Supersonic is Back (Quietly)

The current interest in supersonic flight is something of a surprise, given how many previous efforts have failed. The United States, Russia, France, and Britain have been building supersonic airplanes since the 1950s, but so far nobody has produced one that makes sound commercial sense. Most recently, in 1999 NASA and Boeing abandoned a 10-year, $1 billion joint effort to build a 300-seat commercially viable supersonic airliner. Despite technical successes, Boeing ultimately deemed the project financially impractical and pulled out. Given how difficult it is to launch a relatively straightforward project like the Airbus A380 double-decker jumbo, Boeing bosses balked at selling airlines on something as risky and radical as a supersonic jet.



Nevertheless, supersonic researchers today radiate a newfound optimism: Barnaby Wainfan, an aerodynamicist at Northrop Grumman, says the goal "is to explore the possible and the only slightly impossible."



Lowering the Boom



With all the starts and stops of the past, why has Gulfstream decided to start chasing the supersonic dream? Because it's decided the money's there. Gulfstream's customers, the wealthiest people in the world, are already willing to drop $45 million on a conventional private jet like the Gulfstream V. The G-Five is the company's intercontinental business jet. Built to land on relatively short (6,000-foot) runways, it justifies its price by flying between small, uncrowded airports close to the traveler's final destination. But it flies at the same poky rate as a Boeing 747.




Since time is money, Gulfstream presumes the same people who buy G-Fives would happily pay $80 million to go twice as fast. Richard Santulli, chairman and CEO of Executive Jet, the largest business jet operator in the world, told Wired and The New York Times last year that his company would buy a large fleet of low-boom supersonic business jets as soon as they became available.



But Gulfstream still needs to overcome a major hurdle: the boom. Until recently, most people thought the boom was an inescapable consequence of supersonic flight. Why? A regular, subsonic jet -- say, a 747 -- travels at about 550 miles per hour. That's slower than the speed of sound, which is 760 mph at sea level, and more like 660 mph at the altitudes where jets cruise (sound travels more slowly in thinner air). The air a jet displaces normally flows around the plane the same way a stream flows around a rock. But when an airplane surpasses the speed of sound, the air can no longer easily flow out of the way. Instead, the plane compresses the air as it passes through. The impact then sends a pressure pulse through the atmosphere.



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 the boom is often heard as a double bang.