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Air pressure changes, combined with just the right humidity levels, result in a condensation cloud as this F/A-18 passes through the sound barrier. (John Gay/U.S.Navy)

The Boom Stops Here

Hush, hush, sweet SST. Engineers are inventing a supersonic airplane that won't bust windows.

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LATE IN AUGUST 2003, A NORTHROP F-5E swept down a test range at Edwards Air Force Base, California, the same range where Chuck Yeager first broke the sound barrier more than 50 years earlier. As the airplane flew over the dry lake bed, it shook the ground with a resounding boom. Moments later, another F-5E flew the same course at the same speed, but with a vastly different result: The boom from the second flyover was hushed, dramatically quieter than the first.

The flights were part of an experiment conducted by the Defense Advanced Research Projects Agency, NASA, and Northrop Grumman. By heavily modifying the shape of an F-5E, called the Shaped Sonic Boom Demonstrator, or SSBD, the three organizations surmised they could dampen the powerful sonic boom that normally accompanies supersonic flight. They were right. “We’re going to fix the sound barrier that Chuck Yeager broke,” says Roy Martin, a Northrop Grumman test pilot who flew the F-5E demonstrator.

In the 1960s, two Cornell University aerodynamicists, Richard Seebass and Albert George, proposed that one way to reduce the strength of a sonic boom is to reshape the aircraft (see “Under Pressure,” p. 63). They formulated their theories at a time when governments were spending money to develop a commercial supersonic aircraft. Great Britain and France were beginning to negotiate a joint program, which eventually produced the Concorde, and in 1965, President Lyndon Johnson asked Congress to commit $140 million to fund research and development for a U.S. supersonic transport.

In 1971, political support died and the U.S. program was cancelled. NASA picked up where the failed program left off and, between 1972 and 1981, spent millions of dollars on the Supersonic Cruise Research program. Throughout that decade, NASA worked closely with the government and private industry to solve the twin problems standing in the way of supersonic airliners: noise and high fuel consumption, which is still an issue to this day.

In 1989, NASA formed a High-Speed Research program in an attempt to reduce the environmental impact of a proposed High-Speed Civil Transport. Dominic Maglieri, a member of a panel of experts assigned to the High-Speed Research program, proposed that an essential element of the HSCT would be reshaping the sonic boom’s N-shaped signature. To accomplish that, the aircraft itself would have to be reshaped—exactly Seebass’ theory.

Until this point, NASA’s research into boom shaping had been primarily theoretical, with several small-scale models tested in wind tunnels. But no one had yet succeeded in building a large-scale test bed. Maglieri originally suggested that nose shaping be tested on a Teledyne-Ryan BQM-34E Firebee II, a supersonic remotely piloted vehicle the U.S. Navy used as a target drone. From 1989 until 1992, computational fluid dynamics and wind tunnel tests were performed on the drone, but results were elusive.

“Issues with the Firebee II came down to cost and technology,” says Northrop Grumman’s David Graham, lead for aerodynamic and sonic boom design on the SSBD program. “And, at 28 feet, it just wasn’t long enough to provide a definitive answer as to the duration of the boom signature.”

Researchers also considered modifying an SR-71 Blackbird, but that too presented challenges. “In the early 1990s, they proposed attaching blisters or bumps on the fuselage to modify the area distribution,” says Graham. Making those changes to the SR-71’s cross-section would have helped researchers measure shock waves as the modified parts of the fuselage met the air during supersonic flight. “The problem is that the SR-71 is costly to modify and to operate. If they’d selected it for testing, the program would have become prohibitively expensive.”

In 2000, DARPA launched the Quiet Supersonic Platform program, and asked Boeing Phantom Works, Lockheed Martin, and Northrop Grumman to come up with new concepts in supersonic aircraft design. Northrop Grumman won the competition with a proposal for an SSBD, which was, at the start, based on Maglieri’s earlier findings with the Firebee II. It was Graham who realized Northrop Grumman had a perfect test bed right in its own hangar: the F-5E. Applying Maglieri’s idea of a forebody change to the relatively small and simple F-5E proved to be the right solution.

