Tomorrow, yesterday

WHEN a British Airways Concorde travelling from New York touched down at Heathrow airport, in London, on October 24th 2003, supersonic passenger travel came to an end. Concorde was a technological marvel, but never a commercial success. Only 14 of them entered service. Yet the idea of building a successor has never quite gone away. Aircraft-makers review the idea from time to time. A number of groups are working on small executive jets intended to travel faster than the speed of sound. The trouble is, something else has also refused to go away: the shock wave known as a sonic boom that emanates from a supersonic aircraft.

That boom was one of Concorde’s failings. It rattled windows and frightened animals, which meant the plane’s flights over land were restricted to subsonic speeds. Throttling back an aircraft that is designed to fly fast is inefficient and causes it to guzzle a lot of fuel. If supersonic air travel is ever to return, Concorde’s successors will thus have to quieten their act.

Several groups are trying to do this by tweaking designs to take account of advances in aerodynamics. By 2021 NASA, America’s aerospace agency, hopes to fly a small experimental supersonic plane fitted with some of these modifications, such as a long, slender nose and engines blended into the fuselage. The agency expects this to reduce the sound of the shock wave to what it describes as a “low boom”. But John Schlaerth, an aerospace engineer based in California, thinks he can take such modifications much further. He and his colleagues have filed for a patent on a set of designs which they believe might eliminate the boom’s sound altogether at ground level.

A sonic boom is the product of a series of shock waves arising from various parts of an aircraft—particularly its nose, wings and engines—as it flies faster than the speed of sound (1,240kph, 770mph or Mach 1, at sea level). Those waves are caused because air molecules cannot get out of the way fast enough during supersonic flight, and thus build up in front of these parts of the plane. The consequent change in pressure then propagates through the air and, when it reaches the ground, is heard as a distinctive boom.

Mr Schlaerth’s idea is to reflect and muffle the worst-offending waves. He would do this not by blending the engines into the fuselage, but rather by placing them well forward of the leading edge of the wing. That could be done either by mounting them on pylons extending from below the wing, or by attaching them to the fuselage. Both configurations would cause the engines’ exhaust plumes to reflect any shock wave forming in front of a wing upwards—ie, away from the ground.

Further shock waves, caused by the exhaust’s counter-reflection downwards by the wing’s wave, could be dealt with by modifying the engine casings to create a slower-moving stream of air below the plume. This slower air should form a boundary layer which, Mr Schlaerth says, would act as a “pneumatic cushion” that softened and impeded downward-propagating shock waves. The aircraft’s long nose, meanwhile, would be shaped to direct its shock waves upwards and sideways. Waves from the engine inlets would be directed upwards too, and put to good use. Adding an appropriate downward curve to a wing would trap the wave and create an area of high pressure that would give the wing additional lift.

To find out whether all this would work, Mr Schlaerth recruited two experts in computational fluid dynamics to act as independent consultants. Tim Colonius of the California Institute of Technology and Luigi Martinelli of Princeton University each carried out a series of tests. Using sophisticated computer modelling, one test found that the shock wave from the wing could be reduced by 63% at Mach 1.5, and that a similar reduction would be expected at Mach 2 (Concorde’s cruising speed). Another test showed that shock propagation below the engine was virtually non-existent. Further analysis, Mr Schlaerth says, indicates that the overall shock wave might be almost inaudible at ground level.

The next step is to replicate the computer tests using models in a wind tunnel, a task which the group hopes to take on later this year. Mr Schlaerth and his business partner, Mark Bryan, have founded a firm called New Century Transportation and Aeronautics Research to exploit the idea. If all goes well, it could lead to an experimental aircraft to demonstrate the technology.

Reducing sonic booms to an acceptable level would allow overland flights, which should make the return of supersonic passenger travel more plausible. Much would depend on the cost of building and operating such aircraft. But the prospect of being able to fly from New York to Los Angeles in less than two hours, instead of a tedious six or so, would be welcomed by many a weary traveller.