gas turbine propulsion systems using thrust-specific fuel consumption as a figure of merit. Although the rate of improvement in this commonly used parameter has decreased, incentives remain great to pursue further technology advances in order to meet customer-driven goals. As an example, NASA has successfully worked with industry to develop and verify analytical tools that address design and system evaluation and reduce the number of experiments needed for a successful engine.

Commercial aviation is highly competitive, so minimizing costs is critical to the survival of individual airlines. The propulsion system is not immune to these pressures: Advanced propulsion technologies will rarely be incorporated in operational products unless they reduce costs or are needed to meet some other requirement, such as more stringent noise or emissions standards.

The propulsion systems of commercial aircraft are only a small contributor to the accident rate as a result of tremendous investments and decades of work to improve the reliability of turbomachinery. It is essential that new propulsion systems and components also demonstrate very high levels of safety.

Propulsion research plans should be structured to meet the needs of advanced airframe concepts in the context of the long-term vision for the air transportation system. Concepts such as BWB aircraft, supersonic business jets, and runway-independent aircraft dictate unique requirements and opportunities for advances in propulsion technology. Areas of interaction include extremes in engine size and fan bypass ratio, design for boundary layer ingestion, highly integrated engine-airframes, power extraction for boundary layer manipulation, variable cycle features, and architectures for integration of system controls.

In the 2025 to 2050 time frame, low-cost hydrogen could become attractive as an aircraft fuel that would reduce the environmental effects of aviation. The key challenge to the use of hydrogen as an aircraft fuel is its low energy density compared with hydrocarbon fuels—unless new (high-density) means of storing hydrogen are developed. Even though the committee is not aware of any particularly promising approaches for overcoming this problem, the high potential payoff warrants continued research. Widespread use of hydrogen as an aircraft fuel would also require an economically and environmentally benign method for producing hydrogen, a challenge that is being addressed by broader efforts to enable hydrogen to replace hydrocarbon fuels in ground-based vehicles and industrial uses.

Advances in electric power systems may eventually allow them to replace internal combustion engines. In particular, methane or hydrogen fuels for fuel cell power systems in various forms offer a potentially significant improvement in energy conversion efficiency over today’s gas turbines, and ongoing research programs are addressing both mobile and stationary fuel cell applications. Even so, tremendous advances in the power density of fuel cells would be required to make them technologically feasible as a source of propulsion power for large commercial aircraft. Other technology issues associated with the development of an electric aircraft propulsion system (such as the development of lightweight electric motors using, for example, room temperature superconductors) would also need to be resolved to make a fuel cell energy conversion system into a successful aircraft propulsion system. It might also be feasible to use the electricity produced by fuel cells to add heat to the gas in a gas turbine engine in place of combustion. Since electricity, lasers, and electromagnetic devices can provide volumetric heating in place of combustion of hydrogen or hydrocarbon fuel, exploratory research is in order to determine the conditions under which these alternatives may be attractive. Other alternative approaches are given in Appendix D.

The first application of electric power sources on commercial aircraft is likely to be as auxiliary power units rather than for propulsion. Although fuel cells are larger and heavier than conventional auxiliary power units, they generate water. This would reduce the amount of water that must be carried onboard at takeoff, thereby improving the overall assessment of fuel cells as auxiliary power units, from a systems perspective. Unrelated technology developments, however, may produce aircraft toilets that flush with 90 percent less fluid, reducing the onboard demand for water.

Intermittent combustion concepts, such as pulse jets (a.k.a. pulse detonation engines), have the potential for improved performance relative to traditional turbomachinery systems. In some cases, intermittent concepts may also significantly reduce complexity. However, it seems unlikely that systems based on intermittent concepts will outperform gas turbine aircraft propulsion systems in the foreseeable future. Nonetheless, continued basic research would be worthwhile to better understand the long-term limitations and potential benefits of intermittent combustion concepts.

Nuclear power is unsuitable for aircraft applications for many reasons, including the weight of radiation shielding, radiation exposure during normal operations, and the risk of widespread radioactive contamination in the event of an accident. The committee did not identify any other specific propulsion or fuel concepts of particular interest, although research to explore new concepts would be consistent with NASA’s vision and goals.

Future airframe and propulsion research will lead to a better understanding of the synergies and tradeoffs that exist among system and subsystem concepts, technologies, design characteristics, and performance parameters, including environmental performance parameters—for example, specific fuel consumption, noise, and specific engine emissions. Cur-

Front Matter (R1-R16)

Executive Summary (1-5)

1. Foundation for Change (6-12)

2. Improving Air Transportation System (13-18)

3. System Modeling and Simulation (19-26)

4. Improving Aircraft Performance (27-41)

5. Process for Change (42-44)

Findings, Recommendations, and the Big Question (45-50)

Appendix A: Statement of Task and Study Approach (51-54)

Appendix B: Comparative Assessment of Goals and Visions (55-59)

Appendix C: Biographies of Committee Members (60-63)

Appendix D: Propulsion Taxonomy: Comments on Propulsion Fundamentals (64-66)

Appendix E: Four Levels of Models (67-70)

Appendix F: Acronyms and Abbreviations (71-72)