“It was important that we selected a plane that had the right kind of performance,” says Graham. “It needed enough of a margin that we could add the pelican nose, add drag, and still achieve supersonic performance. It’s just like in Goldilocks and the Three Bears: The Firebee II was too small, the SR-71 was too big, but the F-5E was just right.”

The F-5E’s new “nose glove,” which resembled a pelican’s pouched beak, produced a strong shock wave, but weakened the shock waves produced by the wings and engine inlets, preventing those shock waves from coalescing and creating a powerful N-wave signature.

The first set of flight tests took place on August 27, 2003. As the low-boom F-5 SSBD flew high over the range, microphones on the ground recorded the sonic booms. Shortly afterward, an unmodified F-5E, based at Naval Air Station Fallon, Nevada, repeated each flight. The result: The standard boom was measured at 1.2 pounds per square foot, but the low-boom aircraft registered only 0.8 psf.

The series comprised five test flights. Three of them used both the modified and standard F-5Es, while the other two used a NASA F-15B fighter that had been fitted with a pressure-measuring probe for close-range study of the shocks from the low-boom aircraft. Flight after flight confirmed the teams’ hypothesis: Carefully reshaping the aircraft reshapes the boom signature.

“Our key objective was to understand the factors that determine the magnitude of the pressure rise across a shock, the rate at which smaller shocks coalesce into larger shock fronts, pressure rise time, and overall boom shape,” wrote NASA project engineer Ed Haering, in a memorandum summarizing the test flights.

Still, these flights had taken place on a very hot summer day, which reduced the flight Mach values. Accordingly, NASA decided that it needed to conduct a second series of tests.

In early January 2004, flying at 32,000 feet, the F-5 SSBD hit a speed of 1,050 mph, roughly Mach 1.4. Forty-five seconds later, the F-5E from Fallon flew the same route. The new tests covered a wider range of speeds and altitudes than those conducted in August, and served to confirm their earlier readings of a .8 psf boom. “We can’t really change the physics of a sonic boom,” says Haering. “We’re plowing through the air faster than the air can move out of the way. The solution is to redistribute the energy around the aircraft so the result isn’t so noisy.”

Northrop Grumman hopes to apply the results of the Shaped Sonic Boom Demonstrator tests to its military jets. “Success with the shaped sonic boom flight demo required us to advance supersonic aircraft design tools well beyond state-of-the-art,” says Charles Boccadoro, Northrop Grumman’s program manager of Future Strike Systems. “Supersonic designs represent a very attractive solution for the nation’s next-generation, long-range strike systems.”

“There’s a synergy involved with low-boom and efficient aerodynamics,” says Graham. “A lot of things needed for low-boom design have direct application to a strike system.”

Graham cites laminar flow research as a prime example of research with dual applications: “Whether you apply the principles of laminar flow to a business jet or a military airplane, its improved efficiency means that the aircraft can be smaller and lighter, thus helping the sonic boom problem.”

Northrop Grumman has its own idea of what a low-boom aircraft could look like: It would be extraordinarily slender, with thin, highly swept wings supported by a strut. To shield the shock waves created by the wing as well as additional shocks created by spilled air, inlets designed to spill very little air would be mounted above the wing. The cockpit would be so well faired into the fuselage the pilot would have to rely on TV cameras to see.

By applying these principles, several groups have already prepared low-boom designs for specific types of aircraft. DARPA has come up with a “dual-relevant” concept that could take form either as a military strike aircraft or a civilian business jet. Two features of the design catch the eye: Its wings are so sharply swept that although it is 170 feet long, its width is less than 58 feet. It also is extremely slender, with similarly svelte wings.

