Shock Treatment
by Peter Tyson

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Is there a way to take the boom out of the sonic boom?

Between 1976, when Concorde began flights to the United States, and 2003, when the aircraft was retired, you may have been fortunate enough, as I was once, to see Concorde take off. I noticed it from a distance, while landing at New York's JFK airport in another plane. But it stood out unmistakably from other nearby aircraft, like a giant white swan among so many geese and ducks. As it lifted from the tarmac it did indeed look like a swan rising from a stretch of still water, its wide-spread wings angled steeply, its distinctive slender head bent forwards as if unwilling to lose sight of the Earth even for a moment.

But neither you nor I nor anyone else in the United States ever caught sight of Concorde in its full glory—that is, flying at its cruising speed of 1,350 miles per hour, or roughly twice the speed of sound. That's because, in its quarter century of flying to America, Concorde was never allowed to fly supersonic, or faster than the speed of sound, over any portion of the U.S. Other countries had similar restrictions.

Why? Because of the thunderous sonic boom Concorde trailed behind it whenever it flew faster than Mach 1, the speed of sound. As much as Americans embrace speed and convenience, the Concorde's nerve-jangling bang was unacceptable, especially since most could never afford to fly it.

Yet the desire for speed and convenience has not gone away. A 1997 National Research Council report stated that the potential market for a large passenger airliner that can fly supersonically is on the order of 1,000 jets. And several companies have already announced plans for smaller supersonic business jets. The challenge for manufacturers is to figure out how to subdue the sonic boom enough that the general public will tolerate it and the Federal Aviation Administration will lift its ban.

What is a sonic boom?

Muzzling the boom has everything to do with knowing how it forms. Every plane pushes air molecules away with great force as it flies, much as a boat pushes water away. But when an aircraft exceeds the speed of sound, those molecules can't get out of the way fast enough, and they begin to pile up around the front and sides of the plane, like water in a bow wave. This results in a shock wave of pressurized air molecules, which form a cone-shaped sheath around and behind the plane.

This cone-shaped shock wave wouldn't be a problem if it lost its strength as it moved away from the plane. But it doesn't. With conventional supersonic aircraft, it maintains most of its shape all the way to the ground. And when that shock wave, traveling at the speed of sound, sweeps past you on the surface, you hear and feel the release of the pressure that the shock wave has built up as a great BOOM-BOOM. (The double boom occurs because the various shock waves put off by the aircraft's nose, engine inlets, and other features tend to coalesce into one big shock wave off the front of the plane and another big one off the tail.)

The change in air pressure you sense on the ground is only a few pounds per square foot (psf) above the normal air pressure that surrounds us at sea level, which is roughly 2,000 psf. It's about what you'd feel descending two or three floors in an elevator. The trouble is, it comes all at once, in a few thousandths of a second.

Several factors determine how big a boom will be, including the aircraft's size, weight, and shape as well as its altitude, attitude, and flight path. Atmospheric conditions such as air temperature and turbulence also play a role.

Shape of things to come

Aircraft designers have known for decades that they can work with one of those factors—and, for all intents and purposes, only one—to significantly ameliorate the sonic boom. In 1971, building on work done in the 1960s by NASA's Ed McLean and Harry Carlson, Cornell University professors Albert George and Richard Seebass published a paper describing how distributing the pressure change over the entire body of a supersonic aircraft could alter the shape of the sonic-boom waveform such that the resulting boom packed a much softer punch. This would be done primarily by blunting the nose of the aircraft.

The only question some experts had was, would the shock wave "shaped" by such a new design retain its shape all the way to the ground under real atmospheric and operational conditions? Or would it revert to the standard type of boom-triggering shock wave, known as the N-wave?

“It still has a boom-boom, but it’s kind of a dull pump-pump. Most people wouldn’t even notice it.”

The idea was finally tested in August 2003 during the Defense Advanced Research Project Agency's Quiet Supersonic Platform program. Engineers at Northrop Grumman gave an F-5E fighter jet a radical nose job, replacing its stork-like beak with one more like that of a pelican. In tests, the peak overpressure off the plane's nose was roughly a third less than that off the nose of an unmodified F-5E, and the shaped boom held its own all the way to the ground, flight after flight.

"The Shaped Sonic Boom Demonstrator was not intended to be a quiet plane, just quieter, to prove that we could change the shape of the boom and reduce the intensity, which we proved," says Ed Haering of NASA's Dryden Flight Research Center, who was principal investigator on the project.

Shaping is tricky, however. The more you blunt the nose of an aircraft, the more it can increase drag. The greater the drag, the more engine power needed. The bigger the engines, the greater the weight. And the greater the weight, the greater the shock wave that causes the boom. What you want is a way to have your cake and eat it too, says Peter Coen, Supersonic Vehicle Sector manager at NASA, which conducts research to support companies interested in developing supersonic aircraft. "What we're looking at," Coen says, "is how can we use computational tools and optimization techniques to produce an airplane that is both low boom and low drag?"

Sound of a heartbeat

One company thinks it has done just that with designs for a supersonic business jet. Los Angeles-based Supersonic Aerospace International (SAI) hired Lockheed Martin's fabled "Skunk Works"—known for crafting some of the world's most advanced aircraft, from the SR-71 Blackbird reconaissance plane to the Joint Strike Fighter—to design a supersonic business jet. As conceived, the Quiet Small Supersonic Transport would take you from New York to L.A. in about half the time of a conventional subsonic business jet.

Working with George and Seebass's theory for sonic boom minimization, further enhanced by work by NASA aerospace engineer Christine Darden, the Skunk Works, with SAI funding, has designed a plane that it claims will produce a boom 100 to 400 times quieter than Concorde's. It's all in how the plane shapes the inevitable shock wave that forms. "In order to achieve a minimum shock strength to the ground from a vehicle, you actually want to generate a rather large disturbance right at the nose, followed by a very small, generally fairly constant pressure," the Skunk Works' John Morgenstern says. "Then you switch to an expansion on the back end of about the same strength as the compression at the front end."

The result, he says, is a shaped sonic boom that is 20 to 25 decibels lower than the Concorde. "Because you've taken out the higher frequencies and lowered the sound pressure in general somewhat, it still has a boom-boom, but it's kind of a dull pump-pump," Morgenstern says. "It actually sounds kind of like a heartbeat for the size vehicles that we're talking about. Most people wouldn't even notice it."

Other ideas have been floated for hushing the boom, but none so far has shown the promise of shaping. "Most of the unique ideas tend to be very impractical, or violate the laws of physics, or violate the laws of sanity," says Ken Plotkin, a sonic-boom expert with Wyle Laboratories in Arlington, Virginia.

Down the runway

Even if the Quiet SST and other supersonic business jets in the works, such as one announced by Aerion Corporation of Reno, Nevada, manage to take the bang out of the boom, one major hurdle remains: convincing the FAA to change its regulations to allow supersonic flight over land. In anticipation, the FAA has already begun psychoacoustic testing in sonic-boom simulators to find out how much of a boom people would put up with. Most experts agree the maximum overpressure would need to be no higher than about 0.3 psf (Concorde's was 2 psf).

The experts I spoke with believe manufacturers will clear this hurdle as well as those surrounding environmental concerns, engine noise, and the high cost of flying supersonically. Within 10 to 15 years, they predict, supersonic business jets will likely be crisscrossing the U.S., with a supersonic airliner to follow within several decades. To hear their sonic booms, you'll have to be all ears.