turbofan engine

5/20/2018 turbofan engine

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Turbofan

Turbofan Engine

A turbofanis a type of aircraft engineconsisting of a ducted fanwhich is

powered by a gas turbine. Part of the airstream from the ducted fan passes

through the gas turbine core, providing oxygen to burn fuel to create

power. However, most of the air flow bypasses the engine core, and is

accelerated by the fan blades in much the same manner as apropeller.

The combination of thrust produced from the fan and the exhaust fromthe core is a more efficient process than otherjet enginedesigns, resulting

in a comparatively low specific fuel consumption.[!

A few designs wor" slightly differently and have the fan blades as a radial

extension of an aft#mounted low#pressure turbine unit.

Turbofans have a net exhaust speed that is much lower than a turbojet.

This ma"es them much more efficient at subsonic speeds than turbojets,

and somewhat more efficient at supersonic speeds up to

roughly $ach.%, but have also been found to be efficient when used

with continuous afterburnerat $ach & and above.

All of the jet engines used in currently manufactured commercial jet

aircraft are turbofans. They are used commercially mainly because they

are highly efficient and relatively 'uiet in operation. Turbofans are also

used in many military jet aircraft.
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Turbofan

Turbofan

(chematic diagram of a high#bypass turbofan engine

A turbofanis a type of aircraft engineconsisting of a ducted fanwhich is

powered by a gas turbine. Part of the airstream from the ducted fan passes

through the gas turbine core, providing oxygen to burn fuel to create

power. However, most of the air flow bypasses the engine core, and is

accelerated by the fan blades in much the same manner as apropeller.

The combination of thrust produced from the fan and the exhaust fromthe core is a more efficient process than otherjet enginedesigns, resulting

in a comparatively low specific fuel consumption.[!

A few designs wor" slightly differently and have the fan blades as a radial

extension of an aft#mounted low#pressure turbine unit.

Turbofans have a net exhaust speed that is much lower than a turbojet.

This ma"es them much more efficient at subsonic speeds than turbojets,

and somewhat more efficient at supersonic speeds up to

roughly $ach.%, but have also been found to be efficient when usedwith continuous afterburnerat $ach & and above.

)
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Turbofan

All of the jet engines used in currently manufactured commercial jet

aircraft are turbofans. They are used commercially mainly because they

are highly efficient and relatively 'uiet in operation. Turbofans are alsoused in many military jet aircraft.

To move an airplanethrough the air, thrustis generated by some "ind

ofpropulsion system. $ost modern airliners useturbofanengines

because of their high thrust and good fuel efficiency.*n this page, we

will discuss some of the fundamentals of turbofan engines.

A turbofan engine is the most modern variation of the basic gasturbineengine. As with other gas turbines, there is a core engine,

whosepartsand operation are discussed on a separate page. +n the

turbofan engine, the core engine is surrounded by a fan in the front and an

additional turbine at the rear. The fan and fan turbine are composed of

many blades, li"e the corecompressorand core turbine,and are connected

to an additional shaft. All of this additional turbomachineryis colored

green on the schematic.As with the core compressor and turbine, some of

the fan blades turn with the shaft and some blades remain stationary. The

fan shaft passes through the core shaft for mechanical reasons. This type

of arrangement is called a two spoolengine one -spool- for the fan, one

-spool- for the core. (ome advanced engines have additional spools for

even higher efficiency.

How does a turbofan engine work?

The incoming air is captured by the engine inlet. (ome of the incoming

air passes through the fan and continues on into the core compressor and

then theburner,where it is mixed with fuel and combustionoccurs. The

hot exhaust passes through the core and fan turbines and then out

the no//le,as in a basic turbojet.The rest of the incoming air passes

through the fan and bypasses, or goes around the engine, just li"e the air

through apropeller.The air that goes through the fan has a velocity that is

slightly increased from free stream. (o a turbofan gets some of its thrust

from the core and some of its thrust from the fan. The ratio of the air that

&
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Turbofan

goes around the engine to the air that goes through the core is called

the bypass ratio.

0ecause the fuel flow rate for the core is changed only a small amount by

the addition of the fan, a turbofan generates more thrust for nearly thesame amount of fuel used by the core. This means that a turbofan is very

fuel efficient. +n fact, high bypass ratio turbofans are nearly as fuel

efficient as turboprops. 0ecause the fan is enclosed by the inlet and is

composed of many blades, it can operate efficiently at higher speeds than

a simple propeller. That is why turbofans are found on high speed

transports and propellers are used on low speed transports. 1ow bypass

ratio turbofans are still more fuel efficient than basic turbojets. $any

modern fighter planes actually use low bypass ratio turbofans e'uipped

with afterburners.They can then cruise efficiently but still have high

thrust when dogfighting. 2ven though the fighter plane can fly much

faster than the speed of sound, the air going into the engine must travel

less than the speed of sound for high efficiency. Therefore, the airplane

inlet slows the air down from supersonic speeds.

