The Boom Stops Here
Hush, hush, sweet SST. Engineers are inventing a supersonic airplane that won't bust windows.
- By T.A. Heppenheimer
- Air & Space magazine, November 2005
(Page 2 of 6)
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.”