1.0 INTRODUCTIONOne hundred and two years ago, after the Wright brothers had justattempted another unsuccessful flight, they predicted that it would beanother 50 years before manned flight was achieved. Only two yearslater and 100 years ago this year, Orville Wright achieved the firstpowered flight at Kitty Hawk in North Carolina, illustrating just howdifficult it is to predict the future and at the same time launching thepioneering age of aviation.

Since this memorable day aviation has grown at a phenomenalrate and is now an essential part of our way of life, with societycompletely dependent on the ability to transport both people andgoods safely, in comfort and in short travel times made possible bymodern aircraft and their engines. Nations across the globe have alsobecome highly dependant upon the aircraft as part of their nationaldefence force.

The two major aerospace sectors of civil and defence, from acommon beginning, have taken quite different directions over time.Recently we have seen the military market develop from the coldwar era into one of regional conflicts, where a wide range ofcomplex scenarios present many new challenges for the aircraft andengine designer. Reduced defence budgets have also pushed afford-ability further up the agenda and these trends look set to continue.

The civil sector, on the other hand, is driven by safety, cost ofownership, and passenger choice with environmental factors takingon ever increasing importance as we move towards an age ofsustainable growth. Ultimately this direction will be determined bythe approach that the industry’s governing bodies play in addressingSociety’s concerns over the environmental impact of aviation. It isalso possible that economic factors, such as the availability ofkerosene will, in the longer term, influence the future direction of theindustry.

If the pressure for environmentally clean aircraft becomes domi-nant then we are likely to see accelerated technological developmentof novel aircraft and aero-engine concepts. Whichever direction theindustry takes, both evolutionary and revolutionary underpinningtechnologies will be required, with innovative approaches in aero-engine design continuing to be a major contributor.

This paper will examine both the civil and defence aerospacesectors and open up the debate by highlighting the key factors influ-encing the future direction in these markets. It will address the likelyrequirements for aircraft propulsion systems in the near, medium andlonger-term, together with the underpinning technologies that arelikely to prove essential in delivering the propulsion systems that thecommunity and the customer demand.

2.0 CIVIL AEROSPACE

2.1 Market drivers

The civil aerospace market today is vast and diverse, covering a widerange of different aircraft types from large commercial airliners andsmaller regional jets to supersonic passenger aircraft and business jetsas well as rotorcraft for private and commercial use. Overall themarket continues to grow at a predicted rate of around 5% per annum,with the highest growth coming from Asia and other less developedparts of the world. Although increased use of Information Technologymay moderate the growth of high yield business travel and fast cargoships may divert some of the anticipated freight growth, these effectswill most likely be offset by the general increase in economic activitythat IT and the Internet create. In passenger terms, the market growthis likely to produce a general increase in the size of aircraft in mostsectors as well as forcing some fundamental infrastructure improve-ments in order to tackle issues surrounding airport security, capacity,air traffic management and safety.

Although the propulsion requirements differ according to indi-vidual aircraft types, there are a number of factors that drive aircraftand engine design dependant upon aircraft size, range and utilisation(see Fig. 1). In all cases however, the designer is aiming to minimise

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P. C. RufflesRolls-Royce

Aero engines of the future

Figure 1. Current civil technology drivers.

cost of ownership whilst satisfying environmental and safetyrequirements imposed either by competition or regulation. Forexample in long haul operation, aircraft are normally large placinggreat emphasis on low noise and fuel efficient engine designswhereas in short haul operation the aircraft are smaller with greateremphasis being placed on first cost and cyclic life. Business jets onthe other hand are small and have low utilisation, therefore life andexternal noise are less important but reliability and low cabin noiseare of paramount importance.

Looking firstly at the environmental factors, noise and emissions(CO, HC and NOx) are driven by public acceptability embodied ineither international or local legislation, whereas CO2 emissions relatedirectly to fuel burn and are currently driven by aircraft economics.We have already reached a point with large aircraft where perfor-mance and economics are compromised in order to satisfy noiserequirements. Depending on the level of environmental regulationfacing the industry over the coming decades, and the relative impor-tance attached to each contributor, we may well see the shape of aeroengines and aircraft change significantly. The Advisory Council forAeronautical Research in Europe (ACARE) in recognising thesetrends has set extremely challenging goals for the research agendafor aeronautics by 2020, including aggressive environmental, safetyand economic targets. These targets will drive designs of futuregenerations of aero-engines and aircraft, thereby accelerating techno-logical progress and forcing novel, higher risk solutions than today’srelatively mature products.

Life cycle costs embracing initial cost, life, reliability and main-tainability will, nevertheless, remain important economic drivers asairlines continue to reduce seat costs in order to win market shareand sustain growth by offering air travel to an increasing proportionof the population. In response to these challenges, as well asproviding better aircraft and engines through the introduction ofadvanced technologies and design concepts, manufacturers willincreasingly become service providers by using their product knowl-edge to provide superior services, coupled with taking on greaterresponsibility for the operating costs of their products. This willproduce changes in the way products are designed to ensure lowercosts throughout the life cycle.

2.2 Near term

Due to the lengthy product development cycles and the time taken tovalidate new technologies, the products that will enter service in thenext five or so years will be developments of those that already exist

or are already under development. These products will incorporatetechnology that is already largely proven and understood. In thewide body sector this will include the Airbus A330, A340, andA380, the Boeing 777 and potentially the Boeing 747 all powered bythe Trent engine family (see Fig. 2).

The Airbus A380 is the largest and latest wide body aircraft due toenter service in 2006 powered by the three-shaft, ultra high by-passratio Trent 900 engine — the latest development of the Rolls-RoyceTrent family of turbofan engines (see Fig. 3).

The Trent 900 engine incorporates significant new technologiesover previous Trent engines as illustrated in Fig. 4, notably the firstfully swept fan design, 3D aerodynamics throughout the compres-sors and advanced 3D designs in the turbines, all aimed at improvingefficiency. The swept fan will also reduce noise. The HP (high pres-sure) system will be contra-rotating further improving turbine effi-ciencies (a concept read across from military applications) and theby-pass ratio is increased from previous Trent engines in order toimprove specific fuel consumption and further reduce noise. Muchof this technology was at the research stage when the Trent 700 andTrent 800 engines were first developed early in the last decade.

