Starting in 2009, in its effort to aid in the development of cleaner aircraft fuels, NASA has conducted ground tests and two series of flight test campaigns out of the Armstrong Flight Research Facility in California called ACCESS (Alternative Fuel Effects on Contrails and Cruise Emissions) using a DC-8 flying laboratory, whose four wing-mounted CFM56 jet engines ran on either a conventional JP-8 jet fuel or a 50-50 blend of JP-8 and an alternative HEFA (hydroprocessed esters and fatty acids) fuel that comes from the camelina plant, a fairly hardy weed. In the first stage of the flight tests, which followed extensive ground-based testing of alternative fuels based on animal fats, the DC-8 was followed by NASA’s HU-25C Guardian (Dassault Falcon 20G business jet), which sampled its emissions. In the second stage, ACCESS II, Falcon 20-E5 and CT-133 airplanes operated by partners with the German Aerospace Center (DLR) and National Research Council of Canada (NRC) were part of the data gathering process. The instrumented chase planes recorded 20 different parameters of the exhaust coming from the DC-8 at various distances, altitudes, and engine power settings. JAXA, Japan’s Aerospace Exploration Agency, is also participating in the research effort by helping to analyze data from the flight campaigns.
Dr. Rubén Del Rosario, Project Manager for NASA’s Advanced Air Transportation Technology Project, said the purpose of the testing “as we develop and understand the combustor concepts of future aircraft, is to test those combustor concepts with a series of alternative fuels all the way from the small laboratory to the higher technology readiness level and components testing.” He added, “When we do that, we make a comparison to understand the emission of regular jet fuels versus alternative fuels as they relate to particulate matter, smoke, NOx [nitrogen oxide] emissions, carbon, and so forth, so we can understand what is happening in the emissions of these fuels.”
Thus far, said Del Rosario, the flight campaign test results have been promising. “We have demonstrated that we can reduce particulate matter by about 50 percent with no impact to performance. The engine is operating in the same way as if it were operating on jet fuels … and we even see a little bit of NOx benefits, more in the single digits. That is probably the result of the engine working a little bit cooler than with regular fuel.” Del Rosario added, “Alternative fuels provide an opportunity to reduce the formation of contrails and reduce particulate matter as we fly.” Going forward, NASA is committed to more testing of alternative jet fuels at the Glenn Research Center, but as of yet has not committed to another flight campaign.
Related to the jet fuel work is laboratory work and computational work on how to use fuel more efficiently. “The tie-in to the ultra-efficient arena is marrying, if you will, the efficient airframe with the efficient propulsion system to greatly reduce fuel usage,” said Esker. “Can you utilize the alternative fuels in a way that’s much smarter if you can better predict how they would actually behave within your engine system? So this ties into the ability to computationally model all aspects of the combustion science. What’s going on in that combustion process? Can you predict it? Can you measure it? Can you, with that knowledge, use these alternative fuels much more intelligently? … We’re conducting detailed scientific measurements and using complementary computational models and predictions to better understand the effects the [fuel] spray and burning patterns have in terms of how the fuel burns overall and its resulting emissions.”
Developing Hybrid-Electric Propulsion Systems
While NASA’s alternative fuels work is aimed at the near term, the agency is also focusing on aircraft designs three generations beyond the current commercial transport fleet, or N+3. Concepts being studied include hybrid-electric propulsion, with research on propulsion and power conducted at the Glenn Research Center, and on airframe efficiency at the Langley Research Center. “As part of studies we conducted in 2008, we challenged the aviation community and said by 2035 we want to reduce fuel burn by 60 percent, we want to cut the noise in half, we want to reduce the landing and take-off nitrogen oxides by 80 percent or more,” noted Del Rosario. “From those studies, two teams came up talking about the fact that we needed to move from the traditional cycle of jet engines to hybrid-electric propulsion concepts.” NASA then funded follow-on studies led by teams from Boeing, GE Aviation, the Massachusetts Institute of Technology (MIT), and Northrop Grumman.
Esker describes the investment in hybrid-electric systems as one in which “you can completely leapfrog from where you are by looking at truly novel approaches. The question on the table is: Can you take a system that couples some of the best characteristics of an electrical-motor-based system with the best characteristics of a much more reduced in size turbine system and end up with a capability that allows you to leverage their efficiencies where they make sense in the course of flight? It wouldn’t be completely unlike how a Prius automobile works. Of course there are a lot of challenges to be overcome.” Del Rosario describes the challenges thusly: “The tall poles of this area are that you need higher power density electric motors, higher power density electric materials, you need to better understand what happens when you distribute the propulsion [between an engine that uses fuel and batteries].”
One NASA-funded study effort had a team – led by Boeing Research & Technology and also involving Boeing Commercial Airplanes, General Electric and Georgia Tech – examine various subsonic concepts, under the project rubric of SUGAR (Subsonic Ultra Green Aircraft Research). One design version of a high span, strut-braced wing aircraft (referred to as SUGAR Volt), which adds an electric battery gas turbine hybrid propulsion system, was found to reduce fuel burn by greater than 70 percent and total energy use by 55 percent, when battery energy is included, as well as reduce life-cycle carbon dioxide emissions and NOx emissions. The downside of the concept is the need to significantly improve battery technology.
Another concept NASA pursued was N3-X, a 300-passenger hybrid wing body aircraft with turboelectric distributed propulsion (TeDP). Del Rosario describes the TeDP system as “a jet engine that is generating power and you take that power and transfer it to electric methods to drive a series of distributed fans. When you have this series of distributed fans, you can now increase the bypass ratio faster than traditional in-line engines. You can have more fans so your relative bypass ratio is a lot larger; you can also put the fans in the area where the drag is the worst, so you can reenergize the boundary layer to reduce the drag at the same time to increase the thrust.”
The work on hybrid-electric propulsion is in its initial stages, with Esker noting “these types of systems have to go to much larger scales and we need to start looking at how you control them in a real vehicle with those tests done at Armstrong. Across all the NASA centers, we have about 100 engineers working on the transition to the low-carbon area. It is and continues to be an amazing journey in terms of what this entire technology area can be and the potential that it has with the revolutionary nature of these aviation systems. … I think while the technology is going to have to grow over time, it’s going to be a lot easier to do a smaller system first and a larger system later. In its ultimate manifestation, I think you’re going to be looking at a fleet of highly efficient air vehicles that have little or no impact on the environment ideally.”
This article first appeared in the NACA/NASA: Celebrating a Century of Innovation, Exploration, and Discovery in Flight and Space publication.