Shushing Sonic Booms: Changing the Shape of Supersonic Planes

Traveling by airplane isfast, but traveling by supersonic jet is faster. The trouble is that peskysonic boom caused by breaking the sound barrier, rattling windows and -- ifyou're a military pilot -- alerting potential enemies of your presence duringlow flights.

But a joint program betweenNASA, the military and the aerospace industry is working to take the 'boom' outof sonic booms by changing the shape of supersonic aircraft. The program maylead not only to better military jets, but also to another age of commercialair travel at faster than the speed of sound.

"For the commercialindustry, this is really huge," said Ed Haering, principle investigator ofNASA's sonic boom research at Dryden Flight Research Center (DFRC) at EdwardsAir Force Base, California. "Right now you cannot fly commercialsupersonic aircraft over land."

That's largely due toregulations set by the Federal Aviation Administration to curtail the effectsof supersonic flight on humans and the environment.

"What we want is asupersonic cruise technology demonstrator that could become a business jet or aglobal strike system," said Charles Boccadero, manager of Long RangeStrike Systems at Northrop Grumman Corp., which also worked on the sonic boomsuppression project. "It's an area that offers three times the speed thatyou're traveling today at efficiency levels that are unprecedented."

While supersonic aircrafthave been military workhorses since Chuck Yeager's historic faster-than-soundflight in 1947, there were only passenger supersonic airplanes. The Tu-144,built in the former Soviet Union by aircraft manufacturer Tupolev, made itslast commercial passenger flight in June 1978.The Concorde, a jointBritish-French endeavor, shut down in 2003 due to rising maintenance costs anda slide in passenger revenue.

Shaping a sonic boom

Researchers drasticallyreduced the sonic booms produced a U.S. Navy jet by giving it a nose job.

Under the Shaped Sonic BoomDemonstration program sponsored by Defense Advanced Research Projects Agency(DARPA), NASA and Northrop Grumman, researchers tacked on custom nose-glove onthe front of Navy F-5E jet as well as an aluminum substructure.

The thunderous booms heardby humans when a vehicle flies overhead at speeds faster than the speed ofsound -- about 758 miles (1,220 kilometers) an hour at sea level. The culpritis a change in air pressure -- about the same experienced by humans climbing afew floors of stairs, but much faster -- which makes the sounds audible.

The added volume on themodified F-5E, however, allowed researchers to better distribute the airpressure build-up in front of a supersonic plane, which shapes how the pressureis later released in a sonic boom shockwave as the aircraft breaks the soundbarrier. Modifying that pressure release meant softer sonic booms.

"Frankly, I think thisis going to usher in a new era of aviation," Boccadero told SPACE.com.

Haering said themodifications eliminated about a third of the pressure typically released byunmodified supersonic aircraft, a noticeable difference when the F-5E boom wasfollowed 45 seconds later by one from an unmodified aircraft.

Dryden researchers made1,300 high-quality recordings of sonic booms during a series of January flightsand used other NASA aircraft to take observations within a boom's shockwave.The end result was the largest set of sonic boom data collected in 20 years,including information on different Mach speeds and in different weather.

"We're really justdrowning in data," Haering said.

A new plane

The Dryden-shaped sonicboom flights were confined to an existing airplane that had already undergonemodifications to reduce boom noise. But in order to tailor an aircraft to runas supersonic silent as possible, project researchers will ultimately have tobuild a prototype from the ground up.

But first, researchers needa new plane.

The F-5E test plane used byNASA and Northrop Grumman was returned to the U.S. Navy and won't fly again forresearch. Dryden does have a stable of aircraft that could be used for theproject.

"We're going to takemore incremental steps," Haering said. "Typically, new prototypeplanes are hundreds of millions of dollarswe're going to have to make a casefor that demonstrator."

Instead of spendinghundreds of millions of dollars on a completely new vehicle right away, theNASA program will look at other ways to shape a sonic boom. Modifications to asupersonic aircraft's engine inlets and lift surfaces, for example, could alsohelp shape the sonic boom it creates.

"I think the next stepis to [eventually] build an entire vehicle made for low sonic booms to show howquiet it can be," Haering said. "And then you can solicit the FAA forrule changes."