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Bang goes the sonic boom

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LONDON, England (CNN) -- Concorde was as synonymous with the champagne-quaffing jet-set as much as its ear-splitting din. But by taking the bang out of the sonic boom a new generation of supersonic cruisers aim to rekindle that sense of glamour while making it a quieter and more environmentally friendly 21st-century mode of travel.

Supersonic Aerospace International have been developing a 12-seater luxury aircraft, the Quiet Supersonic Transport (QSST) jet, which they hope will take to the skies in 2013 and reduce the noise of the sonic boom to no more than the noise level of a truck driving past your window.

Before it was retired in 2003, noise restrictions imposed by the International Civil Aviation Organization on Concorde meant it could only fly at supersonic speeds over unpopulated areas, limiting its service to mainly trans-Atlantic routes.

By cutting out the racket, Michael Paulson, CEO of Supersonic Aerospace International, and a number of other companies, believe they will open up overland routes to supersonic air travel as well as providing passengers and those on the ground with a more pleasant experience.

"We've been in a stagnant era. We've been flying subsonic, 400mph to 500mph jets since the late 1950s and early 1960s. Hopefully 30 years from now there will be a fleet of QSST jets able to fly at Mach 1.6 from London to New York in three and a half hours and the technology used for commercial airliners as well," Paulson told CNN.

The key to reducing the boom lies in the design of the jet. As an aircraft flies it creates waves of pressure as it pushes its way through the air.

These waves travel at the speed of sound, but when a jet travels at Mach 1 and above these waves merge into one single shock wave as they cannot get out of the way of each other quickly enough. The result is a sonic boom.

Spreading the area of air pressure over the jet staggers the sound waves and decreases the intensity of the boom.

Ideas of minimizing the noise of a jet as it passes through the sound barrier have been around since Concorde first deafened those living under its flight paths and beyond in 1976.

Early ideas suggested blunting the nose of the plane to prevent the pressure waves joining up to form one intense shock wave, or lengthening the plane without increasing its weight to dampen the noise.

Both have their own problems -- a blunted nose going against aerodynamic considerations. SAI employed Tom Hartmann of Lockheed Martin's Skunk Works to lead their QSST design team.

Their final design of the QSST jet is reminiscent of Concorde in its delta wing shape, but with a number of unique features, including an "inverted V-tail" and tailored nose cone.

"We considered over 30 designs and believe that the QSST is the only design that meets all the desired requirements including range, take-off and landing field length, take-off noise, emissions and sufficiently low sonic boom characteristics to enable supersonic flight over land," he told CNN.

"Its going to be 1/100th quieter than the recently retired Concorde, so to put that in normal terms, it would be comparable to the inside of your car at 70mph or normal speaking volume," said Paulson.

How low can the boom go?

Ilan Kroo of Stanford University's Department of Aeronautics and Aerospace has been working with Aerion to develop another supersonic luxury jet.

"Concorde's boom had a shock wave that created a pressure change on the ground of 2lbs per square foot. Small low boom aircraft create initial shock strengths of 0.5lbs per square foot or less," he told CNN.

Hartmann expects the change in pressure associated with the QSST to be roughly equivalent to the change in pressure a person would experience climbing down a flight of stairs.

Such a small change in pressure should solve the problem of building vibrations on the ground that could shake pictures off walls or crockery off sideboards when a supersonic jet passed overhead.

"The biggest challenge now is to determine what levels are acceptable. Noise around airports is also important and much design work is focused on making supersonic jets that are quiet on take-off and landing," said Kroo.

While taking the bang out of the sonic boom may usher in a new era of supersonic travel for the super-rich, there are still reservations that the aircraft design and technology could be applied to commercial aircraft, primarily because the magnitude of a sonic boom increases with an aircraft's weight.

But there are other problems for supersonic jets. Wear and tear on engines is much greater than for subsonic jets, plus their fuel-guzzling tendencies make them even less environmentally friendly.

Boeing abandoned its plans for Sonic Cruiser after studies revealed the same problems as Concorde; that it wasn't cost effective in terms of maintenance or fuel-economy.

"Even if it becomes cost-effective for commercial jets to travel at supersonic speeds, we'd have to evaluate if this is the best use of the technology. Should we travel faster or travel more fuel-efficiently?" Terrance Scott a spokesman for Boeing told CNN.

For Aerion this is an issue that they're looking at rather than trying to manufacture low-boom technology. They are banking on their design and new engine technology to provide fuel-efficient subsonic cruise speeds of Mach 0.98. So avoiding the compromise of speed for fuel-efficiency or relying on a change in the acceptable noise limits of supersonic jets by the ICAO.

While conceding that financial considerations are key, both Kroo and Hartmann believe that the supersonic jet designs and technology they are separately developing could allow mid-sized commercial jets in the near future that could carry around 40 passengers.

However, at an estimated $80 million for the QSST, stylish speed will come at a high price for airlines and those who will be able to fly on them. The noise from jaws dropping at the cost of a ticket might be louder than the sonic boom itself.

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