The moderngas turbine

how it works and how it’s built

© 2000 Rolls-Royce plc

TS22760

January 2000

Rolls-Royce plc

PO Box 31

Derby DE24 8BJ

England

www. rolls-royce.com

3

Jet propulsion

Though the principle of jet propulsion

had been demonstrated in the first

century AD by Hero of Alexandria,

the practical gas turbine, popularly

called the ‘jet’ engine, had to wait for

the 1930s and the genius of Frank

Whittle. Originally designed for aircraft

propulsion, the gas turbine is now

adapted for marine propulsion, power

generation and gas & oil pumping,

all benefiting from its inherent high

power and small size.

How does a gas turbine work?

Like the motor car engine, the gas

turbine is an internal combustion

engine. In both, air is compressed,

fuel added, the mixture ignited, and

the rapid expansion of the resultant

hot gas produces the power. However,

combustion in a motor car engine is

intermittent and the expanding gas

produces shaft power through a piston

and crank, whereas in a jet engine

combustion is continuous and its power

results from expanding gas being

forced out of the rear of the engine.

One of Newton’s principles is that to

every action there is an equal and

opposite reaction. The expanding gas

flow is an action which creates a reaction

of equivalent force. This force - thrust -

is transmitted through the engine to

the aircraft, propelling it through the

air. An inflated party balloon can

demonstrate this: with the neck closed,

the air inside presses equally in all

directions; open the neck, and the air is

released as an action, creating a

reaction on the opposite surface of the

balloon which drives it forward.

Thrust is measured in pounds force (lbf ),

kilograms force (kgf ), or Newtons (N).

The gas turbine - how it works

54

There are four main types of gas

turbine: turbojet, turbofan,

turboprop, and turboshaft.

The turbojet and turbofan are both

reaction engines which derive power

from the reaction to the exhaust

stream.

The turboprop and turboshaft operate

differently by using the exhaust stream

to power an additional turbine which

drives a propeller or output shaft.

Turbojet

The original concept, the turbojet, is the

simplest form of gas turbine and relies

on the high velocity hot gas exhaust to

provide the thrust. Its disadvantages

today are its relatively high noise levels

and fuel consumption.

Examples: Olympus 593 in the Concorde

supersonic airliner; Viper in a variety of

military aircraft.

Turbofan

In the turbofan or ‘bypass’ engine the

partly compressed airflow is divided,

some into a central part - the gas

generator or core - and some into a

surrounding casing - the bypass duct.

The gas generator acts like a turbojet

whilst the larger mass of bypass air is

accelerated relatively slowly down the

duct to provide ‘cold stream’ thrust.

The cold and hot streams mix to give

better propulsive efficiency, lower noise

levels, and improved fuel consumption.

In the high bypass ratio turbofan, as much

as seven or eight times as much air

bypasses the core as passes through it.

It achieves around 75% of its thrust

from the bypass air and is ideal for

subsonic transport aircraft. A low bypass

ratio turbofan, where the air is divided

approximately equally between the gas

generator and the bypass duct, is well-

suited to high-speed military usage.

Examples: in commercial usage - Trent in the

Airbus A330; RB211-535 in the Boeing 757:

in military usage - RB199 in the

Tornado and EJ200 in the

Typhoon.The vectored-

thrust Pegasus in the

Harrier is a variation of

the turbofan.

Turboprop

As its name implies, a turboprop uses

a propeller to transmit the power it

produces. The propeller is driven

through a reduction gear by a shaft

from a power turbine, using the gas

energy which would provide the thrust

in a turbojet.

Turboprop power is measured in total

equivalent horsepower (tehp), or

kilowatts (kW).

Examples: Dart in BAe748 and Fokker F27;

AE2100 in the Saab 2000.

Turboshaft

The turboshaft is a powerplant for

helicopters. Like the turboprop,

it also uses a power turbine and

gearbox, though in this case the

power is transmitted to the helicopter’s

rotor system. This type of engine is

also used in industrial and marine

applications.

Turboshaft power is measured in shaft

horse power (shp), or kilowatts (kW).

Examples: Allison 250 in the Jet Ranger;

RTM322 in the Merlin.

Main types of gas turbineLayout of the gas turbine

The compressor

The compressor draws air into the

engine, pressurises it, and delivers it to

the combustion chamber. It is driven

from the turbine by a shaft.

There are two types of compressor:

the centrifugal flow impeller type, as

used in Whittle’s designs, and the axial

flow type which has several stages of

alternate rotating and stationary

aerofoil blades. The rotor blades are

mounted on a drum and the stator

vanes in the compressor casing.

Axial compressors can achieve

compression ratios in excess of 40:1.

At full power the blades of the Trent

892 compressors rotate at 1000mph

(1600kph) and take in 2600lb (1200kg)

of air per second.