At the smaller end of the market, two-shaft lower OPR (overallpressure ratio), high by-pass ratio turbofans will remain the propulsionsystem for the majority of narrow body aircraft from small airliners,

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Figure 2. Widest market coverage with the Trent family. Figure 3. A380 and Trent 900.

Figure 4: Trent 900 high technology at low risk

regional aircraft and most business jets, with turboprops providingpower for regional aircraft below about 50 seats (see Fig. 5).

The International Aero Engines (IAE) V2500 engine, poweringthe Airbus A320 family has proven to be an extremely successfulproduct in this fiercely competitive area of the market. Rolls-Roycehas played a key role in this international consortium and willcontinue to do so through the IAE ‘Vista’ product strategy. In thenear-term, this includes both performance and durability improve-ments and modifications to the V2500 engine, whilst longer-termtechnological developments and preliminary design activity willensure that a competitive product is available for the next generationsingle aisle aircraft. The Rolls-Royce two-shaft engine strategy willbe a key contributor to this activity.

Experience gained on the design and development of the V2500compressor has been read across to the development of the BR700compressor. The BR700 series of engines, which employs a similarlow risk derivative approach to the Trent family, seeks a further near-term application on the Global 5000 aircraft in addition to theBoeing 717, Gulfstream V and Global Express (see Fig. 6). TheBR710 variant of this family meets the market requirements of thelarge business jet sector through providing high performance and lowcost of ownership, whilst retaining strong environmental performance.

For the regional market and medium sized business jets, the

AE3007 powers the Cessna Citation X and the Embraer ERJ familyand has found successful military application on the Global Hawk.The engine core is itself a derivative of the AE1107C engine used inthe SAAB 2000 commuter aircraft, which also has military applica-tions and these are discussed later in the paper.

In addition to new product applications, existing products will beprogressively upgraded with new technology as operators attempt tomaximise the value of their assets over several decades of operation.Technology upgrades are aimed at improving efficiency and oper-ating cost while minimising their environmental impact and bringingthe products in line with new environmental legislation.

2.3 Medium term

Although the medium term presents more unknowns, productconfigurations are likely to be evolutionary as the associatedenabling technologies are already largely identified. Again, thelengthy product development cycles and the cost and safety drivenrequirement to validate technologies prior to application, means thatmany of the technologies that will feature in the products in the nextten years, are currently at the validation phase.

The debate continues over the direction that the next generation oflarge civil aircraft will take. Boeing and Airbus have expressed theirdiffering views on the requirements for either faster, more directaircraft flying point to point, or larger aircraft operating fromgrowing hubs. Both requirements have a place provided thepassenger is prepared to pay a premium for the benefit of a shorterjourney time.

Originally, Boeing offered the Sonic Cruiser, which thoroughinvestigation showed to be unviable economically. As an alternative,the 7E7 is now offered (see Fig. 7), aiming to achieve improvedeconomic and environmental performance.

The engine for the 7E7 would be a new generation of engine,building on the success of the Trent family using three-shaft archi-tecture. A significant amount of new technology, designated‘Vision10 technology’' by Rolls-Royce, would be incorporated,possibly including embedded electric starting, integrally bladed(blisk) compressors and significant weight reductions through use ofadvanced materials and reduced parts count (Fig. 8).

In the medium size and range of aircraft, it is likely that we willsee a new generation of products as a replacement for current 180 to280-seat aircraft. Presently this new aircraft type is likely to be alargely conventional aircraft following the classic swept wing designof current subsonic transport aircraft, but with new engine andaircraft technologies and design methods, all aimed at addressing

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Figure 5. Widest market coverage with the two-shaft family.

Figure 6. The BR700 family applications.

Figure 7: The Boeing 7E7.

both the environmental and economic challenges that face theindustry. The engine is likely to have a thrust above 40,000lb wherethe three-shaft architecture is optimum and will feature the sametechnologies as proposed for the 7E7 but the engine thermodynamiccycle will be optimised to the application.

The ANTLE (affordable near term low emissions) engineprogramme will deliver the technology to support this architectureand is very much focussed on the environmental aspects includingnoise and emissions, thermal, propulsive and component efficienciesand weight reduction, in order to deliver fuel burn improvements andhence CO2 production. Reduced through life costs, better reliabilityand maintainability are also being addressed (see Fig. 9).

ANTLE is a European Union programme led by Rolls-Royce basedon a Trent 500 engine. Many of the technologies are applicable rightacross the product range including military applications. ANTLEincorporates a new HP system with a low-emissions combustor incor-porating a novel fuel staging system and an advanced compressorincorporating blisks, an advanced unshrouded HP turbine and a vari-able capacity IP (intermediate pressure) turbine. The HP system alsoincorporates a novel embedded electrical starting system. The LP (lowpressure) turbine is very highly loaded to enable high bypass ratios tobe achieved without increasing the number of turbine stages and a newIP compressor with fewer stages will be introduced at a later date.

The medium-term will also almost certainly feature developmentsof existing models of aircraft and new smaller aircraft developmentsfrom other parts of the world in addition to those from Airbus andBoeing, encouraged by the recent success of the South Americanmanufacturer Embraer and the Canadian Bombardier products in thesmaller regional jet sector. For example a 100-seater aircraftprogramme in China is already underway and will serve to developtheir industry appreciably, although engines will come from Westernsuppliers.

These types of aircraft will be powered by two-shaft high by-passengines adopting technology developed through the German aero-space research programme known as ‘Engine 3E’ (E3E — effi-ciency, environment, and economy), which is fully integrated withthe ANTLE programme (see Fig. 10). E3E features a new advancedcompressor and has a strong focus on improving noise and combus-tion emissions as well as cost which contributes much more to directoperating cost than on longer-haul operations. It has to be notedhowever that short-haul operations are a big contributor to globalwarming which could lead to a reprioritisation of drivers for thissector were a carbon tax or other legislation introduced to controlCO2 emissions.

