NASA/TM—2003-212615 IGTC03-ABS-066b Revolutionary Propulsion Systems for 21st Century Aviation Arun K. Sehra and Jaiwon Shin Glenn Research Center, Cleveland, Ohio October 2003 The NASA STI Program Office . . . in Profile Since its founding, NASA has been dedicated to • CONFERENCE PUBLICATION. Collected the advancement of aeronautics and space papers from scientific and technical science. The NASA Scientific and Technical conferences, symposia, seminars, or other Information (STI) Program Office plays a key part meetings sponsored or cosponsored by in helping NASA maintain this important role. NASA. The NASA STI Program Office is operated by • SPECIAL PUBLICATION. Scientific, Langley Research Center, the Lead Center for technical, or historical information from NASA’s scientific and technical information. 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Sehra and Jaiwon Shin Glenn Research Center, Cleveland, Ohio Prepared for the International Gas Turbine Congress 2003 sponsored by the Gas Turbine Society of Japan Tokyo, Japan, November 2–7, 2003 National Aeronautics and Space Administration Glenn Research Center October 2003 This report contains preliminary findings, subject to revision as analysis proceeds. Trade names or manufacturers’ names are used in this report for identification only. This usage does not constitute an official endorsement, either expressed or implied, by the National Aeronautics and Space Administration. Available from NASA Center for Aerospace Information National Technical Information Service 7121 Standard Drive 5285 Port Royal Road Hanover, MD 21076 Springfield, VA 22100 Available electronically at http://gltrs.grc.nasa.gov Revolutionary Propulsion Systems for 21st Century Aviation Arun K. Sehra and Jaiwon Shin National Aeronautics and Space Administration Glenn Research Center Cleveland, Ohio ABSTRACT improving the quality of travel and allowing aviation to expand to meet public demand. The aero propulsion systems The air transportation for the new millennium will of the future will require operation over a wide range of flight require revolutionary solutions to meeting public demand for regimes, while providing high levels of safety and reliability. improving safety, reliability, environmental compatibility, and They will need to be much more energy efficient throughout affordability. NASA’s vision for 21st Century Aircraft is to their flight envelope while keeping emissions of atmospheric develop propulsion systems that are intelligent, virtually pollutants and noise to harmless levels. In addition, they will inaudible (outside the airport boundaries), and have near zero need to be inexpensive to develop, manufacture, and operate. harmful emissions (CO and Knox). This vision includes Advanced aeropropulsion systems and computational 2 intelligent engines that will be capable of adapting to research tools, as well as discrete technologies, will be the changing internal and external conditions to optimally major contributors to 21st Century air transportation accomplish the mission with minimal human intervention. innovation. Revolutionary propulsion systems will enable The distributed vectored propulsion will replace two to four revolutionary aircraft designs that meet the need and demand wing mounted or fuselage mounted engines by a large number of the future. of small, mini, or micro engines, and the electric drive propulsion based on fuel cell power will generate electric power, which in turn will drive Propulsor to produce the AEROPROPULSION VISION FOR 21ST CENTURY desired thrust. Such a system will completely eliminate the AVIATION harmful emissions. This paper reviews future propulsion and power concepts that are currently under development at NASA proposes a phased aeropropulsion research NASA Glenn Research Center. approach triggered by thresholds of technology revolutions and National needs to realize our vision for 21st Century air transportation (Figure 1). NASA will transfer high-risk INTRODUCTION research and technology from each phase to industry- compatible propulsion configurations. This will be done by The future world economy is envisioned to be truly compounding innovation successes from the preceding phases global, where national boundaries become diffused by to enable new configurations. Revolutionary propulsion ideas interdependent commerce. This vision of the future can only will be enabled as we pursue technology research that be realized if there is a revolutionary change in transportation supports our extended long-term vision with the national systems, enabling greater mobility of people and products research community. with improved timeliness and convenience. Propulsion and The Gas Turbine Revolution (as characterized by power capabilities are the foundation on which future Variable Capacity, Ultra High Bypass Ratio, Intelligent subsonic and supersonic transports