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Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
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8
PROPULSION

INTRODUCTION

Propulsion technology offers the greatest single contribution to the improvement of cruising economy and the environmental impact of commercial aircraft. The past three generations of gas turbine engines have incorporated increased turbine inlet temperature, increased compressor pressure ratio, increased bypass ratio, improved fan and nacelle performance, reduction of noise and emissions, and improved reliability that led to a continued dominance of the world commercial aircraft market. This pace of development in new engine technologies, together with advances in engine-airframe integration, can, with adequate support from industry and the National Aeronautics and Space Administration (NASA), be continued over the next 10 to 20 years, thus providing superior propulsion systems entering service through the 2005–2015 period.

These gains rest, of course, on continued development of new and improved materials and material-processing techniques, the clearly available advances in turbomachine technology, promising progress in combustion technology, and vastly improved utilization of computational fluid dynamics (CFD) in engine design procedures. Finally, there is the unknown impact of novel technologies, such as ''smart engines'' and magnetic bearings, that may completely change the course of engine development.

This wide range of possibilities for engine development is narrowed by the general type of commercial aircraft to which they will be applied. The specific requirements differ between the advanced subsonic transports, the high-speed civil transport (HSCT), and the short-haul class of aircraft. Although these engine classes have many features in common, each has unique criteria that have a major influence on its design.

In light of the factors mentioned above, one section of this chapter is devoted to the energy efficiency, economy, and environmental goals of the engines appropriate to each of the aircraft types. Sections are then included dealing individually with technological issues that require discussion in greater detail. The boxed material summarizes the primary recommendations that appear throughout the chapter, with specific recommendations given in

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
×

Recommendations

General

NASA must vastly strengthen its propulsion technology program to include:

  • much greater emphasis on subsonic transport propulsion systems where the United States has lost its technological edge over foreign competitors;

  • continued support for the HSCT propulsion program; and

  • a strong, broad-based propulsion technology program to position the United States for the post-2000 short-haul markets, including a better balance between vertical takeoff and landing commuter systems and conventional systems.

Specific

  1. NASA should increase its support for generic computational and experimental propulsion research. In addition, NASA must lead in the development of technical communication with industry in computational science applied to propulsion.

  2. NASA should take the initiative in setting up a joint NASA/industry program for innovative subsonic propulsion technology that is at least equivalent to the NASA/industry HSCT propulsion program.

  3. NASA must support the development of specialty materials not currently available as commodities that, if properly developed, will become common in aircraft engines.

  4. The basic research effort at the Lewis Research Center (LeRC) in low nitric oxide combustors for the HSCT has produced excellent results and the momentum should be maintained. NASA should put in place the planned Joint Technology Acquisition Program between LeRC and industry.

  5. NASA should take advantage of its unique position to mount a substantial program in active control technologies for aircraft engines.

  6. NASA's turbomachinery research program, should be strengthened to the point at which NASA can recapture its leadership role.

  7. NASA's LeRC should direct increased effort toward the enabling technologies in compressor, combustor/turbine, and control accessories for engines appropriate to short-haul aircraft.

order of priority, and the benefits that are possible with adequate research and development effort in propulsion.

PROPULSION FOR ADVANCED SUBSONIC AIRCRAFT

Advances in propulsion system technology have been the prime source of subsonic transport performance improvements for more than 30 years and this trend has continued through

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
×

Benefits of Research and Technology Development in Propulsion

Engine Performance

Reduced engine fuel consumption

Reduced emissions

Reduced engine noise

Reduced engine weight

Greater engine thrust

Enhanced reliability

Reduced maintenance requirements

Long-life materials and components

Design for maintenance

Engine Design and Development

Shortened development cycle

Improved computational capabilities for propulsion

Improved testing facilities for propulsion

Technology validation

the evolution of high bypass turbofans. Engine fuel consumption has improved more than 40 percent from the early turbojets; even within the high bypass class, Pratt & Whitney (P&W), Rolls Royce, and General Electric (GE) engines have improved by 16 percent since 1970. Size is a major factor in aircraft productivity and in fuel burned per seat-mile, and the growth in engine thrust has been a major contributor to aircraft size. The high bypass turbofans of 40,000 pounds force (lbf) thrust ushered in the jumbo jet era, and engine growth to 50,000–60,000 lbf thrust has led to even larger, more capable aircraft. Outstanding improvement in engine reliability has resulted in long-range (over water) twin-engine aircraft with engines of 75,000–85,000 lbf thrust under development for mid-1990s service. Growth to 90,000–100,000 lbf thrust is expected.

State of the Art

The state of the art in new large engines is characterized by an installed propulsion system cruise-specific fuel consumption of 0.54–0.56, including nacelle drag but not customer bleed. The installed thrust-to-weight ratio is about 4.5. Cycle pressure ratios are 36–38; the compressor discharge air temperature is 1250ºF; and the turbine rotor inlet gas temperature is 2250ºF, with the mechanical design redline 150ºF higher. Materials include fourth-generation nickel superalloys, high-strength titaniums, and carbon composites. Controls are digital. Most turbomachinery component efficiencies are 90–92 percent; compressors have greater than 87 percent polytropic efficiency. Large engine fans are 8–10 feet in diameter; fan inlets and cowls

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
×

are 11–14 feet in diameter and 15–18 feet long. Prices for large engines are around $100 per pound of thrust, and because of intense competition, industry will make major efforts to reduce this. In the 1990s this technology can be expected to evolve to 50:1 cruise pressure ratio, with turbine inlet temperatures up 100–150ºF as further materials improvements are made. Refinements will continue to be made in component performance, nacelle performance, and weight.

At this point it is worth mentioning a program that exemplifies the type of joint NASA/industry cooperation that can have significant impact on the state of the art and on the overall competitiveness of U.S. products. The Aircraft Efficiency (ACE) program was begun in the 1970s in response to the energy crisis and contained six separate programs aimed at producing real improvements in the efficiency of aircraft. One of the six ACE programs, the Energy Efficient Engine (E3) program, had a goal of 12 percent reduction in engine fuel consumption. The program ran from 1977 to 1982 and was jointly funded by NASA, GE, and P&W. The goal was met and, most importantly, both GE and P&W have aggressively incorporated the component and systems technology that resulted from the E3 program into their current generation of engines, including the GE CF6-80 and the P&W 4000 engines. Also, the GE-90 engine that will be certified in 1994 is very closely related to the engine that was developed and tested during the E3 program. The Committee believes that this type of program can provide tremendous benefits to the U.S. aeronautics industry and should be pursued wherever feasible.

The Future

In 1989, NASA Lewis Research Center (LeRC) sponsored initial preliminary design studies of a new class of high thermal efficiency, high propulsive efficiency engines. Pressure ratios of 75–100:1 and turbine rotor-in temperatures of 3000–3400ºF were explored, using new materials and technologies, for the post-2000 period—far beyond the evolution expected in the 1990s. From this initial work it is apparent that there could be one block of new technology readied in the next 10 years for service around 2005 and a second block, less defined at this time and more dependent on new fiber-reinforced materials, that could be available about 10 years later for service in 2015. Both would permit dramatic advances. The first (2005), would yield a 15–20 percent reduction in fuel burned by mid-1990s engines, resulting from reduction in fuel consumption and reduction of propulsion drag and weight. The aircraft direct operating cost (DOC) could be reduced by 8–10 percent (at a fuel price of $1.00 per gallon). The second (2015) could lead to 30 percent reduction in fuel burned and 12–15 percent DOC reductions from mid-1990s levels. These advances are greater than the high bypass turbofans of the late 1960s.

To achieve these results, higher core engine thermal efficiencies and higher propulsive efficiencies must be combined with improved nacelle and installation technology—all at lighter weight and at affordable costs. Figures 8-1 to 8-3 show trends in pressure ratio, compressor exit temperature, and turbine inlet temperature, respectively. The factors contributing to these three items are summarized in Tables 8-1, to 8-3.

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
×

FIGURE 8-1 Overall pressure ratio for high-bypass ratio turbofans, maximum climb.

The core engine technologies that are key to the improvements listed in Table 8-2 are

  • improved metal materials, coatings, and fabrication techniques (2005);

  • new engineered materials such as titanium metal matrix composites (MMCs), titanium aluminides, and nickel aluminide (2005);

  • fiber-reinforced metal and ceramic high-temperature materials (2015);

  • more efficient turbine blade cooling; possibly cooling air coolers;

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
×

FIGURE 8-2 Compressor discharge temperature for high-bypass ratio turbofans, sea level static.