Early turbofans

2arly turbojet engines were very fuel#inefficient, as their overall pressureratio and turbine inlet temperature were severely limited by the

technology available at the time. The very first running turbofan was the

3erman4aimler#0en/ 40 %56a"a 67#665 which was operated on its

testbed on April , 78&. The engine was abandoned later while the war

went on and problems could not be solved. The 0ritish

wartime $etrovic" 9.)axial flow jet was given a fan to create the first

0ritish turbofan.

+mproved materials, and the introduction of twin compressors such as inthe Pratt : ;hitney high

velocity exhaust better suited to supersonic flight.

The original low-bypass turbofanengines were designed to improve

propulsive efficiency by reducing the exhaust velocity to a value closer to

that of the aircraft. The ?olls#?oyce =onway, the first production

turbofan, had a bypass ratio of 6.&, similar to the modern 3eneral 2lectric9868fighter engine. =ivilian turbofan engines of the 7%6s, such as

8
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Turbofan

the Pratt : ;hitney


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Turbofan

introduced if the turbine inlet temperature is allowed to increase, to

compensate for a correspondingly smaller core flow. +mprovements in

turbine cooling>material technology would facilitate the use of a higher

turbine inlet temperature, despite increases in cooling air temperature,

resulting from a probable increase in overall pressure ratio.

2fficiently done, the resulting turbofan would probably operate at a

higher no//le pressure ratio than the turbojet, but with a lower exhaust

temperature to retain net thrust. (ince the temperature rise across the

whole engine inta"e to no//le would be lower, the dry power fuel flow

would also be reduced, resulting in a better specific fuel

consumption(9=.

A few low#bypass ratio military turbofans e.g. 9868 have Dariable +nlet3uide Danes, with piano#style hinges, to direct air onto the first rotor

stage. This improves the fan surge margin see compressor map in the

mid#flow range. The swing wing 9#achieved a very high range >

payload capability by pioneering the use of this engine, and it was also

the heart of the famous 9#8 Tomcatair superiority fighter which used the

same engines in a smaller, more agile airframe to achieve efficient cruise

and $ach ) speed.

Afterburning turbofans

(ince the 756s, mostjet fighterengines have been low>medium bypass

turbofans with a mixed exhaust, afterburnerand variable area final

no//le. An afterburner is a combustor located downstream of the turbine

blades and directly upstream of the no//le, which burns fuel from

afterburner#specific fuel injectors. ;hen lit, prodigious amounts of fuel

are burnt in the afterburner, raising the temperature of exhaust gases by a

significant amount, resulting in a higher exhaust velocity>engine specific

thrust. The variable geometry no//le must open to a larger throat area to

accommodate the extra volume flow when the afterburner is lit.

Afterburning is often designed to give a significant thrust boost for ta"e

off, transonic acceleration and combat maneuvers, but is very fuel

intensive. =onse'uently afterburning can only be used for short portions

of a mission. However the $ach & (?#5was designed for continuous

operation and to be efficient with the afterburner lit.

Enli"e the main combustor, where the downstream turbine blades must

not be damaged by high temperatures, an afterburner can operate at the

%
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Turbofan

ideal maximum stoichiometric temperature i.e. about

)66F>&5@6?a>&&)69. At a fixed total applied fuelGair ratio, the total

fuel flow for a given fan airflow will be the same, regardless of the dry

specific thrust of the engine. However, a high specific thrust turbofan

will, by definition, have a higher no//le pressure ratio, resulting in a

higher afterburning net thrust and, therefore, a lower afterburning specific

fuel consumption. However, high specific thrust engines have a high dry

(9=. The situation is reversed for a medium specific thrust afterburning

turbofanG i.e. poor afterburning (9=>good dry (9=. The former engine is

suitable for a combat aircraft which must remain in afterburning combat

for a fairly long period, but only has to fight fairly close to the airfield

e.g. cross border s"irmishes The latter engine is better for an aircraft

that has to fly some distance, or loiter for a long time, before going intocombat. However, the pilot can only afford to stay in afterburning for a

short period, before his>her fuel reserves become dangerously low.

