Almost two decades since Concorde retired, interest around supersonic travel has been picking up pace, and several super-fast planes are under development. Airlines seem interested: United has already committed itself to offering supersonic routes as early as 2029.
But what about hypersonic travel, which happens at speeds of Mach 5 -- five times the speed of sound -- and above? That would get an aircraft from New York to London in just 90 minutes, compared to about three hours for Concorde, and between six to seven hours for a regular passenger jet.
Hermeus, an Atlanta-based startup whose goal is to develop hypersonic aircraft, believes so. It's already testing a new type of engine it says will eventually be capable of reaching Mach 5 (over 3,000 mph). The engine is designed for a small, unmanned hypersonic aircraft Hermeus is currently creating for the US Air Force, but scaled to a bigger size, it will be able to power a passenger plane.
That passenger plane is a long way away -- Hermeus hopes to get it in the air for the first test flight before the decade is out, in 2029 -- but because its technology has to be built almost entirely from the ground up, the company is already planning it out.
For a start, it will be much smaller than current airliners and even Concorde, which had a capacity of around 100 passengers.
"To help us size the aircraft, we basically built a business model for an airline," says AJ Piplica, CEO of Hermeus. "We focused on the business class and first class travelers, and then played around with some parameters such as speed and operating costs. What came out of that was an aircraft with a 20-passenger cabin," he adds.
That's not far from the capacity of a large business jet, which means there will be just one class.
"We expect it to be profitable at today's business class prices," says Piplica, with the caveat that it's hard to gauge how much people will be prepared to pay to fly five times faster, because "you can't really answer that question until there's a product out there and you have the real data."
The range of the plane will be about 4,000 nautical miles, enough for transatlantic routes such as New York to Paris, but not for transpacific routes like LA to Tokyo, which would require a stopover.
To understand how daring the idea of a Mach 5 passenger plane is, it's useful to look at flight speed records.
The fastest any aircraft with an engine has ever flown is Mach 9.6 (about 6,800 mph), a record set in 2004 by the NASA X-43A -- an unmanned aircraft measuring about 12 feet in length.
Because that flight only lasted a few seconds, the record for the longest sustained flight above Mach 5 belongs to the Boeing X-51, another unmanned experimental aircraft, which in 2013 flew for over three minutes at Mach 5.1 (about 3,400 mph). Both aircraft had to be launched from altitude by a B-52 bomber, and then brought up to speed by a rocket, highlighting the intricacies of these type of high-speed flights.
For aircraft with humans on board, the current absolute speed record is Mach 6.7 (4,520 mph), set in 1967 by the X-15. It was basically a rocket with a seat, designed to achieve the record, and also had to be launched from altitude by a B-52.
For an air-breathing aircraft -- that is, powered by jet engines rather than a rocket -- capable of taking off and landing by itself, the speed record is "just" Mach 3.3 (about 2,200 mph), set by the SR-71 Blackbird, a military spy plane, in 1976.
The top speed of Concorde, one of only two supersonic passenger planes to have flown commercially, was Mach 2.04 (1,350 mph).
The proposed Hermeus passenger aircraft, therefore, would beat the current record for the fastest air-breathing plane by a large margin, and by flying for an extended time at Mach 5, it would outclass an achievement currently in the realm of unmanned experimental vehicles (of course, other aircraft might beat these records in the future before Hermeus does).