Sound barrier

The retirement of Concorde has re-ignited the quest for a quiet supersonic business jet to fill a niche at the high end of the luxury market

The last-ever flight of Concorde on 27 Nov-ember last year brought to an end an exciting period of supersonic transport, during which those travellers with sufficiently deep pockets could cross the Atlantic in just over three hours. Economics, the downturn in the airline industry and environmental constraints finally grounded the sleek delta-winged aircraft. Few experts believe that there will be a successor before 2050, but experiments continue unabated.

However, the emphasis has shifted from an airliner seating hundreds of passengers to a smaller supersonic business jet (SSBJ), targeting those rich, influential and busy people for whom the demise of Concorde has left a void, and others who crave the prestige and speed, but need more flexibility. The requirements from operators appear to come down to an aircraft that can convey 8-12 people a minimum distance of 9,250km (5,000nm) or non-stop across the Pacific, at a speed of at least Mach 1.6.

Manufacturers have already come to the edge of the subsonic envelope through the development of long-range new business jets, such as the Gulfstream V, Dassault's Falcon 7X and Bombardier's Global Express and Global 5000, all capable of non-stop flights well in excess of 9,250km. The next logical step that still remains is to break the sound barrier. Even if the first SSBJs cannot be designed to fly non-stop across the Pacific, a one-stop supersonic service will still win hands-down over today's subsonic jets cruising below Mach 0.9 and flying the same route without a stop.

By comparison, Concorde cruised at Mach 2, while the fastest of the modern widebody airliners, the Boeing 747-400, comes in at Mach 0.86, with others closer to Mach 0.8. Boeing's proposed Sonic Cruiser aimed to raise speeds to Mach 0.95 or above, but even that proved too little to attract real interest.

Sonic boom

But before such a supersonic business jet becomes reality, there is the small matter of the sonic boom. Unless this can be reduced to acceptable levels, permitting supersonic flight over populated areas, the SSBJ could founder, as Concorde did, more on regulatory resistance than on technical challenges, although the latter remain formidable.

The sonic boom, more than any other consideration surrounding supersonic transport, has aroused such extreme emotions that solving this problem has become a holy grail for aerospace engineers. On 27 August 2003, the Defense Advanced Research Projects Agency (DARPA), in a co-operative programme with Northrop Grumman, NASA's Langley Research Center and the Dryden Flight Research Center, flew a specially Shaped Sonic Boom Demonstration (SSBD) aircraft that clearly demonstrated a measurable reduction in noise. Part of the Pentagon's Quiet Supersonic Program (QSP), the SSBD seeks to reduce the intensity of the sonic boom by reshaping the aircraft, particularly the nose, to break up and redirect the massive shockwave created when the aircraft exceeds the speed of sound.

The SSBD aircraft is an F-5E with an elongated "nose glove" that pushes forward and effectively blunts the nose, thus minimising the sudden jump in pressure. Results from the tests indicated an overpressure of 0.055bar (0.8lb/in2), still some way above the stated QSP goal of 0.02bar, but considerably lower than the average 0.13bar that had been generated by Concorde. While this may represent a quantum leap in noise reduction, it still may not be enough.

The position of the FAA in the USA in terms of licensing an SSBJ for unrestricted overland flight remains ambivalent, although the hosting of a technical workshop on civil supersonic aircraft last November indicates that it is taking a much closer interest in this subject. It may also be more favourably disposed towards a US-built jet than it was to Concorde when it arrived in the 1970s. To replace Concorde with an SSBJ that can fly only across the Atlantic makes no more economic sense than it did for the European supersonic transport.

Design problems

The reduction of the sonic boom to an acceptable level is only one of the problems facing designers, and shaping the airframe will go a long way towards achieving this goal. But an SSBJ will also need radically new engines that are less thirsty and keep noise within at least present Chapter 3 limits. Aerion Corporation (formerly ASSET Research) however, actually claims it can achieve these goals using an off-the-shelf JT8D engine in conjunction with its patented laminar flow wing.

All manufacturers are working on solutions, although no consensus on a preferred line of research has been reached. A high-bypass supersonic engine with highly-swept fan blades is one possibility, but another candidate could be a hybrid engine combining a conventional turbofan with pulse-detonation technology. This would provide a simpler and thermodynamically more efficient engine, which, claim proponents, will eventually make gas engines obsolete. However, thermal fatigue and noise are major challenges still to be overcome before application on aircraft becomes a reality.

Another hurdle to be overcome by the designer is the size and shape of the aircraft. Present technology requires a fuselage that is longer and slimmer than present business jets in order to limit the effects of drag and provide a reasonable operating performance. This would mean more cramped conditions within the cabin and runs counter to the wishes of operators who have been clamouring for ever-wider accommodation. This constraint may be overcome by Aerion's laminar flow wing design, which claims to reduce cruise drag by up to 50% compared with accepted delta-wing technology, while also increasing range. Michael Henderson, chief operating officer of Aerion, says the company is working on a new, more exciting configuration, but details will not be ready until the summer.

