In the almost two decades since the retirement of the supersonic Concorde airliner, jet-setters have had to make their peace with planes that poke along at about 500 miles an hour. But with help from new materials and advanced engine technologies, faster-than-sound air travel may be poised for a comeback.
Scientists and engineers on both sides of the Atlantic are developing airliners that would slash the time now required for long flights. The planes would not only fly faster than conventional airliners but also the vaunted Concorde, whose top speed was Mach 2.04, or a bit more than twice the speed of sound (1,354 miles per hour at cruising altitude).
Some researchers are working on a new generation of supersonic airliners, capable of speeds between Mach 1 and Mach 5. Others are dreaming bigger.
Last year, Boeing unveiled a design for a plane capable of reaching Mach 5, the low end of a realm known as hypersonic speed. And the Stratofly MR3, a 300-passenger airliner under development by a European consortium that includes the German Aerospace Center, the French Aerospace Lab and eight other universities and research organizations, would fly faster still.
With its needle-sharp profile and advanced powerplants, Stratofly would hit speeds of up to Mach 8 — about 5,400 miles an hour — and reach altitudes of more than 98,000 feet, far above the operational ceiling of conventional airliners.
At that heady altitude, there's little turbulence or bad weather. Experts say passengers would experience smooth flights and see the curvature of the Earth — though the view would likely be visible through weight-saving window-like video screens rather than actual windows.
"It would be like going to space," says Michael Smart, a professor of mechanical engineering at the University of Queensland's Centre for Hypersonics in Brisbane, Australia, who is not involved in the Stratofly project.
Shrinking the globe
While the Concorde was used primarily for transatlantic flights, Stratofly would likely be reserved for longer journeys. It could zip from New York City to Sydney, Australia, in about three hours, according to the consortium, or make the trip from Los Angeles to Tokyo in about an hour and 45 minutes.
"We want to go to Mars but still we have huge distances [separating us] here on Earth," says Nicole Viola, a professor of aerospace systems design at the Polytechnic of Turin in Italy and the project coordinator of the Stratofly consortium. With hypersonic flight, she adds, "it will be so much easier to travel to distant destinations."
By flying at altitudes beyond the reach of conventional airliners, Viola says, Stratofly might also help ease the congestion that is expected to be an increasing problem as the population swells and more and more people take to the skies.
Later this year, the Stratofly consortium plans to begin wind tunnel tests of the engine components. But it will take many years to bring Stratofly to life. The key technologies might not be ready until 2035, Viola says, with another decade needed before flight tests are completed and the plane is ready to carry passengers.
One reason for the lengthy timeline is that researchers have little experience with hypersonic aviation. Only a few experimental planes have ever reached hypersonic speed, and the last time a hypersonic plane flew with a person on board was in 1967. That plane, the North American X-15, was powered by rocket engines, which are impractical for commercial air travel.
Stratofly would need more than one kind of engine. For takeoff and landing, the plane would use turbine engines similar to those found in conventional airliners. For cruising, the plane would switch to so-called ramjet and scramjet powerplants. These engines compress the air-fuel mixture not with spinning fan blades, as turbine engines do, but using the aircraft's high speed and the shape of their inlet ducts.
Combining engine technologies in a single plane is a daunting task, Smart says. And the challenges won't end when Stratofly hits hypersonic speed: since the plane will generate tremendous friction and heat as it cuts through the air, it must be able to withstand temperatures well in excess of 1,000 degrees Fahrenheit. That's hot enough to melt the aluminum found in conventional airliners.
To survive the searing heat, the Stratofly team is exploring the use of high-tech materials, including ceramic reinforced with carbon fiber. They're also looking into innovative ways to cool the plane — perhaps by allowing some of the craft's supercold liquid hydrogen fuel to boil off during flight and absorb some of the friction-induced heat, Viola says.
Using hydrogen fuel instead of jet fuel would also help reduce Stratofly's carbon footprint; unlike the jet fuel burned by conventional airliners, the hydrogen-fueled vehicle would produce no carbon dioxide — although it would emit water vapor and nitrogen oxides, which are greenhouse gases.
Concern over the environmental impact of commercial aviation has led some people to avoid flying, and Viola says the consortium is paying close attention to the plane's effect on the environment. "We are analyzing all the operations and all the technologies to minimize the environmental impact," she says.
But such concerns notwithstanding, experts are rooting for hypersonic air travel.
"I'm really excited to see the progress they've been making, even though it's a big challenge," Smart says. "Flying at these types of speeds is something we've all dreamt about being able to do."
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