  • advanced, more efficient, high-speed turbomachinery;

  • turbomachinery with "smart" controls (2015); and

  • combustors to reduce takeoff emissions.

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
×

FIGURE 8-3 Turbine inlet temperature for high-bypass ratio turbofans, sea level static.

The corresponding keys to providing the improvements in low pressure systems as shown in Table 8-2 are

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
×

TABLE 8-1 Factors Providing Improved Core Thermal Efficiency

 

2005 Service

2015 Service

Higher cycle pressure ratio

60–75

>75

Higher turbine rotor-in temperature

2900–3000°F

3200–3400°F

Higher component efficiencies

1–2%

2–3%

More efficient customer power and bleed extraction

Advanced ECS

Power by wire

TABLE 8-2 Factors Providing Improved Propulsion Efficiency

 

2005 Service

2015 Service

Higher fan efficiency

2%

2–3%

Higher fan turbine efficiency at higher loadings

1%

2%

Increased bypass ratio

Yes

Yes

  • integrated fan aeroacoustics for higher efficiency and lower source noise;

  • advanced materials for higher-temperature, lighter-weight fan turbines;

  • composite fan blade development;

  • advanced gearbox systems employing new materials, bearing technology, and lubricants;

  • simpler higher-reliability variable pitch actuation systems; and

  • lower-cost designs and manufacturing.

Current engines have bypass ratios of 5–6 at fan pressure ratios of 1.65–1.8. With the higher turbine temperatures of the post-2000 period we must explore a range from direct-drive, mixed-flow, high-bypass turbofans (1.55–1.7 fan pressure ratio, 8–10 bypass ratio) to geared-drive, very high bypass engines (1.35–1.4 fan pressure ratio, 15–20 bypass ratio) with fixed-pitch fans and variable-pitch reversing fans. The range of engines and applications should also include advanced gear and turbine-driven unducted fans (1.1 fan pressure ratio and 35–50

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
×

TABLE 8-3 Improvements in Nacelle and Installation Technology

 

2005 Service

2015 Service

Reduced fan cowl drag

2%

More ?

Eliminate interference drag

Yes

Yes

Better acoustic treatment—less noise

Yes

Yes

Weight reduction

Yes

Yes

Integrated advanced ECS and laminar flow suction system

Yes

Yes

ultrahigh bypass ratio). As bypass ratio increases and fan pressure ratio decreases, specific fuel consumption improves but fan diameter increases dramatically. Nacelle and installation drag increases and weight increases. The low-pressure fan, fan turbine system, and the composite lightweight nacelles weigh two-thirds of the entire system in today's power plants. In past studies, such trends increased cost, eroded most of the fuel efficiency benefits, and drove up the direct operating cost. The question for the future is whether new gearbox, acoustic, and nacelle technology can shift the optimum bypass ratio for better economics at lower noise. Given the state of the art of large gearboxes, it seems questionable that engines of 15–20 bypass ratio would first enter service in the 60,000–100,000 lbf thrust category or for any long-range twins over water. The same holds for unducted ultrahigh bypass ratio turbofans, which have the lowest fuel burned potential. In neither case should this discourage very careful examination of their potential for the 2005 and 2015 periods.

The keys to providing these improvements in nacelle technologies are

  • integrated nacelle, wing and fan aeroacoustic development;

  • advanced mixers;

  • smart controls;

  • advanced composites and lighter-weight integrated nacelle, reverser, and fan structures;

  • laminar flow nacelle;

  • advanced aircraft environmental control system; and

  • lower-cost designs and manufacturing.

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
×

Future noise reduction is possible but must be balanced with economic improvements. Source noise reduction combined with advanced nacelle acoustic features is necessary. An aggressive target is 10 dB below Federal Aviation Regulation (FAR-36) Stage III restrictions.

Observations for Advanced Subsonic Aircraft

It is clear that through both innovation and evolution in the twenty-first century, propulsion will provide technological improvements to transport aircraft equal to those accruing from aerodynamics, materials and structures, and active controls combined. Very large gains can be made in performance, weight, noise, emissions, and operating costs.

U.S. engine products have the greatest share of the large subsonic propulsion market—about 80 percent in 1990—but the United States no longer has a technology edge.

The U.S. propulsion industry is focusing on the next 10 years—the next new products and derivatives. Manufacturers have committed billions to research and development in the 1990s and to improved manufacturing capabilities. They have few financial resources left to undertake the high-risk, high-payoff technologies for advanced post-2000 products.

The subsonic transport market, which is by far the largest, the most certain, and the most competitive, receives disappointing support from NASA. There is no evidence of a coherent, comprehensive approach. Only a few selected elements of subsonic technology are under way or in planning, and these mostly in-house. There are no goals for maintaining a competitive edge.

The investments made by NASA in subsonic propulsion technology through joint NASA/industry programs in the 1975–1983 period were very effective. The 1989 preliminary design studies with engine companies on high thermal efficiency and advanced materials were a good start, but small and with no follow-on. This valuable type of investment has been drastically reduced over the last eight years and, as a consequence, NASA is not playing a strong "pathfinder role" in subsonic propulsion technology.

Recommendations for Advanced Subsonic Aircraft

It is the belief of the Committee that the technology for subsonic transport propulsion must be vastly strengthened; there is great opportunity here. To do so, NASA should take the initiative in setting up a joint NASA/industry program for innovative subsonic propulsion technology that is the equivalent of the NASA/industry HSCT program. There is no reason to do less.

The first step should be to set up a vigorous preliminary design activity for aircraft propulsion systems—again, the equivalent of what has been underway for HSCT. Systems for two time frames should be examined: year 2005 service and year 2015 service. Improvement goals for fuel burned, DOC, noise, and emissions should be set. Firm conclusions can be reached regarding benefits and the definition of key high-risk/high-payoff technologies described earlier in this section. Programs for enabling technologies can then be planned.

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
×

One of the greatest benefits of this preliminary design effort is that it would get the aircraft industry, the propulsion industry, and NASA working together as a team to address high-risk, high-payoff technology in a disciplined system context, 15 years before products are expected.

PROPULSION FOR HIGH-SPEED CIVIL TRANSPORT

The HSCT's three key requirements, on which the NASA/industry program is focused, are economic viability relative to advanced subsonic transports of the same time period, FAR-36 Stage III noise standards, and insignificant depletion of stratospheric ozone. The consequent principal challenges to the propulsion system are price and fuel consumption, weight and noise, and emissions. Engine weight and noise go together because the thrust level and jet velocities are fixed by the aircraft's takeoff and supersonic cruise characteristics. The engine contenders are in the 650 pounds per second airflow, and 50,000–60,000 lbf thrust class. The HSCT will probably require four such engines, with takeoff jet velocity in the range of 2,400 to 3,000 feet per second.

Four candidate engine types which are being sorted out by the preliminary design process:

  1. a turbine bypass engine—basically a single spool turbojet with a ''variable cycle bleed'' of compressor air around the turbine; this engine would use an ejector nozzle to entrain another 100 percent or so of air to mix with the engine exhaust during takeoff; additional suppression features are included;

  2. a mixed-flow turbofan similar to U.S. Air Force/U.S. Navy fighter engines, but with two-dimensional ejector nozzle of more than 60 percent extra air entrainment, mixers and suppression features;

  3. a variable-cycle engine of the general sort that GE flew in the U.S. Air Force (USAF) Advanced Tactical Fighter prototypes, but with higher bypass and a two-dimensional ejector/ mixer/suppressor nozzle; and

  4. a version of the mixed-flow or variable-cycle engine with special fan features for bringing additional air aboard.

Generally, these engines have cycle pressure ratios between 20:1 and 25:1, consistent with compressor discharge temperatures of about 1250°F, which is an important consideration for long-life supersonic cruise. Takeoff turbine temperatures will be 2900–3000°F, with redline design temperatures 100–150°F higher, and supersonic cruise temperatures about 2700°F. Engine controls will be advanced digital.

To meet the FAR-36 Stage III requirements it is necessary to bring aboard a large additional amount of air during takeoff to reduce high-velocity jet noise through mixing or shielding, in addition to mechanical noise suppression devices. These features add a great deal of weight. The total gross weight penalty charged to propulsion noise reduction is 50,000–100,000 pounds with current concepts. The larger fuel consumption associated with the

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
×

HSCT adds an additional 8,500 pounds for each 1 percent increase in specific fuel consumption (sfc). It is anticipated that the supersonic cruise sfc will be 1.23–1.3 and the subsonic sfc 0.88–0.95. Weight is also challenged by the requirement for the HSCT engine to spend most of its life at high supersonic cruise temperatures. To keep weight in line, the engine and the jet nozzle are being designed around an advanced group of new materials. Without these, engine weight would escalate by 20 percent and fuel consumption would increase accordingly.

