6

Findings, Recommendations, Roles, and Resources

APPROACHES FOR REDUCING CO₂ EMISSIONS

• Advances in aircraft–propulsion integration,
• Improvements in gas turbine engines,
• Development of turboelectric propulsion systems,¹ and
• Advances in sustainable alternative jet fuels.

• Aircraft–propulsion integration research. Advances in the integration of aircraft and propulsion are needed to enable many aspects of low-carbon aviation that are not achievable with the incorporation of discrete improvements in individual component technologies. Areas of interest include both evolutionary

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¹ Turboelectric propulsion systems use gas turbines to drive electrical generators that power electric motors that drive propulsors (fans or propellers). A partial-turboelectric system is a promising variant of the full turboelectric system that uses electric propulsion to provide part of the propulsive power; the rest is provided by a turbofan driven by a gas turbine. In contrast, hybrid electric systems use high-capacity batteries to provide some or all of the propulsive power during one or more phases of flight, and all-electric systems rely solely on batteries for propulsive power. The term “electric propulsion” encompasses all of these concepts.

• configurations such as lower fan pressure ratio engines in nacelles on standard tube-and-wing aircraft as well as significant departures from standard configurations including modified aircraft platforms, distributed propulsion concepts, and boundary layer ingestion configurations.

• Gas turbine engine research. Gas turbine engines have considerable room for improvement, with a potential to reach overall efficiencies perhaps 30 percent higher than the best engines in service today, with a concomitant reduction in CO₂ emissions. This magnitude of gain requires investment in a host of technologies to improve thermodynamic and propulsive efficiency of engines, with each discrete technology contributing only a few percent or less.
• Turboelectric propulsion research. Turboelectric propulsion systems are likely the only approach for developing electric propulsion systems for a single-aisle passenger aircraft that is feasible in the time frame considered by this study. System studies indicate that turboelectric propulsion systems, in concert with distributed propulsion and boundary layer ingestion, have the potential to ultimately reduce fuel burn up to 20 percent or more compared to the current state of the art for large commercial aircraft.²
• Sustainable alternative jet fuels research. Sustainable alternative jet fuels (SAJF) will be able to reduce life-cycle CO₂ emissions, and in some cases the reductions may be substantial. SAJF have the potential for immediate impact on lowering net global CO₂ emissions from commercial aviation because, as drop-in fuels, they are compatible with existing aircraft and infrastructure. Thus, their widespread use will not be limited by the rate at which new aircraft replace existing aircraft. The combustion of SAJF will likely also produce lesser amounts of other harmful emissions, such as oxides of sulfur and particulate matter, than the combustion of equivalent amounts of conventional jet fuel. SAJF are also compatible with and complementary to the three other high-priority approaches recommended in this report for reducing carbon emissions.

Hybrid-electric and all-electric systems were considered but are not recommended as a high priority because the committee determined that batteries with the power capacity and specific power³ required for commercial aircraft at least as large as a regional jet are unlikely to be matured to the point that products satisfying FAA certification requirements can be developed within the 30-year time frame addressed by this report. The same situation applies to technologies associated with superconducting motors and generators, fuel cells, and cryogenic fuels, and other potential approaches and technologies that are not included in the list of high-priority approaches above or the list of high-priority research projects described below.

CHALLENGES

• Economic competitiveness. Aviation is a highly competitive industry for which reduction in fuel burn (and, thus, CO₂) is a major technology driver. Cost considerations can be a challenge and have to be taken into account as new systems are proposed for commercial development.
• Aircraft systems complexity and integration. Commercial aircraft are composed of many distinct systems that are carefully integrated and regulated to maximize performance and safety. Disciplined system integration is required to introduce new technologies so that the improvement of one system does not adversely impact the performance of other systems or the performance of the aircraft as a whole.

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² Although turboelectric concepts include a gas turbine, the potential improvements resulting from research in gas turbine engine research (potentially up to 30 percent) and turboelectric propulsion (potentially up to 20 percent) do not together imply that future aircraft have the potential for improvements up to 50 percent.

³ In this report, “specific power” and “specific energy” refer to power and energy per unit mass, respectively, and “power density” and “energy density” refer to power and energy per unit volume.

Aircraft–Propulsion Integration Research

Technical Challenges

• Propulsive efficiency. Low fan pressure ratios are needed to reduce exhaust velocities and thereby improve propulsive efficiency, regardless of whether the fan is driven by a gas turbine or an electrical motor. For a constant level of thrust, this requires that the effective fan area increase so as to avoid commensurate increases in weight, drag, and integration losses.
• Boundary layer ingestion. To use boundary layer ingestion and wake cancellation to reduce aircraft cruise energy requirements, an aircraft configuration integrated with a propulsor design is needed in which the overall benefits of wake cancellation outweigh the costs in terms of propulsor efficiency, noise, and weight.

Economic Challenge

• Relative economic value. A balanced technology investment portfolio in aircraft–propulsion integration research is needed to ensure that economic uncertainties arising from changes in net fuel price do not slow continued reductions in emissions by commercial aircraft.