$odern low#bypass military turbofans include the Pratt : ;hitney 97,

the 2urojet 2Turbomeca

Adourafterburning in the (2P2=AT


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Turbofan

2. Fan

3. Low pressure compressor

4. High pressure compressor

. Com!ustion cham!er

". High pressure tur!ine#. Low pressure tur!ine

$. Core no%%le

&. Fan no%%le

(chematic diagram illustrating a )#spool, high#bypass turbofan engine

with an unmixed exhaust. Again, the fan and booster stages are driven

by the low#pressure turbine, but more stages are re'uired. A mixed

exhaust is often employed nowadays

The low specific thrust>high bypass ratio turbofans used in todays civil

jetliners and some military transport aircraft evolved from the high

specific thrust>low bypass ratio turbofans used in such aircraft bac" in the

7%6s.

1ow specific thrust is achieved by replacing the multi#stage fan with a

single stage unit. Enli"e some military engines, modern civil turbofans

do not have any stationary inlet guide vanes in front of the fan rotor. The

fan is scaled to achieve the desired net thrust.

The core or gas generator of the engine must generate sufficient =ore

Power to at least drive the fan at its design flow and pressure ratio.

Through improvements in turbine cooling>material technology, a higher

HP turbine rotor inlet temperature can be used, thus facilitating a

smaller and lighter core and potentially improving the core thermal

efficiency. ?educing the core mass flow tends to increase the load on the

1P turbine, so this unit may re'uire additional stages to reduce theaverage stage loading and to maintain 1P turbine efficiency. ?educing

core flow also increases bypass ratio G, or more, is now common.

9urther improvements in core thermal efficiency can be achieved by

raising the overall pressure ratio of the core. +mproved blade

aerodynamics reduces the number of extra compressor stages re'uired.

;ith multiple compressors i.e. 1P=, +P=, HP= dramatic increases in

overall pressure ratio have become possible. Dariable geometry

i.e. stators enable high pressure ratio compressors to wor" surge#free atall throttle settings.

@
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Turbofan

=utaway diagram of the 3eneral 2lectric =9%#% engine

The first high#bypass turbofan engine was the 3eneral 2lectric T9&7,

designed in mid 7%6s to power the 1oc"heed=# 3alaxymilitary

transport aircraft. The civil 3eneral 2lectric =9%engine used a derived

design. *ther high#bypass turbofans are the Pratt : ;hitney 32nxand

the 3P5666, produced jointly by 32 and P:;.

High#bypass turbofan engines are generally 'uieter than the earlier low

bypass ratio civil engines. This is not so much due to the higher bypass

ratio, as to the use of a low pressure ratio, single stage, fan, which

significantly reduces specific thrust and, thereby, jet velocity. The

combination of a higher overall pressure ratio and turbine inlettemperature improves thermal efficiency. This, together with a lower

specific thrust better propulsive efficiency, leads to a lower specific fuel

consumption.

9or reasons of fuel economy, and also of reduced noise, almost all of

todays jet airliners are powered by high#bypass turbofans. Although

modern combat aircraft tend to use low bypass ratio turbofans, military

transport aircraft e.g. =#5 mainly use high bypass ratio turbofans

or turboprops for fuel efficiency.

0ecause of the implied low mean jet velocity, a high bypass ratio>low

specific thrust turbofan has a high thrust lapse rate with rising flight

speed. =onse'uently the engine must be over#si/ed to give sufficient

thrust during climb>cruise at high flight speeds e.g. $ach 6.@&. 0ecause

of the high thrust lapse rate, the static i.e. $ach 6 thrust is conse'uently

relatively high. This enables heavily laden, wide body aircraft to

accelerate 'uic"ly during ta"e#off and conse'uently lift#off within a

reasonable runway length.

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Turbofan

The turbofans on twin engined airliners are further over#si/ed to cope

with losing one engine during ta"e#off, which reduces the aircrafts net

thrust by 6C. $odern twin engined airliners normally climb very

steeply immediately after ta"e#off. +f one engine is lost, the climb#out is

much shallower, but sufficient to clear obstacles in the flightpath.

The (oviet Enions engine technology was less advanced than the ;ests

and its first wide#body aircraft, the +lyushin +l#@%, was powered by low#

bypass engines. The Ia"ovlev Ia"#8), a medium#range, rear#engined

aircraft seating up to )6 passengers introduced in 7@6 was the first

(oviet aircraft to use high#bypass engines.

Turbofan configurations

Turbofan engines come in a variety of engine configurations. 9or a given

engine cycle i.e. same airflow, bypass ratio, fan pressure ratio, overall

pressure ratio and HP turbine rotor inlet temperature, the choice of

turbofan configuration has little impact upon the design point

performance e.g. net thrust, (9=, as long as overall component

performance is maintained. *ff#design performance and stability is,

however, affected by engine configuration.

