SPECIAL FEATURE: FLIGHT

www.iop.org/journals/physed

Gas turbine technologyPeter Spittle

Rolls-Royce plc, Ansty, Coventry CV7 9JR, UK

E-mail: peter.spittle@rolls-royce.com

AbstractGas turbine engines power most commercial flights operating today. Yetmany people are ignorant of the cutting-edge technologies used in thecreation and operation of these engines. This article explains some of theprinciples involved with emphasis on the selection of materials for fanblades and turbine blades, which have to operate reliably in exceedinglyhostile environments.

(Some figures in this article are in colour only in the electronic version)

Simple theory, outstanding performanceThe number of commercial flights has escalatedrapidly since the middle of the last century:currently it is believed that as many as threemillion people may be in the air at any givenmoment. Clearly flight is a technology thatwe have all embraced and are happy to exploit.The ease and efficiency of transport over largedistances, combined with the unrivalled levels ofsafety within the industry when compared with anyother mode of transport, give air travel a distinctadvantage. On the safety front, recent US figuresactually show that you are twice as likely to dieas a result of an incident involving an animal-drawn carriage as opposed to an aircraft. Thesefacts—and perhaps the exhilaration experiencedduring flight—mean that it has become commonpractice for us to climb aboard 100 tonnes ofcomplex machinery and intricate electronics inan understandable effort to experience farawaydestinations.

However, once seated in a cosy window oraisle seat, already wondering what delights willlie below the foil wrappings of the in-flight meal,it surprises me how little thought people generallygive to the technology that they are relying on.There seems to be little attention given to the30 000 components hanging together under the

wing that propel us upwards and onwards, andto the fact that a decent proportion of thesecomponents are operating above their meltingpoint (I based this on personal experience ofpassengers I have sat next to and chatted with, andperhaps left slightly scared and bewildered).

When boarding the aeroplane the only partof the engine clearly visible is the fan set. I amsure most people climb aboard without a glance inthe engine’s direction, yet the fan blades are notcomponents that should be disregarded in blaséfashion. At full speed the 3 m diameter arrayrotates at 3300 rpm, and if a blade were releasedit would have enough kinetic energy to launch asmall car over a seven-storey building. I haveflown eight times within the last year and on eachand every occasion I marvel at the performanceof the engines and airframes. It is amazing to methat the sensation of acceleration we experience,as the 100 tonnes of aeroplane reaches almost300 km h−1 in under 20 seconds, is generated byonly two engines that are simply throwing hot airbackwards at high speed.

During these moments at take-off I usuallyexperience a strange mental confrontation: likemany people there is always a small sense ofunease and a heightened awareness of strangenoises and cabin movement. However, it isabout then that I remind myself of the sound

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Gas turbine technology

Fan – low pressure compressor

–50 to 40 °C

8 stage intermediate pressure compressor

50 – 300 °C

6 stage high pressure compressor

300 – 650 °C

Combustion chamber 2000 °C

Turbine: - HP 1 stage - 1500 °C IP 1stage - 1200 °C

LP 5 stage - 900 °C

Cold bypass air

Figure 1. The principle of gas turbine operation: suck, squeeze, bang, blow.

engineering and unshakeable physical principlesthat support this undertaking. Having workedwithin Rolls-Royce, and fortunately spent timewithin various departments (stress, thermals,materials and ‘lifing’1, design etc), I have gainedan appreciation of the engineering effort, interms of performance, safety and reliability, thatsupports the nine tonnes of rotating machinerybelow the wing, and am as a result alwaysconfident and happy to be onboard.

As suggested above, the fact that we now takeflight for granted has only come to pass due tothe continued drive of aero-engine manufacturersto improve engine performance. Over time therehave been very clear trends in improved engineefficiency, power and safety, all achieved at arelatively low cost to you the traveller. Withinthe following paragraphs I hope to briefly describeto you the workings of this simple machine, anddiscuss some of the technology and ingenuity thatsuccessfully drive it millions of miles a year.

Suck, squeeze, bang, blowThe principle of gas turbine operation is relativelysimple (figure 1):

• Suck: air is sucked in through the fan at thefront of the engine

1 The term ‘lifing’ refers to the analysis of a material’s fatigueproperties, and the determination of a cyclic ‘life’ (cycles tofailure) when it is subjected to a given alternating stress level.

