Summary

The primary human activities that release carbon dioxide (CO₂) into the atmosphere are the combustion of fossil fuels (coal, natural gas, and oil) to generate electricity, the provision of energy for transportation, and as a consequence of some industrial processes. Although aviation CO₂ emissions only make up approximately 2.0 to 2.5 percent of total global annual CO₂ emissions, research to reduce CO₂ emissions is urgent (1) because such reductions may be legislated even as commercial air travel grows, (2) because it takes new technology a long time to propagate into and through the aviation fleet, and (3) because of the ongoing impact of global CO₂ emissions.

The purpose of this report is to examine propulsion and energy technologies; it does not cover research in other areas, such as airframe design or air traffic management systems (e.g., optimizing flight descent paths to save fuel). The report also excludes nontechnology policy approaches, such as the imposition of carbon taxes, the use of carbon offsets, or legislative limits on carbon emissions.

This report focuses on large commercial aircraft—single-aisle and twin-aisle aircraft that carry 100 or more passengers—because such aircraft are the source of more than 90 percent of global CO₂ emissions from commercial aircraft operations. Moreover, while smaller aircraft also emit CO₂, they make only a minor contribution to global emissions, and many technologies that reduce CO₂ emissions for large aircraft also apply to smaller aircraft.

• Advances in aircraft–propulsion integration,
• Improvements in gas turbine engines,
• Development of turboelectric propulsion systems,¹ and

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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 being 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.

• Advances in sustainable alternative jet fuels.²

• 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 evolutionary 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 probably the only approach for developing electric propulsion systems for a single-aisle passenger aircraft that is feasible in the time frame considered by the committee. 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.
• 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.

Many potential approaches and technologies for reducing CO₂ emissions through the use of advanced propulsion and energy systems are not included here in the four high-priority approaches or their associated high-priority research projects. These include all-electric and hybrid-electric propulsion systems, high-power batteries and fuel cells for propulsion, superconducting motors and generators, hybrid compound engines (which combine a gas turbine and another internal combustion engine such as a diesel), engines that use thermodynamic cycles other than the simple Brayton cycle that is used by conventional gas turbine engines, and alternative fuels such as hydrogen or liquefied natural gas. This does not mean that the committee is recommending that all research to support these other approaches should be discontinued, nor does it imply that the committee believes it can predict with certainty how far the state of the art may advance in any of these areas over the next 30 years. Over the long term, only time will tell where and when breakthroughs in various technologies will revolutionize approaches for reducing CO₂ emissions for commercial aviation, and a broad-based program of basic research is more likely to

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² This report uses the term sustainable alternative jet fuels (SAJF) to characterize a family of drop-in fuels that are intended to lower the net life-cycle carbon emissions of commercial aviation. First and foremost, SAJF must meet current specifications for jet fuel, either on their own or when blended with conventional jet fuel. As such, SAJF (blended as necessary) are drop-in replacements for conventional jet fuel. SAJF are alternative in that they are produced primarily from nonpetroleum sources of hydrocarbons using a potentially broad range of biochemical and thermochemical conversion processes. To date, four pathways have been approved for producing alternative fuels that meet the specifications necessary to be considered jet fuel, while others are pending. To be successful over the long term, alternate fuels must be sustainable both in terms of their ability to reduce net life-cycle carbon emissions relative to conventional jet fuel and in terms of environmental, societal, and economic factors. Not all alternative fuels will result in a net reduction in life cycle carbon depending, for example, on their source materials.

make breakthroughs than one that is narrowly focused. On the other hand, the approaches and research projects detailed in this report are necessarily focused on a very particular goal: identifying the most promising propulsion and energy system technologies for reducing CO₂ emissions that could be introduced into service during the next 10 to 30 years, and the committee has concluded that the four high-priority approaches are indeed the most promising approaches, insofar as the other approaches are less likely to be matured to the point that products satisfying Federal Aviation Administration (FAA) certification requirements can be developed for a regional jet or larger commercial aircraft within the 30-year time frame addressed by this report. Also, the national research agenda recommended by the committee could lower the priority assigned to research programs that are developing high-power batteries, fuel cells, and superconducting motors for the purpose of incorporating them in the propulsion systems of large commercial aircraft. Even if that happens, however, a substantial national research investment in each of these technologies is certain to continue because of their potential to benefit a wide array of other applications. Aircraft-specific research in these areas may then take on a higher priority as the general state of the art advances.