As the design overall pressure ratio of an engine cycle increases, it

becomes more difficult to throttle the compression system, without

encountering an instability "nown as compressor surge. This occurs when

some of the compressor aerofoils stall li"e the wings of an aircraft

causing a violent change in the direction of the airflow. However,

compressor stall can be avoided, at throttled conditions, by progressivelyG

opening interstage>intercompressor blow#off valves inefficient

and>or

) closing variable stators within the compressor

$ost modern American civil turbofans employ a relatively high pressure

ratio High Pressure HP =ompressor, with many rows of variable stators

to control surge margin at part#throttle. +n the three#

spool ?0)>Trentthe core compression system is split into two, with the

+P compressor, which supercharges the HP compressor, being on a

different coaxial shaft and driven by a separate +P turbine. As the HP

=ompressor has a modest pressure ratio it can be throttled#bac" surge#

free, without employing variable geometry. However, because a shallow

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Turbofan

+P compressor wor"ing line is inevitable, the +P= re'uires at least one

stage of variable geometry.

[edit]Single shaft turbofan

Although far from common, the (ingle (haft Turbofan is probably thesimplest configuration, comprising a fan and high pressure compressor

driven by a single turbine unit, all on the same shaft. The (B2=$A $&,

which powers $irage fighter aircraft, is an example of a (ingle (haft

Turbofan. 4espite the simplicity of the turbomachinery configuration, the

$& re'uires a variable area mixer to facilitate part#throttle operation.

Aft fan turbofan

*ne of the earliest turbofans was a derivative of the 3eneral 2lectric


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Turbofan

$ost modern passenger and military aircraft are powered by gas

turbineengines, which are also called jet engines. There are several

different typesof gas turbine engines, but all turbine engines have

somepartsin common. All turbine engines have an inletto bring free

streamair into the engine. The inlet sits upstream of the compressorand,

while the inlet does no wor" on the flow, inlet performancehas a strong

influence on engine net thrust. As shown in the figures above, inlets come

in a variety of shapes and si/es with the specifics usually dictated by thespeed of the aircraft.

S!S"#$% $#&ETS

9or aircraft that cannot go faster than the speed of sound, li"e large

airliners, a simple, straight, short inlet wor"s 'uite well. *n a

typical subsonicinlet, the surface of the inlet from outside to inside is a

continuous smooth curve with some thic"ness from inside to outside. The

most upstream portion of the inlet is called the highlight, or the

inlet lip'A subsonic aircraft has an inlet with a relatively thic" lip.

)
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13/47

Turbofan

S(E)S"#$% $#&ETS

An inlet for a supersonicaircraft, on the other hand, has a relatively sharp

lip. The inlet lip is sharpened to minimi/e the performance losses

from shoc" wavesthat occur during supersonic flight. 9or a supersonicaircraft, the inlet must slow the flow down to subsonic speeds before the

air reaches the compressor. (ome supersonic inlets, li"e the one at the

upper right, use a central cone to shoc" the flow down to subsonic speeds.

*ther inlets, li"e the one shown at the lower left, use flat hinged plates to

generate the compression shoc"s, with the resulting inlet geometry

having a rectangular cross section. This *ariable geometryinlet is used

on the 9#8 and 9# fighter aircraft. $ore exotic inlet shapes are used on

some aircraft for a variety of reasons. The inlets of the $ach &J(?#5

aircraft are specially designed to allow cruisingflight at high speed. The

inlets of the (?#5 actually produce thrust during flight.

H+(E)S"#$% $#&ETS

+nlets for hypersonicaircraft present the ultimate design challenge.

9or ramjet#poweredaircraft, the inlet must bring the high speed external

flow down to subsonic conditions in theburner. High stagnation

temperatures are present in this speed regime and variable geometry may

not be an option for the inlet designer because of possible flow lea"s

through the hinges. 9or scramjet#poweredaircraft, the heat environmentis even worse because the flight $ach number is higher than that for a

ramjet#powered aircraft. (cramjet inlets are highly integrated with the

fuselage of the aircraft. *n the K#8&A, the inlet includes the entire lower

surface of the aircraft forward of the cowl lip. Thic", hotboundary

layersare usually present on the compression surfaces of hypersonic

inlets. The flow exiting a scramjet inlet must remain supersonic.

$#&ET E,,$%$E#%+

An inlet must operate efficiently over the entire flight envelope of the

aircraft. At very low aircraft speeds, or when just sitting on the runway,

free stream air is pulled into the engine by the compressor. +n 2ngland,

inlets are called intakeswhich is a more accurate description of their

function at low aircraft speeds. At high speeds, a good inlet will allow the

aircraft to maneuver to high angles of attac"and sideslip without

disrupting flow to the compressor. 0ecause the inlet is so important to

overall aircraft operation, it is usually designed and tested by the airframe

company, not the engine manufacturer. 0ut because inlet operation is so

important to engine performance, all engine manufacturers also employ

&
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14/47

Turbofan

inlet aerodynamicists. The amount of disruption of the flow is

characteri/ed by a numerical inlet distortion inde.. 4ifferent airframers

use different indices, but all of the indices are based on ratios of the local

variation of pressure to the average pressure at the compressor face.