This summer, NASA awarded four industry teams $1 million grants each for a 5-month study to define the technology and design requirements for a quiet supersonic aircraft. The Sonic Boom Mitigation Project, as it is being called, will use the teams’ recommendations to develop a solicitation for proposals for an actual low sonic boom demonstrator.

“It will probably be an X-plane, although we don’t have a designation for it yet,” says NASA’s Bob Meyer, associate director for programs at Dryden. “We’re approaching this fairly aggressively. We hope to award the contract to the winning company early next year and perform flight tests in 2008.”

Meyer says the F-5 Shaped Sonic Boom Demonstrator tests were the catalyst for NASA’s continuing its research into shaping sonic booms. “However, those tests only addressed bow shock mitigation, or shocks from the nose of the aircraft,” he says. “The next step is to look at the whole airplane.”

About a year ago, NASA assembled an alliance of companies called the Super 10—“super” for supersonic and “10” for the 10 industry giants, including airframers and engine companies—and asked them to evaluate a direction for supersonic research and recommend areas that would yield the highest payoff. The group recommended NASA support building a sonic boom flight demonstrator. “There are two important pieces of the program,” says Meyer, who is leading the Boom Demonstrator project for NASA’s Aeronautics Research Mission Directorate. “The first task is to take the design tools developed over the years, validate them with flight demonstrator data, and determine if we really can propagate a low-boom shock wave to the groundfrom an entire airplane at supersonic speeds. The next step will be to use that data to change the current regulations prohibiting supersonic flight over land.”

The length of the piloted demonstrator airplane will be about 80 to 100 feet. “Hopefully, this will be a stepping stone to a larger demonstrator sometime in the future,” says Meyer. “We also have a lot of challenges for supersonic cruise aside from low-boom that we have to address in parallel with the demonstrator, including propulsion, inlets, laminar flow, and materials, just to name a few.” Managing fuel consumption is obviously still a hurdle as well. He adds that the program will be more than just the sum of its test flights: “There will be a lot of analytical work performed, from computer codes to ground work, including wind tunnel tests.”

Boom acceptability work will include “boom boxes,” booths like those used for hearing tests, in which people will experience a replicated sonic boom and then rate the strength of the sound and their reaction to it. “We’ll also rate what we call the indoor response,” adds Meyer, “that will tell us how a boom feeds through a structure and how it affects people inside buildings. These tests will be more complex than outdoor response tests.”

Ultimately, NASA hopes to take its low-boom aircraft beyond the constraints of Edwards’ supersonic corridors and over more populated areas. “We want to expose the cities to the reduced sonic boom and see if they even notice it,” says Meyer.

“We had a lot of grey hair on this project,” says Graham. “When we [at Northrop Grumman] first started working with Dominic [Maglieri], he sent us a résumé that said he had 45 years of experience in sonic booms. I thought it was a typo, but it wasn’t.” There’s a certain triumph in working decades on one goal and finally seeing a pay-off. And in 2008, if NASA’s newest X-plane streaks over New York City at Mach 1.2—quietly—engineers will finally reap the rewards of all those years of research.


Sidebar:

Dennis Shoffner has heard it all. “I want to make a formal complaint about what these sonic booms are doing to my physical body,” declared one irate caller. “I moved to Barstow over a year ago, and I wasn’t obese back then.” She went on to claim that the sonic booms to which she had been subjected in the past 12 months were making her fat. Shoffner listened politely, held in his laughter, and referred the call to the claims department. “A half-hour later, they chased me down the hall for sending them that phone call,” he remembers.

 

Shoffner is the chief of community relations at Edwards Air Force Base in California. Since 1998, he’s been fielding aircraft noise and sonic boom complaint calls from residents in the surrounding counties. Under his guidance, the complaint line has morphed into a “query” hotline because, believe it or not, not all the calls are negative. Sometimes people just want to know what kind of airplane just went boom over their house. Shoffner spends much of his time visiting nearby communities, attending meetings, and giving out his work phone number to everyone he meets. “What I found out when I went to talk to people is that they felt barraged with noise and ignored by us [at Edwards],” says Shoffner. “There’s a science to dealing with the calls. These people are in the mood to talk, not to listen.

