One of the most engaging images the sky presents is that of the straight white arcs of an airplane’s contrails, heralding the voyage of passengers to parts unknown. While contrails have long been viewed as signs of progress, they also represent a form of pollution, the accumulation in the atmosphere of emissions including particulate matter, also known as soot, from the exhaust of jet fuel. NASA’s Terra satellite has captured images of clusters of contrails lasting as long as 14 hours from transatlantic commercial aircraft. The cirrus clouds created when the hot, moist air released from an airplane freezes in the colder and drier air also has an undetermined impact on climate, because they reflect sunlight and trap infrared radiation absorbed by the atmosphere.
In its effort to conduct research that addresses important global trends such as aviation’s carbon footprint, NASA’s Aeronautics Research Mission Directorate (ARMD) is tackling the challenge of enabling the transition of the aviation industry to lower-life-cycle-carbon fuels and alternative propulsion systems that may in the long run reduce the environmental impact of contrails and lead to other efficiencies that benefit society.
Pressure for Technology Improvement
The stakes are clear, as Dr. Jaiwon Shin, NASA’s Associate Administrator for Aeronautics, indicated in testimony before the House Science, Space and Technology Committee’s Subcommittee on Space and Aeronautics in 2012: “The pressure for technological improvement is mounting. Despite impressive improvements to the overall fuel efficiency of the U.S. airline fleets, fuel has about doubled as a portion of their total costs to be the single largest direct operating cost today for airlines, as other operations costs have been reduced. Globally, airlines are demanding more highly efficient aircraft to counter rapidly rising energy costs and uncertainty over new environmental regulations. Airlines also are seeking more efficient air traffic management operations to meet growing demand, make better use of their existing fleets, and reduce operating costs.”
Engineers at NASA’s Langley Research Center in Hampton, Virginia, installed this 15 percent scale model based on a possible future aircraft design by The Boeing Company in its Transonic Dynamics Tunnel. The 13-foot model is “semi-span,” meaning it looks like a plane cut in half. It is being used to assess the aeroelastic qualities of the unusual truss-braced wing configuration. (The “truss” is the diagonal piece attached to the belly of the fuselage and the underside of the wing.) Boeing designed the concept as part of the SUGAR (Subsonic Ultra-Green Aircraft Research) program to help conceive of airplane technologies and designs needed 20 years from now to meet projected fuel efficiency and other “green” aviation requirements. NASA Langley/Sandie Gibbs
But that was back at a time when fuel prices were much higher, a skeptic may say. While the costs of jet fuel may not currently be on people’s minds, as NASA’s Barbara Esker, Deputy Director of NASA’s Advanced Air Vehicles Program points out, thinking our current good fortune of low oil and fuel prices will last is not realistic. “In 1995, the price of jet fuel was about 10 percent of total airline costs, but was about 30 percent by 2011. And with that kind of growth, there’s a reason for the interest. And these energy costs are expected to continue to escalate even though we are currently enjoying lower oil prices. There’s a general feeling that this is a small perturbation against a much longer cost growth curve that’s expected.”
Esker adds that “the other part of the story is the impact on the environment. All propulsion systems, be it your tractor engine, or your car engine, produce emissions as a result of the combustion process,” she said. “The question: How can you increase your efficiency so that you drive down the amount of fuel that is used and thereby drive down the amount of emissions contributed? Right now, air transportation amounts to only about 2 percent of the world’s carbon dioxide emissions, but with the expected growth in air transportation, that percentage is expected to grow if it is not addressed. The global aviation community through IATA [International Air Transport Association] has the goal of reducing world transport aircraft’s carbon footprint 50 percent by 2050. We’re going to try to achieve this through much more efficient operations, not only in propulsion and engine systems, but also through efficiencies in airframes. We’re going to need to look for new fuels and possibly for new propulsion concepts. In general, we need to broaden our leapfrogging capabilities.”
Testing Alternative Fuels: Toward Low-Carbon Propulsion
To address the issues of aviation efficiency and environmental impact, NASA’s ARMD, with flight tests, studies, and laboratory work ongoing at its Armstrong, Glenn, and Langley Research Centers, is accelerating research on alternative jet fuels and hybrid-electric propulsion systems.