will shape the aviation Engines) concentrates on component design and systems landscape and establish this global conduit of commerce. operability that result in propulsion systems that are compact, Propulsion innovations have been the fundamental driver intelligent, and efficient for subsonic and supersonic to progress in air transportation. Enormous advances in transports. Adaptive controls and materials will be an integral propulsion performance and efficiency have made it possible part of the propulsion system design. The Engine for aircraft to travel at higher speeds over longer ranges while Configuration Revolution (as characterized by Distributed carrying larger payloads. This has increased capacity by Vectored Propulsion systems) will focus on smart engine orders of magnitude since the advent of air travel. The FAA operations and distributed vectored propulsion systems. and commercial sectors are forecasting another dramatic Distributed exhaust and engine concepts will be an integral growth in commercial air transportation in the next 10 to 25 part of advanced airframe designs. years. Commercial air transportation will again seek and rely The Fuel Infrastructure Revolution will make possible on advanced propulsion solutions to meet this ever-increasing the use of alternate fuels such as low carbon fuels, hydrogen public demand. and hybrids towards low emissions propulsion concepts. The As we approach the centennial celebration of the Wright Alternate Energy Propulsion Revolution will exploit fuel cells Brothers’ First Flight, the challenge of the new millennium is and other high-energy power sources towards powering to develop and deliver innovative, revolutionary solutions to emissionless, non-gas turbine propulsion concepts. Innovative air transport that will meet public demand. Safety, capacity, technologies, propulsion components, and engine systems will and environmental concerns remain the theme, vision and maximize efficiency and performance of the next generations challenge for NASA and our industry partners in the 21st of subsonic and supersonic aircraft while having no adverse Century. We need to travel faster and cheaper, while impact on our environment. NASA/TM—2003-212615 1 technologies will be focused in the following three strategic Fuel Cell-Wings Ai Alternate H2 ECleactthrooly areas: Energy & Anod -Intelligent Computing and Controls strategies. Power H 2 Revolution Electric Propulsion -Smart Components with active (or passive) control to Fuel enhance/ optimize the performance. Infrastructure -Adaptive Cycles and Systems to optimize the engine Revolution HybridPropulsion performance throughout the entire operation. Engine Architecture Intelligent Computing and Controls Revolution VectoredPropulsion In addition to the current effort on physics based modeling for multi-disciplinary (aero, thermo, and structural) Gas Turbine analysis of propulsion systems, the next generation computing Revolution IntelligentEngines Electrochemical Energy process will have several new features. The intelligent Hybrid Systems Energy Chemical Combustion Energy computational environment will: provide need based information to individuals from different disciplines; compute Figure 1.—Propulsion System Revolutions Enabling Mobility the level of uncertainty in the computed results caused by variability in geometry, operating conditions and numerical error (probabilistic methods); select optimum number of processors for computing; and determine the use of VARIABLE CAPABILITY, ULTRA HIGH BYPASS appropriate code for desired level of fidelity in the design or RATIO INTELLIGENT ENGINES analysis process. NASA Glenn Research Center is currently developing a Advancements in the hydrocarbon-fueled gas turbine computational environment for the design and analysis of any engine are rapidly approaching the limits of integration for conceivable propulsion system (Lytle, 2000), called the conventional transport aircraft. Increases in bypass ratio Numerical Propulsion System Simulation (NPSS). NPSS (BPR) enabled by high temperature, high-pressure cores have (Figure 2) focuses on the integration of multiple disciplines ushered in a sustained era of quiet, fuel efficient subsonic such as thermodynamics, aerodynamics, structures, and heat propulsion. Future improvements in commercial core specific transfer. It captures the concept of numerical zooming power output are limited by the growing sensitivity to NOx between 0-dimensional to 1-, 2-, and 3-dimensional analysis emission impacts and the physical size of the core-powered codes. propulsor (or fan) for any given thrust-class of engine. Historically, the first generation high bypass turbofan engines prior to about 1985 were designed to meet the energy crisis challenge. These propulsion systems introduced early 3- D aerodynamic and computer aided design, and incorporated the first generation of superalloy materials, ceramic coatings and polymer matrix composites (PMCs). The second- generation aircraft gas turbine engine, prior to about 1995, had continued emphasis on fuel-burn reduction but was also designed to meet emerging noise and emissions challenges. These higher BPR turbofan engines incorporated advanced materials for still higher cycle temperatures and pressures, which realized greater core specific-power and overall efficiency. Aiding these turbine engine advancements was the introduction of multi-bladerow computer analysis and modeling of unsteady flow phenomena, supercomputing Figure 2.