IT MUST have made a terrific din. But luckily as the aircraft tore across the spring sky over Woomera, South Australia, last month at more than twice the speed of sound, there was no one around to hear it except a few kangaroos. Shot into the sky on the back of a rocket, the plane was testing a new wing design for what might become the next supersonic passenger airliner, known as the âson of Concordeâ.
The Japan Aerospace Exploration Agency (JAXA), which built the prototype, hopes the next generation of supersonic jets will be carrying passengers within 15 years. The aim is to build a 300-passenger aircraft that could whip you from London to Tokyo in a little over 4 hours.
And the JAXA project, which is coordinated by the Society of Japanese Aerospace Companies (SJAC) and the French aerospace industries association, is by no means the only one. Just two years after Concorde was retired from service, a global race to launch the next supersonic passenger jet is under way.
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The other main competitor is a consortium called the Supersonic Cruise Industry Alliance (SCIA), and includes Boeing, Raytheon, Lockheed Martin and Rolls-Royce. It would like to build a 30-metre prototype at the moment, though technology is not the consortiumâs only concern. It is hoping to persuade the regulators to lift the ban that prevents supersonic aircraft flying over your house.
These two groups could be pipped to the post by any one of a number of smaller firms, including Supersonic Aerospace International (SAI), which has secretly contracted another firm to develop its design, and Aerion and Gulfstream, which are working on their own. However, these companies are focusing their efforts on business jets, and at around $80 million each, they will be exclusively for ĂŒber-rich celebrities and time-poor executives. âThere is no question we will see supersonic business jets before we see supersonic airliners,â says Richard Tracy of Aerion. He says Aerionâs jet will be in the air within six years.
Many of the new designs will be based on Concorde, though they will have various tweaks and modifications. These are vital because, while it was ahead of its time, Concorde had two critical flaws: it made a lot of noise and no money. The aircraft was horribly wasteful of fuel, burning almost five times as much per kilometre as a jumbo jet. To tackle this consumption problem, the JAXA group is starting out with modest speed ambitions: its first supersonic aircraft will not reach Concordeâs Mach 2.04. âThe main obstacles were noise and [fuel] economy,â says Akira Yanagida of SJAC. The group has a target of Mach 1.6 to 1.8, which will make the engines more fuel efficient, and stop the aircraft heating up as much as Concorde â at Mach 2, Concordeâs nose reached 127 °C. Lower temperatures mean lighter materials such as epoxy resin and plastic reinforced with carbon fibre can be used, another benefit for fuel efficiency.
Noise was Concordeâs other major problem. It created a huge din at take-off and an ear-splitting sonic boom once it passed the sound barrier. The International Civil Aviation Organization banned Concorde from supersonic flight over populated areas, and because its engines were even less efficient at subsonic speeds, it burnt even more fuel. The net effect was that Concorde was restricted to flying routes that took it predominantly over water, such as London to New York, severely limiting its market.
The boom is a problem faced by all supersonic jets. As an aircraft flies, it pushes the air in its path out of the way, producing pressure waves in the air that radiate out from it like waves on the surface of a pond when a pebble is dropped into it. If the aircraft is travelling faster than the speed of sound, the sound waves become compressed into a single shock wave (see Diagram). This tsunami of noise trails behind the aircraft just like the wake created by a speedboat.
In 1972, long after Concordeâs design had been finalised, Richard Seebass and Albert George at Cornell University in Ithaca, New York, came up with an idea that could have drastically reduced the aircraftâs sonic boom. Based on a theoretical analysis of how sonic booms form, they suggested a theory that described how to minimise the boomâs intensity. The implication of the theory for aircraft designers trying to reduce the sonic boom is profound: the lift force that the aerodynamics of the aircraft produces must be evenly distributed along its length. Conventional aircraft have regions where the lift is concentrated, such as under the wings, and this causes a sudden change in air pressure at these points along the length of the aircraft. This concentration of pressure is the main culprit in producing sonic booms. Spreading the pressure change more evenly along the length of the aircraft staggers the pressure waves, and the intensity of the boom is reduced.
One counterintuitive consequence of the Seebass and George theory is that to dampen the sonic boom, the nose of the aircraft must be more blunt than pure aerodynamic considerations would suggest. This is because producing a slightly larger initial pressure change at the front of the aircraft helps prevent the pressure waves down the length of the aircraft coalescing into a single intense shock wave. Unfortunately the theory required modern-day computer-aided design software to make it possible to design an aircraft that obeys this theory. As a result, although well known in the aeronautics community, it never got further than an academic journal.
But in 2000 the US Defense Advanced Research Projects Agency (DARPA) returned to it, teaming up with aircraft manufacturer Northrop Grumman to test sonic boom reduction on a modified F-5E fighter jet, called the Supersonic Boom Demonstrator (SSBD). Their design turned the aircraftâs nose into something that looked like a pelicanâs beak and in tests, sponsored first by DARPA and then by NASA, reduced the pressure of the shock wave the sonic boom produced by more than 30 per cent (see Diagram). Buoyed by these results, NASA planned to fund separate groups at Northrop Grumman, Boeing and Raytheon to help build a 30-metre-long demonstration aircraft, but in a spending review a few months ago, funding was suddenly cut and the project suspended.