• Squeeze: the air is then squeezed to manytimes atmospheric pressure

• Bang: within the combustion chamber fuelis mixed with the air and is ignited

• Blow: the hot air expands and is blown outof the rear of the engine.

Sir Frank Whittle designed and built the firstjet engine in 1941. Because of their majoradvantages in terms of power, efficiency at highspeed and simplicity, jet engines rapidly becamethe preferred option for air travel. Rolls-Royce hashad many firsts within the commercial gas turbineindustry since Whittle’s innovation, a trend that wemaintain to this day. For example, Rolls-Royceproduced the Dart, the first gas turbine in civilflight, and the Conway, the first Bypass engine.Our current products include the Trent seriesof gas turbines—engines that incorporate manycutting-edge technologies and that have becomesynonymous with reliability and efficiency. TheRB211 family of engines (the RB211-524G is usedto power the Boeing 747, and is the precursor tothe Trent family) has accumulated over 75 millionhours of flying, a figure that equates to more than46 billion km, or flying to the sun and back over150 times. The Trent has helped us to securea large proportion of new engine sales, and assuch Rolls-Royce is now recognized as the secondbiggest aero-engine manufacturer, and currentlylists 38 of the top 50 airlines amongst its customers.

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HHPP ssyysstteemm 66 ccoommpprreessssoorr ssttaaggeess

11 ttuurrbbiinnee ssttaaggee >>1100 000000 rrppmm

IIPP ssyysstteemm 88 ccoommpprreessssoorr ssttaaggeess

11 ttuurrbbiinnee ssttaaggee >>77550000 rrppmm

LLPP ssyysstteemm 11 ffaann ssttaaggee

55 ttuurrbbiinnee ssttaaggeess >>33000000 rrppmm

1 2

3

Figure 2. Rolls-Royce three-shaft engine configuration.

The Trent engine is a three-shaft engine(figures 2 and 3), a system that allows the designerto more efficiently match the thermodynamic andaerodynamic requirements throughout the engine.This is a significant step from the two-shaft enginesoriginally employed by Rolls-Royce and which arestill designed by our competitors. From figure 2one can see that there are three systems:

• The low pressure system (one compressorstage, the fan, driven by five turbine stages).

• The intermediate pressure system (eightcompressor stages driven by one turbinestage).

• The high-pressure system (six compressorstages driven by a single turbine stage).

Within the gas turbines generally seen onAirbuses and Boeings, the expanding hot gasesprovide only a small fraction of the engine thrust.In figures 2 and 3 one can see that the gas is passedthrough various stages of blading. Firstly, air thatis pulled in by the fan through the core of theengine is compressed by the intermediate and highpressure compressor blades. After fuel injectionand ignition the expanding gas exits the enginethrough various sets of turbine blades. Theseblades are turned (like the sails on a windmill) and,in turn, they rotate the fan and compressor bladesat the front via shafts that pass through the centreof the engine. As such, it is the turbine blades that

Figure 3. The Trent 500 engine, four of which areused on the Airbus A340.

turn the fan, which pulls the air through to developthe thrust. I mentioned the ‘core’ of the enginepreviously to emphasize the distinction betweenthis region and the ‘bypass’ air (figure 1). In realityonly about a quarter of the air pulled through thefan enters the core of the engine: the remainingcold air is thrown backwards, around the outsideof the core to develop the majority of the engine’sthrust (see figure 1).

The choice between a ‘bypass’ and a ‘turbojet’(in which 100% of thrust is developed throughthe core of the engine) configuration is mainlya function of desired engine efficiency andmanoeuvrability. High ‘bypass’ ratio enginesare desired for commercial travel to maximizeefficiency (see box): high jet velocity and

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High by-pass or turbojet?To understand this statement we may considerthe flux of momentum entering and leaving theengine, and a general equation that defines itsefficiency.

Flux of momentum entering the engine

= mairV

Flux of momentum leaving the engine

= (mair + mf)Vjet

where mair is the mass flow of air, V is thevelocity of air entering the engine, mf is themass flow of fuel and Vj is the velocity of airleaving the engine.

