Times have changed - flight times, that is. "Look at the continued global growth in air travel and the increased distances that need to be covered," says Peter Coen, the head of the High Speed Project at Nasa's Aeronautics Research Mission Directorate. "In this culture, people see great attractiveness in getting to places faster, of saving time. The idea of spending essentially a day and a half to get from the US to Asia, for example, is not the way to go."

Coen may be "a blue sky thinker", but it is his job to develop the technology that commercial partners may then make reality. And, as the title of the US$30 million-a-year project suggests, that technology is all about passenger aircraft going supersonic again.

Indeed, progress is such that he expects the supersonic flight demonstration of a ground-breaking aerodynamics technology within the next three to five years.

"After that, the market will play a bigger part," he says. "And there is a market for it. Any business that gets into it is going to be a leader. The likes of Boeing and Airbus have already said that supersonic travel is the next exciting opportunity in the aviation business - though no one will jump in until the technology is more mature."

Yet, that is happening. Indeed, much as computing was pushed forward by many small private enterprises until just a few global companies controlled the market, so too more small, independent companies are looking to how to bring back commercial supersonic flight, with small business jets expected to lead the way. The numbers stack up - there is a pool of very comfortably-off individuals ready to buy one - but also because, as Spike Aerospace's CEO Vik Kachoria says: "if you think smaller, the technological problems are easier to tackle. Easier, but not easy - some ideas will be shown to work and others won't."

Spike Aerospace, of Boston, US, may be just three years old, but it expects to launch its S-512 supersonic jet for 18 passengers over the next eight years - while major technical hurdles mean it expects no 300- to 400-seat supersonic aircraft to launch much before 2035. Not every company is thinking small. Reaction Engines - based in Oxfordshire, England - has, for example, developed a concept that has proven to be technically feasible - for a 300-seat aircraft capable of flying at Mach 5, or about 5,600km/h, using engines fuelled by liquid hydrogen. But Spike's "business jet first, bigger jet later" model seems to be dominant.

Gulfstream Aerospace, one of the world's largest makers of business jets, has worked on a supersonic project with the Sukhoi Design Bureau of Moscow, and over recent years has applied for various patents on advances such as a telescoping aircraft nose, highly-sloped fuselage and variable-geometry wings. British company HyperMach is developing a Mach 4 business jet. Meanwhile, Reno, Nevada-based Aerion has proved its high-speed drag-reduction theories by strapping a scaled wing to the underside of a Nasa F-15B research jet flying at twice the speed of sound.

Aerion has also partnered with Airbus, which has taken US$4 billion-plus in letters of intent to buy its proposed AS2/SBJ supersonic business jet, each of which is expected to cost about US$100 million. Aerion just needs to raise the capital to make it happen.

"Our belief is that the demand for supersonic passenger flight is there now as it ever was - the market is impatient for more speed," says Aerion chief executive Doug Nicols. "There are a lot of people who value time above all else: the time to recreate, or tend to business, or even tend to some emergency."

The commercial aircraft industry "has largely been locked at sub-supersonic speeds for decades now", he adds, "but it's never really lost sight of the desire for supersonic flight - it's always been a question more of economics and operating costs. But new technology is making overcoming those obstacles a reality."

Indeed, the biggest obstacle to the deployment of a supersonic passenger aircraft now is not commerciality or safety, but the noise; or, more specifically, the characteristic sonic boom created as such aircraft pass through the sound barrier. It was for this reason that Concorde was essentially restricted to a trans-Atlantic route: US and other national authorities prohibited its flight at supersonic speeds over land-masses, which put other companies off developing supersonic tech.

There have been signs of a realistic solution. A now-defunct organisation, Supersonic Aerospace International, for instance, trialled a v-tail design for its proposed Quiet Supersonic Transport aircraft - tested by the Lockheed Martin's Skunk Works - that produced a boom longer than Concorde's, but at 1/100th of the volume. And Nasa has its so-called aerodynamics-based Quiet Spike technology, a retractable lance for the nose of an aircraft that effectively swaps a loud sonic boom - actually always two, one at the nose and one at the tail of an aircraft - for three much quieter ones.

"The sonic boom has a big impact on society - it came almost exactly with the rise of the environmental movement, when the idea of technology for technology's sake was wearing thin," Coen says. Since then the sonic boom has remained "an extremely challenging" problem, he adds.

"We've needed more powerful computing to simulate air-flow over and around any proposed aircraft, but also new thinkers who take a fresh approach," Coen explains. "And we think now we've reached a breakthrough in which the noise would be low enough that you couldn't really call it a boom, more of a 'supersonic cruising noise'."

Indeed, this new tech might not work for a Jumbo Jet-sized aircraft - the heavier the aircraft, the less well the new thinking on aerodynamics can be applied - but could well work for perhaps a large, 70-100 seat supersonic business aircraft. There are other issues that need addressing: reducing take-off and landing noise; a push towards a dramatic reduction in emissions; and how a supersonic aircraft would fit into the current air traffic control system. But few in this fledgling market are anything but certain supersonic flight will return soon.

"Because there's growing opportunity there," Coen explains. "Supersonic flight makes the world smaller. It allows us to do more, to get somewhere, get it done and get home faster too."