Aeronautical Technologies for the Twenty-First Century (1992) / Chapter Skim
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8 Propulsion
8 Propulsion
Pages 149-186
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From page 149... ...
The specific requirements differ between the advanced subsonic transports, the high-speed civil transport (HSCT) , and the short-hau} class of aircraft.
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From page 150... ...
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
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From page 151... ...
Cycle pressure ratios are 36-38; the compressor discharge air temperature is 1250°F; and the turbine rotor inlet gas temperature is 2550°F, with the mechanical design redline 150°F higher. Materials include fourth-generation nickel superalloys, high-strength titarliums, and carbon composites.
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From page 152... ...
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.
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From page 153... ...
. 1990 2000 2010 Year of Service FIGURE 8-1 Overall pressure ratio for high-bypass ratio turbofans, maximum climb.
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From page 154... ...
co ,_ 1 1 00 1000 900 <: ~` . 1 ~ 1 1 1970 1980 1990 2000 2010 Year of Service FIGURE 8-2 Compressor discharge tempera re for high-bypass ratio turbofans, sea level static.
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From page 155... ...
. 1990 2000 2010 Year of Service FIGURE 8-3 Turbine inlet temperature for high-bypass ratio turbofans, sea level static.
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From page 156... ...
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)
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From page 157... ...
. As bypass ratio increases and fan pressure ratio decreases, specific fuel consumption improves but fan diameter increases dramatically.
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From page 158... ...
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.
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From page 159... ...
Advanced Tactical Fighter promotes, but win higher bypass and a two-dimensional ejector/ mixer/suppressor nozzle; and a version of the mixed-flow or vanable-cycle engine with special fan features for bringing additional air aboard. Generally, these engines have cycle pressure ratios between 20:]
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From page 160... ...
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.
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From page 161... ...
PROPULSION 161 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 · let nozzle performance, acoustics, durability Effective ejector systems let mixing Chute and mixer suppressor technology Acoustic suppressor lining for jet nozzles CMC material ThNSt 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 · Advance digital condors Must handle twice the functions of current commercial turbofans Maximize performance, operability, life, reliability, and maintainability
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From page 162... ...
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.
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From page 163... ...
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 cn~ising at Mach I.6-2.0 below 50,000-foot aI`citude.
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From page 164... ...
. 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.
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From page 165... ...
The next generation of advanced turboprops win 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 over combination of fuel efficiency, high speed, low noise, lower emissions, arid superior cabin comfort and nde.
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From page 166... ...
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.
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From page 167... ...
40:1 40-50:1 . 2400 °F 2500°F 2250°F 2500°F 2600°F 2800 °F 2800 °F 3000°F Specific Fuel Consumption .
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From page 168... ...
"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.
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From page 169... ...
LeRC is very active in He development of CFD analysis codes for application to turbomachinery. However, the communication of NASA results to the propulsion industry is not always successful, so I~eRC analysis codes are not always appropriate or easily adapted to industry needs.
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From page 170... ...
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.
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From page 171... ...
Specific advanced composite systems needed for advanced engines are shown in Table 8-5, along with propulsion applications for the HSCT and He 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.
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From page 172... ...
Manufacturing Issues Continued development of gas turbine engines in the next century will depend on To 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
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From page 173... ...
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.
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From page 174... ...
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 He needs of the manufacturing community.
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From page 175... ...
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.
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From page 176... ...
Active Fan and Compressor Stabilization One of the most troublesome phenomena in jet engine design and operation is compressor surge and sew. These are large-sa3le oscillations in air flow that result in abrupt thrust loss and can inflict severe mechanical damage.
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From page 177... ...
Active control of these instabilities has yielded both a IS-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.
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From page 178... ...
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.
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From page 179... ...
These machines must operate at high Mach numbers and lower Reynolds numbers, the latter in low-pressure turbines. The cooling air arid turbine inlet temperatures are expected to be higher, whereas the amount of cooling and secondary air will decrease.
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From page 180... ...
preferential migration of hot and cold streaks 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.
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From page 181... ...
Essenhal work remains to be done in the design of the stator, improvement of He stage performance, and the mechanism of stardng. ~ _: LeRC has undertaken a cooperative research program with Allison on an advanced two-stage compressor with a pressure ratio better Man 5:1, handling a through-flow of about 10 pounds per second.
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From page 182... ...
Another area in which additional effort would be welcome is the problem of short blades that win 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.
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From page 183... ...
Research on the chemistry and gas dynamics of NOX reduction continues at LeRC. The produchon of nitrogen oxides increases with pressure, temperature, and residence time ~n the combustor.
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From page 184... ...
Airport NOX emissions will be a challenge for the advanced subsonic engine cycles with 70: ~ pressure ratio and 3000°F turbine inlet temperature. With current combustor types, the
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From page 185... ...
This will require very advanced combustors employing variable geometry, new materials, and smart controls.
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