—Numerical Propulsion System Simulation of a advancements, and the introduction of parallel processing. Large Commercial Turbofan Engine The current generation of turbofan engine technology research focuses on meeting an increasing diversity of The vision for NPSS is to create a "numerical test cell" applications requirements (civil, fighters, high Mach, high enabling full engine simulations overnight on cost effective altitude, etc.). This research places emphasis on the computing platforms. Numerical zooming between NPSS environment and affordable performance gains in the wake of engine simulations and higher fidelity representations of the lost US market share and declining research budgets. The engine components (fan, compressor, burner, turbines, etc.) global economy and ecology are driving more physics-based has already been demonstrated. Future augmentations will modeling of the component-integrated propulsion system, address the above mentioned intelligent computing concepts. with greater emphasis on reduced computational time. This In the area of sensors and controls, future research will also design/analysis capability is presently compounding with transform recent successes in physics-based multidisciplinary fundamental research advancements from traditional modeling into real-time propulsion health monitoring and disciplines (aerodynamics, materials, controls, etc.) to usher management for improved safety and reduced maintenance in a next generation of technology innovations. costs. Further developments in adaptive on-board engine To further reduce fuel burnt and harmful emissions and models, advanced component design techniques coupled with noise, the next generation turbine engine (intelligent engine) material-embedded nano-sensors and evolving information- technology capabilities (computational processing speed, data NASA/TM—2003-212615 2 acquisition and dissemination, etc.) will allow for real-time speeds. As mentioned in the previous subsection, trailing edge engine condition monitoring and performance optimization. blowing and circulation control for turbomachinery will Such progress will provide several benefits including provide improved loading and efficiency as well as reducing continuous real-time trending of engine health, synthesized wake-induced acoustics. Similar application of this sensor values which can be used in sensor validation logic and technology to airfoil leading edges will likewise contribute to estimates of the unmeasurable engine parameters such as virtual camber changes as well as improving operability thrust and component stability margins which can be used in margins by reducing aerodynamic stall of high-performance, feedback control logic. sharp edges. Future vision is to make various engine systems function Flow Control and Management: Active and passive more autonomously from the cockpit using biologically redistribution of boundary layer flows within the engine will inspired “intelligent engine” controls akin to the involuntary have a profound adaptability effect on the overall propulsion nervous system. This will enable event or outcome-based performance and weight by minimizing mechanical actuation decisions from the voluntary cockpit control for safer aircraft and associated life and leakage losses. For example, turbine operation of increasingly complex aviation systems. flow area control through fluidics and active seals will enable re-optimization of the engine BPR between takeoff and Smart Components for Noise and Emission Reduction cruise. This will reconcile design constraints for reduced The research effort for smart components is directed at takeoff emissions and improved cruise fuel-efficiency across active and passive control strategies to improve performance the transport aircraft flight envelope. Computational modeling (increased loading and operability), and reduce noise and will be extremely taxed for these designs to minimize the harmful emissions. For performance improvement, NASA’s losses associated with low Reynolds number flows for research has demonstrated rotating stall and surge instability fluidics. New propulsion configuration concepts, employing can be significantly delayed by actively or passively reverse-flow components or concentrically