Current estimates of engine weight, including a 4,000–5,000-pound jet nozzle, are in the range of 11,000–12,500 pounds, but much design effort, and materials and noise research are required to establish the weight with confidence. Present weight uncertainty is probably about 2,000 pounds per engine; an extra 1,000 pounds of weight in each engine would add approximately 24,000 pounds of takeoff gross weight.

Reducing nitrogen oxide (NOx) emissions from an index of 40 g/kg fuel or so to the current index target of 5 g/kg fuel involves a completely new combustor design, new materials, and a major research and development program. There are two concepts, each fundamentally sound in its principle of NOx reduction. Early progress is encouraging.

NASA and industry recognize these major HSCT challenges and are concentrating the early phases of propulsion work on aggressive noise, emissions, and materials programs.

Much of the turbomachinery technology involved is well grounded in previous and current military programs for low bypass turbofans and variable-cycle engines and in recent progress in improving component efficiencies of subsonic transport engines. Integrated supersonic inlet, basic engine, and jet nozzle work is critical for optimizing supersonic performance.

In general, there appears to be less risk in meeting goals for specific fuel consumption than in other challenges mentioned previously. It is important to note, however, that the fuel burned per passenger seat per trip will be somewhere between 2.5 and 3 times that of improved/advanced subsonic transports and that to ensure economic competitiveness, this factor must be offset by increased productivity of the aircraft.

The price of engines and engine parts is a function of the manufacturing cost and the amortization of the development, certification, and flight test program costs required to ensure commercial guaranteed performance and to meet commercial standards of safety, reliability, durability, and maintainability. The prices will be well above the $100/lbf thrust of current 60,000 lbf thrust subsonic engines, and the development cost will probably be several times that of a subsonic engine, perhaps $3 billion to $5 billion. P&W and GE have teamed together on the HSCT propulsion program in order to pool resources, maximize progress for the funding available, and bring more ideas and talent to bear on the challenges.

Propulsion Technology Challenges

The Committee has identified the following challenges that must be faced to meet the propulsion needs of the HSCT:

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
×
  • Turbomachinery

    • Fan, compressor, high-pressure turbine, low-pressure turbine efficiency improvements through CFD and experimentation

    • Inlet, engine, control systems compatibility; operability

    • Turbine life and cooling

    • Component reliability

  • Combustor

    • Development of a viable commercial design: new fuel injection, mixing, variable geometry, and high-temperature walls with new ceramic matrix composite (CMC) materials

    • Validation of technology readiness

    • Combustor reliability and life

  • Jet nozzle performance, acoustics, durability

    • Effective ejector system

    • Jet mixing

    • Chute and mixer suppressor technology

    • Acoustic suppressor lining for jet nozzles—CMC material

    • Thrust reversing

    • Nozzle coefficients exceeding 0.98 at cruise, including leakage

    • Actuation systems

    • Validation of technology readiness

    • Nozzle reliability and life

  • Materials

    • A family of six new materials: highest risk are CMCs for combustor and jet nozzles; somewhat less risky—but vital for weight and life—are fan, compressor, and turbine materials

    • Validation of technology readiness

  • Advanced digital controls

    • Must handle twice the functions of current commercial turbofans.

    • Maximize performance, operability, life, reliability, and maintainability

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
×
  • Weight

    • Very careful, comprehensive, and clever mechanical design to accommodate new materials and jet nozzle/acoustics technology

  • Manufacturing technology

    • Large components using new materials

  • Long-term HSCT growth

    • CMC turbine blade materials and higher turbine temperatures, smart engine/controls for inlet/fan/compressor weight savings.

Observations for High-Speed Civil Transport

The Committee was very impressed by the major joint NASA/industry propulsion effort on the HSCT, funded mainly by NASA. In fact, the primary features of this collaboration could serve as a model for other technology development efforts:

  • It is a high priority effort.

  • NASA is playing the leadership role.

  • It is a carefully planned and focused effort.

  • It has a well-established timetable that is flexible enough to account for unexpected results.

  • Vigorous preliminary research and design work has been done.

  • There is excellent and enthusiastic teamwork among the LeRC, P&W, and GE.

The Committee believes that high-speed research is well funded through Phase I of the HSCT program, but that adequate technology validation will be an important issue in the 1997–2000 time frame.

Many major problems remain to be solved, including engine/nozzle noise, engine emissions, and the advanced materials such as CMCs for the combustor and nozzle that will be required to make the HSCT viable. Also, efforts to maintain the weight of the propulsion system and keep the cost of the systems to a reasonable level must be addressed. Furthermore, there are some important uncertainties that are beyond the control of either NASA or industry. These include problems in defining and alleviating concerns about ozone depletion, stratospheric pollution, and global warming, and the price of fuel in the early parts of the next century. The Committee believes that NASA has good programs under way for most of these concerns, but definitive results may be years away.

An important determination that has yet to be made, is a preliminary evaluation of the economic competitiveness of an HSCT compared to new and improved long-range 400–800-seat

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
×

subsonic transports of the 2005 period and beyond. Clearly, without a compelling economic reason to proceed with the development of the HSCT, it—like the supersonic transport—will run the risk of being a technological success but an economic failure. If such an economic justification exists, the development of a successful HSCT without U.S. leadership will severely affect the overall competitiveness of the U.S. aircraft industry. Thus, it is extremely important to maintain an adequate design and research and development program to establish the viability and technology readiness of the HSCT concept.

Recommendations for High-Speed Civil Transport

The Committee recommends that NASA:

  1. NASA should continue to maintain an adequate design and research and development effort within the HSCT program. Problems yield to creative effort and new knowledge from research. The key work is still in an early stage.

  2. NASA should broaden the preliminary design program to include serious evaluation of alternative HSCT systems with lower overall propulsion risks and penalties:

    • Operate lower in the stratosphere. Reduce stratospheric NOx emissions by cruising at Mach 1.6–2.0 below 50,000-foot altitude.

    • Increase cruise lift-over-drag (L/D) ratio as far beyond 9 as practical through increased span, laminar flow control, etc. Include consideration of oblique wing configurations with L/D ratio of 13. The aerodynamic design of the vehicle will be more difficult, but the size and weight of the propulsion system will be reduced.

    • Make special additional efforts to enhance takeoff lift, thereby reducing takeoff thrust requirements and allowing lower specific thrust cycles with less jet noise suppression.

    • Make a very careful study of three-point noise trade-offs. Consider raising the airport sideline noise a few decibels and doing better than FAR-36 Stage III at the other two stations. Overlay noise contours on communities near airports where HSCTs will operate. See whether large propulsion risks and economic penalties can be reduced.

  1. NASA should include growth studies in the preliminary design program. The HSCT is less flexible than subsonic aircraft when it comes to growth and derivatives. How will the range be increased to 6,500 miles; how are additional passengers accommodated; what new technologies are involved?

  2. NASA should invest more effort in establishing preliminary prices for economic evaluation.

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
×

PROPULSION FOR SHORT-HAUL AIRCRAFT

The current fleet of short-haul or regional transports covering the 19–50 passenger market has undergone dramatic changes over the past 10 years as a result of airline deregulation. Deregulation in Europe is expected post-1992. Most scenarios suggest three large, major airlines and three to five midsize and niche players by the year 2000—all having absorbed the regional carriers to support the behemoth hub-and-spoke systems. In general, this points to larger, route-tailored aircraft with particular emphasis on cost of operation and passenger comfort. Also, the large projected increase in passengers will drive simultaneous adjustments in airline route structure, aircraft performance, and airport infrastructure.

40–75-Passenger Aircraft Market to 2005

The 40–75-passenger market is poised for the largest adjustment in the short-haul market. As the stress on the infrastructure of the hub-and-spoke system has grown, regional carriers have reacted aggressively with a trade-up to aircraft for more than 40 passengers. The trend is expected to continue, with sales of 40–75-passenger regional transports exceeding 2,500 units over the next 10 years.

Regional transport equipment in the future will follow the existing trend toward a new generation of 100–200-passenger transport airplanes. The trend is to use higher aspect ratio wings and very high bypass (VHB) turbofan power (bypass ratio > 10). During the 1990s, regional transport for more than 60 passengers will move toward jet transport using higher bypass ratio turbofan engines to provide the best overall solution for fuel efficiency, high-speed, low noise, lower emissions, and superior cabin comfort and ride. Operationally by 2005, the 40–75-passenger machines will be predominantly jet transports using high-bypass or VHB turbofans.

