Breaking the sound barrier—again

December 15th, 2005  

Topic: Breaking the sound barrier—again

Breaking the sound barrier—again

Dec 8th 2005
The Economist
Transport: A new breed of supersonic business jets, without Concorde's drawbacks, could soon be taking to the skies

IMAGINE being able to cross the Atlantic in less than three and a half hours. You could fly from London to New York for a meeting and still be home in time for dinner. It used to be possible, before the demise of Concorde, of course. But while Concorde has passed into history, the dream of supersonic travel is alive and well. Several firms are now racing to develop a new breed of supersonic passenger aircraft. These planes will use new technology to ensure that they are quieter, more efficient and capable of turning a profit—something Concorde struggled to do throughout its 27-year lifespan. The new aircraft will differ in another way, too. To start with, at least, they will probably not be airliners, but much smaller business jets.

That is because business-jet users are most likely to value time over money, says Preston Henne of Gulfstream Aerospace, a big business-jet manufacturer based in Savannah, Georgia. Supersonic business jets make sense from an engineering and financial point of view, too. “Historically, if you go back and look at how today's aeroplane systems evolved, it started with smaller aircraft and affluent customers,” says Mr Henne. Concorde was an exception to this rule, which may explain why it failed.

That said, those rushing to build supersonic business jets fall into two camps, taking two very different approaches. Those in the first camp, which includes Gulfstream, are aiming to overcome what was arguably Concorde's biggest drawback: the sonic boom it created during supersonic flight. The resulting noise pollution ultimately led to international regulations banning commercial aircraft from flying supersonic over land. This in turn severely restricted the flight paths Concorde could follow, since it was so inefficient at subsonic speeds that routes had to be designed to minimise the distance flown over land. Gulfstream plans to tackle this problem head on, by getting rid of the sonic boom. It sounds ambitious, but 40 years of research suggests that it should in fact be possible.

According to a theory developed by two researchers at Cornell University, Richard Seebass and Albert George, the sonic boom can be minimised by altering the shape of the plane to redistribute the shockwaves that cause it. Sonic booms actually consist of two parts, each caused by a shockwave—one at the front of the aircraft and the other at the tail. The shockwaves arise when the plane is travelling faster than the sound waves it is producing. Unable to dissipate, these sound waves build up instead and form a shockwave. Dr Seebass's theory suggested that shaping the fuselage appropriately could reduce the boom by causing the sound waves to spread out, or better still by causing them to interfere with each other, so that some sound waves cancel others out.

But it was only in 2003 that these theories were finally shown to be correct. The complex shapes required to reduce the boom proved to be very difficult to design by hand. Eventually engineers at America's space agency, NASA, used a supercomputer to simulate the airflow around an F-5 fighter jet and determined how to alter its shape to reduce the sonic boom. In flight tests, the modified F-5—often likened to a “pregnant pelican”—proved that it was indeed possible to reduce the first part of the sonic boom by 25%. Mr Henne believes that Gulfstream can reduce the second part of the boom too, and so drastically reduce the “sonic signature” of a supersonic plane passing overhead. Supersonic flight over land might then become a possibility.

Of course, it is not quite that simple. It will be necessary to persuade regulators—the Federal Aviation Administration and the International Civil Aviation Organisation (ICAO)—to reconsider the current ban. Furthermore, to make the new aircraft commercially viable, there would also have to be changes in the way that supersonic aircraft are handled when coming in to land at airports. There is little point in having a supersonic aircraft if it gets put into a holding pattern for an hour when it arrives at its destination. These are ambitious goals which might seem foolhardy if Gulfstream were the only firm pushing for them, but it is not alone. Also looking for reform is an alliance of ten large aeronautical firms—including Lockheed Martin, Boeing Phantom Works, Raytheon and Rolls-Royce—collectively known as the Supersonic Cruise Industry Alliance (SCIA). This group is working with NASA to build a prototype supersonic aircraft.

But with NASA and SCIA having only just begun their partnership earlier this year, such an aircraft still seems a long way off. Developing the technology will take time, as will lobbying to get the regulations changed. For companies such as Gulfstream or SAI (which is taking a similar approach, in conjunction with Lockheed Martin), there is a danger that other firms could take to the skies first.

That is because those in the second camp are taking a different approach, designing aircraft that fly at supersonic speeds over sea and subsonic speeds over land, but far more efficiently than Concorde once did. Leading the pack is Aerion, based in Reno, Nevada. It hopes to have its supersonic business jets airborne by 2010 or 2011, by which time SCIA is unlikely even to have made a dent in the ICAO regulations.

Aerion's approach is based on the massive improvements in engine and airframe design since Concorde, says Richard Tracy, the company's chief technology officer. Its aircraft will be powered by two Pratt & Whitney engines, normally found on far larger aircraft, that can be reconfigured to optimise their performance for both subsonic and supersonic flight. The aircraft will meet airport noise requirements—Concorde used to make a huge amount of noise on take-off and landing—and will also produce a smaller sonic boom than would normally be expected for a plane of its size. That is an unexpected benefit of putting the engines close to the fuselage and above the wings, explains Mr Tracy, which means that most of the sonic boom is directed upwards, not down.

So, which camp is right? Perhaps both. Mr Henne admits that the first camp could lose out to rivals that are not dependent on changes to the current rules. But he is adamant that deregulating supersonic flight is the way to go. According to a survey carried out by Gulfstream, only a quarter of all journeys by business jets are over water. This, says Mr Henne, severely limits Aerion's market. Aerion does not dispute that figure, but argues that the 25% of flights that are over water are still a significant market. Indeed, notes Mr Tracy, the market is big enough that Gulfstream has developed long-range subsonic aircraft to service it, so there should be room for supersonics too.

Nor is Aerion alone in this belief. Earlier this year an agreement was signed between the Society of Japanese Aerospace Companies (SJAC) and France's Aerospace Industries Association to develop technologies jointly that could be used to produce supersonic airliners. Like Aerion, this partnership is concentrating on adapting existing technology to produce a supersonic aircraft that can fly subsonic over land, in accordance with existing regulations. The aim is to have an airliner capable of carrying 300 passengers 6,000 nautical miles, equivalent to the distance from Tokyo to New York.

It is still early days for this joint-venture, but it seems to mean business. In October, researchers at Japan's space agency, JAXA, carried out flight tests of a prototype airframe in Woomera, Australia. It achieved speeds in excess of Mach 2.5, or two and a half times the speed of sound. That said, Akira Yanagida, general manager of engineering at SJAC, says that at speeds above Mach 1.6 the economics simply do not work. As a result, none of the runners in the new supersonic race aims to fly faster than Concorde, which used to cruise at Mach 2. Whoever wins the race, this much seems clear: supersonic aircraft are about to stage some sort of a comeback. And this time they will be here to stay.