configured controlling (steady or fluctuating) the compressor bleed. It has flowpaths will benefit most from fluidics, because of the short also been recently demonstrated that blade loading and ducting distance and natural radial migration of flow from the efficiency can be significantly enhanced by flow injection/ high-pressure core outward. Advancements in ejector design suction on the airfoil suction surface. methods will also contribute the fluidic engine adaptability by For noise reduction, the key engine components that exploiting natural aerodynamic aspiration rather than active need to be addressed are Fan, Inlet, and Exhaust Nozzle. energy-debit pumping of boundary layer control flow. Aspirated fans with trailing edge blowing (passive) have Acoustical fluidic control of inlet and nozzle boundary layers shown significant reduction in rotor-stator interaction as well could be teamed with actively pulsed noise attenuating liners, as broad band noise. Inlet and nozzle technologies thereby maximizing the dual applicability of a single will focus on noise reduction and propulsion system integrated technology. operability impacts. These advanced modeling techniques will Morphing Structures: Mechanical and structural also allow designers to capitalize on natural acoustic variability will also undergo a revolution with the advent of phenomena (such as ground reflection/dissipation of noise) to active/passive shape-memory materials and tailored reduce the observable noise footprint of future aircraft to less aeroelastic design capability or “morphing”. Similar in effect than that of the surrounding community. Enhanced mixing to the fluidic virtual shape technologies, future shape-memory technologies (such as chevrons and naturally-aspirating materials will be employed in a variety of component areas. ejectors) will be optimized to passively reduce nozzle jet Inlet lip radius/ sharpness, coupled with anti-icing technology, noise without sacrificing performance. Additional active noise could be made to change shape between takeoff and cruise, suppression (such as pulsating acoustic liners) will also be enabling high takeoff airflow without compromising the high employed in future inlet and nozzle systems. cruise efficiency and low drag afforded by a sharper inlet lip. While reduction is fuel burn directly translates to CO2 Shape memory inlet and nozzle contraction area variability reduction, major advances need to made in the area of will improve engine performance and operability without the combustors to reduce NOx emissions. As the latter weight from mechanical actuation. Application of shape- requirement becomes more stringent, the combustor designs memory materials to turbomachinery will yield camber move towards a “lean” burning solution where the fuel/air reshaping (for loading and efficiency optimization and mixture is richer in air to allow for complete combustion of operability) and leading edge sharpness versus operability the fuel. Active control of fuel/air mixture will help to reduce improvements similar to those described for fluidic virtual the NOx emission. Such combustor designs are prone to shaping. Research in the area of fluidic and shape-memory instability due to thermo-acoustic driven pressure oscillations. adaptable airfoils will also be applicable to the aircraft Active control of such oscillations will allow for more configuration (particularly for viscous drag reduction, trim efficient combustor designs. drag reduction, and circulation control for wings and empennage). The large internal changes in engine temperature Adaptive Cycles and Systems environments and speeds readily provide untapped thermal Adaptive technologies for turbine engines will center on and centrifugal forces from which to team passive, structural performance and operability, utilizing research in fluidics, shape control. Variable-speed gearboxes are another fertile structures and material system capabilities, and advanced application of shape-memory materials, providing optimum variable cycle engine configurations. Fundamental fluidic matching of engine high and low pressure spool speeds technology will enable “virtual” aerodynamic shapes, throughout the flight envelope, and maximizing the utility of providing inlet and nozzle area control and peak compressor the gearbox. and turbine efficiency operation over a wide range of flight NASA/TM—2003-212615 3 Adaptive Materials: Future material systems will not missions. These modified Brayton cycles also intrinsically only be designed for their properties but also for their unique offer leaner combustion and reduced emissions, but challenge functionality. Crystalline grown metallics optimized for their state-of-the-art stability practices. application-specific grain boundary properties may contain lattice-encoded DNA-like properties. These will be capable of changing grain boundary