About 12 years later, the U.S. Air Force’s X-15 became the first piloted aircraft to achieve hypersonic flight, generally defined as reaching a minimum of Mach 5 (five times faster than the speed of sound) or 3,800 mph.
Yet the notion of a business jet operating at Mach 5 remained a sci-fi fantasy. That is, until recent advances in design and technology have brought the concept closer to a workable reality. “We’re experiencing a renaissance of hypersonic that will lead to a revolutionary era in the transport of people and products around the globe,” says Zachary Krevor, CEO and president of Stratolaunch LLC. The California-based aviation company has created the Talon-A, a reusable rocket-powered flying machine that, after being released at 35,000 feet from Stratolaunch’s Roc—the world’s largest aircraft—approached Mach 5 in initial flight tests last year. Stratolaunch’s prototype was designed with a research focus for government, education, and business customers, but not as a commercial airliner, and is backed by private funds.
“Most of the technology has been around since the ’50s, but materials and design capabilities are so much better [now],” says AJ Piplica, cofounder and CEO of Hermeus, an aerospace firm based in Atlanta.
Among the advancements made by engineers are practical, cost-effective solutions to the extreme heat encountered in hypersonic flight, during which friction can cause the leading edges of an aircraft to reach 1,800 degrees Fahrenheit. Hermeus, for example, will use titanium and a nickel-chromium superalloy for the exterior of its 20-seat Halcyon, while other outfits are employing thermal-resistant carbon fiber or thermal tiles similar to those on NASA’s space shuttle. Wind-tunnel testing and computational fluid dynamics have helped give shape to those materials, including smooth delta-winged craft with no blunt edges.
Specific aerodynamic configurations also impact experimental ramjet and scramjet systems, which are operational only above Mach 3. Australia-based Hypersonix designed its Dart flight demonstrator, scheduled to be tested with the U.S. Defense Innovation Unit at NASA’s Wallops Flight Facility later this year, around its 3-D-printed Spartan scramjet engine. Hypersonix will be partnering with Rocket Lab, which will provide the rocket for the Dart’s initial launch to Mach 5.
Hermeus, meanwhile, has devised and tested a dual-mode hybrid engine for the Halcyon, one that allows its turbine propulsion to operate at higher temperatures and therefore generate enough speed to activate a ramjet.
Then there’s Texas-based Venus Aerospace, which will equip its 12-seat Stargazer with a hybrid system that kicks over to a ramjet after breaking the sound barrier with a rotational-detonation rocket engine. “The concept was patented in the ’80s, but the technology wasn’t there,” says Andrew Duggleby, chief technical officer for Venus. “Around 2018, I saw signs that it was ready and started development. It’s a massive leap forward in the use of combustion.”
Despite the innovations in technology and design, hypersonic applications for commercial aircraft are in their infancy, with both engines and airframes remaining years away from implementation in that category. A major factor slowing progress is, not surprisingly, money. Switzerland’s Destinus, which last year promised to have a hypersonic option for consumers by 2030, recently suspended that program and is focusing instead on defense initiatives. “The technology is there,” says CEO Mikhail Kokorich. “The funding is not. If we were offered $1 billion, we would continue our program.” But he suspects the market size might never provide the desired payback.
Still, this fiscal turbulence, while acknowledged, doesn’t seem to be dissuading the other half dozen players. “We’ve made a lot of progress… but we need the cadence to continue picking up,” says Krevor, while Piplica asserts that “it’ll be the 2030s before we get there.” With only that nebulous countdown to rely on and sustained financing up in the air, the fledgling hypersonic sector certainly can’t afford to, well, cool its jets. J. George Gorant
The Thrust of the Matter
Different engine configurations have powered the current advances in hypersonic travel, each employing disparate technologies. As to which is better, it all comes down to the desired application— whether it’s intercontinental jet-setting at otherworldly speeds or actually blasting out of this world. And while such commercial flight is at least a decade away, here are the three current types of propulsion to make it possible. Michael Verdon
Rocket Engines
Representing the oldest tech of this trifecta, a rocket engine contains both fuel (liquid hydrogen, RP-1 kerosene, or solid propellants) and an oxidizer (liquid oxygen or ammonium perchlorate) so it can operate in the vacuum of space. After the combination mixes and ignites in the combustion chamber, the resulting high-pressure gases produce thrust and eventually accelerate the rocket into the hypersonic realm.
Pros: Ideal for use in space flight, since it does not require the intake of air.
Cons: A heavier and more limited option within the atmosphere due to the carriage of its own oxidizing agents.
Ramjet
As the name implies, a ramjet relies on ramming air through the intake via “shock waves” created by the engine’s shape. The air, compressed to subsonic speeds, mixes with fuel (typically hydrogen) and ignites in a combustion chamber. Ramjets are usually best suited to work at supersonic levels from Mach 2 to Mach 4.
Pros: Lighter than a rocket engine, it has a simple design devoid of any moving parts that could fail.
Cons: It’s not able to function in space, as it’s reliant on the intake of compressed air for combustion.
Scramjet
Unlike the standard ramjet, the supersonic combustion ramjet—or scramjet—is configured to compress higher-velocity air (typically Mach 5-plus), then pair it with hydrogen fuel for ignition in the combustion chamber. This uber-accelerated flow contributes to the capability of propulsion to Mach 10 or higher.
Pros: Faster airflow translates to a faster aircraft compared to one using a ramjet system.
Cons: The scramjet needs a booster rocket in order for the aircraft to reach Mach 4 prior to initiating hypersonic flight, and—as with the ramjet—it can’t perform at a subsonic rate of travel or above the Earth’s atmosphere.
