The final Concorde supersonic flights touched down in 2003.

For almost two decades, travellers – regardless of their wealth – have been limited to subsonic aircraft.

But could plans for a high-speed civil aircraft for the 21st century now be taking off?

Miami to London in under five hours: that is the promise from American Airlines when it takes delivery of a new generation of supersonic jets.

The giant carrier, based in Dallas, has announced an “agreement to purchase up to 20 Overture aircraft, with an option for an additional 40”.

Its rival, United, has already paid a deposit on 15 of the new planes.

They are to be made by Boom Supersonic, based in Denver.

But will it ever happen? These are the key questions and answers.

What went wrong with Concorde?

The Anglo-French jet flew its first test flight in 1969, and carried paying passengers for Air France and British Airways from 1976 until 2003.

The plane flew up to 100 passengers at a time at Mach 2.04 – equivalent to 1,350mph at its cruising altitude of 60,000 feet. Travellers from London and Paris to New York and Washington DC arrived earlier than they had departed, local time.

But Concorde was fitted with noisy and thirsty military engines. The 1960s technology used for the Anglo-French project was extremely challenging, with the aluminium airframe extending by about 15 inches due to the heat generated by friction during supersonic flight.

The aircraft was grounded by a combination of high oil prices, low demand and concerns about safety following the 2000 crash in Paris in which 113 people died.

What’s the latest plan?

Denver-based Boom Supersonic is developing a new passenger aircraft known as Overture.

The proposed “SST” – supersonic transport – will have a range of 4,890 miles, about one-third more than Concorde, and will fly at Mach 1.7.

At 60,000 feet that equates to a ground speed of around 1,050mph – slower than Concorde, but around twice as fast as current short-haul aircraft. They tend to fly at Mach 0.85.

“Sustainable supersonic travel enables three-day business trips to become one-day hops, long-distance relationships to thrive, and humanitarian missions to save more lives,” says the company.

When will it be ready?

“Overture is slated to roll out in 2025, begin test flights in 2026, and carry passengers by 2029,” says Boom Supersonic.

In 2017, though, the company said it would be flying scheduled services by 2023.

How many passengers will be on board?

That is one of several curious uncertainties. The announcement of the American Airlines order said: “Overture is being designed to carry 65 to 80 passengers.”

Initially Boom said it would seat 55 travellers, but the company now says it will hold up to 88 – which is approaching the capacity of Concorde.

How much space will they have?

“Ample,” according to Boom. The company says: “Imagine a large personal window, allowing you to take in the stunning views of the Earth from 60,000 feet. You’ll enjoy ample space for work or relaxation, a large entertainment screen, and dedicated underseat storage.”

The aircraft has more than a passing resemblance to Concorde, which offered very little personal space.

Helen Coffey, head of travel for The Independent, reviewed the design plans last month. The aircraft will have “gull wings” to increase speed and stability. In addition, a newly designed fuselage that’s larger at the front of the plane and smaller at the back should help reduce drag.

Where will Overture fly?

Boom says there are 600 possible city pairs. Feasible routes will involve long stretches over oceans; Overture will have to fly subsonic over land because of the “sonic boom” as it breaks the sound barrier.

Because of the range constraints, it will be much more suited to transatlantic routes than transpacific journeys – it could not reach Sydney nonstop, and the only US-Japan possibility, just within range, is Seattle-Tokyo.

Journeys from US east coast cities such as Boston, New York and Miami to London, Paris, Amsterdam and other European hubs are likely to comprise the key market. Miami to key South American destinations, plus Los Angeles, San Francisco and Seattle to Hawaii, could also be feasible.

How much time will it save?

That all depends. At the launch of Overture at the Farnborough Air Show last month, the company claimed it would fly from London to New York in 3.5 hours, the same as Concorde. That looks frankly impossible given the top speed. Even four hours looks ambitious.

The problem for Overture, though, is that a route needs to be towards the limit of range for the speed advantage to become clear. Saving two hours on the usual five-hour flight from California to Honolulu may not be substantial enough differentiation for supersonic travel.

On routes such as London to Chicago or Toronto, the most direct routes pass over more than 1,000 miles of land, meaning the speed advantage is sharply reduced.

Could it fly US-Australia?

When Boom first announced its plans, the design speed was Mach 2.2, making it well over twice as fast as conventional aircraft, with the 7,500-mile Los Angeles-Sydney run expected to take under seven hours even with a refuelling stop: “Leave LA at 7am, enjoy an operetta in Sydney, and be back before midnight,” promised the company.

What happens when the plane breaks the sound barrier?

Nothing, from the point of view of passengers and crew. On the surface, it sounds like a thunderclap.

How much more will a flight cost than conventional aircraft?

No details have been released on fares; airlines will decide the pricing. Boom says: “We’re designing Overture to allow airlines to offer fares comparable to today’s business class.”

Will it be safe?

American Airlines says: “Under the terms of the agreement, Boom must meet industry-standard operating, performance and safety requirements as well as American’s other customary conditions before delivery of any Overtures.”

The disaster in Paris in 2000 involving an Air France Concorde that had just taken off for New York was unrelated to supersonic flight.

Which airlines will fly it?

American Airlines and United Airlines have said they will buy the jet and have paid deposits.

British Airways and Virgin Atlantic have not expressed interest in acquiring the jet. Were they to do so, it would be best suited for US and Caribbean routes.

Before Concorde took to the skies, airlines including Air India, Lufthansa and Qantas were in the running to order the Anglo-French supersonic plane. But their interest never translated to actual sales.

What about the environmental footprint?

The makers say Overture is “optimised for speed, safety and sustainability” – even though the carbon footprint per passenger is far heavier than for economy travellers on conventional aircraft.

“Overture is the first supersonic airplane designed with a focus on sustainability from day one,” says Boom. “We are optimising the airplane to accommodate 100 per cent sustainable aviation fuels (SAF) and facilitate net-zero carbon operations.”

Hypersonic flight: fact or science fiction?

Every couple of years, it seems, British travellers are told that they will soon be able to travel to Australia in two hours flat (the fastest current journey is 17 hours).

The key, we are assured, is hypersonic travel. The idea is: propelling a passenger-carrying craft beyond the earth’s atmosphere, where it flies at perhaps 5,000mph before coming in to land. But there is a distinct lack of hard cash and political will for such a project.

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