(CNN) – Nearly two decades after Concorde retired, interest in supersonic travel is gaining momentum, with several ultra-fast aircraft under development. Airlines seem to be interested: United has already committed to offering supersonic routes as early as 2029.
But what about hypersonic travel that takes place at Mach 5—five times the speed of sound—and beyond? This will take the plane from New York to London in just 90 minutes, compared to about three hours for Concorde and six to seven hours for a conventional passenger plane.
Is it even possible?
Hermeus, an Atlanta-based startup focused on developing hypersonic aircraft, thinks so. He is already testing a new type of engine that he says will eventually be capable of Mach 5 (over 3,000 mph). The engine is designed for a small unmanned hypersonic aircraft that Hermeus is currently building for the US Air Force, but scaled up to a larger size it could power a passenger aircraft.
This passenger jet is still a long way off – Hermeus hopes to have it airborne for its first test flight before the decade is out, in 2029 – but since its technology must be built almost entirely from scratch, the company is already planning it.
First, it will be much smaller than current airliners and even Concorde, which can accommodate about 100 passengers.
“To help us determine the size of the aircraft, we built a business model for the airline,” says A.J. Piplika, CEO of Hermeus. “We focused on business class and first class passengers and then experimented with some parameters such as speed and operating costs. The result is a plane with a cabin for 20 passengers,” he adds.
This is not far from the capacity of a large business jet, which means there will be only one class.
“We expect it to be profitable at today’s business class prices,” Piplika says, noting that it’s hard to estimate how much people would be willing to pay to fly five times faster because “that question can’t be answered as long as there is a product and you have real data.”
Faster than ever
The NASA X-43A is the world’s fastest jet aircraft.
The range of the aircraft will be about 4,000 nautical miles, which is sufficient for transatlantic routes such as New York-Paris, but not for trans-Pacific routes such as Los Angeles-Tokyo, which would require an intermediate landing.
Overland routes, such as from New York to Los Angeles, are out of the question due to noise regulations: breaking the sound barrier is accompanied by a loud bang, which should usually occur over water.
To understand how audacious the idea of a Mach 5 passenger plane is, it is helpful to look at airspeed records.
The fastest powered aircraft to ever fly is Mach 9.6 (about 6,800 mph), a record set in 2004 by the NASA X-43A, an unmanned aircraft about 12 feet long.
Since this flight only lasted a few seconds, the record for the longest sustained flight over Mach 5 is held by the Boeing X-51, another unmanned experimental aircraft that flew over three minutes at Mach 5.1 (about 3,400 mph) in 2013. ). Both aircraft had to be launched from a height by a B-52 bomber and then brought up to speed by a rocket, emphasizing the intricacies of this type of high-speed flight.
For occupied aircraft, the current absolute airspeed record is Mach 6.7 (4,520 mph), set in 1967 by the X-15. It was essentially a rocket with a seat, designed to achieve the record, and also had to be launched from a height on a B-52.
For a jet aircraft, i.e. equipped with jet engines rather than a rocket, capable of taking off and landing on its own, the speed record is “only” Mach 3.3 (about 2200 mph) set by SR. 71 Blackbird, military reconnaissance aircraft, 1976.
The top speed of the Concorde, one of only two commercial supersonic passenger aircraft, was Mach 2.04 (1,350 mph).
Thus, the proposed Hermeus passenger aircraft will by a wide margin break the current record for the fastest air-jet aircraft, and with a sustained flight at Mach 5, will surpass the achievement currently in the field of experimental unmanned aerial vehicles. of course, other aircraft may break these records in the future before the Hermeus).
Hermeus uses hybrid technology in its engines.
Therefore, it is not surprising that the company’s initial focus was on the engine. In February 2020, testing began on a new type of engine based on an existing model used in fighter aircraft and manufactured by General Electric.
It will be a hybrid of two traditional technologies: a turbojet engine, similar to that used in airliners, and a ramjet engine that operates only at supersonic speeds and above. The engine will initially power the Quarterhorse, a sleek hypersonic drone that Hermeus is developing in a $60 million partnership with the US Air Force.
Interestingly, when designing a jet engine to make it run faster, parts are removed rather than added. In a turbojet engine, air enters from the front and is first compressed (to increase its energy potential) by rotating blades, then mixed with fuel and ignited. The resulting hot gas is expelled through the rear of the engine, propelling the aircraft forward.
Above Mach 3, however, there is no need to compress the air: it compresses as it enters the engine, simply because it has to slow down so much. Therefore, for speeds above Mach 3 and up to Mach 6, a type of engine called a ramjet is often used, so named because it literally crashes into the air. It has no moving parts, unlike turbojet engines, but does not operate at speeds below Mach 3 at all.
Hermeus will use its hybrid engine in turbojet mode during takeoff and landing, as well as at subsonic speeds. The engine will then gradually reconfigure into ramjet mode as it reaches Mach 3 and up to Mach 5.
“Turbojet and ramjet are themselves mature technologies that we have been using for 50 years. The trick is to connect them together, so we developed our own architecture based on a standard turbojet engine and then built from there,” says Piplika.
Hermeus will have to withstand extreme temperatures.
There are a number of issues that Hermeus isn’t even working on at the moment, such as what type of sustainable fuel to use – as consumption would be much higher than current jet engines – and the extreme temperatures that a hypersonic aircraft fuselage must withstand.
The speed of the Concorde, which was moving at less than half the calculated speed of the Hermaeus, was limited precisely by the temperature: towards the end of the flight, the windows and other internal surfaces became warm to the touch.
The SR-71 Blackbird, on the other hand, was to be made of titanium, a rare metal capable of withstanding extreme temperatures, and the cockpit glass was to be made of quartz, which reached 600 F during the mission.
In response to skepticism about Hermeus’s chances of success and the need for potentially huge amounts of funding, Piplica draws an analogy to Elon Musk’s SpaceX.
“I think people were asking the same questions about the new space industry in the early days of SpaceX,” he says. “People looked at going into orbit and said it should cost a billion dollars, but SpaceX did it for $90 million with the Falcon 1.”
Hermeus plans to fund itself by developing various aircraft on the way to its passenger aircraft, similar to how SpaceX is developing its Falcon 1, Dragon, Falcon Heavy and Starship rockets, which ultimately serve the vision of interplanetary spaceflight and also generate revenue by working with NASA and commercial partners.
“There really isn’t anything quite like Hermeus, although a lot of similar projects have come and gone in the past,” says Richard Abulafia, aviation analyst at Teal Group. “It never seems to work. If they can magically create hypersonic vehicles in the late 2030s, and the ticket price is in the business class range, then yes, it will be successful. But the chances of this happening are somewhere in the 1% range.
If and when hypersonic passenger aircraft becomes a reality, what will it be like to fly it?
“It will be very similar to Concorde,” says Piplika. “You will accelerate for a longer period of time than in today’s aircraft, where you feel like you’re leaning back in your seat for 30 seconds to a minute or so.
“This experience will last 10 to 12 minutes. But once you hit Mach 5 at 100,000 feet or so, it’s a really smooth ride. There’s not a lot of air traffic and the atmosphere is relatively benign.”