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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.

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.”

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