High-speed business jets will prove the concept of next-generation supersonic flight, according to research being carried out in Japan

Japan's National Aerospace Laboratory (NAL) is determined to press ahead with its Next Generation Supersonic Transport (Nexst) research and development project to prepare the country for full partnership in any international programme to develop a successor to Concorde. While some in the industry do not share NAL's optimism that large supersonic jets will be whisking passengers across the Pacific at twice the speed of sound as early as the middle of the next decade, several manufacturers are studying supersonic business jets.

NAL next generation SST project-centre director Kimio Sakata believes such business jets will serve to prove the concept of next-generation supersonic flight because the traditional problems of take-off noise and sonic boom suppression will be easier to achieve in smaller aircraft. "It should take time to solve, but smaller aircraft are less severe. The sonic boom is proportional to the aircraft's weight," he says.

The Nexst project was launched in 1997. It brought together strands of SST research work and market studies performed by individual companies and academic establishments in Japan. The decision to focus on a next-generation SST was born of Japan's desire to elevate its aerospace industry from being a mere parts supplier on international collaborative programmes, to a leader in design, development and manufacture.

With little prospect of penetrating world markets with a conventional subsonic design competing with current Airbus and Boeing products, Japanese policy makers decided to focus on a second-generation SST. One argument in its favour is that Japan would be a strong beneficiary of such an aircraft, given its relative geographic isolation from its trading partners in Europe and North America.

NAL was awarded funding to start a nine-year SST research programme in 1997, and, in April this year, gained more autonomy from the government in its decision-making as an "independent administrative institute". Due to finish in 2005, completion of the $280 million project will almost certainly slip by a year after the new Japanese administration of Prime Minister Junichiro Koizumi decided to review all the government's spending priorities. A decision on funding the remainder of Nexst is scheduled for year-end, but Sakata says he is confident the project will survive the budget review.

The conceptual four-engined SST being studied by NAL would carry 250 passengers over a range of around 11,000km (5,950nm) at a nominal speed of Mach 2.0. The cranked-arrow, natural-laminar-flow wing design would be about 100m (33ft) long, with a maximum take-off weight of about 360t.

Performance characteristics

To produce an aircraft with performance characteristics economic enough to be compatible with commercial operation, NAL is developing state-of-the-art computational fluid dynamics (CFD) software. The use of CFD techniques is one of the lynchpins of the Nexst project and represents a radical departure from traditional approaches to aircraft design, which rely heavily on windtunnel testing and empirical databases.

The so-called "inverse design method" developed by NAL enables engineers to define precisely the desired aerodynamic characteristics of the aircraft. The CFD software takes these performance values and works "backwards" to design the aircraft shape that will achieve those characteristics. Taking the theory a step further, the "optimised inverse design method" involves the CFD software attempting to calculate the "best" value for a given performance characteristic. "Then the computer will design a shape to achieve that value," says Sakata.

The CFD software is run on a Fujitsu supercomputer with 160 parallel processors operating at a speed of 200 Gigaflops, and a full aerodynamic calculation takes about a week's worth of processing time. "We are thinking of replacing that computer next year with one that is more than 100 times faster and has more memory," he adds. A key target is to achieve a lift/drag ratio of 9, which Sakata estimates is "30% better than Concorde", and which would allow wing area to be reduced, giving substantial weight savings. "If we have optimisation technology, we can have [a lift/drag ratio] better than nine. I think 10 should be the next target for that," says Sakata. "Low drag is the most important thing, so the low drag effect is the key for the design technology," he adds.

The Nexst project is approaching a milestone with the first flight of an unpowered experimental one-tenth scale model of the SST design due to take place by June at the Woomera test-range in Australia. The instrumented 11.5m (38ft)- long model will be rocket-boosted to a speed of Mach 2.0 at an altitude in excess of 59,000ft (18,000m), and make a supersonic gliding descent to validate the basic performance of the design. A parachute system is incorporated to enable recovery of the test article at the end of the flight.

A second unpowered flight will be attempted shortly afterwards, followed by two more around six months later. According to Sakata, a full analysis of initial flight-test results will take about a year. Critical to drag-reduction is understanding the behaviour of the "boundary layer", or turbulent air making contact with the aircraft's surface - the test flights will perform an important role here. Friction drag makes up about one-third of the aircraft's overall aerodynamic drag.

"The boundary-layer issue is not easy to introduce into the CFD. After the flight experiments we will have very good data for predicting the boundary-layer characteristics. We can simulate the external flow outside the boundary layer, but the boundary-layer phenomenon is very much based on molecular movement," says Sakata. The wing is designed to produce a pressure distribution that induces natural laminar flow. "There is a very steep pressure gradient. Rapid expansion of the air in the vicinity of the leading edge means that cross-flow instability is suppressed," he adds.

So-called cross-flow instability, or the tendency of the air flowing over the wing to be deflected spanwise, can induce a turbulent boundary layer, and hence prevent laminar flow and increase drag. The design of the wing for M2.0 cruise flight will have to be compromised to deliver the required high lift at low speeds for take-off and landing, which Sakata calls "multi-point optimisation". High lift will be achieved using leading-edge slats and trailing-edge flaps.

Another key aspect in the development of a second-generation SST will be the application of advanced composite materials. NAL envisages the aircraft incorporating both a composite fuselage and wing, although major load-bearing elements such as the wing-body join would still be made from aluminium. "The target reduction of the weight is about 20% compared with present technology. The outer surface would be all composite, which has a thermal expansion ratio less than aluminium," says Sakata. NAL is studying carbon-fibre reinforced plastic composite materials constructed with three-dimensional interwoven matrices featuring lateral fibres, rather than conventional two-dimensional layered matrices. In conjunction with the materials work Fuji, Kawasaki and Mitsubishi Heavy Industries have received government funding to develop the complex manufacturing techniques required to produce such materials.

The programme's culmination will be the start of powered flights from mid-2005. At least 20 flights using a pair of one-tenth scale twin-engined aircraft are planned. The models will be dropped from an Orbital Sciences Lockheed L1011 TriStar. The fact that the models are equipped with only two engines rather than the four of the full-size design means that the "interaction between the nacelles is not dealt with", says Sakata.

Even if it can be demonstrated that a large second-generation SST with transpacific range could be operated profitably, there are still several major and potentially prohibitive obstacles to be overcome. These include the problems of take-off noise, NOx emissions and the sonic boom. "Low noise is one of the most difficult and important features to be studied," says Sakata. He envisages the use of ejector nozzles combining the exhaust flow with ambient air to reduce take-off noise. "The concept is clear, but it's heavy and expensive," he adds.

NOx emissions could be difficult to tackle because the bypass engines of the SST would operate at 100% of their thrust-rating during the cruise, and therefore at higher combustion temperatures compared with conventional aircraft, which require maximum engine-thrust only during take-off and climb. However, Sakata believes that with advances in engine technology, NOx could be reduced to acceptable levels.

On the sonic boom issue, Sakata thinks supersonic flight over inhabited areas by such a large aircraft may be possible, but not at M2.0. "I think that higher than M1.4 is not good for populated areas. If we obtain 'low boom' technology we can fly over at something like M1.5 or M1.6," he says.

Source: Flight International