size through active and/or passive DISTRIBUTED VECTORED PROPULSION stimuli thereby preventing component failures. Similar chemically encoded properties for coatings and compliant With the advent of the high bypass ratio turbofan, layers will passively provide self-healing protection against research has promoted higher temperature more thermally surface delamination, oxidation, and spalling. efficient smaller cores to power larger and larger fans for Future matrix fibers (used in MMC, CMC, and PMC propulsion. These smaller ultra-efficient cores will someday materials) will not only provide structural reinforcement but reach practical economic limits in manufacturing size. also serve as an embedded conduit for information exchange Similarly the larger fans will also reach limits in their to and from the intelligent engine control, as described under manufacturability and aircraft integratability. At present the Fundamentals. current state-of-the-art design BPR continues to grow, resulting in larger fans (eventually requiring gearing), increased aircraft integration challenges (necessitating high Nano tube wing aircraft designs, etc.), and growing fan acoustic challenges. To circumvent these eventual limits, technologies affording highly integrated propulsion and airframe configurations must be pursued. Airframe-integrated propulsion and power configurations centered on distributed propulsion and capitalizing on technologies realized through the Gas Turbine Revolution will usher in the future air transportation system. The distributed propulsion concept is based on replacing the conventionally small number of CCeelllluullaarr discrete engines with a large number of small, mini, or micro propulsion systems as defined in the following table. Nanommtuaabtteeerr iiaall rope Table 1.—Maximum Thrust of Various Engine Class Engine Micro Mini Small Mediu Large class m Max <10 10 to 100 to 1000 to 10000< Thrust <100 <1000 <10000 Figure 3.—Nanotechnology Materials (lb) High conductivity fibers such as carbon nano-tubes (Figure 3) will simulate nerve ganglia to passively collect Distributed propulsion broadly describes a variety of component diagnostic data. These same fibers may also be configurations that can be classified into three main used to supply messages and adjust the configuration to categories: Distributed Engines (including small, mini, and optimize operating characteristics or to prevent/control micro engine systems), Common-Core Multi-Fans/Propulsors, component failures. and Distributed Exhaust. In all three categories, the forward Adaptive Cycles: Thermodynamic cycle modifications thrust delivered by the propulsion system remains as the and accompanying structural flowpath changes will also conventional large engine counterpart (mass flow times produce propulsion system adaptability. Bladerow by exhaust velocity). Strategic distribution of the exhausting bladerow counter-rotating, concentric spool engines mass flow affords direct and indirect propulsion and airframe employing blade-on-blade technologies and advanced system performance benefits that can ultimately enable new materials will stretch the limits of the variable cycle engine. aircraft missions beyond what is achievable with the state-of- These propulsion systems will enable the use of extremely the-art turbofan concepts. low-weight (strength-compromised) composites by turning In general, all three categories will produce lower the turbomachinery “inside out”. This will put the blades in thermal efficiencies using state-of-the-art technology (due compressive rather than tensile stress. Bladerow counter- principally to reduced component efficiencies from size, rotation will further reduce the required rotational spool speed increased transmission losses, increased internal nozzle & per turbomachinery loading, enabling acoustically superior inlet viscous losses, etc.). Through infusion of innovative tip-shrouded counter-rotating fans. Other modified Brayton propulsion technologies the losses associated with each cycle adaptations will include off-axis cores powering ultra- individual propulsion system thrust will be mitigated and the high pressure combustion and serving as topping cycles for airframe/mission benefits enabled by distributed propulsion peak-power takeoff thrust without compromising the optimum will be fully realized. The most profitable research investment cycle operation for cruise. Inter-turbine and even inter-stage areas to mitigate these losses are those technologies that can turbine combustion configurations are being investigated for only (or most fully) be realized in the small scale their large impacts on cycle adaptability over diverse (i.e., flow/circulation control through micro-turbines, foil/air NASA/TM—2003-212615 4 bearings, concentric engines/core). The most profitable As much as 3-5% total aircraft fuel burn reduction might systems