NASA's Contributions to Aeronautics, Volume 1 by National Aeronautics & Space Administration. - HTML preview

CASE

12

Case-12 Cover Image: The F-16XL-2 was one of two test aircraft that NASA Dryden used in the 1990s to test the application of laminar flow at supersonic speeds. NASA.

Although NASA’s aeronautics budget has fallen dramatically in recent years,[1] the Agency has nevertheless managed to spearhead some of America’s biggest breakthroughs in fuel-efficient and environmentally friendly aircraft technology. The National Aeronautics and Space Administration (NASA) has engaged in major programs to increase aircraft fuel efficiency that have laid the groundwork for engines, airframes, and new energy sources—such as alternative fuel and fuel cells—that are still in use today. NASA’s research on aircraft emissions in the 1970s also was groundbreaking, leading to a widely accepted view at the national—and later, global—level that pollution can damage the ozone layer and spawning a series of efforts inside and outside NASA to reduce aircraft emissions.[2]

This case study will explore NASA’s efforts to improve the fuel efficiency of aircraft and also reduce emissions, with a heavy emphasis on the 1970s, when the energy crisis and environmental concerns created a national demand for “lean and green” airplanes.[3] The launch of Sputnik in 1957 and the resulting space race with the Soviet Union spurred the National Advisory Committee for Aeronautics (NACA)—subsequently restructured within the new National Aeronautics and Space Administration—to shift its research heavily toward rocketry—at the expense of aeronautics—until the mid-1960s.[4] But as commercial air travel grew in the 1960s, NASA began to embark on a series of ambitious programs that connected aeronautics, energy, and the environment. This case study will discuss some of NASA’s most important programs in this area.

Key propulsion initiatives to be discussed include the Energy Efficient Engine program—perhaps NASA’s greatest contribution to fuel-efficient flight—as well as later efforts to increase propulsion efficiency, including the Advanced Subsonic Technology (AST) initiative and the Ultra Efficient Engine Technology (UEET) program. Another propulsion effort that paved the way for the development of fuel-efficient engine technology was the Advanced Turboprop, which led to current NASA and industry attempts to develop fuel-efficient “open rotor” concepts.

In addition to propulsion research, this case study will also explore several NASA programs aimed at improving aircraft structures to promote fuel efficiency, including initiatives to develop supercritical wings and winglets and efforts to employ laminar flow concepts. NASA has also sought to develop alternative fuels to improve performance, maximize efficiency, and minimize emissions; this case study will touch on liquid hydrogen research conducted by NASA’s predecessor—the NACA—as well as subsequent attempts to develop synthetic fuels to replace hydrocarbon-based jet fuel.

NASA’s Involvement in Energy Efficiency and Emissions Reduction

The goal of improving aircraft fuel efficiency is one shared by aerospace engineers everywhere: with increased efficiency come the exciting possibilities of reduced fuel costs and increased performance in terms of speed, range, or payload. American engineers recognized the potential early on and were quick to create a center of gravity for their efforts to improve the fuel efficiency of aircraft engines. The NACA established the Aircraft Engine Research Laboratory—later known as NASA Lewis and then NASA Glenn—in 1941 in Cleveland, OH, as the Nation’s nerve center for propulsion research.[5] The lab first worked on fast fixes for piston engines in production for use in World War II, but it later moved on to pursue some of America’s most forward-leaning advances in jet and rocket propulsion.[6] Improving fuel efficiency was naturally at the center of the laboratory’s propulsion research, and many of NASA’s most important fuel-saving engine concepts and technology originated there.[7] While NASA Glenn spearheaded the majority of aircraft fuel efficiency research, NASA Langley also played a critical role in the development of new fuel-saving aircraft structures.[8]

NASA’s efforts to develop aircraft technology that both increased fuel efficiency and reduced emissions reached their nadir in the 1970s. From the time of Sputnik to the late 1960s, space dominated NASA’s focus, particularly the drive to land on the Moon. But in the late 1960s, and particularly after introduction of the wide-body Boeing 747, the Agency turned increasing attention toward air transport, consistent with air transport itself dramatically increasing as a means of global mobility. Government and airline interest in improving jet fuel efficiency was high. However, NASA Lewis struggled to reenter the air-breathing propulsion game because the laboratory had lost much of its aeronautics expertise during the Sputnik crisis and now faced competition for Government support.[9] Aircraft engine companies had developed their own research facilities, and the U.S. Air Force (USAF) had completed its propulsion wind tunnel facility at Arnold Engineering Development Center in Tullahoma, TN, in 1961.[10] NASA scientists and engineers needed a new aeronautics niche. Luckily for them, they found it with the arrival of the oil embargo of 1973 and the coinciding emergence of a national awareness of environmental concerns. NASA’s “clean and green” research agenda had been born.

