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Improving the Efficiency of Engines for Large Nonfighter Aircraft 8 Technology Preparedness and Insertion INTRODUCTION Some benefits of re-engining programs are due to the fact that newer engine models typically embody more advanced technologies and design and manufacturing processes than were available for the original engine. These new technologies provide improvements in aerothermodynamics, structures, materials, and controls and result in an engine that is more powerful and lighter in specific weight, consumes less fuel, and is more durable, reliable, and maintainable than the engine being replaced. For large nonfighter aircraft for military applications, engine usage (flight hours per year) is low enough that several generations of technology may evolve during the lifespan of any given engine. The performance, fuel consumption, and support cost differences between a baseline engine and a re-engining candidate may therefore be very large. Although the benefits of re-engining with newer technology engines are clear, the costs associated with procurement and the nonrecurring development cost required to integrate the new engine can be prohibitively high. There is a clear trend away from new centerline engines and toward longer service life for fielded, on-wing engines, so classic re-engining opportunities will become rarer. It is thus important for national turbine engine science and technology (S&T) efforts to recognize this trend and place greater emphasis on the continual transition of new technology to fielded engines, in particular, technology that can reduce fuel consumption. The former Engine Model Derivative Program (EMDP) is a noteworthy example of successful technology transition from research and development programs to fielded engines.1 1 An EMDP was created in 1978 to develop an alternative engine to the F100. The Air Force was concerned about early operational and supportability problems with the F100 engine fleet and wanted to make the F-16 engine purchase competitive to obtain cost and support savings (Irwin, 2006).
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Improving the Efficiency of Engines for Large Nonfighter Aircraft TURBINE ENGINE SCIENCE AND TECHNOLOGY OVERVIEW Until 2005, U.S. turbine engine research was guided by the Integrated High Performance Turbine Engine Technology (IHPTET) program. IHPTET was initiated in 1987 with very aggressive technical goals, essentially achieving a 100 percent improvement in turbine engine capability based on a 1987 state-of-the-art baseline engine by the turn of the century. IHPTET featured specific goals in each of three engine classes—namely, turbofan/turbojet, turboprop/turboshaft, and expendable engines. For the turbofan class, the primary goal of IHPTET was to double the engine thrust to weight ratio (T/W). Although IHPTET made significant progress toward its goals, the program focused on low-bypass-ratio, fighter/attack-class engines, and payoffs for large-bypass-ratio, transport-class engines occurred primarily as a spin-off from the fighter/attack application. Thrust-specific fuel consumption (TSFC) was regarded as important, but not as critical as T/W. With the conclusion of IHPTET in 2005, the Versatile Affordable Advanced Turbine Engines (VAATE) program became the nation’s premier turbine engine S&T program.2 VAATE has been structured to take advantage of the features that made IHPTET successful. These include coordination between DoD, NASA, academia, and industry, with the Federal Aviation Administration (FAA) and DOE joining the effort as well. The breadth of this integrated team allows the VAATE program to coordinate gas turbine technology development strategy at the national level while leveraging funding of the constituent organizations. A fundamental goal of VAATE is to advance overall air system capability with a capability-focused investment strategy. The scope of VAATE is thus significantly greater than the rotating machinery focus of IHPTET and will encompass the entire propulsion/power system, including inlet/nozzle integration, thermal and power management, integrated controls, and prognostics and health management. This approach requires optimization of integrated propulsion capability at the aircraft system level, rather than optimization of just the engine turbomachinery. To this end, the major aircraft manufacturers are full partners on the VAATE industry team. The VAATE program goal is to realize a 10-fold improvement in the affordable capability of a turbine-engine-based propulsion system. Here, “affordable capability” is defined as the ratio of propulsion system capability to cost. Capability in this context measures technical performance parameters, including thrust, weight, and fuel consumption. Cost is the total cost of ownership and includes development, procurement, and life-cycle maintenance cost (excluding fuel). These improvements are to be realized relative to a baseline representative of year 2000 state-of-the-art systems. Specific measurable technical improvements, such as thrust, weight, TSFC, and life-cycle cost, are called “goal factors” in the VAATE lexicon. The overall VAATE goal, expressed as a capability-to-cost index (CCI), is defined by the following function of the goal factors, where each factor is expressed as a ratio between the subject and a contractor-chosen reference system: While each VAATE contractor is free to determine the specific combination of goal factors and product application that comprise its offering to achieve the CCI goal, it is important to note that the fundamental physics-based turbine engine technology barriers, such as temperature, pressure ratio, and 2 For additional information on the VAATE program, please see the American Institute of Aeronautics and Astronautics (AIAA) VAATE position paper at http://pdf.aiaa.org//downloads/publicpolicypositionpapers//VAATE.pdf. Last accessed on September 11, 2006.