Other technology demonstration programmes include the quiettechnology demonstrator (QTD), which is a joint programme withBoeing, aimed specifically at noise technology, reducing both jet andfan noise through the use of serrated nozzles and advanced acousticlinings. The complimentary SILENCE(R) (significantly lower aircraftenvironmental noise community exposure) programme is a Europeanprogramme, led by Rolls-Royce, which also aims to validate furthernoise reduction technologies within the 10-year time frame.

2.4 Longer term

The longer-term view on the direction that aero-engine manufac-turers follow is far more difficult to foresee. As the industry enters anew era of sustainable growth with enforced, highly stringent envi-ronmental targets being the primary concerns of aircraft and enginedesigners, a step change in aircraft and engine designs, as well as thekey technologies within them, will be required. The specific ACAREgoals for 2020 include cutting by half the current perceived averagenoise levels, reducing CO2 by 50%, reducing NOx by 80% andreducing accident rates by a factor of five. Of these goals, thatrelated to CO2 is undoubtedly the most arduous and will require amore radical approach to the way the air transport system, includingaircraft and engines, is designed and operated.

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Figure 8. Conceptual engine for future 250-seat aircraft.

Figure 9. ANTLE — proving technology.

Figure 10. E3E — proving technology.

As Fig. 11 shows, 50% of the world’s aviation fuel is currentlyused on sectors of less than 1,200nm. In view of this and with theACARE goals in mind, the issue of the design of short haul aircraft,which are optimised around overall economics, passenger comfortand convenience rather than fuel burn, is immediately raised. Forjourneys of this short distance, the time penalty through switching toslower aircraft would be minimal allowing aircraft and enginedesigns to be focussed on fuel efficiency at the expense of overalloperating economics. Aircraft are likely to be of conventional layoutwith up to 250 seats, using engines of very high by pass ratio(including open rotor engines) and overall pressure ratio with highcomponent efficiencies. Aircraft could become larger at the expenseof frequency whilst more direct routing by better air traffic manage-ment could yield further benefits. Future aircraft concepts in thissector might include the ‘low noise concept aircraft’, where enginesare mounted on the top of the aircraft to reduce noise effects on theground (see Fig. 12).

However in the long haul market, the need to maintain speeds atcurrent levels is the most likely outcome but using novel aircraft (seeFig. 13) and engines to further reduce aircraft drag and enginespecific fuel consumption (SFC). Concepts being evaluated include

the ‘blended wing body’ (BWB) aircraft, the ‘lifting body’ styleaircraft and the ‘three-surface lifting aircraft’. The most promising ofthese is the BWB, which offers significant aerodynamic benefits dueto its reduced wetted area and friction drag. This would produce thetype of step change required to meet potential fuel burn improve-ments. Design constraints, notably the wing depth determined bypassenger height, limit the minimum size of a BWB aircraft to abovethat of wide-body aircraft. With this configuration the optimumengine solution may well be quite different to today’s large turbo-fans. In fact much work has been done in assessing the contra-rotating aft fan (see Fig. 14).

This design improves fuel consumption, weight and noise. The aftfan configuration lifts the air intake clear of the wing and so enablestop-mounted (rather than underslung) engines to be located closer tothe fuselage. The top mounted engine allows the wing surface itselfto act as an additional noise absorber.

However, even with these new engine architectures there is a ther-modynamic limit to the improvements that can be realised by using aconventional Joule cycle engine. A recent study, based on a conven-tional Joule cycle, predicted that SFC improvements from continuingto improve cycle parameters, component efficiencies and lower

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Figure 11. Global fuel consumption breakdown (reproduced with kind permission of ACARE).

Figure 12. New aircraft concepts – ‘low noise’ concept (reproduced with kind permission of ACARE).

Figure 13. New aircraft concepts.

Figure 14. Novel aircraft and engine concepts.

cooling flows are likely to be no more than ten per cent. This ismuch less than in the past, although the most rewarding area is byfurther increasing by-pass ratio provided the installation penalties ofincreased drag and weight can be avoided.

As solutions are sought to provide cleaner, quieter and more effi-cient engines, these highly interactive design trade-offs becomeincreasingly important considerations. They also serve to highlightthe significance of the approach of the governing environmentalbodies in establishing an effective legislative and economic balance,aimed at minimising the overall environmental impact of theindustry.

Advanced cycle engines embodying constant volume combustionwould also provide significant efficiency improvements if the sealingand vibratory stress problems associated with this concept could beovercome. Recent studies on concepts for high Mach number vehicleshave yielded some very promising results in this area(6).

In looking beyond the horizon of the next 25 years, it is necessaryto consider alternative fuels both from the viewpoint of emissionsand security of supply. Recent studies predict that supplies ofkerosene, or a synthetic substitute from natural gas, would preventthe industry from facing a kerosene shortage until 2090(2). However,environmental pressure may expedite a move towards alternativefuels that are cleaner than kerosene.

Hydrogen and methane are the most obvious alternatives, withmethane producing significantly less CO2 as a combustion by-product and hydrogen producing none at all. However, the result isan increase in the production of water vapour, the effects of whichare not yet fully understood. Resulting contrails and increased cirrusformation may also have a detrimental effect as regards climatechange. Together with the practical problems posed in terms of fuelstorage, manufacture and safety issues, it is unlikely that we will seethe commercial use of such alternative fuels for some time.However, the picture in the longer-term must not preclude thesefrom the frame.

3.0 DEFENCE AEROSPACE

3.1 Market drivers

The defence market today encompasses a wide range of differentaircraft roles from combat to reconnaissance, from helicopters totransports, from tankers to missiles and from light combat andtrainers to the emerging, yet increasing market for unmanned aircraftof all types. The removal of the pilot from vehicles where humaninteraction is not necessary provides numerous advantages including;more stealthy shape, non-pressurised cabins with more space, uncom-promised longer duration missions and a significantly reducedtraining demand. These factors are leading to increased focus on thedevelopment of unmanned air vehicles and unmanned combat airvehicles (UAVs and UCAVs) opening up brand new opportunities tochange the shape of military air strategies into the future.