The Organization of the Petroleum Exporting Countries (OPEC) oil embargo led Americans to realize that the Nation’s economy and military were far too dependent on foreign sources of energy. In 1973, 64 percent of U.S. oil imports came from OPEC countries.[11] The airline industry was particularly hard hit; jet fuel prices jumped from 12 cents to over $1 per gallon, and annual fuel expenditures increased to $1 billion—triple the earnings of airlines.[12] During the oil crisis, fuel accounted for half the airlines’ operating costs,[13] and those operating costs were rising faster than the rate of inflation and faster than efficiencies in the airlines’ own operations could reduce them.[14] The airline lobby descended on Capitol Hill, warning that its struggles to maintain profitability in the face of rising fuel costs were a bellwether for the Nation’s entire economy. Lawmakers turned to NASA to for help.

In 1975, the U.S. Senate asked NASA to create the Aircraft Energy Efficiency (ACEE) program, with the twin goals of lowering the fuel burn of existing U.S. commercial aircraft and building new fuel-efficient aircraft to match foreign competition.[15] The 10-year, $670 million ACEE yielded two of NASA’s greatest contributions to aircraft fuel-efficiency research. The most significant was the Energy Efficient Engine (E Cubed) program, which spawned technology still used in gas turbine engines today. The second key element of ACEE was the Advanced Turboprop (ATP), a bold plan to build an energy-efficient open-rotor engine. The open-rotor concept never made it into the mainstream, but aircraft propulsion research today still draws from ATP concepts, as this case study will later explain. Other technology developed under ACEE led to improved aerodynamic structures and laminar flow, as well as the design of supercritical wings, winglets, and composites.

Around the same time as ACEE, NASA began to sharpen its focus on the reduction of aircraft emissions. Space exploration had opened the Nation’s eyes to the fragility of the planet and the potential impact that humans could have on the environment.[16] The U.S. Congress pushed NASA to become increasingly involved in projects to study the impact of stratospheric flight on the ozone layer following the cancellation of the Supersonic Transport (SST) in 1971. The Agency provided high-altitude research aircraft, balloons, and sounding rockets for the Climactic Impact Assessment Program (CIAP), which was launched by the Department of Transportation (DOT) to examine whether the environmental concerns that helped kill the SST were valid.[17]

DOT and NASA’s CIAP research led to the discovery that aircraft emissions could, in fact, damage the ozone layer. CIAP results showed that nitrogen oxides would indeed cause ozone depletion if hundreds of Concorde and TU-44 aircraft—the Concorde’s Russian cousin—were to fly as planned. Following the release of CIAP, Congress then called on NASA to conduct further research into the impacts of stratospheric flight on the ozone layer, prompting NASA and DOT to move forward with a series of studies that by the 1980s were pointing to the conclusion that SSTs were less dangerous to the ozone layer than first thought.[18] The findings gave NASA reason to believe that improvements in combustor technology might be enough to effectively mitigate the ozone problem.

After conducting its breakthrough ozone research, NASA has fairly consistently included clean combustor goals in many of its aeronautics projects in an effort to reduce aircraft emissions (examples include the Ultra Efficient Engine Technology program and Advanced Subsonic Technology program). Today, NASA has broadened its aeronautics research to focus not only on NOx (the collective term for water vapor, nitrogen oxide, and nitrogen dioxide), but also carbon dioxide and other pollutants.[19]

NASA’s research in this area is seen as increasingly important as the view that aircraft emissions harm air quality and contribute to climate change becomes more widely accepted. The United Nations International Panel on Climate Change (IPCC) issued a report in 2007 stating that aircraft emissions account for about 2 percent of all human-generated carbon dioxide emissions, which are the most significant greenhouse gas.[20] The report also found that aviation accounts for about 3 percent of the potential warming effect of global emissions that could impact Earth’s climate.[21] The report forecasts that by 2050, the aviation industry (including aircraft emissions) will produce about 3 percent of global carbon dioxide and 5 percent of the potential warming effect generated by human activity.[22]

