When British and French aeronautical engineers designed the Concorde in the early 1960s, they thought they understood supersonic transport. They put a long, pointy nose on the fuselage and tucked four two-speed engines under a big delta wing to reduce drag. With a cruising speed of Mach 2.04---1,354 miles per hour---the Concorde was fast and beautiful.
It was also, famously, loud. When the Concorde crossed the sound barrier, topping 786 miles per hour, the double sonic boom it generated eventually led to a worldwide ban on commercial supersonic aircraft. On takeoff, those big turbo-charged engines generated hundreds of complaints from residents around Washington and New York airports.
Maybe even worse, from an economic perspective, the Concorde was a gas guzzler. It burned 2 percent of the fuel in its tanks just taxiing onto the runway.
So NASA’s announcement this week that it was resurrecting a commercial supersonic aircraft called the Quiet SST (“QueSST”) seemed like a jump into the way-back machine. NASA administrator Charles Bolden promised the new airliner would be “cleaner, greener and faster” than existing commercial jetliners, whisking passengers anywhere in the world in six hours. But the laws of physics haven’t changed. So...how’s that going to work, exactly?
It turns out NASA and industry researchers have been working on the sonic-boom problem for the past couple of years. They think they’ve cracked it. “The trick to making airplanes quiet is to change the way the air flows around the airplane,” says Juan Jose Alonso, professor of aeronautics and astronautics at Stanford University who worked on the X-Plane design at NASA headquarters from 2006 to 2008.
At supersonic speeds, the air flowing around the plane generates several shockwaves, each one emanating from anything that sticks out along the airframe---the nose, cockpit, engines and tail section. As the shockwaves reach the ground, they compress into two distinct waves that go “boom-boom.”
But angle and contour those surfaces correctly and you can reduce the noise from the Concorde’s 106 decibels down to 65 or 70 dB---about the loudness of a car door slamming. “The airplane still produces shockwaves and expansion waves, but you are tailoring how they are produced so when the noise reaches the ground it has a specific signature,” Alonso says.
Thanks to the relatively recent ability to integrate 3D modeling into computer simulations, the engineers were able to experiment more widely. That gives them more options, like trying out different nose shapes to minimize the leading edge of a shock wave. They're also looking at putting the air intake on top rather than underneath the engine, and entirely eliminating the forward-facing cockpit window. (Pilots will navigate with the help of video cameras.) It's also possible that the airframe itself might help dissipate shocks rather than form them. “If you mindlessly try to decrease the sonic boom, you will have a very poor performer,” Alonso says. “The trick is doing everything at the same time.”
The final design and shape of the plane will be the job of Lockheed Martin, which got a $20 million NASA contract to come up with the blueprints. An existing F-18 jet will be the test vehicle; NASA officials say a working prototype of the quiet X-Plane will cost at least $300 million, and could be flying by 2019 or 2020.
Even if you shush the plane, though, engines powerful enough to go faster than Mach 1 are inherently thirsty. Going faster takes more energy than going slower. “I personally would have been happier if the administrator had said ‘green and fast’ when talking about the supersonic X plane and ‘greener’ in reference to the other planned subsonic flight demonstrators,” says Peter Coen, supersonic project manager at NASA Langley. QueSST, Coen says, is designed to deal with the boom, not the gas mileage.
“When you go supersonic, you take a hit of two or three times or more in fuel burn,” says Mark Drela, an aeronautics engineer at MIT. “You want to make the airplane as thin, light and small volume as possible.”
That’s good for physics, but bad for business. A tiny, skinny airplane can’t carry a lot of people, and might not make economic sense, unless airlines charge a ton of money for each passenger. “As far as who will pay, I can see some sheik having heart problems and you have to get him to an American hospital,” Drela says. “How big that market is I don’t know.”
Coen thinks the problem is solvable, though. He says the agency is researching new kinds engine inlets and nozzles, that along with lighter composite airframe materials and avionics will result in a plane that burns one-third as much fuel as the Concorde did. “We are chipping away at the efficiency gap between subsonic and supersonic aircraft,” Coen wrote in an e-mail to Wired. “But we keep in mind that supersonic flight also delivers a value that subsonic cannot: Time!”
Saving time was the Concorde’s big selling point---New York to Paris in 3.5 hours---but that didn’t save it from extinction in the early 2000s. If NASA engineers can turn off the boom, a supersonic jet could fly over the continental US instead of being confined to transoceanic travel. Imagine: New York to LA takes only 2.5 hours. That might be just the ticket.
