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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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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.


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