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In the 1960s, it seemed like the future of air travel was supersonic, but in 2003 Concorde made its final flight.

Now, hopes for super speedy journeys by air have been rekindled as a number of companies have unveiled concepts for supersonic jets of the future.

The concepts coincide with Nasa tests that are hoping to find a way to create an aircraft that is quieter than Concorde.

It is hoped new supersonic aeroplanes designed to carry civilian passengers could be ready within the next 15 years.

Is the future supersonic? Hopes for super speedy journeys by air have been rekindled as a number of companies, including Boeing (pictured) have unveiled concepts for supersonic jets of the future, and Nasa has begun tests to work out a way of creating an aircraft that is not as noisy as Concorde was

Aeronautics engineers at Nasa are working to define a new standard for low sonic booms and are presenting their work at Aviation 2014, the annual event of the American Institute of Aeronautics and Astronautics, in Atlanta.

They have been busy gathering data in order to create new, quieter planes that could help overturn the current ban on supersonic flight over land.

'Lessening sonic booms - shock waves caused by an aircraft flying faster than the speed of sound - is the most significant hurdle to reintroducing commercial supersonic flight,' said Peter Coen, head of the High Speed Project in Nasa's Aeronautics Research Mission Directorate, Washington.

'Other barriers include high altitude emissions, fuel efficiency and community noise around airports.'

Aeronautics engineers at Nasa are working to define a new standard for low sonic booms. They have flown F/A-18 mission support aircraft (pictured) to create low-intensity sonic booms in a bid to gauge the public's tolerance to noise at the agency's Armstrong Flight Research Centre in Edwards, California

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In a supersonic aircraft, shockwaves from the nose, cockpit, inlets, wings and other features coalesce as they move through the atmosphere. As these shockwaves pass over someone on the ground, air pressure rises sharply, declines, then rises rapidly again. This produces the 'double-bang' sonic boom

Engineers are investigating how to design a low-boom aircraft, as well as measuring the loudness and the annoyance of the boom, by asking people to listen to the sounds in a specially designed noise test chamber.

The space agency recently flew small planes at supersonic speeds at Nasa's Armstrong Flight Research Centre in Edwards, California, to gauge the public's response to the noise.

Scientists are also working on how to design aircraft that reduce the noise of the sonic boom.

Mike Park, a fluid mechanics engineer at Langley, said: 'We are working to understand the worldwide state of the art in predicting sonic booms from an aircraft point of view.

'We found for simple configurations we can analyse and predict sonic booms extremely well. For complex configurations we still have some work to do.'

Designs including a needle-like nose, sleek fuselage, and a delta wing have been tested in wind tunnels and seem to result in lower booms, according to the experts.

It's more than a decade since Concorde (pictured) was retired from service. No company will build a supersonic passenger plane unless it is allowed to fly supersonically over land - something that was off-limits for Concorde

'Lessening sonic booms - shock waves caused by an aircraft flying faster than the speed of sound - is the most significant hurdle to reintroducing commercial supersonic flight,' said Nasa's Aeronautics Research Mission Directorate. Pictured is Lockheed Martin's design for a supersonic aircraft

Sonic boom noise is measured in perceived decibel level (PLdB).

Concorde's boom was a window-rattling 105 PLdB. Researchers believe 75 PLdB would be an acceptable level for unrestricted supersonic flight over land, but Nasa is aiming more ambitiously for 70 PLdB or lower.

This looks achievable for a small supersonic business jet, because boom is proportional to aircraft weight, but is a much greater challenge for a heavier airliner.

In wind-tunnel tests, designs from both Boeing and Lockheed Martin - funded by Nasa - which would carry between 30 and 80 passengers, achieved boom levels as low as 79 PLdB.

At this level, the sonic boom would be more of a thump than a loud bang. Nasa is setting the bar even higher - it thinks 70 PLdB is achievable with more refinement. Studies are still taking place to try to reduce the volume level even further.

Nasa and industry engineers claim supersonic research has progressed to the point where the design of a practical low-boom supersonic jet is within reach.

Nasa and industry engineers say they believe supersonic research has progressed to the point where the design of a practical low-boom supersonic jet is within reach. Here Lockheed Martin's design, with a long needle-like nose and delta wing intended to cut the noise of a sonic boom, is tested in a wind tunnel

The space agency is not the only firm working on building the supersonic jet of the future.

Companies such as Aerion and Spike Aerospace are looking to take business jets supersonic.

Boston-based Spike Aerospace has designed a supersonic jet called the Spike S-512 that it claims could carry 12 to 18 passengers at 1,100mph (1,700 km/h), or Mach 1.6.

It claims the craft could cut flight times in half so passengers could fly from New York to London in just three hours, of from LA to Tokyo in six hours.

Lockheed Martin unveiled its vision for a supersonic future at the aviation event, with a concept featuring two engines under a sleek aircraft's wings and one on top of the fuselage.

Once Virgin Galactic is fully operational, Richard Branson has also set his sights on supersonic travel and has plans to create supersonic planes aircraft enough to travel from New York to Tokyo in less than an hour, CNBC reported.

'After we've done the space program, we will be producing supersonic planes, which will go far, far, faster than Concorde…You could be traveling at 19,000 miles per hour orbitally,' he said.

COULD THE FUTURE OF AIR TRAVEL BE HYPERSONIC?

Supersonic could be superseded by something even faster.

Mach 2.5 is about the speed limit for gas-turbine engines. Any faster and the temperature and pressure of air entering the engine is too high for the turbo machinery inside. To fly at hypersonic speed - Mach 5 and above - requires a different type of engine.

A supersonic-combustion ramjet, or scramjet, has no moving parts. Instead of the rotating compressor and turbine in a jet engine, air is compressed and expanded by complex systems of shockwaves under the front of the aircraft, inside the inlet and under the fuselage at the rear.

Scramjets have been under development for decades, but a breakthrough came in May 2013, when the U.S. Air Force Research Laboratory's Boeing X-51A WaveRider flew for 240 seconds over the Pacific on scramjet power, reaching Mach 5.1 and running until its fuel was exhausted.

The next step is to build a high-speed cruise missile, able to strike distant targets in minutes, not hours. Lockheed Martin's Skunk Works - builder of the Mach 3.5 SR-71 Blackbird spyplane - has unveiled plans to develop a successor, dubbed the SR-72 (pictured above).

Designed for reconnaissance and strike missions, the SR-72 would combine turbojet and ramjet/scramjet engines to enable the aircraft to take off from a runway, accelerate to a Mach 6 cruise, and then return to a conventional runway landing.

If it can secure funding from the U.S. Defense Department, Lockheed Martin believes a prototype could be flying as soon as 2023 and the SR-72 could enter service by 2030, potentially paving the way for commercial applications of scramjet technology.