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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.

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.

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