Materials Needs and R&D Strategy for Future Military Aerospace Propulsion Systems (2011)

The ongoing development of military aerospace platforms requires continuous technology advances in order to provide the nation’s warfighters with the desired advantage. In 2006, a report entitled A Review of United States Air Force and Department of Defense Aerospace Propulsion Needs from the National Research Council’s (NRC’s) Air Force Studies Board (AFSB) concluded that airplane propulsion systems designed to approach Mach 5 would require the development of materials technology solutions that are as yet unavailable.¹ A related 2006 AFSB report, Future Air Force Needs for Survivability, describes challenges to improving propulsion and signature, or stealth, that are materials-intensive and must be addressed if the Air Force, and the other services by extension, are to move ahead toward the development of high-Mach manned or unmanned air vehicles.²

The NRC’s 2006 Aerospace Propulsion Needs report concluded that “additional emphasis must be placed on propulsion research or the technological lead of the United States will almost certainly cease to exist.”³ It also concluded that the way forward for the materials technology development base is still not fully defined

and that top priority should be given to overcoming the technology barriers that will have the largest impact on future weapons systems. As indicated in that report, these barriers include the following: compressor discharge temperature limits; turbine inlet temperature limits; high-temperature, high-heat-sink fuels for thermal management; lightweight structures; and signature control.

The technology challenges in developing future military aerospace propulsion systems are significantly materials challenges. Overcoming these challenges will require focus on a systematic materials approach and materials R&D specific to the needs of the subsystem involved. Such considerations would include the following:

• Materials for atmospheric propulsion systems, both air-breathing and alternate systems;

• Materials for space propulsion systems;

• Materials for alternative fuel engines;

• Materials for the development of lightweight and multifunctional systems;

• Materials methodologies for stealthier (signature controlled) systems; and

• Strategies to coordinate the development of materials, composites, and interactive materials systems that will work together to create an effective and efficient multifunctional materials palette.

These materials issues will require that the DOD take new R&D directions and, in that context, the NRC was asked by the DOD to conduct the present study to assess the needs and directions for a national materials R&D strategy to respond to the challenge of developing materials for future military aerospace propulsion systems and to keep the United States on the leading edge of propulsion technology.

Capabilities-based planning addresses the uncertainty in the threat environment by using a wide range of scenarios to bound requirements for future systems. The DOD introduced this approach several years ago as the planning approach to be used for justifying military needs, but at the present time this planning approach is not sufficiently mature to have identified stated needs. However, the 2006 NRC study referred to above—A Review of United States Air Force and Department of Defense Aerospace Propulsion Needs—identified global strike, global mobility, airborne C4ISR (command, control, communications, computers, intelligence, surveillance, and reconnaissance), and next-generation space access as required capabilities. These capabilities require technology advances in high-speed turbine engines, ram/scramjet/pulse detonation engines, rocket propulsion, combined-cycle engines, and ultra-efficient propulsion. Therefore, the committee used these capabilities as the required system improvements for the purposes of this study.

Each of these required advances in propulsion technology is strongly dependent on materials development activities. Improving the efficiency and performance of a jet engine requires higher operating temperatures in order to improve thermodynamic efficiency. To improve efficiency further, engine weight must be reduced while preserving thrust. All of these improvements require new materials with higher melting points and greater strength and durability. Similar advances are required in rocket casing and nozzle throat materials. To address these required advances, the DOD identified the tasks listed in the following section to be carried out by the present study, conducted by the NRC’s Committee on Materials Needs and R&D Strategy for Future Military Aerospace Propulsion Systems.

The statement of task for this study is as follows:

To fulfill its statement of task, the committee held five meetings (see the “Acknowledgments” section in the front matter of this report for a list of the committee’s guest speakers) and made a site visit to the AFRL Propulsion and Power Directorate at Wright-Patterson Air Force Base in Ohio. The committee received presentations from the sponsor and government research agencies, key industry participants in the propulsion and materials science domains, and academics from the materials science and metallurgy fields, as well as from General Kenneth Eickmann, chair of the 2006 NRC study A Review of United States Air Force and Department of Defense Aerospace Propulsion Needs.

In addition, the committee was given access to a document titled Materials for Advanced Aerospace Propulsion and Power Systems (AFRL-RZ-WP-TM-2008-2171). Restricted by the International Traffic in Arms Regulations (ITAR), that document (hereinafter referred to as the plan) contains the current plan for materials development within the AFRL and served as the point of comparison for the committee’s first task (see the preceding section), which requires an assessment of “whether current and planned U.S. R&D efforts in materials for aerospace propulsion are sufficient (a) to meet U.S. military needs and (b) to keep the U.S. on the leading edge of propulsion technology.”

The committee addressed the statement of task in the following manner. Chapter 1 presents the origin and background of the study. Chapter 2 addresses the process of materials development and the second task. The first task is addressed in Chapter 3, and the committee’s specific assessments of the AFRL plan are included in Appendix D, the content of which is governed by ITAR/Export Administration Regulations restrictions and so is not included in the publicly released version of this report. Chapter 3 also contains a limited assessment of international materials development efforts, to aid in the determination of which nations are near the “leading edge” of propulsion technology. The issue of intellectual property protection is covered in Chapter 4, which addresses the third task. Chapter 5 outlines the recommended path forward by describing the key elements of an effective R&D strategy, addressing the fourth task.

Appendix A presents the committee’s statement of task. Appendix B provides an overview of the work being done throughout the world at the leading edge of aerospace propulsion as described in open-source literature. Appendix C presents the biographies of the committee members. The text of Appendix D, which contains the committee’s ITAR-restricted analysis of the current plan for materials development within the AFRL, is not releasable to the public under ITAR. Appendix E offers some materials development case studies. Appendix F defines the acronyms used in this report.