Thus the net thrust, FN, that is available inflight is given by the difference between the twomomentum fluxes, that is

FN = (mair + mf)Vjet − mairV.

This equation tells us that for a high net thrustthere must be either a high jet velocity, Vj, or alarge mass flow, mair.

Propulsive efficiency compares the rate ofwork done on the aircraft to the rate of kineticenergy increase of the flow through the engine.It may be approximated for the typical casewhen the mass flow (of fuel) is much smallerthan that of the air, by [1]

ηp = 2V

V + Vj.

From this second equation we can see thatpropulsive efficiency (and therefore net engineefficiency) is maximized if the jet velocity,Vj, is minimized. Therefore if we wish tomaximize efficiency (i.e. low Vj) but maintainthrust (FN) we must maximize the mass flowof air through the engine. Therefore wewant high bypass engines for commercial andfreight air travel.

manoeuvrability are not required because a largeproportion of time is spent at a single altitudeand speed. Military applications, however, willuse turbojet type engines, such as the Olympus(Concorde) and the Pegasus (Harrier), because

they desire many rapid changes in power, attitudeand speed and care a little less about propulsiveefficiency. However, as for many things in life,the decision on engine design is not necessarily ascut and dried as suggested above, but is a trade-offbetween many interacting parameters.

Although large bypass engines are efficientthere are some tangible negatives. They are heavybits of kit that provide considerable drag, and costan appreciable amount of money to purchase andmaintain. Furthermore, not all military aircraftscream around the skies at maximum power allday long: many military jets that spend significantamounts of time cruising around, needing alittle extra only for the occasional chase, areusually equipped with low bypass engines. Fuelconsumption is important to the military, not forcost, but for reasons of range and weapon loadcapability.

As a metallurgist I am concerned withoptimizing materials to meet the various stringentrequirements of the engine. Clearly the conditionsvary dramatically throughout the engine, butthere are at least a couple of underlying drivingforces when choosing the material to do thejob: weight and cost. It would be ideal toconstruct everything from a cheap material suchas steel. However, with a density of 7.6 g cm−3,employing steel, as a compressor disc materialfor example, would cost appreciably more in

Figure 4. A Trent 800 fan set comprising 25 blades.The fan has a diameter of approximately 3 m and themass flow of air through the engine is approximately72 tonnes/min, or 1.2 tonnes/second.

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1. 2. 3.

4. 5. 6.

Figure 5. Trent fan blade production. Left: a six-step breakdown of wide-chord fan blade production for theTrent engines using the DB-SPF technique. The fan blades for the Trent engines are manufactured from threesheets of titanium (1). The two outer sheets will form the aerodynamic surfaces of the blade and the thicker rootsection. The middle, thinner, sheet will form the ‘Warren–Girder’ structure internally, providing the bladestiffness and impact resistance required. Once the pack of three plates has been formed a masking pattern ispainted onto the internal faces of the outer plates (2). The plates are then re-stacked, with a small tube attached tothe end, welded together around the edge, evacuated and heated above 950 ◦C (3). At this temperature diffusionoccurs across the metallic surfaces in intimate contact within the pack and at its edges. Thus, after heat treatmentthe three plates are now a single unit. The pack is then shaped into its approximate aerofoil morphology (4),before it is inflated (5). To enable inflation the component is re-heated above 900 ◦C and argon gas is injected, atpressure, through the tube. After inflation the aerofoil cross section seen in images 5 and 6 is created. The girderconfiguration is developed because diffusion bonding is unable to occur across the ‘masked’ regions, and theseunbonded areas are expanded during argon injection.

Right: stage 4 being completed. The operators are removing the shaped, diffusion-bonded pack from afurnace at ≈950 ◦C.

terms of fuel consumption than the current optionand would impart significantly higher stresses onitself and the surrounding structures. Converselymagnesium- or aluminium-based alloys mightbe an ideal option with regard to weight, butgiven their relatively low tensile properties anda propensity to become liquid around 660 ◦C (atemperature encountered within the high pressurecompressor section), they are not necessarilythe metals of choice. The following sectionsdetail two components that are key to efficientengine operation, and demonstrate the effortsmade to maximize performance through material,chemistry, design and manufacture.

