Traveling by airplane is fast, but traveling by supersonic jet is faster. The trouble is that pesky sonic boom caused by breaking the sound barrier, rattling windows and -- if you're a military pilot -- alerting potential enemies of your presence during low flights.
But a joint program between NASA, the military and the aerospace industry is working to take the 'boom' out of sonic booms by changing the shape of supersonic aircraft. The program may lead not only to better military jets, but also to another age of commercial air travel at faster than the speed of sound.
"For the commercial industry, this is really huge," said Ed Haering, principle investigator of NASA's sonic boom research at Dryden Flight Research Center (DFRC) at Edwards Air Force Base, California. "Right now you cannot fly commercial supersonic aircraft over land."
That's largely due to regulations set by the Federal Aviation Administration to curtail the effects of supersonic flight on humans and the environment.
"What we want is a supersonic cruise technology demonstrator that could become a business jet or a global strike system," said Charles Boccadero, manager of Long Range Strike Systems at Northrop Grumman Corp., which also worked on the sonic boom suppression project. "It's an area that offers three times the speed that you're traveling today at efficiency levels that are unprecedented."
While supersonic aircraft have been military workhorses since Chuck Yeager's historic faster-than-sound flight in 1947, there were only passenger supersonic airplanes. The Tu-144, built in the former Soviet Union by aircraft manufacturer Tupolev, made its last commercial passenger flight in June 1978.The Concorde, a joint British-French endeavor, shut down in 2003 due to rising maintenance costs and a slide in passenger revenue.
Shaping a sonic boom
Researchers drastically reduced the sonic booms produced a U.S. Navy jet by giving it a nose job.
Under the Shaped Sonic Boom Demonstration program sponsored by Defense Advanced Research Projects Agency (DARPA), NASA and Northrop Grumman, researchers tacked on custom nose-glove on the front of Navy F-5E jet as well as an aluminum substructure.
The thunderous booms heard by humans when a vehicle flies overhead at speeds faster than the speed of sound -- about 758 miles (1,220 kilometers) an hour at sea level. The culprit is a change in air pressure -- about the same experienced by humans climbing a few floors of stairs, but much faster -- which makes the sounds audible.
The added volume on the modified F-5E, however, allowed researchers to better distribute the air pressure build-up in front of a supersonic plane, which shapes how the pressure is later released in a sonic boom shockwave as the aircraft breaks the sound barrier. Modifying that pressure release meant softer sonic booms.
"Frankly, I think this is going to usher in a new era of aviation," Boccadero told SPACE.com.
Haering said the modifications eliminated about a third of the pressure typically released by unmodified supersonic aircraft, a noticeable difference when the F-5E boom was followed 45 seconds later by one from an unmodified aircraft.
Dryden researchers made 1,300 high-quality recordings of sonic booms during a series of January flights and 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 just drowning in data," Haering said.
A new plane
The Dryden-shaped sonic boom flights were confined to an existing airplane that had already undergone modifications to reduce boom noise. But in order to tailor an aircraft to run as supersonic silent as possible, project researchers will ultimately have to build a prototype from the ground up.
But first, researchers need a new plane.
The F-5E test plane used by NASA and Northrop Grumman was returned to the U.S. Navy and won't fly again for research. Dryden does have a stable of aircraft that could be used for the project.
"We're going to take more incremental steps," Haering said. "Typically, new prototype planes are hundreds of millions of dollarswe're going to have to make a case for that demonstrator."
Instead of spending hundreds of millions of dollars on a completely new vehicle right away, the NASA program will look at other ways to shape a sonic boom. Modifications to a supersonic aircraft's engine inlets and lift surfaces, for example, could also help shape the sonic boom it creates.
"I think the next step is to [eventually] build an entire vehicle made for low sonic booms to show how quiet it can be," Haering said. "And then you can solicit the FAA for rule changes."