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Improving the Efficiency of Engines for Large Nonfighter Aircraft materials, are the same regardless of the exact architecture, configuration, and application of the engine. Without guidance, VAATE contractors may choose to emphasize performance (T/W) improvement for clean-sheet, high Mach or fighter/attack-class engines rather than specific fuel consumption (SFC) and cost improvements for the subsonic, high-bypass-ratio engines necessary for large nonfighter aircraft. Several technologies that are of specific interest to this class of application are shown in Figure 8-1. Other advanced technologies, such as unducted or geared high-bypass-ratio fans and low-pressure spool power extraction, have been shown to yield fuel efficiency improvements and might be amenable to eventual re-engining applications. Environmental constraints, such as noise and emissions levels, are additional considerations for the technology planning process. Finding 8-1. Engine fuel efficiency was an important consideration but not a primary focus of previous S&T programs such as IHPTET. VAATE allows greater leeway for emphasis on fuel efficiency, but it is not clear that such focus will materialize without specific DoD direction. Recommendation 8-1. The Air Force should review and amend the VAATE plan and its engine development programs, as appropriate, to provide an explicit emphasis on technology to improve fuel efficiency and reduce operational costs, to transition those improvements to fielded, high-bypass-ratio engines, and to consider research aimed at the reduction of particulate, hydrocarbon, sulfur, carbon monoxide (CO), carbon dioxide (CO2), nitrogen oxides (NOx), and noise emissions by the DoD systems. FIGURE 8-1 Key advanced technologies for large turbofan engines that will be developed through the VAATE program. ISR, intelligence, surveillance, reconnaissance. SOURCE: Personal communication from Timothy Lewis, Air Force Research Laboratory, to committee member Jeffrey Hamstra on August 24, 2006.
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Improving the Efficiency of Engines for Large Nonfighter Aircraft POTENTIAL FOR SPECIFIC FUEL CONSUMPTION IMPROVEMENT Outside the gas turbine engine community, many believe that turbine engine performance and efficiency have reached a natural limit. In fact, just the opposite is true: There remains substantial potential for improving the current state of the art and more nearly attaining theoretical limits. The ideal cycle based on optimum stoichiometric combustion properties is a sign of how far the gas turbine engine remains from its theoretical limits in terms of the key fuel efficiency metric. Turbine engine fuel efficiency has improved dramatically from the 1940s to today. Each new technology improvement, whether in component aerodynamics, materials, or turbine cooling, has allowed increments to overall pressure ratio and turbine inlet temperature, resulting in improved fuel efficiency across a diverse range of engine applications. As shown in Figure 8-2, current state-of-the art engine technology reaches only approximately 38 percent of the stoichiometric limit of gas turbine engines. Through a combination of technologies offered by the VAATE program, a 25 percent improvement in fuel efficiency is anticipated, a substantial improvement over today’s state of the art. FIGURE 8-2 Progress in gas turbine thermodynamic cycle fuel efficiency. SOURCE: Harrison (2006).
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Improving the Efficiency of Engines for Large Nonfighter Aircraft Finding 8-2a. Current state-of-the-art engine technology has closed only 38 percent of the fuel efficiency (i.e., TSFC) gap between 1940s-vintage jet engines and theoretical limits. Finding 8-2b. Through a combination of technologies offered by further development, an additional 25 percent improvement in turbine engine fuel efficiency is anticipated. Recommendation 8-2. As an additional facet of VAATE, the Air Force Research Laboratory (AFRL) should establish a technology insertion plan for SFC improvements integrated across the top fuel-consuming DoD systems. COMPONENT IMPROVEMENT AND ENGINE MODEL DERIVATIVE PROGRAMS This section discusses the issues and opportunities associated with a large and growing portion of the DoD aircraft propulsion systems. Figure 8-3 shows that approximately $4.2 billion per year of the total $6.6 billion DoD yearly gas turbine engine budget is spent on the sustainment of existing engines (AFRL, 2005). The cost of fuel burned by the existing fleet is currently estimated at $11.75 billion annually based on a fuel cost of $2.50 per gallon (Connelly, 2006; Harrison, 2006). The projected weapon system force structure for the next 15 to 20 years indicates that currently fielded systems will continue to dominate FIGURE 8-3 DoD investment in turbine engines. A/C, aircraft. SOURCE: AFRL (2005).
