Supersonic Business Jet Still a Faraway Dream

 - May 5, 2008, 11:01 AM

Economical, practical, environmentally friendly supersonic flight is the next big thing in commercial aviation. Or is it? From where aeronautical technology stands today, practical supersonic flight (and by “practical,” we do not mean the Anglo-French Concorde, which generates noise and atmospheric pollution levels that preclude all but the smallest volumes of operation) is far off.

However unattainable it may look now, clean, efficient and boom-free supersonic flight would make its inventor(s) very rich and famous indeed. That’s why three major players, Gulfstream, Dassault and Sukhoi are working quietly on aviation’s version of the better mouse trap.

Designing a supersonic transport is a chicken-and-egg thing. The law of inverse squares dictates that as drag exponentially increases at Mach-plus speeds, so does the need for greater amounts of thrust. More thrust means bigger engines, which consume more fuel, which means a bigger airframe, which means more drag and so it goes, in a vicious circle.

Even worse, the larger the SST, the bigger the sonic boom. Because of the boom, Concorde, the world’s only operational SST so far, can break Mach only over the ocean.

Many manufacturers have pondered the problems of environmentally sound, economically viable supersonic flight. Spurred on by pledges from big buyers such as fractional czar Richard Santulli, who has promised to buy a fleet of cost-effective, low-boom jets for his NetJets program, Gulfstream Aerospace is one of only three airframers (in addition to Dassault and Sukhoi) worldwide looking at the challenge.

Size is one way to beat the sonic boom, Gulfstream researchers report. The 202-ft-long Concorde seats up to 108 passengers. An SSBJ would likely seat only a dozen or so passengers. Quid pro quo, Gulfstream says, smaller would be quieter. And there are other ways to keep the noise down. Aircraft shape is just as important as size. The shock wave generated by a supersonic aircraft strikes the ground in the shape of a letter “N.” The intensity of the pressure pulse decreases as it moves away from the airplane. But at the same time, the pulse changes shape, coalescing into an N-shaped wave. Within the N-wave, the pressure rises sharply, declines gradually and then snaps back up to the normal atmospheric pressure. Meanwhile, a wall of compressed air, moving at the speed of the airplane, spreads out from the wave. As the wall of air passes over the ground, it is heard and felt as a sonic boom. The human ear picks up the pressure increases at the front and back of the N-wave, which is why a sonic boom is often heard as a double boom.

If the airplane is long in proportion to its weight, the N-wave pattern is spread across a greater distance and the peak pressure will be lower. In addition, if the wings are spread along the body, not concentrated in the center as in a conventional aircraft the pressure pulse bulges less and there’s a smaller boom. One prospective design from Gulfstream (tentatively dubbed the QSJ, for quiet supersonic jet) would be about 140 ft long but carry only eight to 14 passengers. And in the words of NetJets executive vice president Richard Smith, such an aircraft would have to be quiet enough to operate over land. “If you can’t fly supersonic over land,” he said, “it’s not worth developing.”

Whatever shapes emerge from Gulfstream’s designers, they will be heavily influenced by research financed by the U.S. Department of Defense’s Advanced Research Projects Agency (DARPA), which awarded close to $35 million in contracts last year to the big three U.S. aviation companies–Boeing, Lockheed Martin and Northrop Grumman–for supersonic research. If DARPA gets more money from Congress, it will select one of the three to build a prototype low-boom supersonic airplane–a so-called X-plane. While the military would obviously have first dibs on such a design, technological spillover onto the civil side is expected.

Together with the British, the French aerospace industry has more operational experience with sustained supersonic flight than most of the other advanced western nations combined. This is almost entirely due to its participation with Concorde. However, Dassault, with its decades of development expertise in designing, building and flying some of the world’s fastest and most aerodynamically advanced fighter jets, has long expressed an interest in combining that technology with its long-lived line of business jets to repeat its own version of the Concorde triumph.

Admitting in 1997 it was beginning the vast amount of preliminary work needed before it could even seriously think of cutting metal for an SSBJ, Dassault crunched numbers until March 1999, when it shelved the plan, expressing a new focus on the more prosaic problems of selling business aircraft of conventional capabilities. Since then, however, Falcon has been quietly meeting with engine makers and other possible program partners, keeping the flame alive while admitting to little.

Nothing happens quickly in peacetime aircraft development and when the aircraft to be developed represents a quantum leap forward in performance and technology, then timetables can be expected to span decades. In a spasm of optimism during the 1999 Paris Air Show, Sukhoi predicted this year as the date for first flight of its SSBJ, dubbed the S-21. That estimate has since been stretched to 2010, followed by an entry into service by 2012.

Earlier this year Russian officials announced that the S-21 program is being folded into a broad-based entity known as the “new” Sukhoi Aviation Corp., which includes an assortment of design and manufacturing entities, among them OKB Sukhogo (the historic Sukhoi design bureau) and the production plants at Irkutsk, Novosibirsk, Komsomolsk-na-Amure and Taganrog. As the divisions shake out their various duties, the task of development and design is expected to fall within the fiat of the Sukhoi Civil Aircraft Corp. Executives at SCAC freely admit that the scope of a supersonic business jet program is far beyond their company’s abilities, which is why SCAC is canvassing aerospace firms worldwide and is preparing memorandums of understanding with possible joint partners.

Target specifications for the S-21 call for a subsonic cruise speed of Mach 0.95; supersonic cruise speed of Mach 1.80; range (subsonic or supersonic) of 5,000 nm; and mtow 124,000 lb.

Runway length for takeoff: 6,500 ft. Proposed number of passengers: six to 10. Real chance of finding sufficient funding and actually making it into production: anybody’s guess.

At one time the Sukhoi SSBJ was to have been propelled by a trio of Soyuz VK21 turbofans, generating a total of some 64,000 lb of thrust. That design has reportedly been abandoned in favor of a two-jet arrangement, with those engines most likely to be made by Rolls-Royce.