APART from a gentle nudge in the back as the pilots opened the throttles of its four Rolls-Royce Olympus jet engines, passengers had little sensation that Concorde was accelerating through the sound barrier. Not so for those on the ground. A sonic boom trailing behind the aircraft would rattle windows and dislodge roof tiles in Devon and Cornwall, two western British counties under the aircraft’s flight path from London to New York, a journey it could complete in just over three hours.

Supersonic passenger flights came to an end in 2003 after a downturn in air travel and a fatal crash in Paris three years earlier. But Concorde, although a technological marvel for its time, was never a commercial success: the 14 aircraft that saw service were heavily subsidised by British and French taxpayers; they had limited range and guzzled fuel flying subsonically, which they were largely forced to do over land because of their sonic booms. Yet the idea of a Concorde successor has never quite gone away. If a supersonic airliner were to fly again, however, such a noisy footprint would have to be toned down.

Now a group of engineers at NASA, America’s aeronautics agency, think they have found a way to do that. Lockheed Martin, an American aerospace firm, has been commissioned by NASA to carry out a $20m design of a small “low-boom” experimental supersonic aircraft (illustrated above). If all goes well, it will be test-flown in 2019 and then begin a series of trials to establish if the sudden and intrusive bang of a supersonic jet passing overhead can be turned into a sound that resembles a soft thump in the distance.

Punching air

Concorde’s supersonic boom, and for that matter those from jet fighters and the Space Shuttle, was caused not by one shock wave but the interaction of a series of shock waves radiating from various parts of an aircraft as it flies faster than the speed of sound, which is around 1,240kph (770mph, or Mach 1) at sea level. (These multiple shock waves leaving a supersonic jet fighter can be seen in a schlieren image (below), a photographic method which captures variations in the density of a liquid or gas.)

Below Mach 1 the molecules of air in front of an aircraft are pushed out of the way, much as a boat travelling through water creates a bow and stern wave. Once an aircraft accelerates beyond the speed of sound, however, the air molecules simply can’t get out of the way fast enough but pile up at certain points on the aircraft. That creates an instantaneous change in pressure, resulting in a shock wave that contains a huge amount of sound energy. The first shock wave occurs at the aircraft’s nose and the others at places such as the leading edge of the wings and the engine inlets. At the rear of the aircraft, a “recompression shock” is formed when the rapid change in air pressure switches back to normal atmospheric levels. As they radiate away, the waves tend to coalesce, forming two main shock waves. This is why a supersonic jet passing overhead is often heard as a distinctive double boom. If plotted onto a graph, the two peaks in pressure from a sonic boom resemble an “N” shape in time. The idea at NASA is that by tweaking the design of a supersonic jet in various ways it should be possible to smooth out the N-wave of a sonic boom so that it resembles a softer “U” shape. “We are not shooting completely in the dark,” says Peter Coen, head of commercial supersonic technology at NASA’s Langley Research Centre in Hampton, Virginia. In an earlier trial the agency used a modified F-5 fighter which had been fitted with an extended nose shaped like a pelican’s, to help reduce the noise of a sonic boom. Tests have also been carried out using sonic-boom simulators to see what sorts of sounds people find less intrusive.

The new design is for a single-seat, single-engine jet. Its most obvious feature is a long and slender triangular nose, which is supposed to modify the shape of the shock wave at the front of the aircraft and help disperse it. Other design features include an engine intake sculpted into the upper wing to reduce the intensity of the shock wave that forms there. To quieten things further, the aircraft would also fly a bit slower than Concorde, which had a cruising speed of Mach 2.

To measure the sudden noise of a sonic boom, NASA uses a scale called perceived decibel level, or PLdB. Mr Coen says the experimental low-boom aircraft should produce a sonic boom with a noise level below 70 PLdB, compared with about 100 PLdB for Concorde. The result, he adds, will be a broader, softer sound that people on the ground should find tolerable, if indeed they notice it at all amid the cacophony of modern living. The proof will come in how people react to the noise during the aircraft’s test flights.

If the aircraft’s quieter boom does prove to be acceptable, then its sound “signature” could become a certification standard that any future supersonic passenger aircraft would have to meet. But is there a realistic possibility that supersonic air travel could ever return?

Mr Coen thinks it will, at first with small, supersonic business jets. A number of groups have plans for such aircraft at various stages of development. Among them is Aerion, a company based in Nevada, which is developing a Mach 1.5 executive jet called the AS2. Airbus, a giant European aircraft-maker, has been lending engineers to assist Aerion. The plan is to have a prototype ready for test flights in 2019.

A supersonic airliner with 100 or more seats would be commercially more risky. And yet, technology moves on. Reducing the sound of a sonic boom is made possible with computational fluid dynamics, which relies on powerful computer systems. Concorde was born in an age of slide rules. New ways of making aircraft, especially with lightweight, carbon-fibre composites, have also been developed.

Ultimately, whatever the technological advances, there has to be a big enough market. With some passengers now seemingly prepared to pay handsomely for super-luxury first-class and even small apartments on long-haul flights, getting to the other side of the world in just six hours might be a tempting proposition for some.