HIGH-PRIORITY RESEARCH PROJECTS

The committee identified 12 high-priority research projects that it recommended for consideration by agencies and organizations in government, industry, and academia with an interest in developing propulsion and energy system technologies that could reduce CO₂ emissions from global civil aviation and could be introduced into service during the next 10 to 30 years. As indicated below, two of the high-priority research projects address aircraft–propulsion integration, three address gas turbine engines, three address turboelectric propulsion, and four address SAJF.

Aircraft–Propulsion Integration

Nacelles for Ultrahigh-Bypass-Ratio Gas Turbines

This project would develop nacelle and integration technologies to enable ultrahigh-bypass-ratio propulsors. It 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.

Aircraft–propulsion integration refers to the aerodynamic, structural, and subsystem (fuel, pneumatic, hydraulic, electrical, control, etc.) interfaces between engines and airframe. Today, commercial aircraft share a configuration known as “tube and wing,” characterized by thin wings mounted to a roughly circular cross-section tubular body. Engines on current turbofan-powered commercial aircraft are mounted on pylons, which isolate the engine and airframe aerodynamic characteristics. The engines are enclosed by fairings known as nacelles, which contain many of the subsystems important to the operation of the aircraft such as the electrical generators. The aerodynamic, structural, and subsystem integration of the engine and nacelle with the airframe is a technology important to determining aircraft performance and optimum engine characteristics such as propulsor diameter and fan pressure ratio. Achieving the goal of this research project will require compact nacelles with lighter weight and lower drag to increase propulsive efficiencies. Key research topics for this project include internal and external aerodynamics, acoustics, thrust reversing, operability, manufacturing, and overall weight.

Boundary Layer Ingestion

This project would pursue technologies that can enable boundary layer ingestion (BLI) to reduce the velocity defect in the aircraft wake (also known as wake cancellation) and thus reduce cruise energy consumption.

Several proposed advanced aircraft designs include configurations in which the boundary layer developing along the aircraft is ingested into the propulsor resulting in wake cancellation, which reduces the power needed to propel the aircraft and so reduces energy consumption and CO₂ emissions. BLI requires a much higher level of aircraft–propulsion integration than is common today and imposes new constraints and requirements on both the airframe and the propulsion system.

BLI configurations have been proposed using a variety of propulsor drive systems, including direct-drive turbofan engines, geared mechanical drives, and electrical drives. BLI holds a theoretical promise to significantly reduce aircraft fuel consumption, all else being equal. Most proposed BLI configurations with respect to the propulsor–aircraft integration, are significantly different than conventional designs, so all else is not equal. For example, BLI configurations bring a very high distortion level into the fan, which impacts efficiency, fatigue life, and noise, at least partially offsetting potential gains from wake cancellation. The benefits and costs of BLI are also confounded by many other significant aircraft and propulsion changes in advanced designs. Therefore, careful, detailed aircraft design studies are needed to guide investments in component and subsystem technologies.

Key research topics for this project include (1) exploring the aerodynamic, structural, subsystem, control, and safety implications of BLI configurations, including detailed systems analyses of alternative approaches such as various advanced propulsion system options and (2) developing technologies for propulsion fans for operation in highly distorted flow fields characteristic of BLI configurations, including detailed assessment of the penalties inherent to current technology as well as pursuing design and technology approaches that mitigate such penalties. This research project requires both analysis and testing at representative Mach numbers.

