LA  RECHERCHE  BRITANNIQUE

 

John Chisholm

Chief Executive Defence

Evaluation & Research Agency

            Introduction

 

            Good afternoon ladies and gentlemen. It was a pleasure and an honour to be invited by so distinguished and successful a company as SNECMA to participate in this conference to celebrate their anniversary. May I congratulate SNECMA on reaching this 50th anniversary landmark. I cannot resist mentioning that last year my organisation registered its 75th year in aeronautics research. But since SNECMA can point to antecedents going back 100 years, maybe we should all take pleasure in the continuity of European aeronautics technology and devote ourselves to equal success in the future.

 

            My task is to say something of aero-engine research in the UK and how we are shaping our programmes to meet the challenges of the 21st Century. The UK has a comprehensive programme and I will only have time to touch on a few major thrusts.

 

            Let me first explain how aeronautics research is organised in the UK . All the government aeronautics establishments with which you may be familiar in the UK , at Farnborough, Pyestock, and Bedford, now belong to the Defence Evaluation and Research Agency of which I am Chief Executive. I shall refer to it as DERA. The main research arm of DERA is the Defence Research Agency, which I shall refer to as DRA. Most of the work I shall describe is carried out in DRA and in our partners. Our emphasis is very strongly on partnership. Nowhere is research partnership more strongly developed than in the aero-engine field. The DRA, in consultation with industry, puts together research programmes and proposes them for funding by the Department of Trade and Industry and/or the Ministry of Defence. A large part of the successful programmes are carried out in DRA’s own laboratories. But a substantial proportion - 50% in the case of aero-engines - is passed on to industry and the universities in extramural contracts. The programmes contracted to industry are normally jointly funded, with the companies supplying matching funding. Both DRA and the company have a direct stake in the work, and the result is a jointly planned and coordinated national effort. Finally, we are becoming increasingly involved in collaborative programmes supported by the European Community. These programmes follow the same principles of partnership and shared funding and they neatly fit the UK way of working.

 

            So, what are the major challenges facing us ? Historically, better performance - more thrust and less fuel consumption - and reduced weight, particularly on the military side, have been the main drivers for advancing engine technology. These demands will always be with us. Indeed, as the big civil turbofans get larger, engine weight is becoming a significant preoccupation in the civil arena also.

            But, in today’s difficult economic climate, « affordability » has become the definitive watchword. Firstly, advanced technology must deliver lower development costs and shorter timescales. The reasons are evident. In the military sphere development costs now have to be spread over fewer and smaller projects. The civil engine maker must be ready to react to a rapidly changing market situation and cannot be locked into long development programmes.

 

            Secondly, affordability means engines must be cheaper to buy and cheaper to maintain, with longer component lives and lower spare parts costs. These cost challenges are at least equal to performance and weight in importance. They could become the dominant themes in future research thinking.

 

            Our other important challenge is environmental - the problems of noise and emissions. The emissions issue is relatively new, but it is having an increasing impact on civil engine design and many of the technical problems remain to be solved. I will return to this later. But first let me say something about performance and affordability.

 

 

            Materials

 

            Materials technology, particularly at the hot end, is the key to most advances in engine capability. It naturally occupies a prominent place in the UK engine research programme. This chart shows how nickel alloys for turbine blades have improved over the years. The advance has occurred as we have moved from conventionally cast polycrystalline materials through to blades cast as a single crystal.

 

            Hitherto, we have tended to exploit these alloys almost entirely to increase engine power, moving up the temperature limit boundary as the technology has improved. But this approach is not without penalty. The latest high temperature materials involve difficult alloying and casting processes. Unit costs are becoming very high - perhaps an order of magnitude greater than current production materials. There are also other disadvantages, like poorer impact resistance and higher density. The UK will continue to work on maximum temperature alloys. But with the accent on cost reduction, we have equal if not greater interest in alloys optimised for long life and reduced manufacture costs, rather than the maximum possible temperature.

 

            The traditional metal alloys - titanium as well as nickel - still have untapped potential and research on them will continue. But much of our emphasis is now on the new material systems depicted on this chart. They offer either much greater temperature capability, like the ceramics, or much greater specific strength, which is very attractive for weight reasons. Whether manufacture costs can also be reduced is another question !

 

            Ceramic composites are beginning to find application for small non-structural components like nozzle petals. But finding the right combination of strength, reliability and (above all) cost is more difficult. We have a long term research programme dedicated to this.