The ratio of the average total pressure at the compressor face to the free

stream total pressure is called the total pressure reco*ery.

Pressure recoveryis another inlet performance indexL the higher the

value, the better the inlet. 9or hypersonic inlets the value of pressure

recovery is very low and nearly constant because of shoc" losses, so

hypersonic inlets are normally characteri/ed by their "inetic energy

efficiency. +f the airflow demanded by the engine is much less than the

airflow that can be captured by the inlet, then the difference in airflow is

spilled around the inlet. The airflow mis#match can producespillage

drag on the aircraft.



different typesof jet engines, but all jet engines have somepartsin

8
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15/47

Turbofan

common. All jet engines have a compressorto increase the pressure of the

incoming air before it enters the burner. =ompressor performancehas a

large influence on total engineperformance.

There are two main types of compressors used in modern jet enginesL

axial compressors , and centrifugal compressors.

+n the a.ial compressorthe air flows parallel to the axis of rotation. The

compressor is composed of several rows of airfoil cascades. (ome of the

rows, called rotors, are connectedto the central shaft and rotate at high

speed. *ther rows, called stators, are fixed and do not rotate. The job of

the stators is to increase pressure and "eep the flow from spiraling around

the axis by bringing the flow bac" parallel to the axis. +n the figure on the

right, we see a picture of the rotors of an axial compressor. The stators ofthis compressor are connected to the outer casing, which has been

removed and is not shown. At the upper left is a picture of a single rotor

stage for a different compressor so that you can see how the individual

blades are shaped and aligned. At the bottom of the figure is a computer

generated figure of an entire axial compressor with both rotors and

stators. The compressor is attached to a shaft which is connected to

thepower turbineon the right end of the blue shaft. Here is an animated

version of the axial compressorG
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16/47

Turbofan

How does an a.ial compressor work?

The details are 'uite complex because the blade geometries and theresulting flows are three dimensional, unsteady, and can have

important viscous and compressibilityeffects. 2ach blade on a rotor or

stator produces a pressure variation much li"e the airfoilof a

spinningpropeller.0ut unli"e a propeller blade, the blades of an axial

compressor are close to one another, which seriously alters the flow

around each blade. =ompressor blades continuously pass through the

wa"es of upstream blades that introduce unsteady flow variations.

=ompressor designers must rely on wind tunneltesting and

sophisticated computational modelsto determine the performance of an

axial compressor. The performance is characteri/ed by the pressure ratio

across the compressor %(), the rotational speed of the shaft necessary to

produce the pressure increase, and an efficiency factor that indicates how

much additional wor" is re'uired relative to an ideal compressor. There

are additional important compressor topics, li"e stall and surge, that will

be added to these pages in the future.

%
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17/47

Turbofan

$ost modern passenger and military aircraft are powered by gasturbineengines, which are also called jet engines. There are several

different typesof gas turbine engines, and all turbine engines have

somepartsin common. All turbine engines have a combustor,

or burner, in which the fuel is combined with high pressure air

andburned.The resulting high temperature exhaust gas is used to turn

thepower turbineand produce thrustwhen passed through a no//le.

0urners are also used on ramjetand scramjetpropulsion systems. The

design of ramjet and scramjet burners are slightly different than the

burners used on gas turbine engines, although the basic thermodynamicprinciplesare the same.

*n this page, we discuss the operation of a gas turbine burner. The burner

is shown in red on the computer graphic at the lower right of the figure.

The burner sits between the compressorand the power turbine. The

burner is arranged li"e anannulus, or a doughnut, as shown by the three

burner configurations at the lower left. The central shaft that connects the

turbine and compressor passes through the center hole. 0urners are made

from materials that can withstand the hightemperaturesof combustion. Aburner usually has an outer casing, shown in red, and an inner liner,

shown in orange. The liner is often perforated to enhance mixing of the

fuel and air, as shown in the photo at the upper right.

There are three main types of combustors, and all three designs are found

in modern gas turbinesG

. The burner at the left is an annularcombustor with the liner sitting

inside the outer casing which has been peeled open in the drawing.

$any modern burners have an annular design.

). The burner in the middle is an older canor tubular design. The

photo at the top left shows some actual burner cans. 2ach can has

both a liner and a casing, and the cans are arranged around the

central shaft.&. A compromise design is shown at the right. This is a can-

annulardesign, in which the casing is annular and the liner is can#

shaped. The advantage to the can#annular design is that the

individual cans are more easily designed, tested, and serviced.