Currently the defence aerospace market is growing from the lowlevels reached after the end of the cold war, although time has seen ashift in the particular requirements as a result of the changing shapeof recent conflicts. Whilst new requirements emerge, life-cycle costsof existing and new equipment and affordability will determine whatnew equipment is acquired. Increased international collaboration, thedevelopment of network-centric defence systems, technology read-across from the civil aerospace sector, together with the develop-ment of multi-use engine cores are all seen as essential means ofmaking the required developments affordable.

Thrust to weight ratio, increased Mach number, safety, reliability(including prognostics and diagnostics), fuel burn, survivability,maintainability, emissions and noise remain key drivers in thedefence sector, although the relative importance of each is highlydependent upon the application (see Fig. 15).

In the area of manned combat, which is likely to be one of thelargest market sectors over the next 20 years, high thrust to weightratio remains an important design requirement in order to provide airsuperiority and good low altitude strike capability. There is also acontinued need for such aircraft to be ‘carrier capable’. Emphasis inthe transport sector is on heavy lift capability, deployment, and shortfield performance requiring engines with low fuel burn and lowoverall product life cycle cost. Helicopters will continue to be usedfor troop carrying capability, support and heavy lift especially in hotand high environments, although offensive roles will increasinglymove toward UCARs (unmanned combat armed rotorcraft). Inengine terms, power to weight ratio, robustness and durability arethe crucial requirements in the helicopter sector.

In the trainer and light aircraft sector, cost is the major driver asaircraft are often complementary to relatively inexpensive simulationtools. There is still, however, a large market in the developing worldwhere trainer aircraft often perform a light combat role.

The primary market drivers in the UAV and UCAV sector arestrongly dependent on the mission requirements. For unmannedreconnaissance aircraft very high altitude and long duration are key,requiring low specific thrust engines whilst, UCAVs on the otherhand, require very high specific thrust. The requirement forautonomous operation, shared by all engines powering this newfamily of aircraft, provides a unique new requirement for the devel-opment of new technologies and processes and in particular thedemand for lightweight electrical power. A further new requirementin this area is the need for long-term storage. Missiles, also a partic-ular form of UCAV, have the additional requirement for the propul-sion system to be expendable.

Product development cycles in the military market are generallylonger and less frequent than their civil counterparts. As a result,recent times have seen more technology transfer in the civil to mili-tary direction, which is a dramatic change from the position 20 to 30years ago.

3.2 The near term

In the near-term engines will be largely conventional embodyinglargely proven technologies developed in the late 1980s and early1990s but will be updated during their life with new technologycurrently under development (see Fig. 16).

The EJ200 engine for the Eurofighter Typhoon manned fighter isone of only three new combat engines satisfying the market for aversatile manned fighter with excellent air superiority. The EJ200

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Figure 15. Current military technology drivers.

contains significant innovative technology including all blisk LPand HP compressors, single-stage shroudless, single crystal HPturbine blades as well as brush seals and an airspray combustorderived from the Trent civil engine.

The Rolls-Royce/Turbomeca RTM322 engine for the EH101,NH90 and Apache helicopters provides an excellent example of anear-term product aimed at the growing military helicopter sector,where innovative design and state-of-the-art technology were essen-tial in penetrating a highly competitive market. Technologies includea unique inlet particle separator, which prevents FOD (foreign objectdamage) and erosion but has no moving parts, a three-stage bliskaxial compressor, compact annular combustor and a highly efficientgas generator turbine using single crystal material.

The V-22 Osprey tilt rotor aircraft (see Fig. 17) fulfils a uniquerequirement in the military transport arena and required the develop-ment of the Rolls-Royce AE 1107C turboshaft engine. The wing-tiptilting nacelles and self-contained oil system were designed toaccommodate vertical and horizontal operation and provided majortechnological challenges during development.

This core has since been used in the Rolls-Royce AE 2100 turbo-propellor engine for the Lockheed Martin C-130J military transportaircraft. The turboprop’s inherently better thermal efficiency and

high take off thrust provide both the low fuel burn and short runwayfield performance essential to this type of aircraft. The final memberof this family of engines to share the same core is the Rolls-RoyceAE 3007 turbofan, powering the Northrop Grumman Ryan Aeronau-tical Centre Global Hawk unmanned surveillance aircraft. Theengine, incorporating a wide chord single-stage fan to provide therequired fuel efficiency and noise performance was initially devel-oped for the business and regional jet markets in the civil sector.However its versatility has made it capable of satisfying the require-ments of this emerging military unmanned market sector.

3.3 Medium term

In the medium term, the emphasis will be on versatility in order tocontain costs by making one basic aircraft design satisfy severalroles. This philosophy has been applied to the Joint Strike Fighter(JSF) multi-role aircraft, the most important fighter for the next 30years, where CTOL (conventional take-off and landing), VSTOL(vertical or short take-off and landing) and carrier variants aim toprovide all military services with versatile and affordable aircraft inlarge production volumes.

The STOVL (short take-off and vertical landing) variant of theJSF provides its forward lift with the novel Rolls-Royce LiftFan®system, incorporating significant innovative technology in both itsaerodynamic and mechanical design. Rearward lift is provided by aninnovative three bearing deflecting nozzle fitted to the main propul-sion engine. The fan comprises a two-stage contra-rotating highflow, low-pressure ratio blisked fan rotor driven by the main propul-sion engine and provides around 19000lbs thrust vertically. Themulti-plate clutch and gearbox transmit nearly 30,000hp whilst thefan exit flow is directed through a variable area vane box nozzle(VAVBN) with a considerable range of deflection. It can bedeployed at relatively high aircraft speeds in conditions of high inletdistortion and at high clutch engagement speeds (see Fig. 19).

As part of the JSF programme Rolls-Royce has also teamed upwith General Electric in order to deliver the F136 — the alternativeengine to the baseline Pratt & Whitney F135. The F136 engine willagain include many new technologies and will find applications inother military aircraft in this thrust range (see Fig. 20).

