IN OCTOBER 2003, the age of supersonic passenger travel came to an inauspicious end. That month British Airways withdrew from service its last Concorde jet, a Franco-British aircraft from the 1970s that could fly at twice the speed of sound. Since the 1940s executives in the aerospace industry had predicted that the future of passenger travel would be supersonic. But since the retirement of Concorde there have been no passenger jets that can fly that fast in service. Worse still, even conventional sub-sonic jetliners these days fly slower than their equivalents from the 1960s.

In 2017 the race to break the sound barrier gained new momentum. In December Aerion, an aerospace start-up from Nevada, Lockheed Martin, a defence giant, and GE Aviation, an enginemaker, announced a joint venture to develop what they are calling the world’s first supersonic business jet, the AS2. Aerion’s executive chairman, Brian Barents, has said that he hopes the jet will be able to carry up to 12 passengers at 1.4 times the speed of sound—about 60 percent faster than typical aircraft today. “We believe the conditions are ripe for a supersonic renaissance.”

Aerion is not the only firm with such hopes. Japan Airlines announced earlier the same month that it will invest $10m in a rival supersonic planemaker, Boom Technology of Denver, and will pre-order up to 20 of their jets. Boom hopes that the first ones will enter service in 2023 and be able to carry 40 passengers at a speed of up to 2.2 the speed of sound. Several potential customers have already bought options for Boom’s jets, including the Virgin Group from Britain. “The economics of our plane are much better than Concorde’s, driven by 50 years of evolution in the aerospace industry,” claims Eli Dourado of Boom:

Whereas Concorde was designed on paper, built with aluminium, and powered by noisy and dirty turbojet engines, we are designing with computational fluid dynamics, building with carbon fibre, and powered by a highly efficient and quiet turbofan engine. This adds up to significantly improved fuel efficiency on a seat-mile basis and means the economics make sense on more than 500 routes globally.

The promoters of both schemes say that their projects will slash the time it takes to jet around the world. Mr Barents claims the AS2 will shave 2.5 hours off the typical flight between New York and London, and more than five hours off trips between America’s West Coast and Singapore or Sydney.

But this kind of accelerated travel won’t be for everyone. Boom says that its supersonic jets will be able to offer round-trip tickets for $5,000 on routes which would have cost $20,000 on Concorde in today’s dollars. Still, that could be five to 10 times as much as an economy ticket on a normal plane—or a premium of perhaps $1,000 per hour saved. That would make sense for time-poor and highly-paid business executives. But supersonic travel would still be out of reach for most leisure travellers except the super-rich.

And in spite of Aerion and Boom’s best efforts, there are still technical barriers to overcome. No engines suitable for passenger jets travelling at supersonic speeds are currently on the market. And fears about the noise created by the sonic booms they create—the result of flying faster than the speed of sound—have led to regulations that restrict supersonic flights, most significantly in the United States, which bans them over its landmass. That should concern Boom, Aerion, Lockheed Martin, and GE Aviation, which are all American companies. Boom says that it is “not banking on any regulatory changes,” but hopes that regulators will eventually choose to restrict noise levels rather than speeds, which could allow supersonic planes that make less of a racket than Concorde used to. New technology could come to the rescue. NASA, America’s aeronautics agency, and Lockheed Martin have been experimenting with ways of reducing the intrusive bang of a sonic boom into a sound that resembles a soft thump in the distance.

But if that turns out not to be feasible, supersonic airlines will need to rely on transoceanic routes, just as they did with Concorde. A big problem is that Aerion and Boom’s aircraft will be only able to fly just over 8,000km without refuelling—not enough to cross the Pacific Ocean in one hop. That will not get them from Los Angeles to Beijing, or from the West Coast to Singapore. Boom, on its website, makes this problem sound trivial: “On routes longer than 5,179 miles (4,500 nm or 8,334 km), the aircraft will need a simple tech stop to refuel. The tech stop will take less than an hour, and passengers will not need to deplane or even wake up.” But when passengers are paying a premium to get to their destination quickly, a refuelling stop could defeat the purpose, if it results in a flight time that is only slightly lower than on a conventional aircraft.

Clearly, supersonic travel is not about to take over commercial aviation. But just as clearly, we seem to have passed a tipping point of sorts, with planemakers and top airlines buying into what they see as a forthcoming trend. And engineers increasingly see problems with engines and supersonic booms as fixable. Flying faster than the speed of sound may not be cheap or affordable for everyone, but it does seem to be on course for a revival.