The fan bladeThe fan blade sees temperatures from ambient to−50 ◦C (at cruise altitude), rotation at 3300 rpmwith a tip speed of 1730 km h−1, and a centripetalforce through its root equivalent to 900 kN (afigure usually quoted by the airline pamphletsas equivalent to the weight of 13 African bullelephants). Furthermore, this component mustlast for 80 million km (10 000 flights), displaying

sufficient flexibility and damage tolerance tocontinue uninhibited operation within wind, rain,snow and the occasional bird impact. All ofthese parameters are met by a single piece ofmetal about 10 kg in mass, approximately 1.0 mhigh by 0.4 m wide (figure 4). In this instancea titanium alloy is used—titanium 6/4 (titanium+ 6% aluminium + 4% vanadium)—a materialthat meets the mechanical requirements stipulatedabove, with the minimum mass possible (densityabout 4.5 g cm−3).

In addition, this material has the specialproperties necessary to incorporate the requiredcomplexity during the manufacture of thecomponent. In an effort to achieve theblade stiffness required at minimum mass Rolls-Royce has developed a unique hollow fan blade(figures 4 and 5). This structure proffers asignificant weight saving (50%) over the solidblade alternatives, reducing the engine’s specificfuel consumption, thereby saving the airlinesmoney. The internal structure of this blade canonly be manufactured, with sufficient integrity,using the diffusion bonding, super-plastic forming

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TrentRB211-524G/H

RB211-535E4RB211-624B4

RB211-524G2

RB211-22CSpey

Conway

AvonDart

DerwentWhittle

600

800

1000

1200

1400

1600

1930 1940 1950 1960 1970 1980 1990 2000

Tu

rbin

een

try

tem

pera

ture

( °C

)

Year

Figure 6. Development of turbine entry temperature (TET) with time.

(DB-SPF) technique (figure 5). Production ofthese components is an exacting and onerous task,one that Rolls-Royce has met by constructing apurpose-built facility for the job, a facility thatmaintains a cleanliness regime beyond that of theelectronics/microchip industry.

Cleanliness during production is key becauseof the ‘stress raising’ effects that impurities andforeign particulates would have within the internalstructure of the blade. Such point defects couldinitiate a fatigue crack that may propagate tofailure under service conditions. When weconsider the kinetic energy of a fan blade releasedat maximum revolution (remember the scenarioof a small car launched very high. . . ) it can beseen that the expense of such a clean facility isa necessary one. On this front, however, we canbe reassured that extensive testing and modellinghas been completed to demonstrate that, if suchan event were to occur, the engine shrouds andcasings would not be compromised and no highenergy debris would exit the engine. Indeed theaviation authorities demand that a complete engine‘blade off’ test is completed for all new engineprojects—necessary, but a multi-million poundengine is fit for little else afterwards. To completethis test a small explosive charge is attached to theroot of one fan blade within the set. Then, oncethe engine has been powered up to full speed, thecharge is detonated and the blade released. Highspeed cameras film the event from a variety of

positions and help to demonstrate that, apart from alarge flame and some low energy material, nothingis released from the engine that might compromisean airframe’s integrity.

Turbine bladesA second key area of expertise for Rolls-Royceis within its turbine blade manufacturing facility.As stated earlier, there has been a clear trend inimproved engine performance with time. In aneffort to improve efficiencies, weight must be shedfrom the construction and operating temperaturemust be increased.

From the graph in figure 6 it can be seenthat within 60 years the temperature of the gas

Temperature

Spe

cifi

c st

reng

th Nickel

alloy

Steel Aluminium

alloy

Titanium alloy

Figure 7. Specific strength versus temperature for avariety of common aero-engine materials.