5
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18/47

Turbofan

The details of mixing and burning the fuel are 'uite complex and re'uire

extensive testing for a new burner. 9or our purposes, we can consider

the burneras simply the place where combustion occurs and where the

wor"ing fluid air temperature is raised with a slight decreasein

pressure.




somepartsin common. All gas turbine engines have a powerturbinelocated downstream of theburnerto extract energy from the hot

flow and turn thecompressor. ;or"is done on the power turbine by the

hot exhaust flow from the burner.

/escription of $mages

The bottom of the figure showsG

computer drawings of a turbojetwith the location of the turbine

relative to the other engine components, on the right

@
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19/47

Turbofan

the turbine section alone with the central shaft attached to the

turbine, on the left.

+n both drawings, the turbine is magenta in color and the shaft is colored

blue. The left end of the shaft would be attached to the compressor,whichis colored cyan in the drawing on the right. Here is an animated version

of the turbine sectionG

The upper left of the figure shows an actual power turbine. The turbine,li"e the compressor, is composed of several rows of airfoil cascades.

(ome of the rows, called rotors, are connectedto the central shaft and

rotate at high speed. *ther rows, called stators, are fixed and do not

rotate. The job of the stators is to "eep the flow from spiraling around the

axis by bringing the flow bac" parallel to the axis.

4epending on the engine type,there may be multiple turbine stages

present in the engine. Turbofanand turbopropengines usually employ a

separate turbine and shaft to power the fan and gear box respectively.(uch an arrangement is termed atwo spoolengine. 9or some high

performance engines, an additional turbine and shaft is present to power

separate parts of the compressor. This arrangement produces a three

spoolengine. The power turbine shown on the upper left of the figure is

for a two spool, turbofan engine.

/esign /etails

7
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20/47

Turbofan

There are several interesting turbine design details present on this slide.

(ince the turbine extracts energy from the flow, the

pressure decreasesacross the turbine. The pressure gradient helps "eep

theboundary layerflow attached to the surface of the turbine blades.

(ince the boundary layer is less li"ely to separate on a turbine blade thanon a compressor blade, the pressure drop across a single turbine stage can

be much greater than the pressure increase across a corresponding

compressor stage. A single turbine stage can be used to dri*e multiple

compressor stages'0ecause of the high pressure change across the

turbine, the flow tends to lea" around the tips of the blades. The tips of

turbine blades are often connected by a thin metal band to "eep the flow

from lea"ing, as shown in the picture at the upper left.

Turbine blades exist in a much more hostile environment than compressor

blades. (itting just downstream of the burner, the blades experience flow

temperatures of more than a thousand degrees 9ahrenheit. Turbine blades

must be made of special materials that can withstand the heat, or they

must be actively cooled. At the upper right of the figure, we show a

picture of a single, actively cooled turbine blade. The blade is hollow and

cool air, which is bled off the compressor, is pumped through the blade

and out through the small holes on the surface to "eep the surface cool.

)6
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21/47

Turbofan




somepartsin common. All gas turbine engines have a no00leto

produce thrust,to conduct the exhaust gases bac" to the free stream,and

to set the mass flow ratethrough the engine. The no//le sits downstream

of thepower turbine.

A no//le is a relatively simple device, just a specially shaped tube

through which hot gases flow. However, the mathematicswhich describe

the operation of the no//le ta"es some careful thought. As shown above,

no//les come in a variety of shapes and si/es depending on the mission of

the aircraft. (imple turbojets,and turboprops,often have a fixed

geometrycon*ergentno//le as shown on the left of the

figure. Turbofanengines often employ a co-annularno//le as shown at

the top left. The core flow exits the center no//le while the fan flow exits

the annular no//le. $ixing of the two flows provides some thrustenhancement and these no//les also tend to be 'uieter than convergent

)
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22/47

Turbofan

no//les. Afterburning turbojetsand turbofans re'uire a variable

geometry con*ergent-di*ergent - %/no//le as shown on the left. +n this

no//le, the flow first converges down to the minimum

areaor throatthen is expanded through the divergent section to the exit

at the right. The variable geometry causes these no//les to be heavierthan a fixed geometry no//le, but variable geometry provides efficient

engine operation over a wider airflow range than a simple fixed no//le.

?oc"et enginesalso use no//les to accelerate hot exhaust to produce

thrust. ?oc"et engines usually have a fixed geometry =4 no//le with a

much larger divergent section than is re'uired for a gas turbine. Iou can

explore the design and operation of no//les with our interactive no//le

simulatorprogram which runs on your browser.