In addition to NASA’s growing interest in climate change, the Agency’s research on improving the fuel efficiency of aircraft has also continued at a relatively steady pace over the years, although it has seemed to fluctuate to some extent in relation to oil prices. The oil shocks of the 1970s spurred a flurry of activity, from the E Cubed to the ATP and alternative fuels research. But interest in ambitious aircraft fuel-efficiency programs seemed to wane during the 1990s, when oil prices were low. Now that oil prices are high again, however, fuel-efficiency programs seem to be back in vogue. (Several alternative fuels research efforts now underway at NASA will be discussed later in this case study.)

One example of the correlation between oil prices and the level of NASA’s interest in fuel-efficiency programs is the ATP, NASA’s ambitious plan to return to open-rotor engines. The concept never made it into mainstream use, partly because of widespread concerns that open-rotor engines are too noisy for commercial airline passengers,[23] but also partly because fuel prices began to fall and there was no longer a demand for expensive but highly energy-efficient engines. “We were developing the ATP in the late ’70s and early ’80s during the fuel crisis. And while fuel prices went up, they didn’t continue to escalate like we originally thought they might, so the utility just went down; it just wasn’t cost effective,” said John Baughman, Manager of Military Advanced System design at General Electric (GE).[24] With oil prices once again on the rise today, however, there are several new initiatives underway that take off where E Cubed and the ATP left off.

Lean and Clean Propulsion Systems

NASA’s efforts to improve engine design stand out as the Agency’s greatest breakthroughs in “lean and green” engine development because of their continuing relevance today. Engineers are constantly seeking to increase efficiency to make their engines more attractive to commercial airlines: with increased efficiency comes reduced fuel costs and increased performance in terms of speed, range, or payload.[25] Emissions have also remained a concern for commercial aviation. The International Civil Aviation Organization (ICAO) has released increasingly strict standards for NOx emissions since 1981.[26] The Environmental Protection Agency has adopted emissions standards to match those of ICAO and also has issued emissions standards for aircraft and aircraft engines under the Clean Air Act.[27]

NASA’s most important contribution to fuel-efficient aircraft technology to date has arguably been E Cubed, a program focused on improving propulsion systems mainly to increase fuel efficiency. The end goal was not to produce a production-ready fuel-efficient engine, but rather to develop technologies that could—and did—result in propulsion efficiency breakthroughs at major U.S. engine companies. These breakthroughs included advances in thermal and propulsive efficiency, as well as improvements in the design of component engine parts. Today, General Electric and Pratt & Whitney (P&W) continue to produce engines and evaluate propulsion system designs based on research conducted under the E Cubed program.

The U.S. Government’s high expectations for E Cubed were reflected in the program’s budget, which stood at about $250 million, in 1979 dollars.[28] The money was divided between P&W and GE, which each used the funding to sweep its most cutting-edge technology into a demonstrator engine that would showcase the latest technology for conserving fuel, reducing emissions, and mitigating noise. Lawmakers funded E Cubed with the expectation that it would lead to a dramatic 12-percent reduction in specific fuel consumption (SFC), a term to describe the mass of fuel needed to provide a certain amount of thrust for a given period.[29] Other E Cubed goals included a 5-percent reduction in direct operating costs, a 50-percent reduction in the rate of performance deterioration, and further reductions in noise and emissions levels compared to other turbofan engines at the time.[30]

The investment paid off in spades. What began as a proposal on Capitol Hill in 1975 to improve aircraft engine efficiency ended in 1983[31] with GE and P&W testing engine demonstrators that improved SFC between 14 and 15 percent, exceeding the 12-percent goal. The demonstrators were also able to achieve a reduction in emissions. A NASA report from 1984 hailed E Cubed for helping to “keep American engine technology at the forefront of the world market.”[32] Engineers involved in E Cubed at both GE and P&W said the technology advances were game changing for the aircraft propulsion industry.

