& Research Agency
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
Let me first explain how aeronautics research is organised in the
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
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 technology, particularly at the hot end, is the key to most
advances in engine capability. It naturally occupies a prominent place in
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
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
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.
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.
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
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
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.
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
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
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
In the face of scarce research resources, collaboration is essential
for survival. We have already shown in the
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
Thank you very much.
Copyright www.stratisc.org - 2005 - Conception - Bertrand Degoy, Alain De Neve, Joseph Henrotin