The Test: The results of Northrop Grumman's August 2003 flights: The red line shows the boom produced by a stock F-5E. As the pressure wave expanded away from the airplane, the strong shocks from the engine inlets and wings migrated forward and aft to produce the classic N-wave--a sharp pressure rise, a straight-line drop and another sharp rise to ambient pressure, all in 80 milliseconds. The ear hears the sharp rises, front and rear, as a double bang. The blue line is the boom from the modified airplane. By adding volume to the nose, the designers spread the pressure more evenly from nose to midbody. As the shock wave expanded toward the ground, the added pressure pushed back against the shocks from the inlet and wings. Result: The front spike of the N-wave was flatter. (In this test, the designers did not modify the tail shock.) Ultimately engineers will want the pressure change, known as overpressure and measured in pounds per square foot, to measure between 0.3 and 0.5 psf, rendering it barely noticeable from the ground. (Concorde's boom measures about 2 psf.) Photo by John MacNeill

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Aerospace engineer David Graham and his three colleagues had a deadline, and a little brown tortoise was putting it in jeopardy. In a few hours, as the sun rose over the Mojave Desert on an August morning last year, two Northrop Grumman F-5E fighter jets would come racing over the horizon. Flying 30,000 feet above Harper Dry Lake and traveling at 920 mph, the airplanes would be trailing long sonic booms–the distinctive aural signatures of supersonic flight that ordinarily make high-speed passages over land impossible.


The engineers, all members of a Northrop Grumman?led research team working to make those signatures significantly less distinctive, expected the two booms would be different from one another–a difference too slight to hear, even with your ear cocked to find out whether a 30-year-old theory aimed at mitigating supersonic shock waves worked in the real, turbulent and bubbly atmosphere, but one big enough to be detected by the instruments in the back of their SUV.


But this SUV, crammed with gear that had to be set out across the lake bed, wasn't going anywhere until the desert tortoise moved its reptile rear out of the way. The Bureau of Land Management's instructions were strict: Startling the endangered animal could threaten its life. The predawn hours are the male desert tortoise's time to roam in search of water, food and female company. That is arduous work, as every tortoise knows, and sometimes a guy just needs a rest. It was 15 long minutes before the beast waddled on its way.


Finally on the lake bed, NASA investigator Ed Haering supervised the placement of the portable instrument packages he'd designed, each containing an ultrasensitive Brel & Kjaer 4193 microphone, in an array about 2.5 miles wide. Away to the north, Northrop test pilot Roy Martin lined up his F-5E, which Graham had disfigured until Welko Gasich's elegant 1956 design was barely recognizable. Martin pushed the stick forward and the pelican-nosed F-5E began to pick up speed in a shallow dive, accelerating through the sound barrier.


Pointing the aircraft accurately wasn't easy. Graham, Haering and Wyle Laboratories boom expert Ken Plotkin had chosen dawn for the test because, later on an August day, thermals rise from the desert floor and the atmosphere gets turbulent. The chosen course put the rising sun smack in Martin's face. Squinting at the instruments, Martin lined up and locked in the test speed–Mach 1.36, 36 percent above the speed of sound.


The cone of the sonic boom trailed miles behind Martin's jet, and by the time the pressure wave swept across Haering's array, he was slowing down, turning for a possible second run. Down the course after him, trailing by 45 seconds, came a standard F-5E from Fallon Naval Air Station in Nevada. Plotkin had reckoned that the comparison between the two booms would be fair if the two fighters were more than 30 seconds and less than two minutes apart.


The second boom arrived on the lake bed with its usual authority–a thunderous double bang (one for the front of the aircraft, another for the aft) audible for miles–and the data was complete. "I definitely heard a difference," Graham recalls. That might be debatable. The two shock waves of each boom were less than 1/10 of a second apart, and the team had not even tried to alter the second half of the boom from the modified airplane. But within moments, the engineers were viewing the two booms on a laptop computer perched on a car trunk. A blue line showed the pressure wave of the modified F-5E; a red line represented the Navy fighter.


It was a dead-on match for the predictions. Ken Plotkin, who'd been in the boom business longer than anyone else present, danced a little jig. Graham says he saw tears in his eyes. Plotkin placed a call to Cornell University in Ithaca, New York. "It worked!" he said. The reply was calm: "I knew it would."



The August test flights over the Mojave Desert have answered a critical question about low-sonic-boom design: Engineers now know that they can predict how the sonic boom develops as it travels from the airplane to your ear. That means there's much less speculative risk involved in designing and building a low-boom airplane. Within a few years, a low-boom X-plane could be paving the way for a supersonic business jet–surveys have consistently shown strong demand for such an airplane, even at a $100 million price tag, and Boeing and Gulfstream are known to have SBJ efforts under way–or a quiet supersonic bomber, capable of sneaking into enemy territory at high speed without having its presence betrayed by piercing sonic booms. There are even signs that Lockheed Martin, with its long history of flying airplanes with capabilities that most observers deemed unattainable, could already be capable of building an operational supersonic business jet, under development in a code-locked vault and bankrolled by an unidentified sponsor.


Major obstacles remain, however, including manufacturing engines that are up to the task and can lower their noise output to tolerable levels during takeoff and landing, and refining the quiet supersonic design to be more visually acceptable than the admittedly ghastly-looking F-5E mod. But the biggest challenge when it comes to clearing supersonic flight for overland travel: Nobody knows how low is low enough for people on the ground, whether the boom comes from an Air Force jet or a billionaire's express ride. And that is ultimately a question of politics, not engineering.


Still, the final political hurdle could be easy work, compared with everything it took to get to it. It has been a 30-year race that began with the voice that Plotkin spoke to from the Mojave lake bed, which belonged to Cornell's Albert George. George was Plotkin's thesis adviser at Cornell in the late 1960s, when sonic booms were a hot issue. The Concorde supersonic airliner and its Russian counterpart were flying, and Boeing was designing a 300-foot-long, 1,800 mph monster. But these aircraft were hamstrung by their shattering booms. Working from pure acoustic theory, George had found a way to reshape the boom from the sharp-edged double bang into a soft, harmless pressure wave. His colleague Richard Seebass–"Seebass would have come up with the theory about 30 seconds later," Plotkin says–built a mathematical structure behind it. The result became the Seebass-George theory.
The researchers published their theory in January 1971. Congress scrapped Boeing's supersonic transport two months later, and for more than 30 years the theory would remain exactly that. The math was complicated: It worked for simple shapes but nobody knew how to use it to design a practical airplane. If you tried, the only way to know if you'd got it right, even in part, was to test a model in a wind tunnel, observe mistakes, modify the model and test again–a long, expensive effort with no real assurance that the next test would produce better results instead of different problems. Seebass-George "is an ideal," says Plotkin. "When you get into a real aircraft, the method is not as precise as you need."