All of the no//les we have discussed thus far are round tubes. ?ecently,

however, engineers have been experimenting with no//les with

rectangular exits. This allows the exhaust flow to be easily deflected,

or vectored, as shown in the middle of the figure. =hanging the direction

of the thrust with the no//le ma"es the aircraft much more maneuverable.

0ecause the no//le conducts the hot exhaust bac" to the free stream, there

can be serious interactions between the engine exhaust flow and the

airflow around the aircraft. *n fighter aircraft, in particular, large drag

penalties can occur near the no//le exits. A typical no00le-afterbodyconfiguration is shown in the upper right for an 9# with

experimental maneuvering no//les. As with the inletdesign, the external

no//le configuration is often designed by the airframer. The internal

no//le is usually the responsibility of the engine manufacturer.

Contents

[hide]

1 Introduction

2 Early turbofans

3 Low bypass turbofans

4 Afterburning turbofans

5 igh!bypass turbofan engines

" #urbofan configurations

o "$1 %ingle shaft turbofan

o "$2 Aft fan turbofan

))
http://www.grc.nasa.gov/WWW/K-12/airplane/aturba.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/mflchk.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/mflchk.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/rocket.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/rockth.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/ienzl.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/ienzl.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/vecthrst.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/inlet.htmlhttp://toggletoc%28%29/http://en.wikipedia.org/wiki/Turbofan#Introductionhttp://en.wikipedia.org/wiki/Turbofan#Early_turbofanshttp://en.wikipedia.org/wiki/Turbofan#Low_bypass_turbofanshttp://en.wikipedia.org/wiki/Turbofan#Afterburning_turbofanshttp://en.wikipedia.org/wiki/Turbofan#High-bypass_turbofan_engineshttp://en.wikipedia.org/wiki/Turbofan#Turbofan_configurationshttp://en.wikipedia.org/wiki/Turbofan#Single_shaft_turbofanhttp://en.wikipedia.org/wiki/Turbofan#Aft_fan_turbofanhttp://www.grc.nasa.gov/WWW/K-12/airplane/aturba.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/mflchk.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/mflchk.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/rocket.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/rockth.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/ienzl.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/ienzl.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/vecthrst.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/inlet.htmlhttp://toggletoc%28%29/http://en.wikipedia.org/wiki/Turbofan#Introductionhttp://en.wikipedia.org/wiki/Turbofan#Early_turbofanshttp://en.wikipedia.org/wiki/Turbofan#Low_bypass_turbofanshttp://en.wikipedia.org/wiki/Turbofan#Afterburning_turbofanshttp://en.wikipedia.org/wiki/Turbofan#High-bypass_turbofan_engineshttp://en.wikipedia.org/wiki/Turbofan#Turbofan_configurationshttp://en.wikipedia.org/wiki/Turbofan#Single_shaft_turbofanhttp://en.wikipedia.org/wiki/Turbofan#Aft_fan_turbofan