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(a) (b)

Multipass cooling air

+ thermal barrier

coating

Multipass cooling air

≈ 11

5 m

m

Figure 8. Gas temperature: (a) 1425 ◦C, (b) >1550 ◦C.

leaving the combustion chamber and passing overthe high pressure turbine blades, i.e. the turbineentry temperature (TET), has risen by nearly800 ◦C. To accommodate this advance significantchanges have been made to the component’sdesign, material and structure. Because of thehigh temperatures involved turbine blades aremanufactured from nickel-based alloys. Figure 7highlights how well nickel alloys maintain theirmechanical properties with increasing temperaturewhen compared with the alternatives. Thiscurve takes account of the material’s density, andindicates that even though nickel is practicallytwice as dense as titanium (about 8.4 g cm−3—contrary to the requirements to reduce engineweight) it is the only material that retains sufficientintegrity at high temperature. Indeed standardtitanium alloys must be avoided towards the hotend of the aero-engine, not only because oftheir diminishing properties, but also because attemperatures in excess of 600 ◦C, and underfriction, they can rapidly ignite and burn aggres-sively, covering the rearward stages in moltenmetal.

The turbine blades see the most aggressiveconditions within the engine, but as with allcomponents, they must readily deliver efficientperformance, whilst incurring minimal overhauland repair costs. The blades that Rolls-Roycecurrently manufactures can withstand a tempera-ture of about 1550 ◦C, rotate at 10 000 rpm, remove

560 kW each from the gas stream (slightly betterthan your average Ford Focus) and last up to5 000 000 flying miles.

Figure 8 demonstrates a couple of thetechnological advances that Rolls-Royce hasemployed in its successful turbine blade designand material definition, thereby allowing it toemploy a TET above the melting point of thealloy (TET > 1500 ◦C, alloy melting pointabout 1350 ◦C). The key advances have beenthe manufacture of single-crystal blades, withinternal cooling channels, and, latterly, thermalbarrier coatings. Cooler air (air at 700 ◦C) isbled off from the compressor and passed throughthe turbine blades. Small laser-drilled holes inthe surface of the blade allow the cooler air toflow over the working surfaces, protecting themfrom the hot gas stream. In a later developmentceramic materials have been deposited onto theblade surfaces, further protecting them from theaggressive environment and allowing yet higherTETs to be achieved.

At high temperatures the major componentfailure mechanism is creep, i.e. over time underhigh temperatures and loads the blade will deform,lengthen and rupture. The creep mechanisms andelongation to failure are caused by atom migrationand diffusion. A key path for this atom movementis along grain boundaries (these are areas of latticediscontinuities and therefore a relative increase inspace). Thus, by removing this easy diffusion path

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Aluminium

Titanium

Steel

Nickel

Figure 9. Engine materials.

for the atoms by developing single-crystal blades,we increase the creep strength of the material.This is a process requiring exact casting conditionsand ingenious mould designs. The mould designmust ensure that only a single grain, in a preferredcrystallographic orientation, is allowed to grow.

OverviewOne can see that throughout this conceptuallysimple machine the engine manufacturer hasworked strenuously to maximize the potentialof each component. The result is a machinecombining a fantastic array of materials thatgenerates the maximum performance at minimumweight with the current technology. Figure 9 is ageneral arrangement of a Trent engine that depictsthe range of materials employed.

Materials technology continues to drivefurther advancements in aero-engine performance.There are additional innovations that cannotbe included in current designs because of theprohibitive cost, both internally and to theairlines. Major improvements will probably resultfrom material development, towards lighter andstronger materials. Innovations such as metalmatrix composites and engineering ceramics area real source of interest and continued researchand development. In addition, research into burn-resistant titanium alloys, allowing them to be used

further into the hot end of the engine, and nickelalloy compositions will further refine the engines’performance. During your next flight it is worthspending a moment to appreciate the cutting-edgetechnology that is sending you skywards, and tofeel safe and contented that it is a technologybacked by excellent know-how and millions ofmiles of experience.

AcknowledgmentsD Rugg, P Withey, C Hanley and C Woodward.

Received 4 June 2003PII: S0031-9120(03)64371-6

Reference[1] Cumpsty N 2003 Jet Propulsion (Cambridge:

Cambridge University Press)

Peter Spittle is a Materials Technologistat Rolls-Royce. He joined as a graduatetrainee in 1999 with a degree inMaterials Science and Technology fromthe University of Birmingham. Sincecompleting his training period he hasworked as a Materials Technologistwithin the aero-engine business and as aCore Metallurgist within Rolls-Royce’snaval marine business, supporting thedesign and manufacture of reactors forthe UK’s nuclear submarine fleet.

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