Reshaping Flight for Fuel Efficiency: Five Technologies on the Runway
Beyond Boeing Dreamliner 787, a new thrust to curb soaring emissions.
A half-mile per gallon might seem like dreadful fuel economy, but for vehicles carrying hundreds of passengers at high altitude, it marks a huge advance in efficiency.
For U.S. airlines, domestic flights now average 0.54 aircraft-mile per gallon of jet fuel (0.23 kilometer per liter), an increase of more than 40 percent since 2000. There's also been progress for the heavier jets on international flights: a 17 percent improvement to 0.27 mpg (0.12 km/l.)
And yet, there's a need to do far more. (Learn more about advances in airplane efficiency or take the related quiz: "What You Don't Know About Flights and Fuel.")
Air traffic worldwide is increasing so rapidly that global carbon dioxide emissions from aviation, which now represent just 2 to 3 percent of all CO2 pollution, could jump as much as 500 percent by 2050, according to one forecast. And for most airlines, fuel costs have surpassed labor costs as their largest expense, about 40 percent of operations, or $47.3 billion last year for U.S. carriers. Renewable jet fuel is available, but currently it is even more expensive than the petroleum-based kerosene it replaces. (See related stories: "As Jet Fuel Prices Soar, A Green Option Nears The Runway" and "First Commercial U.S. Biofuels Flight Takes Off.")
That's why the aviation world is looking at technologies, shapes, and materials that would transform flight far more dramatically than the advances embodied in Boeing's 787 Dreamliner, which before it was grounded in January was one of the world's most fuel-efficient commercial airliners. The Dreamliner uses 20 percent less fuel per mile than the similar-size Boeing 767, thanks mainly to improved aerodynamics and the use of lightweight composite materials.
Boeing also relied on a powerful lithium-ion battery so it could replace some mechanical components with electronics to cut the plane's weight. But two nasty battery incidents—one overheated on a runway in Boston, while another caught fire, forcing an emergency landing in Japan—led to the grounding of all 50 Dreamliners in operation. Boeing has taken steps to rewire the batteries to prevent them from overheating, and will also encase them in heavy-duty steel boxes that vent outside the aircraft.
On Friday, the U.S. Federal Aviation Administration approved the modifications, and the Dreamliners are expected to be back in the air soon. The U.S. National Transportation Safety Board is holding investigative hearings on the battery this week.
While the Dreamliner moves closer to retaking the skies, here are five new technologies—including one that will debut within months—that could soar far higher in fuel efficiency.
Geared Turbofan Engine
Connecticut-based engine-maker Pratt & Whitney, a division of United Technologies, tried a radical approach for making turbofan engines more efficient—adding a gear. The resulting fan-drive gear system engine, more than a decade in the making, can cut fuel use by up to 16 percent. "That's huge," says Magdy Attia, a professor of aerospace engineering at Florida's Embry-Riddle Aeronautical University. "It's a real game-changer."
It wasn't a new idea. Honeywell years ago used geared engines for very small private jets, but never advanced the technology. That's probably because it's difficult to do, Attia says. The 250-pound (113.4-kilogram), 18-inch (45.7-centimeter) gearbox has 30,000 horsepower passing through it, meaning there is a lot of heat to manage and expel quickly. And most aeronautical engineers aren't experts in gearboxes. But Pratt focused on making a geared engine that works, and the payoff has been huge. Five manufacturers—including Airbus, Bombardier, and Embraer—have so far placed orders for 3,500 PurePower engines. According to Attia, an engine-maker typically has to sell 350 units before it starts recouping its investment in developing a new engine. So already the geared engine is a success, well in advance of its June debut aboard the new Bombardier C Series planes.
Pratt & Whitney says the PurePower line of engines will be able to cut carrier operating costs by 20 percent (or about $1.7 million per plane per year), dampen noise levels by half, and cut CO2 emissions by 3,600 metric tons a year.
Why does the gear improve efficiency? Modern turbofan engines create thrust by expelling fast-moving hot gases from their core. But they also use their fans to push slower air around the outside of the engine, so it mingles with the faster hot gases at the rear, increasing thrust. Typically, engines have a bypass ratio of 8:1; eight pounds of the air hitting the engine bypass the core for every pound that enters. The higher the bypass ratio, the greater the engine's thrust and efficiency. Pratt's geared engine has a ratio of 12:1. A jet engine's fan works more efficiently at slower speeds than does the core's turbine, and the gearbox allows the two to spin independently, each at its optimum speed. Accordingly, PurePower engines have larger fans, and smaller, lighter turbines.
