Newer engines perform most efficiently at relatively slow speeds, Drela says. Indeed, the "double bubble" is designed to fly about 8 percent slower than today's commercial airliners. But a wider body with two aisles could allow faster loading and unloading of passengers—meaning overall trip time could be less despite the flight speed, he says.
Research into biofuels promises to reduce fuel consumption and emissions with only marginal changes to planes that have remained basically unchanged for decades. Boeing's N+3 proposal focuses on a hybrid jet fuel–electric propulsion system. Such an engine could reduce fuel consumption by 70 to 90 percent compared with typical aircraft today, Boeing spokesman Daryl Stephenson says. Such a hybrid engine could be especially useful in shortening takeoff distances, he adds, providing extra kick as the vehicle goes airborne. During takeoff and climb, the aircraft would use both jet-fueled turbines and an electric drive to provide the needed power, says Marty Bradley, the Boeing engineer in charge of the subsonic design effort. While cruising, the plane would rely on either its gas turbines or electric drive.
Northrop Grumman's N+3 approach calls for a 120-passenger plane built from ceramic composites and "smart metals" that can deform and return to their original shapes. Although the aircraft resembles a conventional jet, it is designed to take off and land on runways 1,500 meters long, compared with 2,400 meters or longer for today's jets. Northrop is hoping this shorter runway space will enable these planes to be used at smaller regional airports, relieving some traffic from large international fields.
Changes on the horizon
NASA has much work ahead of it to coax big changes out of an industry that has maintained conventional approaches to engineering for so long. What's more, achieving the meaningful reductions in emissions would require that these ecofriendly changes be widespread across the entire aviation industry—not limited to one or two aircraft-makers.
In the short term the push to develop viable biofuels, which are said to be carbon-neutral, likely will dominate efforts to make commercial aviation easier on the environment. "Already three airlines have had successful trials with a mix of kerosene and various biofuels, such as Jatropha and camelina," and fuels based on algae also have shown promise, Green Aviation's Pozniak says. Europe is pushing biofuels over the next couple of decades through emissions trading rules that require airlines to pay for their carbon use, he adds.
And the most innovative aircraft redesigns are yet to come. The truly green plane of the future will run on solar energy, requiring no fossil fuels and generating zero emissions, Pozniak says. A group called the Solar Impulse project, led by the balloonist Bertrand Piccard, successfully tested its Solar Impulse HB-SIA, on a 26-hour voyage in Switzerland earlier this month.
"We are setting the bar rather high," says Jay Dryer, director of NASA's Fundamental Aeronautics Program Office, who is overseeing N+3. "We're really trying to push as many [advances] simultaneously as we can." Dryer doubts that one design will be able to meet all of the agency's targets. More likely, certain elements of the various proposals will appeal to manufacturers and eventually make their way into airplanes, he adds. Solar flight might be the most ecologically friendly technology, but Dryer predicts it will not be mature enough to ferry large numbers of passengers for a half century or more.