19–35-Passenger Aircraft Market to 2005

To carry fewer than 40 passengers, the most efficient propulsion system for the regional transport will likely remain turboprop-powered aircraft, which will continue to serve routes of less than 300 nautical miles. However, the airframe and the engine must be uniquely designed for the needs of the regional transport, rather than being a derivative of today's military and general aviation turboprop/turboshaft engines. These new airframes and engines must focus on reducing the cost of operations by 20–30 percent by 2005, with moderate improvements in fuel efficiency (10 percent per decade).

Long-Term Regional Aircraft Market (2005–2020)

Gradually, the demarcation between regional carriers of 10–75 passengers and the new wave of 75–130-passenger VHB jet transports will disappear, and a fully integrated system of

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
×

modern aircraft will service the short-haul market with substantially better levels of flexibility and economy.

The deregulation of Europe will promote longer point-to-point routes as barriers within Europe disappear after 1992. The similar needs for larger regional transports with speed and efficiency will prevail and also provide the best charter aircraft solutions for weekend traffic, when most regional equipment would go unused.

Regional transports tend to have a viable life of 15 years or more; hence, the next have of major technical improvements in airframes is not expected until 2020, given that 2005 is when the introduction of new equipment is likely. VHB turbofan power will penetrate further down into the 30-passenger size as the cruise thrust-specific fuel consumption drops below 0.58 by the year 2015, but turboprops will remain dominant in the 19–-40-passenger segment even beyond 2015, due to their lower cost of operation.

Short Takeoff and Landing and Vertical Takeoff and Landing Systems

Shortened field lengths and high, hot-day performance for regional aircraft will set the requirements for short takeoff and landing (STOL) vehicles. The engine requirements for conventional and STOL aircraft will be similar, because STOL requirements will be met by high-lift wings and advanced thrust reversing systems.

Engine Requirements

By 2000 there will be a need for a continuum of engines to support then-existing regional aircraft. This continuum includes turboshaft from 5,000–8,000 specific horsepower (shp), turbofans from 5,000–14,000 lbf thrust, and turboprops from 3,000–8,000 shp. Both conventional and STOL aircraft will be supported by the same engines since the STOL aircraft will utilize high-lift wings and advanced thrust reversing systems to reduce takeoff and landing distances.

As discussed earlier, the 35–60-passenger aircraft will be supported by either turboprop engines between 4,000 and 8,000 shp and high bypass/VHB ratio fan engines between 10,000 and 12,000 pounds. The gas generators or core engines of high-speed turboprops are very similar in size to those of regional jets, 7,500–9,200 shp. The burn per hour of the turboprop is roughly the same as that of the regional jet, whereas the latter offers a 70–150-knot speed advantage along with a higher level of passenger comfort. Additionally, installed costs and weight of large turboprop engines are higher than for equivalent turbofan engines. However, existing engines (the 6,000–7,000 shp turboprop or 6,000–9,000 lbf thrust turbofan) are not designed with regional transportation in mind. These engines are simply available as a by-product of military or business jet applications. The next generation of advanced turboprops will need rugged cores optimized for regional aircraft.

The regional transport for more than 60 passengers will likely move toward higher bypass ratio turbofan engines to provide the best overall combination of fuel efficiency, high-speed, low noise, lower emissions, and superior cabin comfort and ride. To drive the VHB turbofans and

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
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provide low specific fuel consumption, overall compressor pressure ratios of 40–50:1 and turbine temperatures approaching 3000ºF will be required.

The regional transport holding less than 35 passengers will continue to use turboprop engines. During the 1990s, the existing fleet will be replaced by a new generation of axial/centrifugal turboprops designed for the rigor of regional service and optimized for low-cost operations.

The key drivers for gas turbine propulsion technology for the short-haul transports are (1) cost, (2) range/fuel consumption, (3) reliability/dispatchability, (4) noise, and (5) emissions. These translate into engines with higher overall pressure ratio, increased turbine inlet temperature, higher bypass ratios, and low-cost, high-reliability controls and accessories.

Enabling Technologies

Shown in Table 8-4 are state-of-the-art cycle parameters for axial centrifugal engines in these size classes and their future goals.

Other key technologies for short-haul aircraft propulsion systems are as follows:

  • Compressor: Continuing to pursue high-compression systems will be key to reducing fuel consumption. By 2000, a combination of advanced three-dimensional CFD techniques and new material systems should allow 30:1 pressure ratios in three axial+centrifugal compressor cores.

  • Combustor/Turbine: Improved combustor materials, fuel injection systems, and turbine materials will allow turbine inlet temperatures up to 3000ºF. Coupling increased gas temperatures and advanced turbine work designs will further decrease the size and stage count of engine cores.

  • Controls and Accessories: As turbofan engines are required to deliver more economical power, controls and accessories development will require increased emphasis. Controls and accessories are typically one-fourth to one-third the cost of the engine. By exploiting advanced computer and fiber-optic/signal concentration technology, the size, weight, and cost of these components can be reduced substantially.

  • Materials: Key to most improvements in gas turbine performance and reliability are improved metallic materials and advanced intermetallics/nonmetallics. Advanced materials development includes titanium MMCs, titanium intermetallics, second- and third-generation single crystal alloys, and polymeric composites.

Observations for Short-Haul Aircraft

The Committee believes that a strong, broad-based technology program is necessary to position the United States for the post-2000 short-haul markets. In general, NASA lacks awareness of the direction in which the short-haul/commuter market is heading and thus is not necessarily applying its resources appropriately. Currently, NASA seems to have much more interest in vertical takeoff and landing commuter systems than in conventional systems.

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
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TABLE 8-4 Cycle Parameters for Axial Centrifugal Engines

Parameter

State of the Art

2000

2007

2015

Overall Pressure Ratio

Turboshaft/prop

20:1

25:1

30:1

30–40:1

Turbofan

30:1

35:1

40:1

40–50:1

Turbine Inlet Temperature

Turboshaft/prop

2400°F

2500°F

2600°F

2800°F

Turbofan

2250°F

2500°F

2800°F

3000°F

Specific Fuel Consumption

Turboshaft/prop

Base

-7%

-16%

-20%

Turbofan

Base

-8%

-11%

-15%

Power- or Thrust-to-Weight

Turboshaft/prop

Base

10%

16%

26%

Turbofan

Base

15%

35%

44%

Cost of Ownership

Turboshaft/prop

Base

-5%

-10%

-20%

Turbofan

Base

-5%

-10%

-15%

The Committee believes that a better balance should be struck between the two. For example, a great deal of work has been done on large engines for subsonic aircraft. Some of this technology is applicable to the lower thrust and horsepower class of engines for the short-haul markets, but many areas that are unique to small engines deserve attention as well.

Recommendations for Short-Haul Aircraft

Cost of ownership and low initial purchase are key factors for the health of the short haul market. The Committee believes that NASA should undertake research activities that lead to lower-cost propulsion systems to benefit the general aviation and commuter markets.

The Committee also recommends that the LeRC direct its efforts at the "enabling technologies" discussed in earlier sections of this chapter and that NASA begin agency-wide

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
×

design efforts to identify configurations that are most suitable in the development of those technologies.

HIGH-SPEED COMPUTATION FOR PROPULSION

The Committee believes that CFD and high-speed computation in general will be essential for new engine development in the coming decades, in order to shorten the time for development and to discover new design optima.

The government, including Congress, is strongly committed to enhancement of national computational capability through the High-Performance Computing Initiative (HPCI). The NASA-managed Numerical Aerodynamic Simulator (NAS) already provides a national aeronautical capability. However, it is not widely appreciated how formidable a challenge adapting computation to the needs of propulsion is. The complexity and range of important scales in a turbojet engine, for example, make it impossible to foresee achievement, in the next 30 years, of complete computational simulations of propulsion processes.

Therefore, for the foreseeable future in propulsion, both CFD and experiments will be needed to efficiently apply the results to meet design goals. Because no single group can accomplish this integration, a new spirit of cooperation is needed among industry, NASA, and universities, based on broadly shared perceptions of the capabilities and interests of each.

"Analysis codes" are necessary to provide the basic predictive ability, at the finest level of detail, for the physics and chemistry underlying propulsion. "Design codes" are the essential direct tools for engine development; they should embody the best capabilities of analysis codes but, because of their different purpose, be far faster, easier to apply, and therefore necessarily less comprehensive and precise. There is a crucial need, obviously, for the developers of analysis and design codes to work in harmony, across organizational lines. Experiments to elucidate physics, generate new ideas or understanding, and evaluate component performance must go hand in hand with code development; yet money, manpower, and time requirements increasingly hamper such initiatives.