âThey want supersonic jets to be able to jump the queue when coming in to landâ
There are other ways of reducing the boom. Seebass and George found that increasing the length of the aircraft without increasing its weight can reduce the intensity of the boom, and perhaps even dampen the noise to the point where no boom is heard (51¶ŻÂț, 18 August 2001, page 26). So Gulfstream is testing the feasibility of temporarily lengthening the aircraft during supersonic flight using spikes extending from the nose and tail. And though it is only planning to build small jets, the design principles should translate to larger aircraft, says Preston Henne of Gulfstream. He believes building a smaller aircraft first will provide crucial lessons. âWith supersonic aircraft we never took that step, we immediately jumped to Concorde.â
Tom Hartmann, project manager of the Quiet Supersonic Transport (QSST) programme at Lockheed Martinâs Skunk Works in Palmdale, California, agrees with Henne. âOnce you solve the problem of supersonic boom suppression for business jets, you can scale up to larger aircraft and that opens the possibility for supersonic flight for everyone. Thatâs my ultimate goal,â he says. But he thinks Gulfstreamâs extendable spikes design is a blind alley. âThere is a big question about whether the spike design really works,â he says. Itâs unclear in practice what benefit extendable spikes will have. Itâs not clear why youâd ever want to contract the nose and tail in flight, says Hartmann, so why not design it to that fixed length from the start?
For the past four years, Hartmann and his team have been working on a secret project to design a low-boom supersonic business jet. They wrote a computer program to help them create aircraft designs that comply with Seebass and Georgeâs theory to reduce the noise of the sonic boom, while maintaining aerodynamic performance and taking into account design constraints by modelling the balance of the new design and the behaviour of different aircraft materials, for example.
Hartmann and his team started by scribbling cartoon-like wing designs on a scrap of paper, then modifying them to account for the fuselage and engines, and finally plugged each one into the computer program to see if they produced the desired results. âWeâd go through hundreds of tweaks to a design and then find it was simply unworkable and have to start from scratch,â says John Morgenstern, one of the designers on Hartmannâs team. After testing hundreds of designs in the computer, the team settled on a shape that fitted all the aerodynamic, low-boom and cabin-space requirements, built a 2-metre-long scale model and tested it in a wind tunnel. The result was the Quiet Supersonic Transport jet, which reduces the boom by redistributing the areas that create lift more evenly over the entire length of the aircraft.
Hartmann calculated the design would cut the boom noise to 62 decibels on the ground. âIt would make less noise than a truck passing on the street outside my house,â he says. But he admits that the vibrations caused to buildings by a QSST flying overhead might still be a problem. âItâs the âgrannyâs Chinaâ problem: you might not hear the boom, but things attached to walls would probably still shake violently as the shock waves from aircraft passing over head vibrate them,â he says.
âWe are now ready to build a full prototype that could be flying in three years,â Hartmann says. Earlier this year, SAI revealed it was Lockheed Martinâs secret customer and said it expected to have a prototype QSST flying by 2012. It is now choosing a manufacturer.
The sonic boom is only really a problem when flying over land. The JAXA consortium has decided a higher priority is to focus on reducing noise at take-off using quieter, more efficient engines. According to Yoshiyuki Fujitsuna, an engineer at the Engineering Research Association for Supersonic Transport Propulsion Systems in Tokyo, and part of the JAXA research effort, one of the groupâs designs can reduce noise at take-off from Concordeâs 110 decibels to around 92 decibels.
And some jet makers are focusing on improving the planeâs fuel efficiency at subsonic speeds. Aerionâs plan incorporates a novel wing design that produces more lift at lower speeds, without compromising the aerodynamics of the aircraft at supersonic speed, which means it can fly more efficiently than Concorde at subsonic speed, but can cope with speeds of Mach 1.6 for a dash across water. âWe are not anticipating flying supersonic over land,â says Tracy.
Henne thinks this is a mistake. âIf you donât address the sonic boom then you have severely restricted your market,â he says. Only 25 per cent of all Gulfstreamâs flights are over water. For Aerion this would mean flying subsonic for the majority of journeys, Henne says. Along with eliminating the speed benefits, this also means the aircraft wonât be as fuel efficient. âWe can do better today than in the time of Concorde,â he says.
While engineers work on the design challenges, another battle is going on behind the scenes. As international regulations stand, flying at supersonic speeds over populated areas of land is forbidden. So companies including Gulfstream and the members of the SCIA group are lobbying the International Civil Aviation Organization for a change. Reviews of the regulations happen every three years â the next is scheduled for 2007. Bob Meyer of NASAâs Dryden Flight Research Center at Edwards, California, says that, before funding was cut, SCIAâs plan was to have supersonic aircraft with a reduced sonic boom ready for the ICAO review in 2010. That now looks unlikely to happen in time.
âWe are now ready to build a prototype that could be flying within three yearsâ
Hartmann hopes regulators will relax the laws about flying supersonic over land but thinks they might resist, because of the cost of verifying new regulations. âThey donât have that money right now,â he says.
And lobbyists wonât be stopping there. They want air traffic controllers to allow supersonic jets to jump the queue when coming in to land. They argue that forcing supersonic aircraft to slow down on approach to their destination and circle the airport forces them to operate in a very inefficient and polluting manner.
Whatever happens, it seems there will be a market for supersonic flight. âWeâd love to see the Virgin Atlantic colours flying at supersonic speed and weâve been watching recent developments with interest,â says Richard Branson, who was keen to buy the Concorde fleet before it was retired. âThereâs no doubt in my mind that there would be plenty of takers for shorter flights to the US and especially the Far East and Australia,â he says. One way or another, those kangaroos better get used to the sound of supersonic flight.
