Supersonic planes may fly over towns

By Irene Mona Klotz

Just as the Concorde heads toward the scrapheap comes a wakeup call from across the Atlantic: supersonic flight over populated areas may not be idle fancy.

Sporting a modified nosecone

During a highly successful demonstration over the Californian desert, a modified F-5E supersonic jet fighter provided proof that a change in an aircraft's shape mitigates the intensity of its sonic boom.

"This technology could eventually enable unrestricted supersonic flight over land," says Charles Boccadoro, programme manager with Northrop Grumman, which modified and flew the jet under a $7m US government-backed research program.

For the test, Northrop modified the fuselage and nose of an F-5E fighter jet, giving it a slight pelican-beak appearance.

After months of preparation, the modified plane, as well as a regular F-5E, left the firm's Palmdale, California plant on 27 August and flew to Edwards Air Force Base, located in the Mojave Desert.

Full throttle

The planes then sped down the same supersonic corridor flown by Chuck Yeager 56 years ago when, for the first time, an aeroplane flew faster than the speed of sound.

Chasing and listening

"We were all blown away by the clarity of what we measured," says Peter Coen, with Nasa's Langley Research Center in Virginia, a partner in the programme.

Sensors on the ground and aboard other aircraft showed the modified jet produced a sonic boom with one-third less intensity than the standard plane.

"There was no change in performance," added Northrop's Jim Hart. "They were both flying at full-throttle - about Mach 1.36."

A quieter boom

All aircraft flying through the atmosphere create pressure waves, similar to the waves created by the bow of a boat as it moves over water.

When airplanes travel faster than sound waves, which move at about 1,200 km/h (750 mph) at sea level, the pressure waves merge to form shock waves, which are heard as sonic booms when they hit the ground.

The test showed that designing aircraft to a particular shape will keep pressure waves from merging, reducing the intensity of the sonic boom.

Discussions to continue the Shaped Sonic Boom Demonstration programme are under way, said a representative with the Defense Advanced Research Projects Agency (Darpa), which manages the programme as part of its Quiet Supersonic Platform (QSP) initiative.

QSP's aim is to identify and develop technologies that could allow military and business aircraft to operate with reduced sonic booms.

Darpa's goal is to cut sonic boom noise by 75%.