Gas Turbine Engines

Low-Pressure-Ratio Fan Propulsors

This project would develop low-pressure-ratio fan propulsors to improve turbofan propulsive efficiency. It is closely related to the aircraft–propulsion integration research project on nacelles for ultrahigh-bypass-ratio gas turbines, and work on these two projects should be closely coordinated.

However it is produced, shaft power is converted to propulsive power with a “propulsor,” which is either a fan in a duct or a propeller. All else being equal, the lower the pressure across the propulsor, the lower the exhaust velocity will be and, hence, the higher the propulsive efficiency. The relevant design parameter for turbofan engines is the fan pressure ratio. At constant thrust, as fan pressure ratio is reduced, more airflow and thus a larger fan or more fans are needed.

The penalty for encasing the propulsor in a duct is an increase in weight and drag. As the duct diameter grows, the propulsor efficiency increases, but at some point the increased weight and drag of the fan, duct, and nacelle cancel the efficiency gain. As a result, further increasing the size of the fan reduces efficiency. Also, larger diameter fans and nacelles may require longer and thus heavier landing gear. Therefore, at a given level of engine, nacelle, and aircraft technology, there is an optimum fan diameter for minimum fuel burn. Key research topics for this project include turbomachinery design, duct losses, acoustics, aeromechanics, nacelle aerodynamics and weight, manufacturing, and aircraft integration.

Engine Materials and Coatings

This project would develop materials and coatings that will enable higher engine operating temperatures. Advanced materials for gas turbine engines have been a particularly fruitful investment area because a successful material can often be used to improve existing engines as well as to enable new concepts. The system-level benefits of new materials may come from reduced weight, higher temperature capability, or reduced cooling requirements and thus higher thermodynamic efficiency. For example, advanced materials for compressors can enable the higher compression ratios needed to improve engine thermal efficiency.

Advanced materials for combustors and turbines can also improve engine power-to-weight ratios and can improve part durability to keep fuel burn from increasing as an engine ages. Key research topics for this project include advanced materials that provide viable approaches to greatly reducing or eliminating turbine film cooling as well as compatible coatings for environmental protection, erosion prevention, ice rejection, and thermal barriers.

Small Engine Cores

This project would 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.

Improved aircraft efficiency means that smaller engine cores would be needed since less power would be required for the same mission. Historically, engines with cores having a small physical size are less efficient than large engines, and new technologies are needed to overcome this disparity. Key research topics for this project include improvements in turbomachinery aerodynamic performance, manufacturing, tip clearance control, secondary flow losses, combustion, and the operational life of turbine airfoils.³

Turboelectric Propulsion

Turboelectric Aircraft System Studies

This project would 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.

Motors, generators, and electrical distribution systems lie at the heart of electrical systems, and most electric propulsion research is understandably focused on these key elements. As these electric propulsion technologies advance, it is essential that research to advance capabilities in related aircraft systems is properly directed. Studies to date of the benefits and challenges of turboelectric propulsion have paid insufficient attention to all contributing aircraft systems. One key research topic is thermal management, because thermal management systems are essential to the performance of the electric propulsion system and because they may affect aircraft flight performance. Other key research topics include aircraft structure and optimized aerodynamic integration. Establishing cost targets, thermal management targets, and reliability targets at an early stage in the research and development (R&D) process would help define the research plan. In addition, certification plans would be most effective if established in active collaboration with the U.S. Federal Aviation Administration and other certification authorities such as the U.K. Civil Aviation Authority and the European Aviation Safety Agency.

Core Turboelectric Technologies

This project would develop the core technologies that are required for megawatt (MW)-class turboelectric propulsion systems: motors, generators, inverters, power distribution, and circuit protection.

Turboelectric propulsion concepts are heavily dependent on the advancement of aircraft electrical power system technologies. These technologies include generator systems for electrical power generation; power electronics for power conversion, conditioning, and distribution; high-power aircraft distribution that includes circuit protection; motors; and energy storage. A key issue is how to address higher distribution voltages designed for operation at altitude.