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Improving the Efficiency of Engines for Large Nonfighter Aircraft and that new systems will be acquired at slower rates and in smaller numbers than the legacy fleets they replace. This will lead to an ever-increasing aging of the DoD gas turbine fleet. By 2020 most of the existing gas turbine engines will have reached or exceeded their design life and will need service life extensions. To address this reality, component improvement programs (CIPs) focus on several key areas, including decreasing Class A mishaps, increasing time on wing, decreasing fuel burn, and extending service life (Fecke, 2006). In many cases, new business models will be required to capture opportunities for incorporating new technologies or component improvements into the existing fleet to reduce the cost of sustainment and fuel for current engines. Current business practices, with different colors of money for CIPs, depot maintenance and support, and operational fuel cost, create artificial roadblocks to maximizing DoD’s return on S&T investments. New business models, coupled with modest investments, could be leveraged for the existing fleet. DoD aircraft systems are continually modernized in order to remain viable and responsive to the warfighter. These upgrades occur to address identified performance deficiencies or to grow existing systems to provide new mission capabilities. Figure 8-4 indicates that more capable aircraft typically weigh more, generate more drag, and have larger electrical/mechanical power loads, leading to increased demands on the aircraft propulsion system. A proven cost-effective and efficient approach to increasing propulsion capability is the development of derivative versions of existing engines. Derivative engines are developed by transitioning newer technology into existing legacy propulsion systems, improving performance and power. Such engines have been used very successfully for both military and commercial applications since they offer significant cost, schedule, and risk advantages in comparison to new centerline engines. There is currently no active programmatic vehicle for increasing the performance of legacy propulsion systems by developing derivative engines. The EMDP, which was canceled in 1998, was just such an effective vehicle, and its cancellation resulted in two significant gaps in the DoD engine development process. The first gap is the inability to conduct timely studies of propulsion system enhancement or to develop technology transition roadmaps to support and complement aircraft modernization and capability growth studies prior to acquisition Milestone A. The second gap is the lack of a propulsion technology demonstration process to mature technology from Technology Readiness Level (TRL) 6 (demonstration in a relevant system) to TRL 7 (demonstration through initial flight test). This gap results in either increased risk or the inability to incorporate new technology into the propulsion system past acquisition Milestone B. In the past, EMDPs were a very cost-effective method to improve capabilities and decrease the cost of sustainment. For example, the EMDP for the F100-PW229, developed for the F-15E, increased capabilities and decreased the cost of supportability (i.e., lowered the cost per flying hour by reducing both scheduled and unscheduled shop visits and reducing the rate of Class A mishaps). Similarly, the F101, F110, and F108 common core design, shown in Figure 8-5, has been a very cost-effective method using derivative engines to provide power for a wide range of aircraft. Across DoD, a new capability-based assessment process has been implemented to define the requirements for upgrades to existing weapon systems or for an entirely new weapon system. After a capability gap or shortfall has been identified, determining how best to provide the desired capability begins with the weapon system studies. The initial capability document is developed bearing in mind the “art of the possible,” based on available technologies. Possible solutions are assessed using a concept of operations (CONOPS)-based Analysis of Alternatives, and a concept is chosen for further refinement and potential technology development. The capability assessment sets forth requirements for the weapon system. However, the propulsion system capability requirements are derived from the aero performance requirements of the weapon system and the subsystem functional interface requirements. The systems engineering process may not
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Improving the Efficiency of Engines for Large Nonfighter Aircraft FIGURE 8-4 Engine model derivative program provides propulsion capability increases for F-15 and F-16 aircraft. SOURCE: Personal communication between Mark Amos, Agile Combat Support Wing, Wright-Patterson Air Force Base, and NRC staff member Carter Ford on July 7, 2006. generate quantitative propulsion system requirements until the weapon system enters systems design and development. This delay, coupled with the lack of funding because the EMPD was dropped, is limiting the benefit that DoD could achieve from derivatives of existing engines. Finding 8-3. Unless strong action is taken, the growing proportion of the DoD propulsion budget needed for sustainment of the existing fleet and fuel for it will lead to a “death spiral” in which the share of budget available for technology development and transition will be continuously shrinking. Recommendation 8-3. The Air Force and DoD should improve synergy between DoD and commercial upgrade programs by improving the tracking of commercial upgrades and using the down-time during aircraft depot maintenance as an opportunity to upgrade engines to more commercial configurations, as appropriate. Finding 8-4. The CIP, like the EMDP before it, is highly effective in transitioning technology to fielded engines.