benefits are those that result from the airframe be realized from boundary layer ingestion employing small to configurations that are realized by these propulsion mini engine distributed propulsion. This performance benefit configurations (i.e. tailless propulsion controlled aircraft, may be enhanced in a hybrid system utilizing micro engines noise mitigation, supersonic cruise aircraft weight & drag to energize the low-momentum boundary layer flow. This reductions, etc.). benefit can only be realized if the micro engine fuel consumption is low (again scavenging of waste heat would be Distributed Engines advantageous as described by the Distributed Exhaust The category of Distributed Engines encompasses concept). Because of their small size, extremely high specific- decentralized propulsion systems and utilizes separate smaller strength composite materials may be used in small and mini powerplants strategically deployed over (or embedded) the engines with less statistical failure due to defects. The reduced aircraft. Examples of this type of distributed propulsion might size allows practical, cost-effective manufacturing of these include small or mini engines (Figure 4) deployed across the advanced-material structures. Success of the small and mini wingspan and fuselage, and micro-turbine engines (Figures 5) engine propulsion deployed laterally across the wing is embedded in the aircraft surface for flow/circulation-control dependent on exploiting technologies that are best realized in and thrust. Severe performance penalties manifest in mini- the reduced sized. engine systems are principally due to boundary layer effects Micro engines themselves can provide distributed of the fluid being on the same geometric scale as the propulsion and exhibit large thrust to weight potential. propulsion system. The challenge of manufacturing tolerances Because of their size, low Reynolds number fluid effects, that can be economically observed in these engines also engine manufacturing tolerances and corresponding impacts severely impacts their performance and cost. Therefore mini on seals and clearances, 3-D turbomachinery shapes, and and micro engine propulsion must “buy its way on” the combustion efficiency are primary technical challenges for aircraft. It must afford greater benefits in other areas, such as these propulsion systems. Currently, parts at the micro-scale noise and drag reduction, or by enabling a superior integrated can only be produced in two dimensions, resembling extruded aircraft/engine system. parts. This efficiency limitation on the rotating components will be overcome with material and manufacturing technologies. These will enable three-dimensional shaping of airfoils and allowing new micro-scale engine configurations with reduced stress concentrations inherent in the current two- dimensional prototypes. Other factors affecting the structural/mechanical design of these micro-engines are the typically high rotational speeds, which may exceed 2 million RPMs. These high speeds are achievable due to reduced-scale inertial loads, but will demand non-lubricated air-bearings to surpass the common modes of failure observed in research prototypes. Though the physical engine scale is decreased, the chemical reaction times remain constant and will require technology innovation to regain lost combustion efficiency. A Figure 4.—Distributed Engines embedded very general rule for mini- and micro-engines is that both in the wing and body specific fuel consumption (SFC) and thrust-to-weight ratio increase as thrust and size decrease. To become a viable Laterally distributed engines will afford similar primary propulsion source, SFC reductions to near current aerodynamic and acoustic benefits as those described for the macro-engine levels must accompany the increased thrust-to- high aspect-ratio wing trailing edge nozzle. Additional aircraft weight ratios already achievable in mini and micro engines. integration of supporting fluidic technologies using Distributed engine concepts will enable a variety of distribution engines could provide more dramatic transport attractive airframe configurations affording both performance mission impacts. and operational benefits. Large engine production rates, lower development cost and cycle time, and line-replaceable-unit elimination of on-the-wing engine maintenance could reduce the life cycle cost by as much as 50%. Aircraft safety will be enhanced through engine redundancy and semi-redundant propulsion control of the aircraft. Dual use of the airframe structure will dramatically reduce the overall system weight, and afford holistic system noise reduction opportunities beyond those attainable with discrete engines. Principle technologies that will afford the greatest potential for realizing micro engine propulsion success include: innovative combustion techniques, processing of SiC and other advanced micro engine material for improved 3D designs, and integral autonomous controls coupled with sub-micro sensors assuring Figure 5.