Aircraft industry experts think we could be getting closer to seeing a replacement for Concorde flying soon|
John Frassanito & Associates
NASA’s view of how a second-generation SST might look
Ten years ago, aviation was about to take a rare backwards step. The end of Concorde services at the end of 2003 meant it was no longer possible to have breakfast in London, lunch in New York and dinner back in England on the same day.
Concorde was a first-generation [url=http://en.wikipedia.org/wiki/Supersonic_transport]Supersonic Transport (SST), designed in the 1960s. The airline industry knew the end would come sooner or later, and despite serious proposals for a replacement, such as BAE Systems’ AST concept of 1990, no second-generation SST has ever made it off the drawing board. Instead, the airlines demanded low operating costs instead of speed. The new queen of the skies was the massive Airbus A380 – impressive, but just as slow as every other airliner.
BAE’s AST was a 1990 concept by Concorde’s makers to build a 250-seat successor
Why is it so difficult to replace Concorde? A new SST would have to overcome serious technical obstacles, while being cheap enough to operate to satisfy the airline bean-counters.
Concorde had its limits – it could only carry 100 people; it was spectacularly noisy to get off the ground; its sonic booms meant it couldn’t operate over populated areas; and its thirsty engines limited its range – it could manage the North Atlantic, but not greater distances such as trans-Pacific routes. And climate change wasn’t an issue in Concorde’s heyday. Now upper-atmosphere greenhouse gases are another hurdle to overcome.
Technology has moved rapidly since Concorde was developed in the 1960s. So has the aerospace industry found ways to overcome these problems? The answer is yes. Research into SSTs has continued in America, Europe and Japan. And there are plenty of reasons to be optimistic that we might see a Concorde successor in the not-too-distant future.
NASA Michelle M Murphy
NASA is testing this Boeing SST design model in its wind tunnel in Cleveland, USA
Shaping the future
A future SST would have to be much bigger than Concorde – most projects seem to agree broadly on a 200-250 seat airliner with a range of at least 10,000km – which would give it the ability to fly direct from Los Angeles to Tokyo, Seoul or Beijing. The extra capacity would let airlines offer two classes of service, making supersonic travel much more affordable.
The first problem is getting off the ground without earth-shaking noise. Concorde literally blasted off the runway with four afterburning Rolls-Royce-SNECMA Olympus 593 engines. A new SST would have to be cleverer than that.
While Concorde was still in service, Airbus and BAE Systems backed a European project called Epistle, whose brief was to investigate the low-speed performance of fast jets, especially during take-off and landing.
The European Epistle project did the ground work for high-lift SST wings 10 years ago
Using advanced modelling and computer simulations, Epistle concluded that it would be possible to develop a smarter wing with advanced leading edge devices that would get a 200-seater “super-Concorde” off the ground without afterburners. Project leader Dr Ulrich Hermann claimed a replacement for Concorde could be “40% quieter and more efficient on take-off”, which would make the aircraft no louder than a Boeing 747-400.
But this would be at the expense of top speed. Rather than travelling at Mach 2, like Concorde, Epistle concluded that a wing optimised to work well at low speeds would not be able to fly faster than around Mach 1.6. That’s 20% slower than Concorde, but still twice as fast as today’s typical jetliner.
The optimum wing wouldn’t be an elegant Ogee delta like Concorde – instead a cranked wing, sharply swept-back toward the front of the plane, but with a less swept-back outer section, was proposed. Canard foreplanes – extra small wings near the aircraft’s nose - would also be used to give extra lift.
Boeing’s Sonic Cruiser of 2001 could still form the basis of a future SST
If that layout sounds familiar, take a look at Boeing’s 200-seater Sonic Cruiser concept from 2001. This follows all the rules for creating a new SST – except Boeing designed the aircraft to fly at Mach 0.98, just below the speed of sound. That’s 20% or so faster than a subsonic jet, but to many experts it made little sense – the airflow round the plane at “transonic” speeds is especially turbulent, so passengers would be in for a bumpy ride.