The geared engine does come with its own unique set of problems, however. With a larger fan, it cannot be retrofitted beneath the wings of existing aircraft. All the planes that will use it will be new, designed especially to accommodate the fatter fan. Moreover, gearboxes have metal-on-metal moving parts, which jet engines don't have. Attia expects that will require carriers to schedule more frequent inspections and require them to look for cracks and fatigue that they don't have to worry about now.
The first PurePower engines are designed for single-aisle jets. But Attia says their "true potential" would be fully realized if they were used on larger, 777-size aircraft. "At that size, the savings would be just astronomical." Pratt, in an email response to questions from National Geographic News, certainly indicated larger engines are in the pipeline. Eventually. Said Pratt: "We have not yet announced a definitive timetable for a Pratt & Whitney Geared Turbofan widebody engine."
While Pratt opted for a major design change to improve jet engine efficiency, other researchers are focused on materials, especially those that would allow for hotter combustion. In fact, one of the primary ways engine-makers have improved jet efficiency so far has been finding ways to burn the fuel and air mix inside turbofan jet engine combustors at hotter temperatures, while developing nickel-based alloys that can withstand the heat. But there's been a problem with this approach. "We have hit their thermodynamic limit," Robert O. Ritchie, a materials scientist at the U.S. Department of Energy's Lawrence Berkeley National Laboratory in California, says of those alloys. That's around 1,150°C (2,102°F). "If we go much hotter, they (engine turbine blades) will literally melt."
So researchers have turned to a material found in most cupboards: ceramics.
Some engine parts, including turbine blades, have ceramic coatings, but it's not an ideal solution because the coatings can spall off, and they also reduce the blades' efficiency, Ritchie says. But parts made purely from ceramics could withstand temperatures approaching 1,300°C to 1,500°C (2,372°F to 2,732°F).
However, as anyone who has ever dropped a teacup knows, ceramics are brittle. Metals, on the other hand, have ductility; they bend before they break. But materials scientists have developed composite ceramics reinforced with, well, ceramics. They've introduced ceramic fibers to the mix, giving the material ductility.
There are still many unknowns about composite ceramics, including how best to make them and improve their properties. To aid understanding, Ritchie recently developed a scanning device that uses three-dimensional tomography that literally peers through the materials in real time as they're pulled apart amid temperatures reaching more than 1,700°C (3,092°F). His imaging device has a resolution of half a micron. (For comparison, a human hair is roughly 75 microns in diameter). "We test it to the breaking point to see how it fails," he explains. That information can be used to compute the predicted life of the material and how far it can be safely pushed. It also allows scientists to reformulate a composite's mix to improve its microstructure and make it more robust and reliable.
Ritchie predicts that within five to ten years, commercial jet engines with a significant number of parts composed entirely of ceramic composites will be in use, allowing engines to run hotter by several hundred degrees Celsius. "Today we're excited if we can increase temperatures by 5°C (9°F)." Moreover, fuel efficiency will be further improved because the ceramic parts should decrease engine weight by 10 to 30 percent. "This is truly a disruptive technology," Ritchie says.
The "Double Bubble"
Could changing the shape of the fuselage, the iconic tube-like central body of an aircraft, make a difference in fuel economy?
Researchers at the Massachusetts Institute of Technology (MIT) think so. In a project funded by NASA, an MIT team came up with a concept called the "double bubble," which essentially merges two fuselages into one rather chubby one. It also has two rear-mounted engines. According to Rich Wahls, a NASA project scientist, it is designed to provide part of the lift in the fuselage, not just the wings, which allows it to have much thinner, lighter wings made from new-age materials.
Moreover, the rear engines ingest boundary layer airflow, giving the aircraft better drag. The bottom line: The technique allows the engines to use less fuel for the same amount of thrust as a conventional aircraft.
Wahls reckons the double bubble would be 60 to 70 percent more efficient than current passenger jets, although those estimates are based, in part, on assumptions that the materials and structures of choice will, by then, be stronger and lighter. Half of that efficiency gain would be due to the new shape, says Wahls, and nearly half of that projected improvement relies on the assumption that the plane's cruise speed is Mach 0.72, or slightly slower than today's average, Mach 0.8.
A more detailed assessment of the double bubble potential will be developed later this year, when a prototype is to be tested in a wind tunnel at the NASA Langley Research Center in Virginia. The MIT concept emerged from a project that NASA initiated in 2008, challenging engineering teams to restyle jetliners to use considerably less fuel. Boeing also came up with a concept in the NASA program that will be tested later this year at Langley.
