The big news in low-boom research is that it appears possible to reduce the boom intensity simply by changing the shape of the airplane. For one, small planes create less of a disturbance. That's because the sonic-boom problem is caused by air displacement: Planes rest on a column of compressed air, and the lighter the plane, the lower the pressure.
Shape is just as important as size. If the plane is long in proportion to its weight, the N-wave 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 plane, the pressure pulse bulges less and there's a smaller boom. The Gulfstream plane would be about 140 feet long but carry only 8 to 14 passengers.
Pres Henne, Gulfstream's senior vice president for programs, folds his arms to demonstrate these concepts. "There's a sweet spot here," he says, indicating the place where his arms cross. If the airplane is relatively skinny and flies at just the right altitude, there is a point where the N-wave does not form and the peak energy at ground level -- the boom -- is weak.
The question remains: How quiet is quiet enough? Nobody really knows for sure. Researchers have decided, however, that it's really not crucial to find out. Rather than trying to decide where the acceptable sound threshold lies, they've decided to skip a step, simply setting a radically low goal for boom noise. Whereas the pressure at the front of the Concorde's N-shaped boom jumps by 2.1 pounds per square foot, the new designs aim for a boom with a maximum pressure rise of just 0.3 pounds per square foot. That's only one-seven-thousandth of the air pressure at sea level -- you feel a bigger change riding three floors in an elevator.
A Faster Jet for the Jet Set
The quiet supersonic airplane Gulfstream plans to build will cruise between Mach 1.6 (1.6 times the speed of sound, or slightly less than 1,100 mph) and Mach 2 (1,320 mph), and will be able to cover at least 4,600 miles before having to stop for refueling. Gulfstream plans to start manufacturing the plane -- its so-called quiet supersonic jet, or QSJ -- by 2006.
Beyond conquering the boom, one of the biggest obstacles is building a reasonably priced engine that won't inflict front-row-Metallica-decibel noise on airport neighbors (we're talking about regular noise here, not sonic booms). The engines on today's jetliners are quiet thanks to big thrust-producing fans in the front, but they won't work on a supersonic plane because at ultrahigh speeds the airflow through the fans is not fast enough to produce any thrust.