 

            Nearer term, the real break-through is likely to come from the new metals - Titanium-aluminide and especially Titanium metal matrix composites. In this area, we are sharing in a collaborative effort with colleaques in France and Italy . Metal matrix composites and stationary parts, low pressure turbine rotors and stators, and many other ancillary components. What has to be done is to complete our understanding of the material properties, and refine and optimise the material structure and processing route. And, above all, we have to ensure that the research leads to a practicable and economic manufacture route. The principle of concurrent engineering starts right in the research laboraty !

 

            By way of example current alloy technology already allows rotating components to be fabricated as integrally bladed discs, or « bliscs », with some saving in weight. The much higher specific strength of the metal matrix composites will allow the disc to be reduced to a simple integrally bladed ring, or « bling ». Apart from major weight benefits to the component itself, there will be « knock-on » gains from lighter bearing housings, containment casings and so on. If Metal Matrix Composites can be successfully perfected and utilised fully in the engine, the overall weight savings will be dramatic.

 

            Materials research like this still depends to a large extent on experimentally-based metallurgy, and intelligent cut-and-try process development. But the extension to mechanical design rests heavily on advanced computational stressing methods. The continued development and validation of these methods for a wider range of problems is an equally important research topic. This chart illustrates the use of finite element analysis for turbine blade design. The method takes account of all thermal and mechanical loads, and can cope with anisotropic materials.

 

            Here we see two analyses with the crystal « grown » along the blade in two different orientations. The red area in the picture on the left indicates a large region of high strain. The right hand picture shows how reorientating the crystal reduces the strain to an acceptable level, which eliminates the problem. The important point is that Computational analysis not only aids the geometric design of the component, it also provides a valuable guide to the manufacture process. This can greatly reduce development time, cost and technical risk - essentially by achieving « Right First Time » designs.

 

 

            Turbomachinery

 

            The topic of advanced computation takes me into turbomachinery aerodynamics. In recent years aerodynamic design, particularly in compressors, has been revolutionised by the use of computational fluid dynamics - cfd. This picture compares 1965 transonic blading, designed by early 2D throughflow methods, with a research machine designed jointly by DRA and Rolls-Royce using a modern quasi-3D method. It is due to go on test later this year. The more complex shape and much higher twist of the 1995 blade are apparent. What cfd is allowing us to is to design the blade shapes to match much more closely what we want the air to do. The result is reduced losses and much greater work output.

            Current UK effort in cfd is devoted to developing methods which represent the full 3d characteristics of the flow and incorporate viscous loss effects. Experimental validation of each new development is vital. This slide shows a result from one such exercise. The flow visualisation was done using multicolored particles in an oil suspension, which was painted onto the blade. The result is qualitive, but gives a good guide as to how well the flow field is predicted by cfd. The technique supplements quantitative measurements using a range of advanced methods, such as laser anemometry, holography, and so on.

 

            Over the past 20 years, achievable work per stage has roughly doubled ; and there has been a significant gain in efficiency levels, despite the much higher loadings. As the red band indicates, we believe that continued research will bring considerable further rewards. Just as importantly from the costs point of view, the power and accuracy of modern cfd methods is enabling compressor designs to be finalised much more quickly. Development testing is still required. But it is now possible to optimise a design in just two or three builds, rather than the six or more needed previously. This is again leading to « Right first time » design.

 

            Here we see graphically the advantages of high stage loadings - smaller engines and far fewer parts. All four engines are scaled to the same thrust level, so the size comparison is real. The EJ200 is today’s technology. The conceptual « 2015 engine » incorporates technology Rolls-Royce expect to demonstrate during the next few years. All things being equal, the reduction in parts makes each succeeding generation of engine cheaper to make and cheaper to maintain. Given our emphasis on costs, this is an important advantage.

 

            I have added thrust/weight ratio numbers on the right hand side, because this is a commonly-used measure of technology level for military engines. The goals for the 2015 engine require a considerable further step. Weight reduction will come from the turbomachinery developments I have just been talking about, together with application of the new lightweight materials. But thrust levels will also go up, which means hotter engines.

 

            This leads to the subject of turbine blade cooling. As I remarked earlier, metal temperatures are also rising as the technology improves, but to nothing like the same extent. The gap must be closed by the blade cooling system and the job is getting harder by the year !

 

            The left hand picture here gives a measure of the problem. It shows a modern Rolls-Royce Nozzle Guide Vane and the complexity needed to achieve adequate cooling. The manufacture cost of such designs is of course very high. The research problem is simply stated. How do we minimise the heat transfer from the hot gas to the blade, and at the same time, maximise the heat transfer from the blade to the cooling air ? Progress in both is essential, not just to keep up with the growth in cooling demand, but if possible to simplify and cheapen vane and blade manufacture.