“The E Cubed program is probably the single biggest impact that NASA has ever had on aircraft propulsion,” GE’s John Baughman said. “The improvements in fuel efficiency and noise and emissions that have evolved from the E Cubed program are going to be with us for years to come.”[33] Ed Crow, former Senior Vice President of Engineering at P&W, agreed that E Cubed marked the pinnacle of NASA’s involvement in improving aircraft fuel efficiency. “This was a huge program,” he said. “It was NASA and the Government’s attempt to make a huge step forward.”[34]

E Cubed spurred propulsion research that led to improved fuel efficiency in three fundamental ways:

First, E Cubed allowed both GE and P&W to improve the thermal efficiency of their engine designs. Company engineers were able to significantly increase the engine-pressure ratio, which means the pressure inside the combustor becomes much higher than atmospheric pressure. They were able to achieve the higher pressure ratio by improving the efficiency of the engine’s compressor, which condenses air and forces it into the combustor.

In fact, one of the most significant outcomes of the E Cubed program was GE’s development of a new “E Cubed compressor” that dramatically increased the pressure ratio while significantly reducing the number of compression stages. If there are too many stages, the engine can become big, heavy, and long; what is gained in fuel efficiency may be lost in the weight and cost of the engine. GE’s answer to that problem was to develop a compressor that had only 10 stages and produced a pressure ratio of about 23 to 1, compared to the company’s previous compressors, which had 14 stages and produced a pressure ratio of 14 to 1.[35] That compressor is still in use today in GE’s latest engines, including the GE-90.[36]

P&W’s E Cubed demonstrator had a bigger, 14-stage compressor, but the company was able to increase the pressure ratio by modifying the compressor blades to allow for increased loading per stage. P&W’s engines prior to E Cubed had pressure ratios around 20 to 1; P&W’s E Cubed demonstrator took pressure ratios to about 33 to 1, according to Crow.[37]

The second major improvement enabled by E Cubed research was a substantial increase in propulsive efficiency. Air moves most efficiently through an engine when its velocity doesn’t change much. The way to ensure that the velocity remains relatively constant is to maximize the engine’s bypass ratio: in other words, a relatively large mass of air must bypass the engine core—where air is mixed with fuel—and go straight out the back of the engine at a relatively low exhaust speed. Both GE and P&W employed more efficient turbines and improved aerodynamics on the fan blades to increase the bypass ratio to about 7 to 1 (compared with about 4 to 1 on P&W’s older engines).[38]

Finally, E Cubed enabled major improvements in engine component parts. This was critical, because other efficiencies can’t be maximized unless the engine parts are lightweight, durable, and aerodynamic. Increasing the pressure ratio, for example, leads to very high temperatures that can stress the engine. Both P&W and GE developed materials and cooling systems to ensure that engine components did not become too hot.

In addition to efforts to improve fuel efficiency, E Cubed gave both GE and P&W opportunities to build combustors that would reduce emissions. E Cubed emissions goals were based on the Environmental Protection Agency’s 1981 guidelines and called for reductions in carbon monoxide, hydrocarbons, NOx, and smoke. Both companies developed their emissions-curbing combustor technology under NASA’s Experimental Clean Combustor program, which ran from 1972 to 1976. Their main efforts were focused on controlling where and in what proportions air and fuel were mixed inside the combustor. Managing the fuel/air mix inside the combustor is critical to maximize combustion efficiency (and reduce carbon dioxide emissions as a natural byproduct) and to ensure that temperatures do not get so high that NOx is generated. GE tackled the mixing issue by developing a dual annular combustor, while P&W went with a two-stage combustor that had two in-line combustor zones to control emissions.[39]

Ultimately, E Cubed provided the financial backing required for both GE & P&W to pursue propulsion technology that has fed into their biggest engine lines. GE’s E Cubed compressor technology is used to power three types of GE engines, including the GE90-115B, which powers the Boeing 777-300ER and holds the world record for thrust.[40] Other GE engines incorporating the E Cubed compressor include the GP-7200, which recently went into service on the Airbus A380, and the GE-NX, which is about to enter service on the Boeing 787.[41] P&W also got some mileage out of the technologies developed under E Cubed. The company’s E Cubed demonstrator engine served as the inspiration for the PW2037, which fed into other engine designs that today power the Boeing 757 commercial airliner (the engine is designated PW2000) and the U.S. military’s C-17 cargo aircraft (the engine is designated F117).[42]