“During their initial call, I try to capture exactly what it is they want to say, and that requires a lot of work,” continues Shoffner. “When people are upset, they aren’t good at accurately communicating their message. I try to slow them down by reading back what they told me, and I tell them I understand—because I do.”

After Shoffner takes down the complaint—on an official “Complex Noise Worksheet”—he determines which aircraft created the disturbance (it isn’t always a sonic boom that generates phone calls), then sends the information to the unit commanding officer. “The first issue is, Were they breaking the rules when they created the sonic boom?” he says. “I forward the reports, but I don’t get to hear about what happens to the pilots—if there’s any disciplinary action taken.”

Shoffner moved to the town of Lancaster, near Edwards, at the age of nine, in 1956. Back then, Century series fighters were booming the area continually.

“When they were flight testing in those years, the same rules didn’t apply,” he says. “One day, our teachers called us out into the schoolyard to watch a B-52 and some chase planes fly over. That’s when a pod dropped from the B-52. We actually got to see the X-15 launch.” Some years later, when he had a job driving a delivery truck in the same region, a B-52 flew so low over his truck that he had to pull over to catch his breath from the shock.

These days, there are two supersonic “corridors” in the area—one high-level and another for low- and medium-level flight. “We would rather test over water, but we can’t always do that in the winter,” Shoffner says. “When it’s cold and wet out, the percussion of the boom feels stronger and the noise travels better.” In the past five years, the F/A-22 test program was responsible for many of the complaints. “People always ask me why we can’t do our testing elsewhere,” says Shoffner. “But the reality is that all of the available airspace has been carved up and taken. And then you also have more people living in areas that didn’t used to be populated.”

That includes Inyo County, to the north of the base, which comprises both the lowest (Death Valley) and highest (Mount Whitney) elevations in the lower 48 states. “Seventy percent of our noise complaints were coming from Inyo—one of the least populated counties in the region,” says Shoffner. “When I asked [the community at a local meeting] if anyone ever heard any jet noise, the room just exploded.” It wasn’t just Edwards’ jets that were booming over Inyo, it was airplanes and helicopters from naval air stations in nearby Lemoore, Fallon, and Point Mugu, as well as other National Guard and Air Force installations. Again, Shoffner used his skills in public relations to smooth the ruffled feathers of Inyo County residents. “Less than five percent of Inyo County belongs to private owners—the rest is government and public land, so there’s a lot of training going on,” he says. With a little bit of finesse, and some calls to the higher-ups at the surrounding military bases, Shoffner was able to help redirect some of the training flights and ensure a little more peace and quiet for residents. “My workday is between 7:30 in the morning and 4:30 in the afternoon, but people can leave me messages if they need to, and I try to get back to them right away,” says Shoffner. “I get a lot of weird calls around the full moon—especially about strange lights and aliens.” Neither of which is generated by booming airplanes, but then again, according to conventional wisdom, neither is obesity.

—Bettina H. Chavanne

 


Sidebar:

We hear a sonic boom when there is a sudden change in pressure—a shock wave—in the air around us. The shock wave is caused by the continuous buildup and violent release of air pressure along the surfaces of an airplane traveling at supersonic speeds. During subsonic flight, pressure waves continuously move away from the aircraft in all directions, like the waves that spread out from a boat’s bow as it cruises through water. At supersonic speeds, however, the pressure waves emanating from the leading edges and fuselage of the aircraft merge into one large pressure gradient, then release suddenly as they escape from the nose (the bow shock) and tail (tail shock) of the aircraft, resulting in a loud double-boom sound.

The pressure increase of a sonic boom, as experienced on the ground, is only about one to two pounds per square foot, a change someone would feel going down a couple of floors in an elevator. But because the change is so sudden, it not only registers on human eardrums as a loud noise, it also has the power to break glass and cause other structural damage.

 

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