 

            Rolls-Royce are working on the internal heat transfer problem. At the DRA we are working on external heat transfer. The right hand pictures show components of the special testing rig used, which has both stationary vanes and rotating blades, instrumented to measure heat transfer. The aim is to measure and understand the relationship between the heat transfer and the blade external aerodynamics, including for example rotor/stator interactions. This particular exercise is a good example of collaboration. The blades were designed by Rolls-Royce ; the research is being conducted at the DRA ; the experimental techniques were developed by Oxford University ; and much of the work contributes to a joint European programme.

 

 

            The environment

 

            In the interests of time I must pass over the important topic of engine controls and I will close by returning to the environmental challenge. The UK has long maintained a vigorous programme to understand, predict and reduce engine noise levels. This large anechoic chamber at DRA was built originally to support the Concorde programme. It has been used continuously ever since, for exhaust noise research covering many different civil engine concepts. Again, there is no time to describe this now, but the chamber remains an important and unique facility, not just in the UK, but in Europe as a whole.

 

            Turning to the probably more challenging question of emissions, there are two problems. Hitherto, the main impact from the engine design point of view has come fron limits on Nox, unburned hydrocarbons, and carbon monoxide and smoke, at and around airports. These have led to the current international rules for emissions certification of new engines. More stringent limits are to be imposed from 1996 onwards.

 

            More recently, widespread public concern about ozone holes, greenhouse gases and global warming has turned the spotlight onto what happens at altitude. Understanding of atmospheric chemistry is very incomplete, so we do not actually know whether aircraft emissions are having any significant effect on either the ozone layer or greenhouse gases. Developing the necessary atmospheric models is outside the competence of the propulsion engineer. What he can do is help by providing quantitative data on emissions production. Potentially all engine emissions, including water vapour and solid particles, could be factors.

 

            To address this issue several European Community programmes have been initiated, in which DERA and Rolls-Royce are playing a full part. For Aeronox, now completed, we have worked with Rolls-Royce to measure Nox emissions from an RB211 development engine in our Altitude Test Facility at Pyestock. The aim here was to establish a broad Nox emissions database, over a range of simulated flight conditions, well correlated with engine operating condition.

 

            Aerotrace is a follow-up programme aimed at measuring and assessing a wider range of trace emissions. The DRA is contributing a combustion research rig investigation into the characteristics of solid particle emissions. The purpose is to correlate particulate output against detailed fuel/air mixing conditions in the combustor.

 

            Whatever we conclude, the pressures will remain to reduce emissions as much as possible, not just around airports, but over the whole flight regime. Combustor technology now in development is expected to deliver a significant reduction in Nox - this is likely to be needed to meet the new 1996 limits. But there is active discussion about much greater reductions still, especially for a future supersonic transport, whose higher altitude operation will add to the concern over atmospheric effects.

 

            UK combustion research is responding to these pressures. There is still a need to understand and improve the characteristics of the current technology single dome combustors, as well as emerging technologies like the double dome stage combustor shown in the middle here. But to meet the more stringent emissions targets, novel and complicated combustor concepts like that on the right are likely to be needed. The main difficulty will be in achieving low emissions levels without compromising other essential characteristics, like high turn-down ratio and altitude relight capability. The basic aim is to reduce the zones of maximum temperature, shown here in yellow, which is where NOX is mainly produced.

 

            To make progress, advanced computational methods, capable of modelling the complexities of recirculating and reacting flows, are essential. An important research aim is therefore the improvement and validation of suitable cfd codes. The supporting experiments must be carried out at realistic temperatures and pressures, with realistic geometries. The DRA uses a specialist rig for this purpose and we have an on-going programme to develop and use the sophisticated instrumentation and analysis techniques neeled.

 

 

            Conclusions

 

            This has been far too rapid an overview of a large and diverse research programme but my time is up. I have said that we see engine performance continuing to be important, but that the UK spotlight is turning strongly to cost and weight reduction. At the same time we must also resolve the difficult environmental issues.

 

            The research principles that underpin these goals involve major emphasis on the continued development of computation techniques, supported by sophisticated experimentation. Concurrent engineering starts in the laboratory and Right First Time design methods must be achieved in all areas of the engine in order to achieve the performance gains at an affordable price.

 

            In the face of scarce research resources, collaboration is essential for survival. We have already shown in the UK how powerful collaboration can be as a multiplier on research programmes. To advance the technology sufficient to realise the potential in the aeronautics industry into the next century I am sure we are going to have to become equally adept at research collaboration on an international scale.

 

            Ladies and Gentlemen, how could I end with a more fitting message at the international conference to celebrate SNECMA’s 50th anniversary. May I wish our hosts an equally glorious next 50 years and couple with that the hope that future success will be born from the fruits of present day collaboration.

 

            Thank you very much.

 

 

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