Boeing's concept is nicknamed SUGAR, for Subsonic (slower than the speed of sound) Ultra Green Aircraft Research. In SUGAR, wings sit atop the fuselage and are held in place with struts fastened to the body's undercarriage—like small, single-engine Cessnas. Boeing's engineers believe that by using modern computational fluid dynamics they can minimize the extra drag caused by the struts, allowing for the longer, lighter, and higher wings—a weight reduction that saves fuel. The longer wings also lower drag, another fuel savings. Overall, Boeing estimates that the strut-wing design could reduce fuel consumption by about 60 percent.
Boeing is also considering different propulsion systems. The SUGAR Volt would use an electric battery/gas turbine hybrid power system. Conventional jet engines would be used for takeoffs, but the aircraft would cruise using battery power. Another version is the SUGAR Freeze, which would be powered by liquefied natural gas.
The Flying Wing
Imagine commercial flight in an aircraft shaped like a B-2 stealth bomber.
Aviation engineers have long known that the manta-ray silhouette of a "flying wing," also known as a "blended wing," is a highly efficient shape for airborne vehicles, from a lift-to-drag ratio perspective. But the cigar-shaped fuselage with which we're all familiar is easier to design to withstand outside forces while maintaining cabin pressure. A blended-wing aircraft built using conventional materials and frames would be very heavy, indeed. But NASA has been working with Boeing on a blended-wing aircraft using lightweight composites.
For the past six years, in a flight test project that ended just this month, NASA and Boeing have been conducting aerial research on the concept using a drone, Boeing's X-48 remotely piloted blended-wing aircraft. Working with the unmanned aircraft allowed the researchers to address issues such as how to transform the airframe so it does a better job of shielding engine noise from the ground, an improvement that would be key for gaining acceptance for such aircraft in communities near airports.
NASA says that, working with Boeing, it has devised a method to manufacture a 777-size blended-wing plane that would be at least 50 percent more efficient. Strong, but lightweight, carbon-composite rods would be used for the wing's structural skeleton. Its skin would be made from carbon fiber fabric, stitched together, and then coated in epoxy to make it rigid. The design overcomes a big hurdle, because the lighter a fabric is, the less tolerant it is to damage. That's where the stitching comes in. "If you develop a crack in a composite structure, what stops it from growing? The stitching is the low-weight solution," Wahls explains.
While the concepts from other NASA-led projects, the double bubble and SUGAR planes, are unlikely to take to the skies anytime soon, blended-wing jets could become commercially viable within a few years, says Wahls. "Years of research have put us ahead of the game on that one," he says.
High-Speed Heat Exchanger
Since the retirement of the Concorde in 2003, there have been no civilian aircraft in operation that fly faster than the speed of sound (Mach 1). But Britain's Reaction Engine aims to build Skylon, a space plane that would reach speeds of Mach 5 and bring any destination on Earth to no more than four hours away.
Reaction says its Sabre engine, which would operate like a jet engine in the atmosphere and like a rocket in space, could fly far faster than today's military supersonic jets, which are limited to Mach 2.45. (See related story: "First Green Supersonic Jet to Launch on Earth Day.") That's because the air first has to be compressed before it enters the core, and trying to compress enough air to reach faster speeds produces metal-melting heat.
So Reaction has developed a heat exchanger, a pre-cooler, that has a spiral matrix of tubes, each with a wall thickness of only 27 microns, to keep it lightweight. Reaction's exchanger can cool air from 1,000°C to -150°C (1,832°F to -238°F) within a hundredth of a second, or faster than a blink of an eye. Cooling something to that icy a temperature so quickly could, however, cause frost to build up and block the engine. But Reaction has also developed a technology that prevents the formation of frost. For competitive reasons, it's not saying how it's done that, but the technology was certified late last year by the European Space Agency.
What's all that got to do with making planes more efficient? Reaction says the exchanger could be installed inside current subsonic engines to make them 5 to 10 percent more efficient. A jet engine's turbine powers its compressor. But if the air entering the compressor is chilled, it needs less power to compress it. That means that without increasing the temperature within the combustion chamber, the turbine will create excess power that can be used to generate more thrust. Embry-Riddle's Attia says that weight and cost would have to be factored in, "but the principle is very smart, indeed."
A Reaction Engines spokesman says a workable, commercial version of the heat exchanger could be on the market "as soon as anyone wants one." For now, it's just one of the new efficiency technologies waiting on a long runway; history has shown the process of testing and adoption of new ideas in aviation moves a good deal more slowly than the speed of sound.
This story is part of a special series that explores energy issues. For more, visit The Great Energy Challenge.