Technical challenges for high-speed computation include

  • multistage turbomachinery flow with mixing and shock losses;

  • combustor design for low emissions, using codes that integrate chemical kinetics, fluid mechanics, heat transfer, materials, and structures;

  • unsteady phenomena fully represented in codes pertaining to new ideas about the stability and control of propulsion systems; and

  • acoustics and noise, which must—through CFD—be featured in the primary design process.

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
×
Role of NASA

The Committee believes that NASA, probably through LeRC, should be the nation's leader in the effort to meet computational needs such as those described in the previous section for propulsion.

  • NASA must be the advocate for the computational needs of the propulsion community; it must make sure that the HPCI supports propulsion and that the NAS capability is increasingly useful for propulsion research.

  • NASA has a special responsibility to develop analysis codes of the greatest possible scientific scope and sophistication as resources for the propulsion engineering community, and to help the community appreciate and use these codes.

  • NASA should perform the necessary experimental research to support and augment computational studies.

  • Underlying all analysis and design codes are physical models of relevant phenomena; by means of appropriate experimentation, these models must be developed and improved, and NASA should lead this effort.

  • NASA should use its computational capabilities to encourage and develop new propulsion ideas; these should aim for long-term advancements, looking far beyond the present needs of engine manufacturers. An appropriate degree of authoritative judgment and advocacy about the future should be provided by NASA.

  • NASA should develop interdisciplinary technical communication pathways in the propulsion community.

The Committee has found that management at LeRC is very alert to the above issues. Its technical program is well conceived and thoughtfully planned, especially when funding is generous, as in the case of high-speed research for HSCT. The limitations are chiefly traceable to inadequate support for generic research, including subsonic topics, and to difficulties in new staff recruitment.

The duty to lead in providing the propulsion community with access to advanced computational facilities is taken very seriously by LeRC. Access to the NAS is apparently satisfactory, and a push toward parallel processing capability is underway through an Advanced Computational Concepts Laboratory. LeRC, through its Numerical Propulsion System Simulator program, participates in HPCI. However, NASA advocacy of that role is not as strong or successful as it should be.

LeRC is very active in the development of CFD analysis codes for application to turbomachinery. However, the communication of NASA results to the propulsion industry is not always successful, so LeRC analysis codes are not always appropriate or easily adapted to industry needs. On the other hand, the working-level communication, leading to successful incorporation of LeRC algorithms, models, or subroutines into industrial design codes, is very

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
×

good. A better job clearly needs to be done in facilitating the cooperative development and use of NASA and industry codes, with the recognition that even the best analysis codes are not automatically applicable for timely design.

NASA should lead in the development of innovative techniques of technical communication with industry in the general area of computational science as applied to propulsion. Although NASA should take the initiative in this process, it is clearly necessary that industry management enthusiastically join in.

A particularly strong computational effort to analyze multistage machines, using an averaging hierarchy, is unique to LeRC and may well provide important insights for the next generation of advanced subsonic engine designs. The HSCT interest is sustaining code development for predicting unsteady engine performance. CFD with chemical kinetics is a LeRC capability that may soon be joined with the experimental studies of low-NOx combustors at LeRC; this development follows from the HSCT initiative but will surely benefit subsonic engine development in the next decades.

The HSCT program also supports a program of computational acoustics in the Langley acoustics division, in cooperation with engine studies at LeRC. At LeRC, prospects for developing a first-principle solver for jet noise are being pursued energetically. The Committee hopes that this program will have support commensurate with the enormous practical importance of propulsion noise, not just for HSCT but for subsonic developments as well.

Serious efforts are being made to integrate experiments with code development, especially by exploiting laser-Doppler velocimeter (LDV) techniques. The Committee heard comments that LeRC was not providing the experimental component research data that were such valuable NASA (and National Advisory Committee on Aeronautics) contributions in the past. Although funding and staffing reasons for this will remain, it does seem that LeRC is moving toward meeting this need in tandem with code development. In the recent past, criticisms could be made that experimental component studies were abandoned in favor of CFD and that surviving experiments were to be driven only by ''code validation.'' A much more sophisticated view now prevails, which encourages experimental and computational teamwork. The difficulties of such teamwork—especially time and funding on the experimental side—are severe, but one can hope for a trend toward more "component research" of significance in the next decade.

An interesting example of experimental-computational teamwork in furtherance of a new development having system and component consequences is the Supersonic Through-Flow Fan program at LeRC. This program depends on CFD for the next steps in which refinements and modifications will look toward engine application. Equally, the experimental program will have to be extended. A challenge will be to keep experimental time requirements in bounds; no doubt increased professional staff is the chief requirement for timely progress.

The Numerical Propulsion System Simulator (NPSS) is intended, in the very long run, to provide a uniformly valid numerical simulator for propulsion systems, by taking into account all relevant fluid and structural issues. This is certainly an appropriate long-range goal for NASA. The path toward that goal involves the generation of numerical analysis tools, in cooperation with an industry steering group that provides current thinking about engineering design goals. The models on which the developing code structure is to be based themselves

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
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require continuous development so that the various codes can interact at consistent levels of validity. This process can clearly have great power to discipline the entire LeRC propulsion program and to ensure centrality to the national propulsion development effort.

At present, an enthusiastic industry team is in place, and certain component analysis tasks have been identified for intensive study (e.g., a fan-nacelle system). An obvious future need is to develop acoustics codes of the right level to be included in this general scheme. Industry participation is at working level, which is good because this is a program that synthesizes technical information about components. On the other hand, the Committee found that management in both industry and NASA does not have a very vivid picture of this program.

Although a full-blown "numerical test cell" is not likely for the time frame of this study, significant progress in unifying numerical propulsion analysis techniques could certainly be made. Therefore, the Committee would like to see a stronger management response to this significant grass roots initiative from all quarters, a response that recognizes the potential importance of NPSS for the future of both LeRC and the propulsion industry.

MATERIALS AND PROCESSING

Both advanced subsonic transports and the HSCT require materials that exhibit drastic improvements in strength-to-weight characteristics, fatigue life, and high-temperature capability. Many of their applications will necessitate unique procedures to fabricate and repair radically different components. Innovative manufacturing methods must be developed to fabricate components from these advanced materials at affordable costs.

Drivers for Materials Development and Requirements for Application

Drivers for materials development for either the HSCT or the Integrated High Performance Turbine Engine Technology program are (1) specific strength, (2) specific stiffness, (3) damage tolerance, (4) special physical properties, (5) producibility/reproducibility, (6) maintainability/repairability, (7) temperature capability (creep and environmental survivability), and (8) cost: raw materials, fabricability, shop handling.

Specific advanced composite systems needed for advanced engines are shown in Table 8-5, along with propulsion applications for the HSCT and the advanced subsonic transport.

Before any design incorporation and application can be considered, there must be adequate demonstration of both materials and processing technology readiness to support the confidence to launch full-scale engine development. The following are key requirements for this confidence:

  • sufficient property characterization for design and determination of life cycle costs;

  • established design methodology, including life prediction systems;

  • reproducible manufacturing processes;

  • demonstrated repairability/maintainability;

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
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TABLE 8-5 Potential Applications for Advanced Composites

Composite

Application

Polymer matrix composites

Ducts, cases, and structure

Metal matrix composites

Reinforced rims on high-speed turbine and compressor disks or drum rotors, stators and cases, fan blades

Intermetallic matrix composites

Lightweight, low-noise exhaust nozzle, low pressure turbine rotors

Ceramic matrix composites

Low-NOx, high-efficiency combustor; high pressure turbine disk hoop reinforcement; exhaust cases

  • validated technology (by engine test);

  • realistic, substantiated cost estimates for production; and

  • realistic assessment of risk.

Special Economic Problems in Materials

Compared with common materials, aircraft engine materials are generally more expensive and produced in smaller quantities. Even for current-technology engine materials (e.g., titanium) the number of suppliers is limited, and some parts (e.g., ball bearings) are mostly manufactured abroad. Many of the raw materials for the advanced composites listed in Table 8-5 are of the specialty type and of very high value. For some high-temperature applications, the raw materials cost $4,000 to $5,000 per pound, based on individual costs of $2,000 per pound for fiber and $2,000 to $3,000 per pound for matrix. These materials are definitely not commodities, and producers have little interest in manufacturing either existing or new fibers in small quantities. Such materials have a poorly defined market potential because they have an uncertain breadth of applications and a market whose duration and extent are unknown. The result is commercial unavailability. The need is for an effort similar to that undertaken by the USAF in the 1950s to secure titanium as a producible engine material.

