Boeing’s Sugar Volt concept, for example, would use a hybrid-electric propulsion system that combines fuel-burning (turbine) engines, electric motors and electrochemical storage batteries—a propulsion concept not totally unlike that inside a Toyota Prius. The hybrid system would let the operators choose to draw engine power from the turbines or the batteries, whichever provides the most benefit for the specific segment of the flight—takeoff, landing, cruise, and so forth. “You can envision a 737-class airliner using the combination of turbine and electric power for take off and then, depending on the situation, switching over to cruise on one or the other,” says Marty Bradley, principal investigator for subsonic ultra-green aircraft research at Boeing Research and Technology in Bellevue, Washington.
Nasa’s N3-X is also designed around a completely new engine concept, called turboelectric distributed propulsion. It splits the main functions of a standard turbine engine in two - generating power by burning fuel and creating thrust by blowing air rearward with a large fan.
The idea is to use two large turbine engines to drive electric generators that would produce electricity to power 15 electric motor-driven, thrust-producing fans that would be embedded across the top rear of the broad fuselage. Such a configuration could be very efficient, Del Rosario says. The array of small electric propulsion fans at the stern of N3-X enables the designers to cut drag significantly by accelerating the flow of drag-causing air moving over the upper surface of the fuselage, keeping efficiency-sapping air friction at a minimum. Like the D8, the top-mounted propulsor fans would also effectively lower noise emissions because the body would come between them and the ground below.
The airliner concept may have an Achilles’ heel, though. For such a system to reach maximum fuel-efficiency targets, the electronics, generators and motors may need to be built from superconducting (zero-resistance) materials, meaning the jet’s electrics would have to be super-cooled by liquid hydrogen at −253C (−423F) or liquid nitrogen at -196C (321F) to make them work. This cryogenic technology is not yet fully practical and could take decades to prove out. Recent studies indicate, however, that substantial fuel-consumption gains could still be obtained by using existing electrical technology running at ambient temperatures, according to Del Rosario.
If that scheme sounds far out, other manufacturers are looking at developing fully electric systems for the 2050 time-frame. Aircraft engineers and designers at Eads, the parent firm of Airbus, for instance, have proposed a rather extreme concept called the Voltaire. The bulbous, 50-seat fuselage with two, long slender wings and a giant propeller on the tail, make it resemble a submarine. The concept, first put forward in 2011, would use next-generation batteries to power high-efficiency superconducting electric motors that would in turn drive the giant counter-rotating propellers mounted in a cylindrical shroud at the tail. Unlike any of the Nasa concepts, it is designed to be zero-emission.
However, anyone thinking that the electric Voltaire airliner may fly any time soon, needs to think again, says Johannes Stuhlberger, head of the global innovation network, power and flight propulsion at Eads. “The development of electric aircraft not only depends on the speed at which battery technology improves, but also how fast electrical equipment - the motors - get better.” Electric motors would need efficiencies of around 95%, he adds, noting that for any new system to become a reality will require “tremendous improvements in the power-to-mass ratio of the entire propulsion system, while still keeping it affordable.
In the shorter term, engineers at the Airbus group are trying to reduce fuel-consumption emissions by developing novel launch systems, similar to those found on naval aircraft carriers. In one radical concept, a low-slung carriage vehicle with an airliner mounted on its back would accelerate down the tarmac and loft the plane into the air. Such a device would substantially reduce the initial power required for a passenger plane to take off. Airbus envisages the eco-climb system moving into position automatically and assisting airliners to climb steeper and reach cruising altitude faster and from shorter runways.