—Radial inflow turbine of a micro engine engine array reliability. NASA/TM—2003-212615 5 Common-Core Multi- Fans/Propulsors suited to supersonic cruise applications, where noise-sizing The category of Common-Core Multi-Fans/Propulsors for takeoff field length and sustained supersonic cruise drag entails the packaging of multiple thrust fans powered by a are the most dominant and least reconcilable constraints central engine core (Figure 6). The advantage of these (Figure 7). configurations is that they provide ultra high BPR engine with higher propulsive efficiency without necessitating radical airframe changes (such as high wing designs) to accommodate a single large turbofan engine. The principle challenges of this approach are power transmission weight and losses. These challenges may be somewhat mitigated by the variable gearbox technologies previously developed under the Gas Turbine Revolution, or by employing blade-on-blade manifolded tip-turbines on the fans. These challenges could also be circumvented using direct-drive tandem fans (i.e. axially aligned fans with separate inlets for and aft of the common core) rather than the side-by-side configuration. Another potential configuration uses the core exhaust to drive two off-axis turbines that are attached to direct-drive fans. Multi-fan cores will require innovative separate inlets to realize their full BPR and aircraft integration benefit. This will require lightweight structures and possibly flow control Figure 7.—Supersonic Airplane with High to minimize weight and inlet performance losses. Aspect Ratio Nozzle The commonly shared performance challenges associated with all forms of distributed propulsion (low- High aspect ratio nozzles for commercial supersonic Reynolds number flows, boundary-layer interactions, and fuel cruise vehicles promise both noise and nozzle weight management systems) will be surpassed during this research reduction potential. The projected sideline noise reduction phase using those technologies and discipline capabilities using a wing trailing edge 2-D mixer/ejector nozzle with (aerodynamic, mechanical, materials, structures, comparatively small exhaust height may be as much as 10dB manufacturing, etc.) outlined for the Gas Turbine Revolution. (due to increased ambient jet mixing, improved ejector The highly integrated Distributed Vectored Propulsion internal penetration and mixing, and increased liner systems for future subsonic and supersonic transports will attenuability resulting from naturally higher frequencies and incorporate V/STOL and Propulsion Controlled Aircraft surface areas). In addition, the high aspect ratio geometrically (PCA) capabilities, and capitalize on intelligent, self-healing produces a shorter nozzle for an equal nozzle pressure ratio properties. and provides the potential for shared structural loading with the wing. This will culminate in as much as 50% equivalent nozzle weight reduction and propulsion related cruise drag. Increased low-speed lift via wing trailing edge flap blowing and thrust vectoring will also be achieved through this configuration, and reduce the required takeoff field length and affording community and approach noise reductions. Hybrid systems incorporating distributed/thrust vectored exhaust and micro-engine for flow control and actuator power are also attractive. To reduce the performance loss of the increased nozzle surface areas and increased internal flow turning, micro-engines can be incorporated for boundary layer control and cooling. This approach might passively utilize waste heat from the nozzle to power the micro-engines rather than active dedicated micro-fuel/combustors. The scavenging of waste heat will reduce the exhaust temperatures as well as increase the effectiveness of the primary distributed propulsion system. The micro engines might also be configured to facilitate virtual shape control through fluidic Figure 6.—Common-Core, Multi-Propulsor Engines “reshaping” of the primary nozzles. This reduces or eliminates mechanical actuation while reducing internal viscous losses and waste heat. Distributed Exhaust The category of Distributed Exhaust entails using a central engine powerplant with a ducted nozzle(s) for strategic ALTERNATE ENERGY PROPULSION deployment of thrust on the aircraft. Distributed exhaust configurations suffer nozzle viscous losses in performance While the timing remains debatable, the 21st Century and will likely only “buy their way on” to aircraft systems will almost assuredly see the emergence of an all-electric exhibiting extreme sensitivity to low-speed lift and/or cruise economy. In this era, electricity will be the common currency. drag. Therefore, distributed exhaust systems will be better NASA/TM—2003-212615 6