Manufacturing Issues

Continued development of gas turbine engines in the next century will depend on two key factors: the ability to manufacture high-performance propulsion systems at affordable costs and the economical exploitation of advanced materials. Affordability is the major goal of

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
×

manufacturing technology. To ensure economical manufacture of advanced components from new and current materials, new processing methodologies and enterprise systems must evolve into reliable production techniques. Speed, with quality, must be the primary focus of the product development cycle, and the production process must be shortened drastically. Quality enhances affordability; intelligent systems that manufacture parts with no variability will significantly reduce the cost of those parts.

The maturation of intelligent processing will depend on the successful development of full adaptive control; process modeling and verification, insitu measurement with advanced sensors, and integration of all adaptive control systems. Process understanding must be great enough that processing trends can be detected and corrections made in real-time. Precision manufacturing is part of the intelligent processing approach. The intelligent machine must know in advance that it will produce a quality part; hence, tailgate inspection may be eliminated and on-machine inspection minimized. Through precision manufacturing the factory of 2010 will be able to make products with improved performance at lower cost because part variability will be eliminated.

Emerging enterprise systems must be integrated with advanced processes to increase productivity and reduce processing time at the machine tool level, cell level, and factory floor level. Machine tools, for all types of processes, must be self-contained, intelligent processing systems—self-directed, self-monitoring, and fully adaptive, with automatic tooling setup capability. The cell must have the ability to operate as an intelligent business unit capable of precision-manufacturing small lots of parts. Control at the floor level must include the capability to synchronize the operation of all cells and machine tools to provide the necessary components, on time, for final engine assembly. The ability to manage information flow will be critical. The overall system must integrate the performance of the suppliers to ensure timely delivery of all manufactured parts.

Evolution of the optimum manufacturing enterprise will require advancements in four areas: skilled personnel, intelligent computer systems, automated manufacturing equipment, and precision manufacturing processes with full adaptive control. The factory of 2010 will require fewer but more highly skilled people. Optimization will be emphasized from the entire factory perspective. Standardization, as the precursor to integration, will be the focus of computer-integrated manufacturing (CIM) as computer power continues to grow. The machine tool, for all processes, including those that are nontraditional, must be a fully automated, intelligent device. It must have the ability to run continuously with no human intervention; it must be flexible, with adaptive tooling/fixturing and intelligent software that controls maintenance and tool design; and it must have the necessary sensors integrated for full adaptive control. CIM technologies must also enhance responsiveness and reduce the product development cycle. Systems must be put in place to create functional prototype parts on demand.

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
×
Role of NASA in Materials and Manufacturing

NASA should continue its major role in materials research and development and extend it into the area of materials processing. Historically, NASA has limited its involvement in manufacturing technology, based on the assumption that the Department of Defense (DoD) and industry were best able to accomplish this task. Only in certain large NASA projects were materials disciplines incorporated with some form of manufacturing technology to accomplish the project goals. These include the Energy Efficient Engine program, the Engine Component Improvement program, the Composite Primary Aircraft Structures program, the Materials for Advanced Technology Engine program, and the Advanced Composites Technology program. NASA contributed only indirectly to the needs of the manufacturing community.

However, the invention of a new laboratory material and its characterization in laboratories do not guarantee useful innovation. Incorporation of a new material in an engine spans 10–20 years, much of the which is occupied with the process development necessary for continuous, reliable production. NASA's responsibility for materials research and development should be extended into the realm of materials manufacturing to incorporate process development cooperatively with engine producers and materials producers. NASA should ensure validation for full-scale producibility of new materials.

As NASA reexamines its role in support of manufacturing, it should avoid a program modeled on the DoD/USAF MANTECH program. First, NASA is not a large procurement agency for current aircraft engine parts. Second, the MANTECH program is relatively short-term with a required implementation plan that includes a cost/benefit analysis showing how the supported effort will be of near-term benefit to the company. Rather, if NASA is to assume a greater role in manufacturing research and development, it should be in longer term, higher risk, more fundamental technologies for which the immediate payoff is not obvious.

One possible area, suggested by the industry, for NASA to examine is the application of its analytical expertise to the physics and mechanics of particular manufacturing processes, materials characterization, and modeling, as well as intelligent process control research, employing real-time sensors and embedded process models. NASA, industry, and the universities should all be involved. Care must be exercised that the subjects chosen are real, long-range, and not parochial in nature. It is essential, however, that this work be undertaken with an increased budget, not through reduction of existing useful programs that NASA is now funding.

Recommendations for Propulsion Materials

In the opinion of the Committee, NASA should

  • continue in its historic role of materials research and development for propulsion applications;

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
×
  • extend its materials research and development programs into the realm of manufacturing to incorporate process development, in cooperation with engine materials producers, to ensure validation for full-scale production;

  • examine and provide solutions, in the form of program support, for the development of specialty materials that are not available as commodities but, if properly supported and developed, will become common in aircraft engines; and

  • reexamine its general historic role in manufacturing and assess its ability to fund fundamental projects of a long-term, high-risk nature, involving industry and universities, in analyzing the physics and mechanics of particular manufacturing processes, materials characterization, and modeling.

SMART ENGINES

The explosive growth of computer technology, fostered by the microelectronics revolution, is a strong driver for change in aeronautics, from avionics in the air to air traffic control on the ground. The air vehicles themselves have been evolving rapidly as the capability, functionality, and complexity of airborne electronics grow. Full-authority electronic digital controls are now routine on new commercial and military engines. With this resultant increase in computational power, the nature of engine control is moving away from the engine governor toward improved functionality and integration with aircraft flight controls. These are evolutionary changes, however, using processors common to personal computers.

Over the past five years, a number of researchers have been investigating the use of active controls applied to the component or subcomponent (compressor, burner, bearings, structure, etc.) level in gas turbines. This work has introduced the concept of a smart engine requiring several orders of magnitude more computational power than current advanced designs. This revolutionary approach incorporates feedback control within the device that can alter both its static and its dynamic behavior, so that the performance of the component can no longer be extrapolated from the data base generated by years of experience. The impact of active components can come from improved component performance and also from a relaxation of conventional engineering constraints, thus enabling new solutions and new designs.

Active monitoring and control technology employed at the component level can be applied across the engine. Some areas of progress that are currently under investigation include

  • active suppression of fan and compressor surge and stall,

  • active combustor monitoring and control,

  • magnetic bearings, and

  • active noise controls.

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
×

The common theme of these areas is that they represent a radically new approach to problems that seriously limit propulsion technology. The technologies, however, differ in phenomenology and engineering approach.

Active Fan and Compressor Stabilization

One of the most troublesome phenomena in jet engine design and operation is compressor surge and stall. These are large-scale oscillations in air flow that result in abrupt thrust loss and can inflict severe mechanical damage. Current practice is to detune the compressor by 20–25 percent (with concomitant loss of performance) to avoid operating in regions where instabilities occur. Surge and stall also place significant restrictions on inlet design because distorted air flow from the inlets can trigger fan and compressor instabilities.

FIGURE 8-4 Actively stabilized compressor suppresses rotating stall.

In the past two years, laboratory-scale experiments have shown that it is physically possible to actively damp a compressor by using feedback control. The concept is to sense the disturbances when they are small and launch counter disturbances to damp them. The power required to do this is typically 10-5-10-6 of the compressor power. One such implementation, for the control of rotating stall in an axial compressor, is illustrated in Figure 8-4. Here, a circumferential array of transducers is used to detect small-amplitude traveling waves, which are damped by wiggling the compressor inlet guide vanes to launch counter waves. Decreases in the stalling mass flow of up to 25 percent have been demonstrated in this manner (see Figure 8-5).

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
×

FIGURE 8-5 Active compressor stabilization moves the stall point to lower mass flows.

Similar gains have been shown for the control of surge.

There are significant potential advantages to stabilizing the fan and compressor against rotating stall and surge, including (1) lighter fan compressors (fewer stages and shorter chord airfoils), (2) improved compressor efficiency (more freedom in component matching), (3) increased distortion tolerance (shorter inlets, reaction control system considerations), (4) improved inlet-engine matching (reduced spillage drag), and (5) increased compressor design freedom. If the demonstrated gains are realizable in full-scale engines, system study has shown there can be dramatic improvement in aircraft performance or weight.