Policy Challenge

• Certification. Technical and policy issues surrounding certification of aircraft and propulsion concepts and technologies not covered by current certification procedures are important for guiding technology development. In addition, manufacturers must have confidence that these issues will be resolved in a timely fashion before they are likely to begin advanced development of the relevant aircraft and propulsion systems.

Gas Turbine Engine Research

Technical Challenges

• Propulsive efficiency. Low fan pressure ratios are needed to reduce exhaust velocities and thereby improve propulsive efficiency, regardless of whether the fan is driven by a gas turbine or an electrical motor. For a constant level of thrust, this requires that the effective fan area increase so as to avoid commensurate increases in weight, drag, and integration losses.⁴
• Thermodynamic efficiency. Enabling higher operating temperatures is a prerequisite to achieving significant improvement in gas turbine engine thermodynamic efficiency, and a major impediment to achieving higher operating temperatures is the difficulty of developing advanced materials and coatings that can withstand higher engine operating temperatures.
• Small engine cores. Activities being pursued to either improve the thermodynamic efficiency of gas turbine cores or improve overall aircraft efficiency result in smaller core sizes. For single-aisle aircraft, this tendency to core size reduction creates multiple challenges for maintaining and improving efficiencies of the overall engine and engine-aircraft integration.

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⁴ This challenge, which also appears as a challenge for aircraft–propulsion integration, is listed as a challenge for gas turbine research because it is a prerequisite for achieving significant improvement in gas turbine engine propulsive efficiency.

Turboelectric Propulsion Research

Technical Challenges

• Electrical technologies. The state of the art of electrical technologies for motors, generators, power distribution, and power electronics (for example, inverters, converters, and circuit protection) will need to advance to enable turboelectric propulsion concepts for large commercial aircraft.
• Aircraft systems. Turboelectric aircraft propulsion systems present a number of challenges related to other aircraft systems (e.g., thermal management systems). More structurally and aerodynamically efficient configurations can help address these challenges.
• Research infrastructure for electrical technologies. The research and development of megawatt-class turboelectric aircraft propulsion systems is hampered by the lack of development testing facilities.

Sustainable Alternative Jet Fuels Research

Economic Challenges

• Feedstock price and availability. Currently achievable refinery-gate feedstock prices are expensive relative to the final product, which is driven in part by immature or nonexistent feedstock supply chains.
• Industrial sector collaboration. The nascent SAJF industry lacks the inherent elements of collaboration of a fully developed industrial sector; these elements are required for initial matching of supply and demand signals and for subsequent system optimization.
• Technoeconomic factors. Lack of technoeconomic assessments and comparative understanding of various approaches impedes the ability of industry and the researchers and agencies that support SAJF R&D to make practical decisions about the prioritization of R&D and demonstration and deployment efforts.
• Hydrogen price and availability. Hydrogen is needed for almost all SAJF production, and in several conversion processes (those with potentially the lowest feedstock costs) it represents a significant portion of operating cost.
• SAJF development and demonstration projects. Additional, affordable SAJF demonstration and deployment efforts are needed to adequately address economic and technical risks.
• Funding for SAJF capital investments. Uncertainty about economic viability of SAJF production has impeded engagement from the petroleum industry or other large industrial entities that could bring appropriate resources to bear on addressing economic challenges.
• Challenges for small SAJF start-ups. Start-ups are unable to explore and leverage the full range of technical and economic opportunities that might provide sufficient economic benefit to facilitate commercialization.

Technical Challenges

• Feedstock development. Despite progress to date in developing feedstocks, in many cases, SAJF feedstocks are widely dispersed, they are unwieldy (e.g., they may have low bulk density, small seed size, and/or high moisture content), and/or they are not easily collected, transported, stored, or preprocessed with existing equipment.
• Feedstock conversion technologies. Cost-effective conversion technologies are not available for some promising feedstocks.
• SAJF fuel testing, qualification, and certification. It takes longer than it should to commercialize new SAJF production methods, in part because of the cost and time required to complete current fuel qualification and certification processes. Improvements to the qualification process are also needed to enable compositionally based evaluation of additional SAJF production pathways.

Policy Challenges

• Renewable fuel standard. Uncertainties about the long-term impact of the U.S. Renewable Fuel Standard, which provides indirect incentives for the production of SAJF, limit its effectiveness in fostering the development of an SAJF industry.
• Sustainability assessment models and requirements. There is no well-defined, internationally adopted framework for sustainability analysis of alternative jet fuels.

HIGH-PRIORITY RESEARCH PROJECTS

Chapters 2-5 identified high-priority approaches and research projects for developing propulsion and energy systems to reduce commercial aviation carbon emissions globally. The research projects respond to all of the technical challenges, but some of the economic and policy challenges cannot be overcome by research and technology development.

• Aircraft–Propulsion Integration Research

  — Nacelles for ultrahigh bypass ratio gas turbines. Develop nacelle and integration technologies to enable ultrahigh bypass ratio propulsors.⁵

  — Boundary layer ingestion. Pursue technologies that can enable boundary layer ingestion to reduce the velocity defect in the aircraft wake (also known as wake cancellation) and thus reduce cruise energy consumption.