The engine uses a second-generation hollow SPF/DB (super plas-tically formed/diffusion bonded) blisk fan based on technologydeveloped in the civil sector and also used in the LiftFan®. Thecombustion system and HP NGV (nozzle guide vane) are Rolls-Royce design responsibility and feature unique transpiration coolingtechnologies in the form of the Lamilloy® and the advanced Cast-

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Figure 16. Near term technology injection.

Figure 17. T406 family.

Figure 18. Rolls-Royce STOVL technology in action.

Bond® process developed under the United States IHPTET (inte-grated high performance turbine engine technology) defence tech-nology demonstration programme. These technologies will all havewide applicability in the longer-term.

The European requirement for a new military transport will besatisfied by the A400M airlifter being developed by Airbus Military.The propulsion system requirements for heavy lift capability andshort field performance with low fuel burn require a high powerTP400 turboprop of 12,000hp, the largest to be developed in thewestern world (see Fig. 21). The engine will be developed by theAero Propulsion Alliance (APA) in which Rolls-Royce, Snecma,MTU, Fiat and ITP are partners.

A three-shaft configuration was selected to enable a high-pressureratio engine cycle to be used whilst avoiding the handlingcomplexity associated with an arrangement where the propeller shaftand LP compressor are directly coupled. The three-shaft layout alsoprovides for power growth potential by addition of either a rebladedLP compressor of higher-pressure ratio or an additional stage.

3.4 Long term

In the longer-term the market could split into manned and unmannedvehicles, with growth in the unmanned sector increasing rapidly as

further roles are identified where pilots can be replaced. The growthin the unmanned sector will cover reconnaissance, combat (bothfixed wing and rotorcraft) as well as missiles and space access. Themanned sector, however, will predominantly feature growth inreconnaissance and strike.

The focus on affordability and versatility will drive the develop-ment of products that will find applications to satisfy more than onerequirement, as is the case with JSF. In order to contain the cost ofdeveloping new technology, sharing of technologies between mili-tary and civil products will be essential.

The general trend towards increased Mach number, range, perfor-mance and reliability are all consistent with the US VAATE (versa-tile affordable advanced turbine engine) and the UK FOAS (futureoffensive air system) technology demonstration programmes. Theseaim to deliver the future technology requirements of a broad range ofsectors and, as mentioned, focus particularly on delivering multi-usetechnologies and versatile engine cores that can be applied across anumber of specific products.

The unmanned sector presents a number of new challenges thatwill require new engine technologies to meet them, for exampleadvances in prognostic and diagnostic techniques will be drivendirectly from the need for autonomous operation which will leadeventually to an ‘intelligent engine’. Advanced control system and

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Figure 19. JSF lift system.

Figure 20. Applying advanced technologies — F136.

Figure 21. A400M and TP400 engine.

Figure 22. Future defence programmes.

increased electric power requirements will accelerate the use ofmore electric technologies as well as power off-take systems. Thesevehicles also need to be stored for great lengths of time (aircraftmay be stored for many months or even years between missions)and this provides further challenges and adds weight to the case forthe deletion of the oil system in preference for more integrated elec-trical systems such as active magnetic bearings (AMBs).

All operational military aircraft will need to be stealthy particu-larly those that operate in the front line. Whilst realising a stealthyaircraft is primarily related to aircraft design, the propulsionsystem has an important part to play in inlet and exhaust systemdesign and emissions, notably smoke and NOx. Major areas ofimprovements in stealth will come from reducing the infraredsignature from the exhaust plume and hot exhaust components bybetter integration with the aircraft shape as illustrated in the UCAVdepicted in Fig. 23. Reheat engines, whilst good in improvingthrust/weight ratio and combat capability, present major problemswhen trying to achieve the very low infrared signature levelsrequired for UCAVs. Instead, the emphasis will be on the need forhigh specific thrust from dry engines during combat and lowspecific thrust at cruise. One way of achieving this difficultrequirement is to employ a variable cycle concept, although costand complexity currently remain barriers to this approach.

High Mach number vehicles are taking on increased importance inorder to reduce the time between target identification and destruc-tion. These vehicles require engines to operate at high temperatureswith acceptable life and reliability (see Fig. 24). Studies of waveengines employing detonation waves to effect combustion are oneoption being considered for these applications(6).

Across the spectrum of gas turbine engines used for defencepurposes, the future will see the introduction of more advancedmaterials, including metal matrix composites, more electric tech-nologies, advanced turbo machinery and combustion systems. Manyof the technologies will continue to be read across from the civilaerospace sector and vice versa whilst some will be developeduniquely for military purposes.

4.0 UNDERPINNING TECHNOLOGIESThe aero engine has always responded favourably to the applicationof new technology whether related to performance, emissions,weight, life, reliability or cost. The pressures to reduce environ-mental impact and reduce life cycle costs will require continuedsignificant investment in all the underpinning technologies in boththe civil and defence markets.

4.1 Design systems and modelling

Advanced design methods, analysis tools and improved modellingcapability will enable further improvements in many areas withgreater emphasis on even more sophisticated computational modellingduring the product design and development stages, enabling expensiveengine and rig tests to be minimised and ultimately eliminated.

Real time modelling of the complete engine or system isexpected to be the next step delivering both product advances andimprovements in the efficiency of engineering processes. Holisticmodelling will allow interactions or ‘knock on effects’ and risksto be properly assessed and trade studies carried out early in theproduct design phase. Importantly this would enable designers tounderstand the life-cycle cost and time-scale effects of proposeddesign changes, thus improving time to market and reducingproduct cost whilst improving performance. Recent work in theareas of design for manufacture (DFM) and design for assembly(DFA), involving multi-disciplinary teams that cover the entiredesign, manufacture and assembly process are already makingprogress in this area.

4.2 Materials

Materials for gas turbines are a key technology and advances will needto satisfy both enhanced functional requirements and lower manufac-turing costs, a theme that runs throughout all advances in technology.Modelling capability is also critical here, particularly in the new envi-ronment of ‘power by the hour’ contracts for both civil and militaryproducts. These contracts place greater emphasis on understanding therelationship between operational or duty cycle and the associatedstresses and temperatures. This is leading to more accurate predictivemodels that are able to accurately calculate component lives and hencebetter quantify and minimise associated costs.

