Why a Mach 5 Airliner Is a Crazy Idea That Might Work

After nearly two decades since the Concorde retired, interest in supersonic travel has increased and several super-fast planes are in development. Airlines seem to be interested: United has already committed to offering supersonic routes by 2029.

But what about hypersonic travel, which takes place at speeds of Mach 5 – five times the speed of sound – and above that? That would take an aircraft between New York and London in just 90 minutes, compared to about three hours for the Concorde, and between six to seven hours for a typical passenger jet.

Is this even possible?

Hermeus, a startup based in Atlanta, USA, whose goal is to develop hypersonic aircraft, believes so. It is already testing a new type of engine that it says will be capable of reaching Mach 5 (over 4,830 km/h). The engine was designed for a small unmanned hypersonic aircraft that Hermeus is building for the US Air Force, but scaled to a larger size, it will be able to power a passenger plane.

That airliner is a long way off – Hermeus hopes to air it for the first test flight before the end of the decade in 2029 – but as its technology has to be built almost entirely from scratch, the company is already planning it. .

For starters, it will be much smaller than current commercial planes and even the Concorde, which had a capacity of around 100 passengers.

“To help us scale the aircraft, we basically built a business model for an airline,” said AJ Piplica, CEO of Hermeus. “We focused on business class and first class travelers and then played with some parameters like speed and operating costs. The result was an aircraft with a cabin for 20 passengers”, he adds.

This is not far from the capacity of a large business jet, which means there will only be one class.

“We hope it will be profitable at today’s business-class prices,” says Piplica, with the caveat that it’s difficult to gauge how much people will be willing to pay to fly five times faster, because “you can’t really answer that question until that there is a product out there and you have the real data”.

faster than ever

The plane’s range will be about 4,000 nautical miles, enough for transatlantic routes like New York to Paris, but not enough for transpacific routes like Los Angeles to Tokyo, which would require a stopover.

Overland routes such as New York to Los Angeles are out of the question due to noise regulations: breaking the sound barrier comes with a loud bang, which usually must happen over water.

To understand how daring the idea of ​​a Mach 5 airliner is, it’s helpful to look at flight speed records.

The fastest any engine-powered aircraft has ever flown is Mach 9.6 (about 10,945 km/h), a record set in 2004 by NASA’s X-43A – an unmanned aircraft measuring about 3.65 meters in length. .

As that flight only lasted a few seconds, the record for the longest sustained flight above Mach 5 belongs to the Boeing X-51, another experimental unmanned aircraft, which in 2013 flew for more than three minutes at Mach 5.1 (about 5,470 km/h). H). Both aircraft had to be launched from altitude by a B-52 bomber and then accelerated by a rocket, highlighting the intricacies of this type of high-speed flight.

For aircraft with humans on board, the current absolute speed record is Mach 6.7 (7,275 km/h), set in 1967 by the X-15. It was basically a seated rocket, designed to set the record, and it also had to be launched from altitude by a B-52.

For an aircraft powered by jet engines rather than a rocket, and capable of taking off and landing alone, the speed record is “only” Mach 3.3 (about 3,540 km/h), set by the SR-71 Blackbird, a military spy plane in 1976.

The top speed of the Concorde, one of only two supersonic passenger planes to fly commercially, was Mach 2.04 (2,173 km/h).

The passenger aircraft proposed by Hermeus would therefore beat the current record for the fastest jet-engined aircraft with a large margin of advantage and, flying for an extended time at Mach 5, would surpass an achievement currently in the realm of unmanned experimental vehicles (Of course, other aircraft may break these records in the future before Hermeus does).

‘Mature technologies’

It’s not surprising, then, that the company’s initial focus is on the engine. Testing began in February 2020 for a new type of engine design, based on an existing model used in fighter planes and manufactured by General Electric.

It will be a hybrid of two traditional technologies: a turbojet, which is similar to what passenger planes use, and a ramjet, a type of engine that only runs at supersonic and higher speeds. Initially, the engine will power the Quarterhorse, the hypersonic drone that Hermeus is developing through a $60 million partnership with the US Air Force.

Interestingly, when designing a jet engine to go faster, parts are removed rather than added. In a turbojet, 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 from the back of the engine, pushing the plane forward.

Above Mach 3, however, there is no need to compress the air: it will compress as it enters the engine, simply because it has to slow down too much. Therefore, for speeds above Mach 3 and up to Mach 6, a type of engine called a ramjet is often used – so called because it literally hits the air. It has no moving parts, unlike turbojets, but does not run at speeds below Mach 3.

Hermeus will use its hybrid engine in turbojet mode for takeoff and landing, as well as at subsonic speeds. The engine will then gradually reconfigure itself into a ramjet mode as it reaches Mach 3 and up to Mach 5.

“The turbojet part and the ramjet part themselves are mature technologies that we’ve been using for 50 years. The trick is to put them together, so we design our own architecture around a ready-to-use turbojet engine and then build from there,” says Piplica.

SpaceX inspiration

There are a whole series of issues that Hermeus is not working on at the moment, such as what kind of sustainable fuel to use – since consumption will be much higher than today’s jets – and the extreme temperatures that the fuselage of a hypersonic aircraft must be. able to withstand.

The speed of the Concorde, which traveled at less than half the design speed of the Hermeus, was precisely limited by temperature, with the windows and other interior surfaces becoming warm to the touch at the end of the flight.

The SR-71 Blackbird, on the other hand, had to be made of titanium, a rare metal that can withstand extreme heat, and the cabin glass had to be made of quartz, with its external temperature reaching 315°C during the mission. .

In response to skepticism about Hermeus’ chances of success and the need for potentially huge amounts of funding, Piplica brings an analogy to Elon Musk’s SpaceX.

“I think people asked the same questions about the new space industry in the early days of SpaceX,” he said. “People saw it go into orbit and said it was supposed to cost a billion dollars, but SpaceX did it for $90 million with the Falcon 1.”

Hermeus is planning to self-finance by developing several aircraft while making his passenger plane, similar to the development of SpaceX’s Falcon 1, Dragon, Falcon Heavy and Starship rockets, which ultimately serve a vision of interplanetary spaceflight, while also developing generate revenue by working with NASA and business partners.

“There is nothing quite like Hermeus, although many similar projects have come and gone in the past,” says Richard Aboulafia, aviation analyst at Teal Group. “It looks like it will never work. If they can magically create hypersonic transport in the late 2030s, and the ticket price is in the business class range, then yes, it will succeed. But the chances of that happening are somewhere between 1%.”

If and when a hypersonic airliner becomes a reality, what will it be like to fly on it?

“It will be a lot like the Concorde,” says Piplica. “You will be accelerating for a longer period of time than on today’s aircraft, where you feel pushed back in your seat for about 30 seconds to a minute or more.

“This experience will last for about 10 to 12 minutes. But when you get to Mach 5, at 100,000 feet or more, it will be a really smooth ride. There’s not a lot of air traffic up there, and the atmosphere is relatively favorable.”