For those old enough to remember – and who lived under a flight path – the sound of Concorde passing overhead was an unforgettable experience, as if the fabric of the sky had been rent asunder.

Thirty years after its first transatlantic crossing in 1973 (Concorde entered regular service in 1976), the world’s first supersonic commercial passenger jet was retired in the wake of the Air France Flight 4590 disaster in 2000, which killed 113 people – and, with that, supersonic passenger travel ended.

Fast forward to January of this year, when a Boeing 787-9 Dreamliner owned by Norwegian made the 3,470-mile trip from New York’s JFK airport to London Gatwick in five hours and 13 minutes, a new journey speed record. In doing so, it became the fastest subsonic plane to traverse the Atlantic.

However, the Dreamliner did not break the sound barrier (Mach 1, 767mph) during its historic flight; the feat was possible thanks to strong tailwinds rather than technological or engineering innovation.

In fact, despite being more efficient, burning less fuel, and being able to carry more passengers over longer distances, most of today’s commercial jets are actually slower than they were in the 1960s.

“Performance-wise, passenger aircraft have hit something of a limit, as we have seen with the Airbus A380,” confirms University of Salford senior lecturer in aerodynamics and aircraft performance Phil Atcliffe. “They have got bigger, but they are not getting faster, because there isn’t really any point – unless you really go faster, that is. The basic improvement is in fuel efficiency.”

Emission impossible: supersonic aircraft and the environment

That could all change as soon as 2023, however, thanks to a handful of start-up companies who are competing to build a new generation of supersonic passenger jets. How they should be regulated in terms of noise and carbon emissions – and how airports will cope – is the subject of heated debate.

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Supersonic aircraft operate at higher cruise altitudes (15–18km) in the sensitive high troposphere and stratosphere. Cruising above 50,000ft reduces resistance, meaning less heat generation and stress on the airframe. However, it also results in more fuel burn because drag increases with speed.

“We fully recognise that we are going to pump out more greenhouse gases than a subsonic aircraft will – that’s just the laws of physics,” Gene Holloway, chief sustainability officer at Nevada-based Aerion – one of three US companies developing supersonic passenger jets – told Aerospace America.

“Performance-wise, passenger aircraft have hit something of a limit, as we have seen with the Airbus A380.”

Just how much more greenhouse gas will be emitted is difficult to quantify. Supersonic aircraft could generate between two-and-a-half to seven times the carbon emissions of comparable subsonic jets, an estimate that appears to be borne out by a study published in January by non-profit research group the International Council on Clean Transportation, based upon supersonics’ higher emissions per passenger kilometre figures.

The design of the new generation of supersonic jets means they likely carry fewer passengers, meaning more fuel burn per passenger. A supersonic jet may also need to stop to refuel during trans-Pacific flights, and unlike a conventional passenger plane, could not carry belly cargo, meaning additional subsonic flights to meet additional demand.

“Would Concorde be allowed to fly now? In terms of carbon emissions I doubt it because we have to remember we are talking about 1960s engine technology here,” says Atcliffe. “Regulators will insist that any new supersonic aircraft meet the same noise requirements, and probably the same emissions requirements, as a new incoming subsonic aircraft. That’s a serious technical challenge.”

The three US companies leading the supersonic revolution – Aerion Supersonic, Boom Supersonic and Spike Aerospace – are rising to that challenge by improving aerodynamics, developing more efficient engines, and eschewing fossil-fuel-based jet fuel in favour of alternative fuels with lower net lifecycle carbon emissions. Boom, for example, is working with a California start-up Prometheus Fuels on a carbon-neutral synthetic fuel for its supersonic XB-1 demonstrator.

Volume control: noise pollution and aircraft design

In October, Airports Council International (ACI) called upon the International Aviation Organisation (ICAO) to fast-track its development of new standards and practices to regulate supersonic aircraft.

Speaking to coincide with the publication the ACI Reintroduction of Supersonics paper, ACI World director general Angela Gittens outlined the organisation’s stance on a key issue: noise reduction.

“In order to be integrated into the commercial aviation ecosystem, supersonic aircraft must not be noisier than comparable subsonic aircraft of the same maximum take off mass (MTOM) during their subsonic operations,” she said.

“We fully recognise that we are going to pump out more greenhouse gases than a subsonic aircraft will – that’s just the laws of physics.”

Engines optimised for supersonic travel typically have a trade-off between lower noise during take-off (high bypass ratio) and lower drag/higher fuel efficiency in supersonic cruise (low bypass ratio).

“The noise could be the real kicker for supersonic aircraft,” says Atcliffe. “One of the challenges for their design is to combine efficiency at high speeds for long periods, but also produce enough lift so the plane can land at low speeds and use existing runways.

“Concorde’s wing was not that great at producing lots of lift, so it needed afterburners to accelerate it up to the high speeds necessary to take off within existing runway distances – and that is one of the things that made it very noisy. In addition, it used a turbojet, rather than a turbofan engine.

“However, noise requirements have got a lot stricter since Concorde was in service and I don’t think regulators now would be as willing to make an exception for supersonic planes as they did then.”

How will airports adapt to supersonic flights?

The noise and emissions produced from supersonic aircraft operations in and around airports is also an issue, although Atcliffe believes the impact of regulations on ground operators will be marginal.

“The manufacturers, the operators, they will be the ones who bear the biggest brunt of any noise and emissions restrictions because, unlike Concorde, this time around they are not going to get special air-worthiness regulations that apply just to them,” he explains.

“Supersonic aircraft must not be noisier than comparable subsonic aircraft of the same MTOM during their subsonic operations.”

Of the trio of US start-ups developing supersonic jets, Atcliffe believes Boom is the most promising.

“Boom has done something that no other start-up has done in all the years since Concorde, I believe, which is to actually cut some metal,” he says. “We’ve had all sorts of aircraft proposals of all types for years, but none have got beyond the drawing board, whereas Boom is actually producing a technology demonstrator. They have also got around 76 potential orders for the developed aircraft – interestingly, that is around the same number of orders and options that Concorde initially had.”

In 2023, it will be half a century since Concorde made its first transatlantic crossing. If the supersonic flight for the masses is to take off again – and a flight from New York to London is to take just 3 hours 15 minutes, as Boom claims – it must do so without disproportionately damaging the planet.

“Public acceptance of aviation rests on our ability to face the operational and environmental challenges currently posed by noise and emissions from subsonic aircraft,” said ACI World director general Gittens. “The introduction of supersonic aircraft must not destroy that acceptance.”