Another exciting area for the future is a family of technologiesknown as ‘smart technologies’ including shape memory aqlloys(SMA). These materials open up new prospects for both reliabilityand performance improvements. Shape memory alloys will enablea component to alter and indeed optimise its shape according to itsenvironment. One application in the area of noise reduction isdiscussed in Section 4.8. Ultimately, many key engine componentsmight optimise themselves by adapting their shape for variousoperating conditions as opposed to being compromised by off-design operation. Research is currently underway in a number of

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Figure 23. Future power for UAVs.

Figure 24. Future power for high Mach vehicles.

areas in both aircraft and engine design. Figure 25 highlights somepotential applications in aero engine design, while Fig. 26 depicts avery long-term NASA study into a ‘morphing aircraft concept’. Thiswould use SMAs and embedded sensors in order to achieve newlevels of aerodynamics and control.

4.3 Fans and compressors

With the trend for high by-pass ratio turbofans set to continue in thecivil sector, the titanium wide chord fan blade will remain a keycomponent. The focus of activity in the future will be on furtherimproved efficiency and reduced noise as is already evident in thelower tip speed swept fan design of the Trent 900.

For military fans pressure ratios will continue to increase in order toenhance thrust/weight ratio and eliminate the need for reheat. Thenumber of stages will be held to a minimum through improved aero-dynamics and higher tip speeds made possible by better materials andmechanical constructions. Work already underway for the LiftFan®and main propulsion fan for JSF using the wide chord SPF/DB bladeand blisk technology will pioneer this technological area.

Further improvements in fan design will result from aerodynami-cally modelling the whole flow regime from the free stream ahead of

the intake to the exhaust nozzle enabling the fan design, with its inletand outlet systems, to be optimised throughout the flight regime.

Longer-term fan technologies may come through fibre reinforce-ment in the form of silicon fibre reinforced titanium providingincreases in strength of around 50% and greater stiffness overconventional titanium blades. This greater strength could allow theblade chord to be increased further allowing fewer blades to be usedwith improved performance and reduced cost. Currently the costassociated with the manufacturing process is inhibiting development.

In the area of blade containment, future advances in mechanicalmodelling will improve fan blade-off and containment analysisleading to weight reduction. Developments in resistance to FOD(and the detection and inspection of any resulting damage) will alsobe important as a means of avoiding blade failures, especially insingle engine military aircraft.

The performance of multistage compressors in both civil and mili-tary engines is being improved by using state-of-the-art CFD (computa-tional fluid dynamics) flow solvers and ‘optimisers’ (see Fig. 28). Byautomating the design process, the optimum design is achieved in theminimum time possible. Recent examples have yielded six foldimprovements in the time taken to optimise tip clearances.

Although compressor aerodynamic efficiency can be improvedfurther, more significant improvements are likely to be realised

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Figure 25. Potential ‘smart’ applications in engine design.

Figure 26. Smart applications — NASA morphing aircraft concept.

Figure 27. Fan technology.

Figure 28. Compressor technology (multistage CFD).

through increased stage loading to reduce parts count and the use ofbladed disks or blisks to reduce weight, initially using monolithic tita-nium and nickel materials. However, further weight reduction willresult from material advances, including the application of titaniumaluminides (with their inherent low densities) to blades and stators.

Blisks will ultimately be replaced by ‘blings’ or bladed rings whichuse MMC (metal matrix composites) to provide a 70% weight savingand a cost benefit over a conventional design (see Fig. 29). The blingeliminates the bore of the conventional disk by using a fibre reinforcedring to bear the hoop stresses and is likely to be used initially in mili-tary products followed by civil products at a later date.

Titanium, whilst an ideal compressor material in many respects, issusceptible to ferocious fires when rubbing under high temperatures.For this reason it is only used for compressor blades below a limitingoperating temperature determined by fire resistance rather thanstrength. However research work is currently underway to develop aform of non-burn titanium (BuRTi), to counter this effect, whichwould enable titanium blades to replace steel or nickel blades in therear compressor stages thereby further saving weight.

4.4 CombustionCombustion technology continues to receive much attention as thefocus falls on delivering solutions to the emissions challenges forcivil engines and high temperature rise for military engines. Whilstthe two requirements are different in nature, the solutions areremarkably similar. In the civil engine the fuel is burnt as weak aspossible to reduce emissions whilst preserving other operating char-acteristics such as weak extinction and relight boundaries. In themilitary engine, whilst the combustion chamber burns more richly atmax power, it is exposed to the same weak extinction and relightproblems when the engine is throttled back.

Past combustor technology developments have virtually elimi-nated emissions associated with smoke, unburned hydrocarbons andcarbon monoxide. However, NOx formation remains a more difficultproblem as the high pressure and temperature conditions favouredfor its formation are exactly the conditions needed to improveoverall engine efficiency, fuel burn and CO2 generation.

Building on the success of the Trent (Phase V) combustion system,studies initially moved on to examining double annular configura-tions to deliver staged combustion, which accommodates theconflicting requirements of both high power and low power operationby using two separate combustors. However the extra cooling airrequired by these concepts and other compromises, meant that thetheoretical benefits were not fully realised in practice. A new

approach is now being pioneered in ANTLE featuring a simple low-cost single-annular system with fuel staging within a single injector(see Fig. 30). This offers significant benefits over the double annulardesign in terms of cost, weight and reliability, while delivering betteremissions. In this design, the volume of air going into the primaryzone is large thereby reducing the peak temperature during theburning process thus significantly reducing NOx. This system aims toreduce NOx by 50% and if successful, will be applied across the widerange of civil aerospace products in the fullness of time.

In the longer-term and in line with the accomplishment of theACARE goals of an 80% reduction in NOx by 2020, combustionsystems will have to proceed towards a premixed system similar tothat used in land-based gaseous applications. Lean pre-vapourised,pre-mixed concepts using liquid fuel are being researched in order todevelop these systems but major issues remain surrounding auto-ignition and flashback. These issues need to be overcome beforethey are likely to have a practical application in aerospace.