• Gas Turbine Engine Research

  — Low pressure ratio fan propulsors. Develop low pressure ratio fan propulsors to improve turbofan propulsive efficiency.

  — Engine materials and coatings. Develop materials and coatings that will enable higher engine operating temperatures.

  — Small engine cores. Develop technologies to improve the efficiency of engines with small cores so as to reach efficiency levels comparable to or better than engines with large cores.

• Turboelectric Propulsion Research

  — Turboelectric aircraft system studies. Conduct more encompassing studies of aircraft powered by turboelectric systems in order to better understand the benefits, component performance sensitivities, certification issues, and trade-offs related to key aircraft systems, such as thermal management and energy storage.

  — Core turboelectric technologies. Develop the core technologies that are required for megawatt-class turboelectric propulsion systems: motors, generators, inverters, power distribution, and circuit protection.

  — Megawatt-class research facilities. Develop research facilities for megawatt-class electric power and thermal management systems suitable for testing turboelectric aircraft propulsion systems.

• Sustainable Alternative Jet Fuels Research

  — SAJF industry modeling and analysis. Undertake research to enable detailed and comprehensive modeling and analysis of SAJF development efforts and impacts at microscale (individual projects)

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⁵ This research project is closely related to the gas turbine research project on low pressure ratio fan propulsors, and work on the two projects should be closely coordinated.

ROLES AND RESOURCES

It will not be possible to execute the recommended research agenda without commitment, resources, leadership, and focus from relevant agencies and organizations in government, industry, and academia. Within the government, key players include the Department of Defense (DOD), the Department of Energy (DOE), the Federal Aviation Administration (FAA), NASA, the Department of Agriculture, the Department of Transportation, the Environmental Protection Agency, and the National Science Foundation.

Roles

Supporting research in all four of the high-priority approaches is prudent both to reduce current CO₂ emissions and to alleviate the potential consequences of future aviation growth worldwide. The research projects within each high-priority approach would rely on academia and industry to play the same role that they normally play in the development of new technologies and products. In particular, academia would generally participate in the projects at lower levels of technology readiness, while industry would focus on more advanced research and product development.

The FAA would be most directly engaged in the development of certification standards and methodologies for technologies not well covered by current practices.

DOD would have an interest in all four of the high-priority approaches to the extent that they could improve the capability of military aircraft or, in the case of SAJF, address the larger goal of reducing the environmental impact of defense operations.

NASA would contribute primarily by supporting basic and applied research in all four approaches, though it would likely play a lesser role in SAJF development given that much of the research (e.g., on feedstocks and fuel conversion processes) does not concern a NASA mission area.

DOE and its national laboratories would contribute primarily to the development of batteries, fuel cells, gas turbines, and SAJF feedstocks and conversion processes.

The primary contributions of the Department of Agriculture and Environmental Protection Agency would be feedstock development and modeling of the SAJF industry, and the Department of Transportation would also make broad contributions to the SAJF research projects.

The Department of Commerce and National Science Foundation could help primarily in improving fuel testing, evaluation, and qualification processes for SAJF.

Resources

Developing new technology for large commercial aircraft requires substantial time and resources, and there are well-established pathways for doing so, particularly with regard to improving gas turbine technology and aircraft–propulsion integration. Both of these approaches are well established and there are substantial motivations for four organizations with extensive research capabilities—the Department of Defense (and, in particular, its research laboratories), the Department of Energy (and its national laboratories), NASA (and its research centers), and the commercial aircraft industry (and its research centers)—to develop advanced technologies. In fact, these organizations, among others mentioned above, are already developing advanced technologies that are relevant to low carbon aviation. Although the missions of these organizations are very different, there are many points of commonality, and even greater progress could be accomplished by aligning relevant research programs in accordance with the recommended high-priority approaches and research projects.

In contrast, the funding situation for the other two approaches, turboelectric propulsion and SAJF, is somewhat problematic. It is not clear when turboelectric propulsion technology will advance to the point that it provides the performance needed for practical application in commercial aircraft. It is similarly uncertain when SAJF will be able to compete economically with conventional (petroleum-based) jet fuels, especially considering the capital costs of founding a new industry, and the fluctuating prices of conventional jet fuel. Currently available resources are making technological advances relevant to turboelectric propulsion and SAJF. Financial requirements, however, will increase substantially as the level of technology readiness increases and the next step requires, for example, flight tests of prototypes of high-power turboelectric systems or the development of full-scale SAJF production facilities. Even so, the turboelectric research projects will likely be able to maintain momentum as long as they achieve technological milestones for higher power systems. Options for sustaining SAJF research include leveraging the interest of multiple agencies with more focus on addressing global climate and energy concerns, such as the Department of Defense, the Department of Energy, the Department of Transportation, the Office of Science and Technology Policy, and entities interested in fostering rural and economic development, such as the Department of Agriculture, Department of Commerce, and state and local governments and public-private partnerships.