Global pressures are mounting on engineers to squeeze every drop of energy efficiency possible out of greenhouse gas emitters, ranging from buildings to vehicles. But that may be hard to do when it comes to improving the jet turbines that power today's large commercial aircraft.
“They have almost perfected [traditional commercial jet] engines as far as they can go,” said Cheryl Bowman, co-technical lead of the aircraft gas-electric propulsion program at NASA’s Glenn Research Center in Cleveland, Ohio.
So if propulsion systems on large aircraft are approaching the limits of efficiency, what else can be done? One field of research wants to rewrite the book on propulsion systems and develop hybrid or possibly even all-electric systems to move aircraft through the sky.
The research behind new methods of propulsion is complicated, however. One tweak will add weight, which increases fuel consumption, while another might disturb the flow of air across a wing in such a way as to lower fuel consumption.
Right now, NASA wants to find ways to make jet engines more fuel-efficient as part of the White House’s multi-faceted push to drive down greenhouse gas emissions following its signing of a worldwide climate compact in Paris in December. According to the U.S. Environmental Protection Agency, American aircraft make up about 3 percent of the country’s total greenhouse gas emissions — about 11 percent of emissions from the transportation sector.
NASA is working with a long list of corporate partners such as Boeing, General Electric and Rolls Royce, as well as Ohio State University, the University of Illinois and Georgia Institute of Technology to develop different concepts for improving aircrafts’ fuel economy, according to Bowman.
“When we start talking about hybridizing the propulsion system for aircraft, there’s actually no one vehicle concept that has been identified as the clear winner or the first system that might see entry into service,” she said. “There’s actually a lot of trades going on right now for different ways to take advantage of new propulsion-airframe integration.”
One concept is a commercial plane about the size of a Boeing 757 that adds extra electricity-powered propulsors along the length of the aircraft. Researchers have put the fuel savings of the design at 7 to 12 percent.
The plane would take a system already in place on many aircraft — the transfer of electricity generated by wing turbines — and expand its uses to include propulsion. On today’s aircraft, the turbine-produced electricity powers devices on the plane that don’t involve actually move the plane forward. Research shows that decentralizing the propulsion mechanisms on airplanes can increase the ratio of lift over drag.
“So we’re talking about increasing the amount of electricity that we pull off the turbine engines and then using part of that electricity for distributed propulsion or propulsors in a new location so we can get increased … efficiency on the new airplane,” she said.
Other fuel-efficiency ideas involve simply improving the aerodynamics of the aircraft. Bowman pointed to an addition now standard in airplanes that has improved fuel economy: small winglets affixed to the tips of each wing that disrupt air flow. That concept could evolve to increase fuel efficiency further.
“The disturbance of that vortex at the end of the wing really helps your drag coefficient … studies have shown that putting a small propulsor, a small fan, at the end of those wing tips would help even more,” she said.
Last year NASA wrapped up a six-year initiative called the Environmentally Responsible Aviation project, in which researchers worked to document several possibilities for improving fuel efficiency of aircraft. Among the proposals were switching to a lighter composite material for building the body of the planes, and shifting turbines to the back of the plane as part of a wing-streamlining shape.
The administration estimated that the technologies and design alterations explored in the project could cut fuel use in half and save airlines $255 billion from 2025 to 2050.
That’s not taking into account the possibility of hybrid propulsion systems. Some of the work in that area goes so far as to propose demonstrating all-electric propulsion systems in smaller, aviation-class planes.
“The power can either come from something like a battery or a fuel cell or something like that, or it could come from a generator run by a turbine,” said Ralph Jansen, the other co-technical lead on the research project at the Glenn Research Center.
Right now, the all-electric technology is at an early stage. Electric propulsion systems would only have the range necessary to power small planes on regional commuter flights, shorter than the route from New York City to Philadelphia. Even during the next few decades of development, it might not scale up to the point where it could power larger craft, according to Jansen.
While that may seem like a long time, NASA knows you can't rush aircraft research and development.
“The target that we’re looking at is entry into service in a 20-to-30-year time frame,” Bowman said. “What that means is that we have to have these technologies that are ready to go in a 15-to-20-year timeframe. You can’t take those dates to the bank yet, but those are our targets.”