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Improving the Efficiency of Engines for Large Nonfighter Aircraft FIGURE 8-5 A common core approach can be used to propagate technology across a range of engine models. SLEP, service life extension program. SOURCE: GE Aircraft Engines. Recommendation 8-4. The Air Force and DoD should reinvigorate the CIP and the propulsion capability enhancement programs and combine the responsibility for component improvement, sustainment, and fuel burn under one budget authority to allow it to capture opportunities to reduce fuel burn and cost. TURBINE ENGINE SCIENCE AND TECHNOLOGY FUNDING Gas turbine engines will continue to be the predominant military propulsion source for the foreseeable future. To maintain air superiority, it is critical for the United States to maintain technological superiority in gas turbines. The technology developed in the IHPTET program, coupled with NASA turbine programs and the DoD Manufacturing Technology (ManTech) programs, is allowing the United States to field the most advanced gas turbines in the world (e.g., the F119, F135, and GE90 families of engines). With the overall reduction of the NASA and ManTech programs, VAATE must bear the burden of U.S. gas turbine engine S&T advancement. Turbine engine S&T programs properly culminate with first demonstrations of the core technology and then proceed to full-engine laboratory demonstrations to mature the technology to TRL 6 and prove out transition capability. Previous generations (IHPTET program bases) of turbine technology development depended on these tests to be run at least once a year, and often twice, to meet timely and necessary technology transition goals to new and fielded military and commercial engines. However, as
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Improving the Efficiency of Engines for Large Nonfighter Aircraft FIGURE 8-6 Turbine engine technology funding. PBR, President’s Budget Request; DARPA, Defense Advanced Research Projects Agency; LRS, long-range strike. SOURCE: Burns (2005). shown in Figure 8-6, owing to the near elimination of the NASA and ManTech programs and reduced funding for the VAATE program, the funding available in the United States for the development of gas turbine technology is approximately one-third of the funding that produced the technology for the F119, F135, and GE90 engine families. With its greatly reduced budget, VAATE will demonstrate full TRL 6 core and propulsion system demonstrations only once every 3 to 5 years. Finding 8-5. DoD’s planned investment in VAATE is inadequate to sustain a minimally acceptable rate of advancement in U.S. gas turbine engine technology. Recommendation 8-5. The Air Force and DoD should restore turbine engine S&T funding to the original level necessary to execute the VAATE plan (with recommended changes), with particular emphasis on reinvigorating engine demonstration programs aimed at rendering new technologies ready for transition to fielded engines. REFERENCES Published AFRL (Air Force Research Laboratory). 2005. NRC Propulsion Needs Study Report: Supporting Charts for Turbine Engine Technology Investment Needs. May.
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Improving the Efficiency of Engines for Large Nonfighter Aircraft Unpublished Larry Burns, VAATE Program Manager, Air Force Research Laboratory, “Current/future turbine engine technology investment plans and VAATE,” Presentation to the committee on April 5, 2005. Richard Connelly, Director, Defense Energy Support Center, “Defense energy support center,” Presentation to the committee on June 14, 2006. Ted Fecke, Propulsion System Group, Agile Combat Support Wing, “Engine component improvement program,” Presentation to the committee on June 15, 2006. William Harrison, Director, National Aerospace Fuels Research Complex, Air Force Research Laboratory, “Alternative fuels overview,” Presentation to the committee on June 13, 2006. Dave Irwin, Propulsion Systems Squadron Development Engines, Wright-Patterson Air Force Base, “Engine model derivative program,” Presentation to the committee on June 15, 2006.
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