As well as advances in combustion modelling capability, hightemperature materials provide the key to future improvements incombustion design and performance (see Fig. 31). Materials temper-ature capability and wall cooling are vital in realising good combus-tion performance, as air used for cooling is not normally available

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Figure 29. Blisk and bling technology. Figure 30. Forcing down emissions through innovation (ANTLEcombustor).

Figure 31. Combustion system modelling.

for the burning process. Lamilloy material, which incorporates aform of transpiration cooling, is already used in some engines forcombustor walls and will be developed further to meet the stringentrequirements of the F136 engine. It will be more widely deployed asgreater confidence is gained with respect to its integrity.

In the longer-term the use of ceramics or CMCs (ceramic matrixcomposites) offer the potential for significant temperature increaseswith minimal cooling. However questions over life, strength, fibrecapability and fabrication must first be answered. Steps towards thisare already well underway in the shape of technology demonstrationprogrammes. If successful, ceramic combustors may provide therealisation of eliminating cooling requirements thus leading to emis-sions improvements.

4.5 TurbinesTurbines have always provided numerous challenges to the designerand remain an area where further developments are critical toimproving overall engine performance. There need to be advances incomponent efficiencies, temperature capability and cooling airconsumption, particularly the latter, so that cycle advances are noteroded by the use of additional cooling air.

Improvements in aerothermal methods and design capabilityfocusing on the analysis of unsteady phenomena, including detailedfilm cooling analysis for turbine aero and life optimisation, willprovide efficiency benefits. This will build on recent developmentsmade through applying advanced ‘3-D end-wall profiling’ tech-niques to the latest turbine designs in the civil products. Furtherbenefits will come through employing variable turbine geometry andcontra-rotation systems within future engines, a configuration thatwill be demonstrated for the first time on the ANTLE programme. Acontra-rotating, statorless system as used in the GE F136 engine, hasbenefits in improving efficiency and reducing cooling flow in certaintypes of engine architecture.

Developments in high temperature materials are inextricably linkedto advances in turbine designs (see Fig. 32). Following the develop-ment of three generations of single crystal material, we have seenrapid developments in the use of thermal barrier coatings to increaseturbine temperature capability. Further developments are now likely tocome through optimisation of the blade alloy and TBCs (thermalbarrier coatings) as a single system. By improving the TBC itself,achieved by the inclusion of heavy elements providing better thermalproperties (referred to as coloured TBCs), and through optimising theinteraction of the TBC with the elements of the blade base alloy,significant temperature advances should be possible.

In the longer-term ceramics could find their way into turbines,offering increased temperature capability and reduced cooling require-ments, providing the problem of fracture toughness can be overcome.HP NGVs(nozzle guide vanes) are likely to provide the initial oppor-tunity for ceramics with the ultimate challenge obviously lying in therealisation of an uncooled HP turbine rotor blade (see Fig. 33).

4.6 Controls technologyThe control system is now fundamental to delivering the perfor-mance and reliability expected of current and future aero engines butit is growing in sophistication, complexity and cost. A reduction inthe level of complexity, leading to reduced costs (although clearlynot at the expense of safety, reliability or performance) is a target forthe future of control systems, with lessons being read across fromthe mass produced yet reliable systems of the automotive sector.

FADEC (full authority digital engine control) is commonplace onmodern aero-engines, complimenting the ‘fly-by-wire’ advances inaircraft controls. The architectures and technologies of these systemswill continue to be closely linked, yet the requirement to interfacewith a large number of sensors and actuators around the engine makethe level of standardisation being sought in IMA (integrated modularavionics) architectures more difficult to realise. So, particularly onlarge civil engines, the central microprocessor controlling dumbactuators around the engine (controlling variable compressor andturbine geometry for example) will give way to a ‘distributed controlsystem’. Intelligent actuators will interface directly with digital databuses, allowing reduction in wiring complexity and weight, whilesimplifying maintenance. This also allows the level of sophisticationand reporting capability of the associated BITE (built in test equip-ment) to be enhanced. Systems become more fault tolerant, furthereasing the logistics issues of provisioning spares. Thus the controlsystem becomes both easier to manage in its own right and cangreatly contribute to the information being gathered on its own andthe engine’s state of health.

4.7 Predictive maintenance and engine health monitoring A growing area of sophistication in new aero-engine designs and onethat is set to expand further in capability is that of EHM (enginehealth monitoring) (see Fig. 34). Although often closely linked tocontrols technologies, these systems are by definition somethingdifferent. Predictive maintenance, leading to major reliabilityimprovements, are requirements that are needed by both operatorsand service providers alike.

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Figure 32. HTDU (high temperature demonstration unit).

Figure 33. Turbine cooling.

The Trent 900 will make a significant step towards advancedEHM through its QUOTE system. This system provides the capa-bility to analyse engine behaviour on-line, based on inputs from anumber of engine sensors. Using stored data it is then able to detectabnormal engine behaviour, thus highlighting potential componentfailures before they happen. Future advances in such systems willcome from employing an increased number of more sophisticatedsensors (including material ‘inserts’ for example) and improvedanalytical engine models, allowing more sophisticated on-lineanalysis of overall engine behaviour. The stored criteria, which arethen modified as the system learns from experience, are used todiagnose abnormal engine behaviour, predict forthcoming failures,and communicate this information in a timely manner to reduceoperational disruption and enable maintenance to be scheduled asappropriate. The Internet provides the medium for conveying thisinformation off aircraft, and on-line product support is already beingpractised in many key areas.

Further improvements in diagnostics could come through whole-engine modelling advances, integrating with the engine health moni-toring tools. Advances in understanding the material propertiesaffecting component lives and the failure mechanisms themselveswill also lead to more sophisticated failure prediction criteria and soprovide reliability improvements as advanced systems are devel-oped. Interestingly, these advances will become increasingly drivenby the military sector, as they will prove essential in effectiveunmanned aircraft operation.

