Julian Moxon/PARIS
Additional reporting by Guy Norris and Graham Warwick
While the announcement was unexpected, few could have been surprised when Dassault Aviation announced at the 1997 National Business Aircraft Association (NBAA) show that it was "studying the feasibility of a supersonic business jet".
Dassault's extensive supersonic combat aircraft experience, coupled with its highly successful business jet activities, meant that the French manufacturer was already deeply involved in those areas that were most important to such a project. Furthermore, Dassault's advanced "digital windtunnel" - its computerised fluid dynamics (CFD) technology - would provide the means to back up the development effort.
Despite these advantages, Dassault now admits that the programme still faces the most fundamental of problems - finding a suitable engine. At the NBAA show in October, Dassault Aviation's senior vice-president for civil aircraft, Jean-Francois Georges, said that, while the programme to date had a "lot to show for its efforts", the question of a suitable powerplant remained unsolved.
The problem highlights the enormous difficulties in the design of civil supersonic aircraft. Not only must the engine meet all current and planned noise, emissions and safety criteria, it must also provide reliable supersonic thrust for the planned 7,400km (4,000nm) range without using a fuel-hungry afterburner.
Georges points to three possible powerplant solutions: a military engine; a larger, more powerful version of an existing commercial engine; and a redesign of an available powerplant.
The military solution was tried first but quickly rejected, because such powerplants are not designed for sustained supersonic flight. "We found that for an acceptable service life they would require more material changes or improved cooling techniques than we had anticipated," says Dassault. Typically, the company says, a military engine operates for 150-300h between overhauls. "The business jet community requires at least ten times that."
A larger, more powerful engine, although providing the necessary thrust with lower temperatures and hence longer life, would drive aircraft size up, with a knock-on effect on production and operating costs. The resulting design "would have to be monitored very carefully", says Dassault. Finally, the "major redesign of an existing type" would increase development time and therefore cost, while losing some of the reliability advantages of a proven engine.
Dassault says it is talking to engine manufacturers now about solutions, but insists that any development would have to be undertaken on a risk-sharing basis. The problem here is that, lacking any application other than a supersonic business jet (SSBJ), the production run would be too small to justify the enormous development costs associated with a new or even derivative engine. Dassault declines to comment on the idea, already mooted by some, of joining forces with competing SSBJ developers to stimulate powerplant development.
The only other manufacturer to declare an interest in developing a supersonic business jet is Gulfstream Aerospace, which announced at the UK's Farnborough air show in September that it has teamed with Lockheed Martin Skunk Works to conduct an SSBJ technical and market feasibility study. "We signed an agreement with Gulfstream to investigate the feasibility of an SSBJ," says Skunk Works vice-president Paul Martin. "It would be a 10-passenger aircraft with Mach 2.2 capability and a range of 4,000nm. We are working on the engine and shock technology to see if it really works."
Gulfstream sees the SSBJ as an extension of its family of large-cabin, long-range business jets, the IV and V, but cautions that development, if it goes forward, can be expected to take 8-10 years. The two companies are engaged in an 18-24 month first phase, looking at the "...technical, environmental, regulatory, certification and market issues, and cost," says Gulfstream president BillBoisture.
The Skunk Works is leading the feasibility study effort, working with a "modest" number of Gulfstream advanced design personnel who are providing input on the business customers' needs. "We are looking particularly at technologies for efficient supersonic cruise," says Martin. "The Skunk Works brings a big technology base to that." The baseline design unveiled at Farnborough was a twin-engined delta with 11-12m (35-40ft) wingspan, a cabin similar in size to that of the GIV and a 38,500-41,000kg (85,000-90,000lb) gross weight.
Lockheed Martin is looking for support from NASA's HighSpeed Research (HSR) programme, possibly in the form of funding for a demonstration vehicle that could form the basis of an SSBJ. "We are talking to NASA about a subscale aircraft that would be about bizjet size. It would be an X-plane SST," Martin says. Boeing, which has held SSBJ discussions with Dassault, was also asked by NASA to sketch out proposals for an SSTX-plane. The resulting twin-engined, delta-canard configuration was close in size to anSSBJ, but Boeing's proposal was rejected on costs.
Despite delays to the HSR programme and threats to continued funding, the Skunk Works continues to see promise in a supersonic business jet. "We think the SSBJ might be just the thing for Lockheed Martin to get into - it fits well," says Martin. Gulfstream remains cautious, believing there is some "very basic studying" still to be done, but it cites the Skunk Works creativity in solving problems.
Dassault, meanwhile, believes that, compared with the powerplant issue, other aspects of the supersonic business jet present far less of a challenge. "All technologies are within reach," says Georges. "We have at our disposal CFD, fly-by-wire flight controls, mixed composite/titanium structures and even a synthetic vision system to eliminate the weight of a Concorde-like drooping nose".
The company's Rafale future fighter programme yields much of this technology, but Dassault also cites its involvement in the hypersonic Crew Rescue Vehicle for the International Space Station. "The folks at NASA were extremely impressed by our sophistication in CFD", says Georges. "We were able to help them from Mach 20 down to subsonic speeds with performance, stability, flight controls, vehicle temperatures and many other things. We're years ahead of other business jet manufacturers and probably most military manufacturers as well."
The first six months' design work on what Dassault has dubbed the Falcon SST has concentrated on engines, systems and structures, and the next six will bring in low speed and supersonic windtunnel testing and estimation of development costs. This, in turn, will enable an estimate of pricing, helping Dassault to forecast the likely market size and to hold preliminary discussions with potential operators.
As it stands, the supersonic Falcon features a cranked delta wing and canard. The aircraft is powered by three rear-mounted engines, each producing 12,000lb (53kN) of thrust at take-off. Length is 32.3m, wingspan 17m and maximum take-off weight 39,000kg. Of this, around half (53%) is fuel.
A weight saving "smart" landing gear is envisaged, with four-wheel bogies that tilt hydraulically just before lift-off so that the gear is effectively longer and the point of rotation moved rearward, providing enough ground clearance at the 14í take-off angle without needing a longer main gear. The same process happens in reverse during landing.
The passenger cabin is around the same size as that of the Falcon 50EX subsonic business jet, but has an ovoid instead of circular cross-section, "which allows a flat floor with the same headroom as the Falcon 50EX as well as facilitating aerodynamic blending of the wing and fuselage", says Dassault.
The tri-jet configuration, already familiar to Falcon operators, was "an easy decision", says the company, because it allows the engines "to be sized around normal operational profiles instead of around the scenario of a failed engine at take-off". Three tail-mounted powerplants also provide for a centre engine exhaust stream, which Dassault says "minimises the base drag of the fuselage in supersonic flight".
The planned M1.8 cruise is seen as the best compromise between the need for high speed and increasing engine durability and reducing skin temperature. As for the 7,400km range, Georges says this is an "almost magic figure" because it is long enough to fly to many destinations along a direct, "although not necessarily non-stop", flight path yet short enough in terms of time (under four hours) for eight-passenger comfort. Dassault adds, however, that "designing an efficient supersonic aircraft able to cruise even this far is a very difficult challenge".
A decision on whether the supersonic Falcon will become a fully fledged programme is unlikely until May at the earliest. Assuming a go-ahead, Dassault estimates that a five-year development period will be required to adapt an existing engine, longer is a new powerplant is required.
Source: Flight International