High-Speed Research

When NASA decided to start a High-Speed Research (HSR) program in 1990, it quickly decided to draw in the E Cubed combustor research to address previous concerns about emissions. The goal of HSR was to develop a second generation of High-Speed Civil Transport (HSCT) aircraft with better performance than the Supersonic Transport project of the 1970s in several areas, including emissions. The project sought to lay the research foundation for industry to pursue a supersonic civil transport aircraft that could fly 300 passengers at more than 1,500 miles per hour, or Mach 2, crossing the Atlantic or Pacific Ocean in half the time of subsonic jets. The program had an aggressive NOx goal because there were still concerns, held over from the days of the SST in the 1970s, that a super-fast, high-flying jet could damage the ozone layer.[43]

NASA’s Atmospheric Effects of Stratospheric Aircraft project was used to guide the development of environmental standards for the new HSCT exhaust emissions. The study yielded optimistic findings: there would be negligible environmental impact from a fleet of 500 HSCT aircraft using advanced technology engine components.[44] The HSR set a NOx emission index goal of 5 grams per kilogram of fuel burned, or 90 percent better than conventional technology at the time.[45]

NASA sought to meet the NOx goal primarily through major advancements in combustion technologies. The HSR effort was canceled in 1999 because of budget constraints, but HSR laid the groundwork for future development of clean combustion technologies under the AST and UEET programs discussed below.

Advanced Subsonic Technology Program and UEET

NASA started a project in the mid-1990s known as the Advanced Subsonic Technology program. Like HSR before it, the AST focused heavily on reducing emissions through new combustor technology. The overall objective of the AST was to spur technology innovation to ensure U.S. leadership in developing civil transport aircraft. That meant lowering NOx emissions, which not only raised concern in local airport communities but also by this time had become a global concern because of potential damage to the ozone layer. The AST sought to spur the development of new low-emissions combustors that could achieve at least a 50-percent reduction in NOx from 1996 International Civil Aviation Organization standards. The AST program also sought to develop techniques that would better measure how NOx impacts the environment.[46]

GE, P&W, Allison Engines, and AlliedSignal engines all participated in the project.[47] Once again, the challenge for these companies was to control combustion in such a way that it would minimize emissions. This required carefully managing the way fuel and air mix inside the combustor to avoid extremely hot temperatures at which NOx would be created, or at least reducing the length of time that the gases are at their hottest point.

Ultimately the AST emissions reduction project achieved its goal of reducing NOx emissions by more than 50 percent over the ICAO standard, a feat that was accomplished not with actual engine demonstrators but with a “piloted airblast fuel preparation chamber.”[48]

Despite their relative success, however, NASA’s efforts to improve engine efficiency and reduce emissions began to face budget cuts in 2000. Funding for NASA’s Atmospheric Effects of Aviation project, which was the only Government program to assess the effects of aircraft emissions at cruise altitudes on climate change, was canceled in 2000.[49] Investments in the AST and the HSR also came to an end. However, NASA did manage to salvage parts of the AST aimed at reducing emissions by rolling those projects into the new Ultra Efficient Engine Technology program in 2000.[50]

UEET was a 6-year, nearly $300 million program managed by NASA Glenn that began in October 1999 and included participation from NASA Centers Ames, Goddard, and Langley; engine companies GE Aircraft Engines, Pratt & Whitney, Honeywell, Allison/Rolls Royce, and Williams International; and airplane manufacturers Boeing and Lockheed Martin.[51]

UEET sought to develop new engine technologies that would dramatically increase turbine performance and efficiency. It sought to reduce NOx emissions by 70 percent within 10 years and 80 percent within 25 years, using the 1996 International Civil Aviation Organization guidelines as a baseline.[52] The UEET project also sought to reduce carbon dioxide emissions by 20 percent and 50 percent in the same timeframes, using 1997 subsonic aircraft technology as a baseline.[53] The dual goals posed a major challenge because current aircraft engine technologies typically require a tradeoff between NOx and carbon emissions; when engines are designed to minimize carbon dioxide emissions, they tend to generate more NOx.

In the case of the UEET project, improving fuel efficiency was expected to lead to a reduction in carbon dioxide emissions by at least 8 percent: the less fuel burned, the less carbon dioxide released.[54] The UEET program was expected to maximize fuel efficiency, requiring engine operations at pressure ratios as high as 55 to 1 and turbine inlet temperatures of 3,100 degrees Fahrenheit (°F).[55] However, highly efficient engines tend to run at very hot temperatures, which lead to the generation of more NOx. Therefore, in order to reduce NOx, the UEET program also sought to develop new fuel/air mixing processes and separate