The combustion system

The combustion chamber receives air

from the compressor which mixes with

fuel sprayed from nozzles in the front

of the chamber. The mixture is burned

at temperatures up to 20000C to

generate the maximum possible heat

energy. The burning process is initiated

by igniter plugs, isolated after start-up,

and remains continuous until the fuel

supply is shut off.

At cruise the Trent 892 uses about 1000

gallons (4500 litres) of fuel per hour.

The turbine

Each turbine consists of one or more

stages of alternate stationary and

rotating aerofoil-section blades.

The rotating turbine blades are carried

on discs, which are connected by a

shaft to the compressor. The stationary

blades - nozzle guide vanes - are

housed in the turbine casing.

The turbine extracts energy from the

hot exhaust gases to drive the

compressor.

In the Trent 892, the first turbine has to

be air-cooled as it operates in a gas

stream temperature of around 1500ºC

- hotter than the melting point of the

blade material. The total power

generated by the engine is 250,000hp

(200,000kW) and the exhaust gases exit

at 1000mph (1600kph)

The gas turbine has three main sections:

the compressors, the combustion

system, and the turbines.

Airintake

Hot-streamthrust

Combustionsystem Turbines and exhaustCompressors

C o l d - s t r e a m t h r u s t

C o l d - s t r e a m t h r u s t

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Following is a simplified look at some

of the features of a large turbofan

engine and the way it is put together.

The example chosen is the Trent 800,

which powers the Boeing 777 airliner.

The RB211 family, to which the Trent

belongs, features modular construction

in its design. That is to say, it is built up

from a number of large assemblies

known as ‘modules’, each of which has

its individual identity and service

history.

The Trent 800 is built-up from eight

basic modules:

Module 01 LP compressor rotor,

(to which the fan

blades are fitted)

Module 02 IP compressor

Module 03 Intermediate case

Module 04 HP system

Module 05 IP turbine

Module 06 High speed gearbox

Module 07 LP compressor case

Module 08 LP turbine

Modules 01, 02, 03, 04, 05 and 08 form

the core engine which can be replaced

as a complete assembly.

Various items and systems are then

added to complete the engine.

Significant benefits are gained from

modular construction:

� Decreased turn-round time for repair

� Lower overall maintenance costs

� Reduced spare engine holdings

� Maximum life achieved from each

module

� Savings on transport costs

� Ease of transport and storage

� On-wing test capability after any

module change

Engine arrangement

Trent modular breakdown

Variations & additions

Module 01

Module 02

Module 03

Module 04

Module 05 Module 08

Module 06

Module 07

Vectored thrust

Thrust vectoring was developed for

short take-off and vertical landing

(STOVL) aircraft. The Pegasus turbofan,

power for the Harrier ‘jump-jet’, is the

unique example of this concept.

The engine has four linked, swivelling

nozzles to direct the gas stream from

vertically downward for upward lift,

through an arc to horizontally rearward

for conventional forward flight. The

bypass air is discharged through the

two front nozzles and the hot gas

exhaust through the two rear nozzles.

Afterburning

Afterburning, or reheat, increases

engine thrust for short periods to

improve aircraft take-off, climb and

combat performance. Because the fuel

in a gas turbine burns in an excess of

air, sufficient oxygen remains in the

exhaust to support further combustion,

particularly in a turbofan. By injecting

and burning additional fuel in the jet

pipe, the exhaust velocity and

consequently the engine thrust is

increased

Reverse thrust

Thrust reversal, found on most

commercial jets, is used to provide a

braking force to add to the effect of

an aircraft’s wheel-brakes when the

aircraft lands.

It is particularly useful in adverse

weather conditions and is achieved by

mechanically deflecting some or all of

the exhaust stream of a gas turbine in

a forward direction.

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Module 03 Intermediate Case

Module 04 High Pressure (HP) System

Major components of this module are the disc and the

shaft, which are connected by a curvic coupling.

At the rear end of the shaft is a helical spline which

transmits the drive from the LP turbine shaft.

Dove-tail slots machined in the disc locate the 26

wide-chord, hollow fan blades (which are not

modular parts).

Module 01 Low Pressure (LP) Compressor Rotor

This module comprises three casings bolted together, and the

compressor rotor.

First is the front bearing housing (FBH) which locates the roller

bearings for both LP & IP compressors and a row each of fixed

and variable vanes to control the airflow into the compressor.

Next is the variable stator vane (VSV) casing containing a

further two stages of variable vanes. The third casing is an

assembly of bolted rings carrying six stages of fixed stator

vanes.

The compressor rotor is a welded drum assembly with eight

stages of blades. A stub-shaft at the front of the drum locates in

the IP roller bearing, and a curvic coupling extends from the

stage 6 disc to connect to the IP shaft.