4.8 Installations and noise technologies

In the civil context, noise and installation aerodynamics will beparticularly important as by-pass ratios are increased to reduce jetvelocity and improve bare engine fuel consumption. Today theAirbus A380 aircraft carries a small fuel burn penalty in order tomeet noise targets because the additional installed drag and weightassociated with its large diameter nacelle more than offset the funda-mental benefit of its high bypass ratio. Avoidance of this penaltyrequires a different approach to engine installations involving weightreduction using even lighter fan systems and LP turbines, a newapproach to thrust reversing and possibly laminar flow nacelles. Inthe longer-term more novel installations such as the blended bodywing aircraft open up new possibilities.

In parallel with this approach, noise reduction technologies willplay a vital role. The SILENCE(R) programme, previouslymentioned, demonstrates a number of key noise technologies aimedat reducing both fan and jet noise. The negatively scarfed intake

system, which directs the noise upwards and hence minimises thenoise observed on the ground is one such technology. Also beingdemonstrated on SILENCE(R) is a reduced speed swept fan design.The swept design is developed specifically to reduce fan rotor noise,which together with the low noise OGV (outlet guide vane) and ESS(engine section stator) designs and advanced acoustic lining, aim tomake significant reductions in fan noise, see Fig. 35.

The QTD project, already mentioned, used a Boeing 777 aircraftwith modified Trent 800 engines to evaluate a serrated nozzle andadvanced acoustic linings (known as Amax) in the intake of thenacelle. These make further reductions in both jet and fan noiserespectively. The measured result was a reduction in both fan and jetnoise of 4db and 13db respectively, albeit with a small performancepenalty, Fig. 36.

Further refinement of nozzle serrations could be achieved bydesigning them to adapt to the different flight regimes to allow noiseto be minimized during take-off and efficiency maximized at cruise.To achieve this result, the serrations could be stowed at cruise orchange their shape according to the surrounding air temperature byusing SMAs — part of the family of ‘smart technologies’ describedin Section 4.2. In order to further reduce fan and turbine noise,several concepts using active control techniques will become avail-able to reduce the tonal content of turbo-machinery noise.

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Figure 34. Engine health monitoring. Figure 35. SILENCE(R) — Noise technology demonstrator.

Figure 36. QTD — reducing engine noise.

Noise at source can be reduced in several different ways. Anemerging technology is that of flow management on fan and turbineblades to reduce noise. Ideas that are being evaluated include bladetrailing edge treatments and flow devices on the blade surface tomodify wake development and hence rotor/stator interaction noise.Alternatively, sensors and actuators can be built into the intake, toproduce sound waves that cancel the noise signature produced bythe fan. The control technology for this is understood today, butfurther development is required to produce sensors and actuatorsthat are cost-effective and lightweight enough to make such asystem viable.

All of the above ideas become easier to implement if a goodunderstanding of noise source generation exists. Modern CFD toolsnow offer great potential in providing this understanding and forsubsequently optimising designs to reduce noise. Eventually thisanalysis capability will include near field to far field effects so thatgreater use can be made of indoor testing on conventional test beds.

4.9 More electric technologies

Both the civil and defence aerospace sectors are demandingincreased levels of electrical power. This is driven by the need forincreased functionality, reliability, lower weight and cost, whilstreplacing mechanical complexity with elegant electrical solutions.Particular requirements in the civil sector are driven by the demandfor increased passenger comfort and facilities, while militaryaircraft demand increased electrical requirements for the movetoward network-centric systems, weapons and surveillance equip-ment, particularly in the growing unmanned sector.

The MEE (more electric engine) follows on directly from themore electric advances of the ANTLE programme and is expectedto deliver step changes in functionality and reliability, whilstachieving reductions in cost and weight. Reliant upon close engineand airframe integration, these improvements will enable thereplacement of traditional modern engine/aircraft systems (thattoday are individually optimised) with globally optimised electricalsystems. An electrically powered ECS (environmental controlsystem), for example, is particularly attractive as it also providesimprovements in fuel burn, while eliminating potential cabin airquality problems.

The next step in this evolution at an engine level would be toreplace conventional lubrication systems with oil-less, AMBs, ulti-mately leading to the deletion of the entire oil system and gearbox.A generator, mounted directly on the fan shaft, would deliver power

to the airframe systems and all flight control actuators would alsobe electric.

However, developments in this field rely heavily on corre-sponding advancements in electric and magnetic materials in orderto realise the required temperature capability and low weightdesigns. Particular developments in insulation technology, perma-nent magnet materials and power electronics are fundamentalrequirements to achieving the more electric engine and more elec-tric aircraft. These areas are currently being addressed throughextensive research and development activity.

5.0 SUMMARYFollowing a century of progress, the aerospace industry continues topose new challenges, no more so than for the engine designer whohas contributed significantly to progress to date. The civil sector isat a crossroads, on the verge of a new era of sustainable growth,where environmental and social factors will take on increasedimportance compared with the more familiar commercial drivers ofchange. In this era we will see radically new aircraft designsrequiring the development of new and, in some ways novel, propul-sion systems. The defence sector also enters a new era, where anemergence of unmanned aircraft will open up new opportunities inthe changing face of modern conflicts. Again cost, together withperformance and capability, are set to retain a powerful role indictating the future direction.

The propulsion system requirements in the two sectors aremoving in quite different directions although the underpinning tech-nologies remain largely common, providing further opportunitiesfor technology transfer. Materials advances, more electric technolo-gies, sophisticated design methods, environmentally cleaner andquieter technologies and the intelligent engine will all influencefurther developments of the gas turbine, which is positioned tocontinue to dominate the propulsion requirements of future aircraftin both sectors as today no viable alternative exists. Developmentswill be increasingly delivered at a global system level, throughglobal companies and through industry collaboration.

Although no prediction can be certain and there will undoubtedlybe unforeseen factors that will also influence the direction of theaerospace markets, we can be quite certain that the second centuryof aviation will provide as many challenges and as much fascinationas the first.

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Figure 37. Electric engine concepts. Figure 38. Simplifying the aircraft and engine interface (more electricaircraft).

ACKNOWLEDGEMENTSI would like to thank Rolls-Royce for permission to present andpublish this paper. I would also like to thank Will Erith who hashelped with writing the text and with the illustrations.

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