With load factors having dropped sharply in the last two years of Concorde service, is the industry confident that the market really wants an SSBJ? The trend over the last few years gives a pointer to what the future may hold. Richard Aboulafia, aerospace analyst of the Fairfax, Virginia-based Teal Group, believes there could be a market for 250-400 aircraft over a 20-year time-span, depending on performance. As to price, he says that the very high end of the market seems willing to pay any price. "Within a few years," he says, "the top end of the business jet market went from a maximum unit price of $24 million to $46 million, with hundreds of orders for Gulfstream Vs, the Global Express, and jetliners. Clearly, the market elasticity is relatively low at the high end of the market. Suddenly, a $70-80 million SSBJ is a very reasonable proposition."

Fractional focus

The expected cost of an SSBJ will be within the reach of some companies and individuals, but numbers are limited and, given the high development cost of such an aircraft, would certainly be short of an economic production run for one manufacturer, let alone two. Airframers would, therefore, have to do their sums carefully before entering such a high-risk venture, but there is some encouragement for doing just that. The concentration of private jet users into large fractional ownership providers (and perhaps airlines) will help the SSBJ, says Aboulafia, suggesting that such customers would provide necessary launch orders, probably with considerable risk-sharing money, or large down payments. Military and government agencies would also have an interest in acquiring the SSBJ and could help fund development by placing orders.

"This represents a major change from the past, when business jet users were predominantly small customers who could not be counted on for up-front programme funding," Aboulafia says. "Development of an affordable SSBJ would give fractional companies the ability to offer a full range of luxury services based on comfort and speed, a position held by scheduled airlines until the retirement of Concorde." He further emphasises that an expensive aircraft would need high levels of utilisation, so "a fractional concern would be an inherently better market than individual flight departments".

While this makes sense, Judith Moreton, managing director of Bombardier Flexjet Europe and Skyjet Europe, says that although interest in business jets is definitely on the rise, Bombardier is not investing money and time in an SSBJ. "We continue to study the transatlantic market and have a group of customers who would use an SSBJ if available," she adds, "but our Global 5000 offers an excellent alternative to ex-Concorde passengers." Bombardier has analysed and compared total door-to-door journey times and found that on the London-New York route, a Global 5000 can do the trip in seven-and-a-half hours, compared to five hours for Concorde, with better punctuality, she adds. Of course, an SSBJ would provide a more direct comparison with the Global Express, but Moreton says "the costs don't stack up and an SSBJ is unlikely to be economically viable in the present environment".

The management of the world's biggest fractional ownership programme, NetJets, has a more optimistic outlook. In a presentation to the FAA Supersonic Workshop, NetJets stated that several long-time owners indicated "extreme interest in purchasing an one-eighth share at $10 million for a Mach 1.8 jet with a 5,000nm range and a Citation X cabin." Based on a $80 million price tag, the cost per hour of a one-eighth share for 100-hour annual usage would work out at just under $24,000. But to make an SSBJ viable, NetJets feels that "a 4,750nm transpacific range is a must". NetJets quoted a Gulfstream estimate for a market of at least 250 aircraft, of which 50 would be fractionally owned.

But such divergent beliefs have not deterred some manufacturers. Russia's Sukhoi is proceeding with its S-21 and has stated that it could be in service by 2012. Given its recent history of acute funding shortages for the development of new aircraft, this is highly unrealistic. Gulfstream Aerospace, once involved with Sukhoi on a joint-venture SSBJ, continues to search for a viable design of what it calls the Quiet Supersonic Jet, while in France, the Dassault project, although abandoned five years ago for lack of a suitable engine, is likely to be revisited now that Concorde has gone. Boeing, Lockheed Martin, Northrop Grumman and Raytheon have been involved in DARPA's QSP programme.

Technological challenges

The Aerospace Systems Design Laboratory, part of the Georgia Institute of Technology, has created an integrated computer environment (ICE), which was used to conduct an experiment that evaluated various configurations of a possible SSBJ. Even the application of advanced technologies failed to meet the set parameters, with the sonic boom proving the main sticking point. Many configurations, therefore, will still need to be explored and technological challenges overcome before an SSBJ can be built that is sufficiently "hushed" to get legislators and the environmental lobby on its side. But this uncertainty will not deter aerospace engineers from climbing new peaks in the relentless drive through new frontiers.

The desire of the business community to again fly at twice the speed of sound remains undiminished. For some, a supersonic business jet cannot come soon enough. But it may still be a wait of 15-20 years.

Supersonic business jet proposals & fastest airliners
Manufacturer/ agency	Project/ type	Max speed Mach	Range km
DARPA	QSP (targeted)	2.4	11,100
Dassault	SSBJ	1.8	7,400
Gulfstream	QSJ	1.6-2.0	8,900
Sukhoi	S-21	2.0	6,200
Concorde		2.0	6,200
Boeing	747-400	0.86	11,400

 

REPORT BY GÜNTER ENDRES IN LONDON

Source: Airline Business