Module 02 Intermediate Pressure (IP) Compressor

The outer casing of this module links the intermediate case

module and the IP turbine module. It contains the inner casing,

HP compressor, combustion system and turbine, and also

provides locations for fuel spray nozzles and igniter plugs.

The compressor casing is made up of six flanged rings which,

when bolted together, form slots into which the stator vanes are

fitted during the bolting sequence.

Two welded disc assemblies, bolted together, form the drum of the

six-stage compressor into which the rotor blades are fitted.

The first stage disc incorporates a curvic coupling to connect to the

thrust bearing in the intermediate casing and the sixth disc extends

into a drive cone and mini-disc which bolts to the turbine shaft.

Within the inner case are the HP compressor outlet guide vanes,the annular

combustion liner,and the air-cooled nozzle guide vane (NGV) assembly

which directs the hot expanding gases into the turbine blades.

The turbine is a single-stage unit.The hollow air-cooled blades

are attached to the disc by fir-tree roots. A forward extension of

the disc connects the turbine to the compressor mini-disc, and a

stub-shaft for the turbine bearing is bolted to a rearward extension.

The intermediate case is a fabricated, spoked structure

housing the thrust bearings for all shafts, and forming the air

path between the IP and HP compressors. Externally it carries

the A-frame support arms which brace the fan case

(Module 7), and its internal hollow struts provide access for

services such as oil tubes, cooling air, and the radial

drive-shaft to the accessory gearbox.

Disc - carries the fan

blades

Drive shaft

Curvic coupling

Rear part

of spinner

Front bearing housing

Variable stator vane case

Compressor

case and

vanes

Fabricated case

Bevel drive to

external gearbox

Thrust bearings

for all three shafts

Compressor rotor

Combustion

system

Turbine

Compressor

Inner casing Outer casing

1110

The gearbox is mounted on the lower part of the LP

compressor case. It is driven by a radial and angled drive shaft

system from the intermediate case module and provides the

drive for the accessories mounted on its front and rear faces.

These accessories include oil, fuel and hydraulic pumps,

electrical generators and the starter motor. During the

starting sequence the air-driven starter motor drives back

through the gearbox and drive shaft to provide initial

rotation of the HP system.

Module 06 High-Speed (HS) Gearbox

Largest of the modules, this is an assembly of forward and rear

cylindrical casings and the fan outlet guide vane (OGV) ring.

It is often referred to as the fan-case.

The forward casing has to be able to contain a fan blade

should it become detached during engine running. To meet

this eventuality the specially-machined lightweight aluminium

casing structure is wrapped with energy-absorbing Kevlar

fabric. Internally, the casing contains acoustic panels, ice impact

panels and the fan track lining.

The titanium rear casing carries the fancase-mounted

accessories and also contains acoustic linings.

At their inner ends, the fan OGVs are secured to the torsion

ring which locates the IP compressor module, whilst the outer

ends are bolted to the front mounting ring. This assembly is

welded to the titanium rear casing and bolted to the front

casing.

Module 07 Low Pressure (LP) Compressor Case

Following the HP module is the IP turbine module.

This assembly comprises turbine casing, blades and vanes,

the turbine disc and shaft, and roller bearings for the HP and

IP shafts.

In the casing are mounted the IP NGVs and the first-stage LP

NGVs. Both sets of NGVs are hollow, the IP vanes enclosing

service tubes and struts for supporting the bearing housing,

and the LP1 vanes containing thermo-couples for

measurement of gas temperature.

The IP vanes are air cooled.

Fir-tree roots locate the IP blades in the disc of the

single-stage turbine.

Bolted to the turbine disc is the turbine shaft which has helical

splines at its forward end. These connect to the IP compressor

stub-shaft via the thrust bearings in the intermediate case.

Module 05 Intermediate Pressure (IP) Turbine

Rearmost of the core engine modules is that of the LP turbine

containing blades, vanes, discs, and the shaft and its roller

bearing.

The casing is made up of the turbine case and the turbine rear

bearing support assembly. The latter also incorporates

location bosses for the rear engine mount.

Five bolted discs form the turbine rotor, with all blades

located in fir-tree slots.

The turbine shaft is bolted to the rotor via a curvic coupling

and connects to the LP compressor shaft through helical

splines at its forward end. The shaft extends rearwards into

the roller bearing.

Service tubes, electrical harnesses, and pressure sensing for

the engine pressure ratio system are routed through

the hollow vanes of the turbine bearing support assembly.

Module 08 Low Pressure (LP) Turbine

Nozzle guide vanes

Shaft

LP turbine

first-stage

NGVs

Turbine

HP & IP turbine

roller bearings

and housing

Five-stage turbine

Turbine case

Shaft

Turbine rear bearing support assembly

Engine rear mount attachment

Cast casing

FRONT

REAR

Faces for

accessory

mounting

Drive

inputDrive

input

Front casing

Rear

casing

Engine front mount

attachment

Fan outlet guide

vane ring