Since the mythical Concorde stopped flying 15 years ago, several proposals arose to return to having commercial flights with supersonic aircraft.

But now those plans seem closer to reality.

Three US aerospace firms?Boom Supersonic, Aerion Supersonic and Spike Aerospace?are vying to be the first to offer flights faster than Mach 1, the speed of sound (761 mph or 1,225 km/h at sea level).

They all plan to have their aircraft in regular service by 2025.

Technologically, supersonic flight is not difficult to achieve. The challenge is to offer a service that passengers can afford, that is less polluting and, most importantly, that eliminates sonic booms from Concorde.

The enormous thunder-like noise created when an aircraft breaks through the sound barrier can even cause damage to structures.

"What we're seeing right now is a renaissance in entrepreneurship in the aerospace industry," says Blake Scholl, CEO and founder of Boom Supersonic, which plans to build a delta-wing aircraft that will carry 55 passengers at speeds of up to Mach 2.2 (1,451 mph, 2,335 km/h).

"The Concorde was really ahead of its time. It was a great technological achievement, but it was incredibly fuel inefficient and for that reason very expensive," he says.

No sonic boom?

The attitude of aviation regulators will be key in determining whether we will see a return to supersonic flight.

Earlier this year, Lockheed Martin won a $248 million contract from US space agency NASA to build an aircraft with mild sonic booms.

The X-59 QueSST (quiet supersonic technology) will fly at Mach 1.42 (940 mph, 1,512 km/h) at 55,000 feet, and will generate a sound as loud as a car door slamming, NASA says.

The key to eliminating loud noise is in the design of an airframe. In a conventional supersonic jet, the shock waves merge as they expand away from the nose and tail, creating two different sonic booms.

The trick is to shape the plane in such a way that the shock waves stay separate as they travel away from the plane. This means that they hit the ground still far apart, generating a quick series of soft hits.

The aircraft should be completed by the end of 2021, and in mid-2022 NASA will begin flying over several US cities to collect data on how people on the ground respond to the flights.

From 2025 onwards, this will be used by the US Federal Aviation Administration (FAA) and the International Civil Aviation Organization (ICAO) in developing new standards for supersonic flight over Earth.

Current regulations state that civil aircraft can only be supersonic over water.

"You have to be able to fly without a sonic boom disturbing people on the ground," says Vik Kachoria, CEO of Spike Aerospace, whose S-512 commercial jet is designed to fly with 12 to 18 passengers at Mach 1.6 .

Spike says its "quiet supersonic flight technology" will allow it to fly on the ground without unduly disturbing people.

However, the businessman does not disclose how he will do this, although he chose a fuselage design with wings retracted at a 55-degree angle.

Affordable Supersonic?

A flight on Concorde could cost four times a first-class fare. But all three firms say their goal is to keep supersonic travel no more expensive than current business-class fares.

Flying time from Shanghai to Los Angeles, currently about 12 hours, would be reduced to just over 6.

"Instead of a round trip across the Atlantic costing $20,000, (the ticket) is closer to $5,000," says Scholl.

"It's still expensive, but if you can afford to fly in the forward cabin you'll have the benefit of getting to your destination in half the time."

Considering that 4 billion people flew in 2017, of which 12% (480 million) flew in business class, this is a huge market.

Boom is building a supersonic test plane, the one-seat XB-1, which will fly next year.

Scholl insists that supersonic travel doesn't require a lot of new technology.

"You can do it with advanced technology that was developed for other aircraft like carbon fiber composites, turboprop engines and software-optimized aerodynamics."

Concorde used relatively inefficient turbojets, while all proposed supersonic successors will use turbofans that are modifications of engines already in commercial use.

Boom aims to have its commercial aircraft enter service by mid-2020. So far, two airlines have shown interest: Richard Branson's Virgin Group has ordered 10; Japan Airlines asked for 20.

Supersonic Rivals: Past and Present

Which one will be first?

Many say it's Aerion Supersonic that has the best chance of getting its aircraft into service first.

Aerion has partnered with Lockheed Martin to jointly discuss the development of its supersonic business jet, the AS2. Lockheed has a lot of experience with high speed, with fighter jets like the F-35, F-22, and the SR-71 Mach 3 reconnaissance plane.

Aerion says it has already selected an engine for its jet and is working with GE Aviation.

The traditional solution for supersonic flight is the hang glider shape, but Aerion says this can cause "extensive airflow" across the glider tips, causing turbulence and thus increasing air resistance .

That's why it opted for a slim-wing design and a horizontal stabilizer that should reduce overall drag by 20%, the company says. NASA has already tested Aerion's planned airfoil at speeds up to Mach 2, validating the design.

"Even though it's very fast, it's conventional in many ways: construction techniques, material and systems, which reduces risk not only for developers but also for passengers," says Barents.

Amid concerns over climate change and the environmental impact of air travel, Spike's Kachoria believes that complying with current and future regulations will be key to whether or not supersonic travel succeeds.

"You can derail the industry if you don't talk to stakeholders."