Requirements for electrical system components are beyond the current state of the art, especially for large commercial aircraft. The committee’s projection of the state of the art in 20 years is that requirements for specific power⁴ of motors and generators can be met for single-aisle aircraft, and projected power capability is expected to cover the lowest end of the projected range of requirements for regional or single-aisle aircraft. Circuit protection and high-power distribution cabling for MW-class aircraft power systems will also need to be developed. Research for this project could initially focus on technologies for 1 MW systems, with a long-term focus on 1 to 5 MW systems.

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³ “Airfoil” refers to both the stationary vanes and the stators in a turbine as well as the rotating blades.

⁴ 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.

Megawatt-Class Research Facilities

This project would develop research facilities for MW-class electric power and thermal management systems suitable for testing turboelectric aircraft propulsion systems.

The research and development of MW-class machines for aircraft applications are hampered by the lack of facilities. Existing facilities for the development of motors, generators, and other electrical equipment for aircraft were generally designed to support nonpropulsion power for the vehicle. Electric propulsion systems will need to have much higher power capacities than electrical systems currently on aircraft. Facilities to meet these higher power levels have not been developed. No capability for proper simulation of a turboelectric propulsion system for a large aircraft exists at this time. After initially focusing on ground-based facilities, a flight demonstration program for a turboelectric system could be considered to support advanced development of MW-class systems.

Sustainable Alternative Jet Fuels

SAJF Industry Modeling and Analysis

This project would undertake research to enable detailed and comprehensive modeling and analysis of SAJF development efforts and impacts at microscale (individual projects) and macroscale (nationwide or worldwide) levels to support the needs of policymakers and industry practitioners.

The variability in the many different sustainability frameworks relevant to SAJF complicates the process of developing SAJF that can be widely marketed as meeting sustainability needs, and it increases uncertainty about the economic feasibility of SAJF. The variability and uncertainty might also influence policy or decisions that have established relatively high hurdles for various metrics. For example, the U.S. Renewable Fuel Standard program and the Roundtable on Sustainable Biomaterials have established the need for advanced fuels to achieve at least a 50 percent reduction in life-cycle CO₂ emissions. This 50 percent minimum reduction disincentivizes the potential of some synthetic fuel production pathways that could produce lesser but still substantial life-cycle reductions in carbon emissions. Key research topics for this project include conducting comprehensive comparative technoeconomic assessments of potential SAJF feedstocks and conversion processes; enhancing system modeling and analysis capabilities for micro (individual project) and macro (nationwide or worldwide) evaluations of the potential impacts and benefits of SAJF development and commercialization (for policy and business decision support); and advancing the science, application, and harmonization of sustainability analysis, starting with life-cycle CO₂ modeling and then progressing to additional topics of interest.

Low-Cost Feedstocks

This project would support continued development of sustainable, low-cost feedstocks and associated systems that have the potential to enable the large-scale production of economically viable SAJF.

Several processes have been demonstrated to produce SAJF that can be qualified as drop-in fuels, and others are envisioned. The development of feedstocks is one of several technical challenges that must still be overcome to enable economically competitive production of SAJF at scales of significance. 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. Key research topics for this project include (1) identifying and developing feedstocks that could enable economically viable and sustainable production of SAJF; (2) developing strategic approaches for the use of several waste streams that could be used as SAJF; and (3) using the results of past feedstock evaluations to inform and prioritize current feedstock development activities. Waste streams of potential interest include municipal solid waste, human waste and sanitary waste treatment, animal waste, animal processing waste, and gaseous waste.

Conversion Processes, Fuel Production, and Scale-Up

This project would develop technologies and processes for cost-effective feedstock conversion, fuel production, and scale-up from pilot and demonstration facilities to enable full-scale production of SAJF.