Active Combustor Control

Combustors also suffer from instabilities, especially in afterburners and ramjets. Active control of these instabilities has yielded both a 15-dB reduction in rumble instabilities in afterburner geometries and more complete combustion, permitting high power levels and shorter combustors.

Active control has also been demonstrated in can-type geometries of main gas turbine combustors. Here, low-amplitude acoustic waves have been used to decrease the pattern factor

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
×

(homogenize the flow) by 50 percent, potentially improving turbine durability and life. Active control may be necessary for lean-burn, ultralow-emission main burners for advanced subsonic and HSCT aircraft.

Magnetic Bearings

Magnetic bearings suspend the rotating members in magnetic fields, eliminating friction and lubrication requirements. Specific advantages over rolling contact bearings include elimination of the lubrication system, active damping of shaft dynamics and vibration, greatly increased temperature capability (up to 800°C), and large increases of load capability (two to four times).

Magnetic bearings using conventional electromagnets are currently employed in large ground-based turbomachinery. Improved designs are capable of supporting typical gas turbine loads using less than 100 W. Since bearings of this type are inherently unstable, active control is required to maintain position. Active control enables the dynamics of the system to be optimized in software, greatly increasing design freedom and vibration damping. Current estimates are that an engine with magnetic bearings could have a 5 percent advantage in weight and a 5 percent advantage in efficiency over a conventional design.

Active Noise Control

Feedback control can be used to reduce noise either by direct wave cancellation or by influencing the noise-producing phenomena. Exhaust noise reductions of more than 20 dB have been demonstrated on large-scale ground-based gas turbines. Although perhaps further from practical aircraft realization than other smart engine technologies, active noise control offers the promise of a new approach to one of the most vexing of problems facing aviation, especially the HSCT.

Recommendations for Smart Engines

Should the results of these small-scale experiments extrapolate to full-scale engines, active control has the potential to improve aircraft propulsion performance and design in important ways. There is a long way, however, between small experiments and full-scale engine development. These are high-risk, high-reward technologies, with application unlikely before the turn of the century. As such, most of these concepts are beyond the horizon that U.S. industry is prepared to pursue.

The Committee believes that NASA is uniquely positioned to pursue active control technologies for aircraft engines. NASA has both the personnel and the facilities to bring the technology along from university benches to the large-scale experiments necessary to assess concept viability prior to development by industry. NASA also has the breadth of disciplines at its centers and among its contractors to forge the teams (e.g., fluid mechanics, controls, structures) necessary to successfully pursue smart engine technology.

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
×

TURBOMACHINERY COMPONENT TECHNOLOGY

Significant advancements in the design methodology of turbomachines are needed to satisfy the requirements of both the advanced subsonic transport aircraft and the HSCT. Turbomachines for these systems should have fewer airfoils, reduced gaps between airfoil rows, lower aspect ratios, and higher clearance-to-span ratios. These machines must operate at high Mach numbers and lower Reynolds numbers, the latter in low-pressure turbines. The cooling air and turbine inlet temperatures are expected to be higher, whereas the amount of cooling and secondary air will decrease. The necessary solutions for these problems will emerge from the challenging fundamental research and development activity of the next 10–15 years. Continued advancements in CFD, together with improved flow measurement techniques, show the promise of utilizing more reliable analytical methods. However, the availability of advanced computational and experimental methods for turbomachinery design does not, in itself, ensure a more efficient and durable machine. The design process also relies very heavily on experience-based correlations and turbulence models and on design criteria. Implementation of advanced CFD codes without improvements in design criteria may not yield an improved design. The complexity of turbomachinery flows requires a national program to develop design systems for the next generation of gas turbine engines.

Fluid Mechanics of Turbomachine Elements

Among the issues facing turbine and compressor designers are the following:

  • Two-dimensional airfoil optimization: Significant reductions in airfoil losses have been achieved over the past 15 years through the control of boundary layers, specifically by designing laminar flow and controlled diffusion airfoils. In current turbomachines, losses generated on airfoil surfaces constitute between 30 and 60 percent of total losses. Substantial increases in losses are measured for airfoils operating at transonic Mach numbers. Currently, low-loss airfoils in the transonic flow regime are designed by using simple criteria along with extensive experimental data. In the future, accurate numerical calculations from two-dimensional Reynolds-averaged Navier-Stokes codes can be used together with controlled experiments in plane cascades to develop design criteria for transonic flow applications. The guidelines needed to design low-loss airfoils for low Reynolds number application can be developed by using direct numerical simulation to identify transition criteria in separation bubbles.

  • Endwall losses: The endwall regions contain 20–30 percent of the total losses in turbomachines. The physical mechanisms governing these losses are not well understood, and their magnitudes are 1.5–7 times larger than those estimated on the basis of wetted-surface calculations. There is a need to conduct accurate numerical simulations to identify loss generation mechanisms in the endwall region and to identify design criteria to develop low endwall loss passages.

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
×
  • Tip clearance loss control: Tip leakage flows are responsible for 10–40 percent of the total losses in turbomachines. Although flows in the tip clearance regions are highly complex, the loss in efficiency due to tip clearance is linearly related to the fractional clearance. There is a potential to control tip clearance flows by designing airfoils to limit the amount of flow in the tip regions. Numerical simulations can play a very important role in this area if they are verified through experimental programs.

  • Heat loads on airfoil pressure sides: Measured heat loads on the pressure sides of rotor airfoils are two to three times larger than those calculated by using current prediction methods. Since the amount of cooling air for rotor airfoils is controlled primarily by pressure-side heat loads, there is a need to develop reliable prediction methods to estimate them. Low heat load design concepts can potentially reduce cooling air requirements.

  • Shock boundary layer/vortex interaction: Significant losses are generated from the interaction of shocks with boundary layers, and with tip leakage vortices. Shocks in fans cause the separation of boundary layers, which provides a conduit for the low-momentum flow to migrate toward the tip region. Shocks also interact with tip leakage vortices; this interaction can initiate stall in the rotor. Accurate numerical simulations must be conducted to identify the detrimental effect of shock interaction. There is also a need to develop turbulence models to provide accurate estimates of shock-induced losses on airfoil surfaces. Both numerical and experimental programs are required to develop flow prediction methods for high Mach number applications.

Problems of Multistage Turbomachines

Turbomachine flows are highly unsteady due to the relative motion of adjacent airfoil rows and incoming total pressure and total temperature profiles. Steady flow simulation methods, which have historically been used, fail to account for three specific flow features: (1) preferential migration of wakes from upstream airfoil rows; (2) preferential migration of hot and cold steaks toward the pressure and suction sides, respectively, of turbine rotors; and (3) preferential migration of endwall secondary flow and tip leakage vortices from the upstream airfoil row to the downstream airfoil passages, causing substantial effects on heat loads and losses. The effects of these flow phenomena are currently accounted for by using empirical correlations that are not very well grounded in flow physics. This situation will be further aggravated by current design trends. Available empirical correlations will not provide realistic flow behavior in the multistage turbomachines. Work needs to be initiated to develop physically sound models that account for the effects of periodic unsteadiness in multistage turbomachines. A promising multistage flow simulation strategy has been developed at LeRC, and concerted effort should be directed toward further development of this approach.

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
×
Combined Computational Fluid Dynamics for Airfoil Rows

Flow leakage from endwalls is expected to increase in future turbomachines. CFD codes, therefore, need to be developed and validated in order to provide accurate flow simulations for airfoil flow passages that have significant amounts of secondary air leakage. Prediction methods also need to be developed for airfoil rows having a significant amount of cooling air injection from airfoil and endwall surfaces. A rational approach is to develop a procedure that will compute flow through the internal passages of the turbine, a heat conduction code, and a solver for the airfoil passage. Such an approach is likely to provide a more accurate and faster method for predicting both performance and surface temperature for airfoil rows.

Observations on NASA Turbomachinery Program

The turbomachinery technology program at LeRC is extensive and contains some very significant and important elements, but, due largely to lack of funds and personnel, the pace of the work may be too slow to maintain the excitement it merits. It appears that the amount of innovative component research there is substantially less than it was two decades ago, when NASA contributed heavily in ideas and experimental results that led to strong advances in the industry. There are, however, excellent areas of work, of which three are mentioned below:

  • Supersonic through-flow fan: This is a fine example of innovative technology firmly grounded in fundamental fluid mechanics, with the promise of significant performance gains in engines for supersonic flight. The design of the fan and the structuring of the experimental program have made effective use of the LeRC CFD capability, and its successful operation lends credibility to both the concept and the design method. Again, the pace of the work is slower than desirable or appropriate for a project carrying such promise. Essential work remains to be done in the design of the stator, improvement of the stage performance, and the mechanism of starting.