Indeed, many believe the Sonic Cruiser was really an SST in disguise, but back in 2001, Boeing was nervous of a negative reaction in the wake of the Air France Concorde crash a year earlier. In the end, the Sonic Cruiser was shelved in favour of the subsonic 787 Dreamliner. However, it’s not been abandoned: last year Boeing applied for some new patents for an aircraft design that looked remarkably like an evolution of the Sonic Cruiser.
More recent research carried out by NASA backs up the Epistle findings. “These future airplanes will be slightly slower, maybe Mach 1.6 to Mach 1.8,” said Peter Coen of NASA Aeronautics Research Mission Directorate. “We can make a better engine for take-off and landing noise by flying at Mach 1.8 than we can at Mach 2. And the shape of the sonic boom solution is easier to achieve at those lower Mach numbers.”
NASA Carla Thomas
NASA’s SSBD demonstrator was no looker, but its bulbous nose cut sonic boom noise
Solving the sonic boom problem is vital. The sonic boom isn’t the sound of a plane passing through the sound barrier – it’s a shock wave that’s constantly made while an aircraft is flying at supersonic speeds – like the bow wave of a ship in the water. You can’t eradicate a sonic boom completely – but you can make it quieter, and NASA knows how.
One project, the Shaped Sonic Boom Demonstrator (SSBD), used a Northrop Grumman F-5E jet fighter fitted with a new, bulbous nose. This spread out the sonic boom shock wave – so instead of one big ‘boom’, the aircraft made lots of smaller ones. Tested in 2003-2004, it was very successful – flying at Mach 1.2 at a height of 32,000ft, no sonic boom was heard on the ground as the SSBD passed overhead. The only downside was the ugly profile of the new nose. A new SST will probably be a lot less elegant than Concorde.
NASA Lori Losey
The Quiet Spike extendable nose probe was even more effective at beating the boom
Another project, Quiet Spike, gained even more remarkable results. Made of advanced composite materials, this was a telescopic needle nose that was fitted to a Mach 2 Boeing F-15B jet fighter. The Quiet Spike extended from 14ft in subsonic flight to 24ft in supersonic flight. Tested from 2004-06, the Quiet Spike changed the shape of the sonic boom pressure wave to smoother, more rounded waves. This resulted in a softer sound that was measured as being 10,000 times quieter than the Concorde sonic boom – almost inaudible on the ground.
So it’s clear you can get an SST off the ground, and it should be able to fly over land at supersonic speeds with almost no intrusive sonic booms. But what about engine pollution? Recently there has been a lot of progress in engine design, largely driven by advanced military projects such as the F-35 Joint Strike Fighter.
GE’s ADVENT engine promises to be quiet at low speeds and efficient above Mach 1
New forms of propulsion
Engine-maker GE may have the solution. New military-derived ADVENT (ADaptive VErsitile ENgine Technology) engines behave like a quiet turbofan at low speeds, but can be switched in flight to run like a turbojet at high speeds, by diverting air through the turbine, rather than bypassing the turbine through the fan. Combined with advanced intakes, this would allow an SST to ‘supercruise’ supersonically very efficiently at high altitudes, burning less fuel, and emitting less CO2.
By the time a next-gen SST is ready for service, airliner noise regulations will have been tightened up, so the aircraft would have to be a lot quieter than the Boeing 747-400 that the Epistle project used as its benchmark in 2003. But their low-speed ability to run like an efficient turbofan means ADVENT engines should be able to meet future noise rules.
This wind tunnel model shows Boeing’s latest design concept for a future SST
Recently it has emerged that a number of SST projects have at least reached the wind tunnel stage. NASA has been testing a Boeing concept, while the Japan Aerospace Exploration Agency has unveiled a design, which it plans to test as a pilotless scale model. Japan has been working on SST designs since 2005, when it announced a joint research project with France, though this failed to yield a final design.
The Japan Aerospace Exploration Agency plans to flight test this working model SST
A new dawn for aviation?
Realistically, a new SST could be ready by around 2025 – if the airlines can be convinced. And that’s the key. There’s clearly a way – all Concorde’s shortcomings can clearly be solved. But is there a will to make a new SST? For now, airlines are focused on efficiency and costs. But Airbus forecasts the number of airliners on service worldwide will double by 2030, from 16,000 or so at present to more than 30,000. If maybe 500 of those extra airliners were SSTs, the project could be viable.