Cost-effective conversion technologies are not available for some promising feedstocks. Key research topics for this project include creating additional process development facilities to enhance the ability of SAJF conversion technology developers to move expeditiously from benchtop to pilot scale with minimal capital and operating expenses; developing fuel conversion and finishing processes and equipment, focusing first on processes common to multiple conversion processes; and fostering the development of lower-cost hydrogen production to deal with the fundamental hydrogen deficit in converting many biofeedstocks to finished fuels.

SAJF Fuel Testing, Evaluation, and Qualification

This project would improve fuel testing, evaluation, and qualification efforts to lower testing costs, increase throughput, and enhance understanding of fuel properties. It takes longer than it should to commercialize new SAJF production methods, in part because of the cost and time required to complete fuel qualification and certification processes. These processes are costly; fuels required for testing are difficult to produce in sufficient quantities in reasonable amounts of time; and the entities undertaking qualification are typically small, underfunded start-up organizations that also must deal with the many other technical and economic challenges usually encountered by start-up companies.

An industry process has been established using standard specifications and qualification practices established by ASTM International to qualify processes for producing drop-in SAJF. The ASTM qualification process has limited throughput, it is highly dependent on physical testing for validation, and is insufficiently based on chemistry and combustion science.

Key research topics for this project include (1) eliminating or reducing time-consuming and costly physical testing by developing a low-cost, high-throughput approach to meeting ASTM specifications and qualification standards relevant to SAJF; this will likely require determining which molecular components in the family of molecules present in the jet fuel are cause for concern with respect to material compatibility; (2) improving the ability to characterize combustion attributes of properties of various SAJF constituents using analysis and simpler testing; (3) assessing the environmental effects (nearer term) and turbomachinery health and performance benefits (longer term) of potential SAJF pathways; and (4) developing a database, to be made broadly available to members of the SAJF community, of fuel feedstocks, processes, fuel properties, and combustion emission characteristics to facilitate the use of alternative jet fuels.

CHALLENGES TO IMPLEMENTING ADVANCED LOW-CARBON TECHNOLOGIES FOR COMMERCIAL AIRCRAFT

Many challenges need to be overcome to implement the proposed research agenda. Some are systemic—being issues for all of the approaches considered—and others are technical, economic, and policy challenges related to the high-priority approaches recommended by the committee.

Systemic Challenges

• Economic competitiveness. Commercial 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.

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.
• 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.
• 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.
• 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.
• 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.
• 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.

Economic Challenges

• 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.
• 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 a comparative understanding of various approaches impede 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.

Policy Challenges

• 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.
• 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.

COORDINATION OF RESEARCH AND DEVELOPMENT

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 FAA, NASA, the Department of Agriculture, the Department of Transportation, the Environmental Protection Agency, and the National Science Foundation. A coordinated approach would make the best use of the available resources.

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.

Many federal organizations and agencies have important roles to play in reducing CO₂ emissions. 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 to the development of batteries, fuel cells, and SAJF feedstocks and conversion processes. The primary contributions of the Department of Agriculture, Environmental Protection Agency, Department of Transportation, Department of Commerce, and National Science Foundation would be to SAJF research.

CONCLUDING REMARKS

Four high-priority approaches were identified throughout the course of study that have the potential to reduce CO₂ emissions from commercial aviation, particularly from those aircraft that produce the bulk of the emissions: large single- and twin-aisle aircraft. However, developing new technology for large commercial aircraft requires substantial time and resources. Aircraft–propulsion integration and gas turbine engines are both well-established approaches that need to be pursued. 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 also uncertain when SAJF will be able to compete economically with petroleum-based fuels, especially considering the capital costs of founding a new industry and the fluctuating prices of conventional jet fuel. Given the immediacy of the issues, however, research supporting all four approaches is prudent both to reduce current CO₂ emissions and to alleviate the potential adverse consequences of future aviation growth worldwide.