  • Cooperative compressor research with Allison: LeRC has undertaken a cooperative research program with Allison on an advanced two-stage compressor with a pressure ratio better than 5:1, handling a through-flow of about 10 pounds per second. This is a high-performance compressor that is a very aggressive design and situates LeRC at the leading edge of compressor technology.

  • Multistage turbomachine flow computation and design: This effort at LeRC is a highly innovative computational scheme realistic treatment of the fundamentally unsteady flow in multistage turbomachines. It represents an excellent example of scientific engineering, having its foundation in fundamentals while aiming at meaningful approximate calculations for design purposes. The technique recognizes the presence of fluctuations in the frequency range of blade passing and separately in the range of rotative speed, and works with relatively independent time averages in these ranges. To fulfill its promise, method requires experiments and computations focused on the determination of some of these average quantities; these are evidently not being pursued at present.

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
×

These very promising activities suggest that increasing their number and augmenting their support merit serious consideration.

Recommendations for Turbomachinery

It has been the historic role of NASA, and one that industry has found beneficial, to provide a focus for research activity, to undertake innovative component experimental research, and to give leadership to teams consisting of industry, university and government agencies. The current environment requires, in addition, that NASA assist in providing computer resources for numerical simulation and that it cooperate in the development and standardization of various CFD codes to facilitate evaluation against basic data and permit ready incorporation into design codes.

NASA has been slow to respond to the exciting potential of stall and surge control and has not yet taken steps toward development of sensors and actuators required for its implementation. Although the capability of such control has been unequivocally demonstrated, the development of techniques appropriate to operational engines demands a strong and innovative effort. Another area in which additional effort would be welcome is the problem of short blades that will be encountered in latter stages of new, very high pressure ratio core engines. NASA's work on the off-axis independent compressor is very promising but should not constitute its total effort in this area.

COMBUSTORS AND EMISSIONS

Although the emission of nitrogen oxides, carbon monoxide, and unburned hydrocarbons constitutes a significant issue for subsonic commercial transports, and promises to become a larger problem for short-haul aircraft as this traffic increases, the currently intense interest centers on the HSCT. Here, the NOx emissions are a central issue for flight in the stratosphere, where particulate exhaust and the formation of ice crystals together with the oxides of nitrogen constitute a potential threat to the ozone layer. The development and design issues, which are already sufficiently complex, are further clouded by currently inadequate, but slowly evolving, models of the upper atmosphere that will play a role in setting standards for altitude exhaust emissions. Moreover, it is not clear at present just how a known body of engine exhaust information would be coupled meaningfully to the grid of atmospheric models now in use.

Engines for current subsonic transport aircraft emit between 10 and 20 g of nitrogen oxides per kilogram of fuel (see Figure 8–6), depending on whether the combustors are of the conventional design or the more recent dual annular design. Under takeoff conditions, emissions from these same combustors increase to between 20 and 30 g/kg fuel. If this performance is scaled on the basis of the nitrogen oxide ''severity index'' currently employed, the HSCT would be expected to produce between 30 and 40 g/kg fuel at cruise conditions. The goal for HSCT

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
×

FIGURE 8-6 Nitric oxide emissions versus severity index(s), current and future engines.

engines is 5 g/kg fuel—between 12 and 16 percent that of current engines—the requirement could be set even lower, more out of uncertainty than rationality.

Clearly this reduction by a factor of six or more requires heroic measures. Recognizing these facts in the early 1970s, LeRC embarked on a low-emissions burner program that focused on what became known as the lean premixed prevaporized burner. Subsequently, this effort led to NASA sponsorship of separate research and development programs with P&W and GE. Two candidate burner technologies were identified and these continue to be pursued by the two engine manufacturers. Research on the chemistry and gas dynamics of NOx reduction continues at LeRC.

The production of nitrogen oxides increases with pressure, temperature, and residence time in the combustor. Consequently, reduction techniques focus on just what latitude one has with these variables in view of the engine cycle requirements. Not only is temperature the most sensitive of these, but owing to the design of conventional burners, it also offers the greatest possibility for control. In the conventional engine, fuel is initially burned at approximately stoichiometric conditions and subsequently diluted to the desired leaner condition. The high temperatures in the primary combustion zone result in rapid production of NOx during its residence time and set the value of the final emission level. The advantages of this arrangement are that the hot, stoichiometric primary zone provides good stability, ignition, and relight, while the addition of dilution air allows convenient cooling of the combustor liner. The low-NOx burners are consequently designed to avoid the hot stoichiometric and dilution zones, thereby reducing emissions, but at the expense of stability and cooling problems.

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
×

The NOx formation rate shows a strong dependence on mixture ratio, as a result of the dependence of temperature on mixture ratio. In a conventional burner the primary region produces NOx at nearly the peak value, and subsequent air dilution decreases the production rate to less than 1 percent peak value at combustion discharge conditions. The lean premixed, prevaporized concept reverses the procedure by preparing the fuel/air mixture at its desired final value and carrying out the combustion at a very low NOx production rate. The near symmetry of the NOx production rate about the stoichiometric mixture suggests the alternative procedure of burning with a rich mixture and subsequently diluting it rapidly to the desired final lean condition. This technique underlies the second type of low-NOx burner under serious consideration.

A combustor that introduces a lean, premixed, prevaporized mixture into the combustion chamber is being pursued by GE. With this design it is necessary to mix the air and the fuel vapor thoroughly on the molecular scale to achieve the indicated reduction of nitrogen oxides. Moreover, because this mixture volume constitutes a source of preignition and flame flashback problems, the mixing must be very rapid to minimize the mixture volume and residence time. To accommodate the anticipated range of operating conditions, variable-geometry vapor inlet and bypass air controls are required as well as the corresponding sensors. Furthermore, because the larger part of the air is mixed before combustion is carried out, the dilution air conventionally available for combustor liner cooling is much less, accessible. As a consequence, high-temperature materials are essential.

The second option, that of rich burning, is being pursued by P&W and, for reasons that will become obvious, has been designated the Rich Burn Quick Quench burner. In this burner the fuel and a limited amount of air are introduced into the rich stage. Here again, it is absolutely essential that the fuel-air mixing be carried out on a molecular scale before combustion occurs. To the extent that mixing is nonuniform, the reaction will preferentially take place in those regions where the mixture is close to stoichiometric, thus partially defeating the aim of the burner. As dilution air is added, each portion of the mixture passes through the stoichiometric ratio and will produce nitrogen oxides at that very rapid rate. Consequently, rapid mixing during the quench phase is essential. Again, because the temperatures are higher than conventional and because bleed liner cooling is unacceptable in the rich burn stage, high-temperature materials are essential.

These ultralow-NOx burners entail higher than conventional pressure losses due to the requirement for rapid mixing, the use of new high-temperature materials, variable geometry, and the attendant controls and sensors. In addition, it appears that these burners may be somewhat longer than conventional and entail an increased engine weight that is, as yet, undetermined.

The cooperative development of the HSCT engine, between P&W and GE, permits efficient and economical exploration of these two burner types; it is essential that NASA, P&W, and GE get their Joint Technology Acquisition Program in place. In addition, it is of the utmost importance that LeRC continue vigorously its basic research in this area, driving toward NOx levels even lower than the present goal of 5g/kg fuel.

Airport NOx emissions will be a challenge for the advanced subsonic engine cycles with 70:1 pressure ratio and 3000ºF turbine inlet temperature. With current combustor types, the

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
×

NOx per unit of thrust could be tripled under these conditions. A good target would be to decrease NOx emission to an absolute level half that of current engines. This will require very advanced combustors employing variable geometry, new materials, and smart controls.

Suggested Citation:"8 Propulsion." National Research Council. 1992. Aeronautical Technologies for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/2035.
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Prepared at the request of NASA, Aeronautical Technologies for the Twenty-First Century presents steps to help prevent the erosion of U.S. dominance in the global aeronautics market.

The book recommends the immediate expansion of research on advanced aircraft that travel at subsonic speeds and research on designs that will meet expected future demands for supersonic and short-haul aircraft, including helicopters, commuter aircraft, "tiltrotor," and other advanced vehicle designs.

These recommendations are intended to address the needs of improved aircraft performance, greater capacity to handle passengers and cargo, lower cost and increased convenience of air travel, greater aircraft and air traffic management system safety, and reduced environmental impacts.

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