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Turbofan

o "$3 &asic two spool

o "$4 &oosted two spool

o "$5 #hree spool

o "$" 'eared fan

o "$( )ilitary turbofans

o "$* igh +ressure #urbine

o "$, Low +ressure #urbine

( -ycle i.pro/e.ents

* #hrust growth

, #echnical 0iscussion

1 ecent de/elop.ents in blade technology

11 #urbofan engine .anufacturers

o 11$1 'eneral Electric

o 11$2 -) International

o 11$3 olls!oyce

o 11$4 +ratt hitney

o 11$5 A/iad/igatel

12 E6tre.e bypass 7et engines

13 #er.inology

14 8ther .eanings

15 9otes and references

Welcome to the Beginner's Guide toPropulsion

)&
http://en.wikipedia.org/wiki/Turbofan#Basic_two_spoolhttp://en.wikipedia.org/wiki/Turbofan#Basic_two_spoolhttp://en.wikipedia.org/wiki/Turbofan#Boosted_two_spoolhttp://en.wikipedia.org/wiki/Turbofan#Boosted_two_spoolhttp://en.wikipedia.org/wiki/Turbofan#Three_spoolhttp://en.wikipedia.org/wiki/Turbofan#Three_spoolhttp://en.wikipedia.org/wiki/Turbofan#Geared_fanhttp://en.wikipedia.org/wiki/Turbofan#Military_turbofanshttp://en.wikipedia.org/wiki/Turbofan#High_Pressure_Turbinehttp://en.wikipedia.org/wiki/Turbofan#Low_Pressure_Turbinehttp://en.wikipedia.org/wiki/Turbofan#Cycle_improvementshttp://en.wikipedia.org/wiki/Turbofan#Thrust_growthhttp://en.wikipedia.org/wiki/Turbofan#Technical_Discussionhttp://en.wikipedia.org/wiki/Turbofan#Recent_developments_in_blade_technologyhttp://en.wikipedia.org/wiki/Turbofan#Turbofan_engine_manufacturershttp://en.wikipedia.org/wiki/Turbofan#General_Electrichttp://en.wikipedia.org/wiki/Turbofan#CFM_Internationalhttp://en.wikipedia.org/wiki/Turbofan#CFM_Internationalhttp://en.wikipedia.org/wiki/Turbofan#Rolls-Roycehttp://en.wikipedia.org/wiki/Turbofan#Pratt_.26_Whitneyhttp://en.wikipedia.org/wiki/Turbofan#Aviadvigatelhttp://en.wikipedia.org/wiki/Turbofan#Aviadvigatelhttp://en.wikipedia.org/wiki/Turbofan#Extreme_bypass_jet_engineshttp://en.wikipedia.org/wiki/Turbofan#Terminologyhttp://en.wikipedia.org/wiki/Turbofan#Other_meaningshttp://en.wikipedia.org/wiki/Turbofan#Notes_and_referenceshttp://en.wikipedia.org/wiki/Turbofan#Basic_two_spoolhttp://en.wikipedia.org/wiki/Turbofan#Boosted_two_spoolhttp://en.wikipedia.org/wiki/Turbofan#Three_spoolhttp://en.wikipedia.org/wiki/Turbofan#Geared_fanhttp://en.wikipedia.org/wiki/Turbofan#Military_turbofanshttp://en.wikipedia.org/wiki/Turbofan#High_Pressure_Turbinehttp://en.wikipedia.org/wiki/Turbofan#Low_Pressure_Turbinehttp://en.wikipedia.org/wiki/Turbofan#Cycle_improvementshttp://en.wikipedia.org/wiki/Turbofan#Thrust_growthhttp://en.wikipedia.org/wiki/Turbofan#Technical_Discussionhttp://en.wikipedia.org/wiki/Turbofan#Recent_developments_in_blade_technologyhttp://en.wikipedia.org/wiki/Turbofan#Turbofan_engine_manufacturershttp://en.wikipedia.org/wiki/Turbofan#General_Electrichttp://en.wikipedia.org/wiki/Turbofan#CFM_Internationalhttp://en.wikipedia.org/wiki/Turbofan#Rolls-Roycehttp://en.wikipedia.org/wiki/Turbofan#Pratt_.26_Whitneyhttp://en.wikipedia.org/wiki/Turbofan#Aviadvigatelhttp://en.wikipedia.org/wiki/Turbofan#Extreme_bypass_jet_engineshttp://en.wikipedia.org/wiki/Turbofan#Terminologyhttp://en.wikipedia.org/wiki/Turbofan#Other_meaningshttp://en.wikipedia.org/wiki/Turbofan#Notes_and_references


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Turbofan

What is propulsion? The word isderived from two Latinwords:promeaning before orforwards andpelleremeaning todrive. Propulsionmeans to pushforward or drive an object

forward. A propulsion system is amachine that producesthrusttopush an object forward. Onairplanes thrust is usuallygenerated through someapplication of !ewton"sthirdlaw of action and reaction. A gasor working fluid is acceleratedby the engine and the reaction tothis acceleration produces aforce on the engine.

)8
http://www.grc.nasa.gov/WWW/K-12/airplane/thrust1.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/thrust1.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/newton3.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/newton3.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/newton3.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/thrust1.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/newton3.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/newton3.html


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Turbofan

A general derivation of the thrust e#uationshows that the amount of thrust generateddepends on the mass flow through the engine and the e$it velocity of the gas. %ifferentpropulsion systems generate thrust in slightly different ways. We will discuss four principalpropulsion systems: the propellertheturbine &or jet'engine the ramjetand theroc(et.

Why are there different types of engines? )f we thin( about !ewton"sfirst lawof motion we

reali*e that an airplane propulsion system must serve two purposes. +irst the thrust fromthe propulsion system mustbalancethe drag of the airplane when the airplane is cruising.And second the thrust from the propulsion system must e$ceedthe drag of the airplanefor the airplane to accelerate. )n fact the greater the difference between the thrust and thedrag called thee$cess thrustthe faster the airplane will accelerate.

,ome aircraft li(e airliners and cargo planes spend most of their life in a cruise condition.+or these airplanes e$cess thrust is not as important as high engine efficiency andlow fuel usage.,ince thrust depends on both the amount of gas moved and the velocitywe can generate high thrust by accelerating a large mass of gas by a small amount or byaccelerating a small mass of gas by a large amount. -ecause of the aerodynamicefficiency ofpropellersand fansit is more fuel efficient to accelerate a large mass by asmall amount. That is why we find high bypass fans and turboprops on cargo planes andairliners.

,ome aircraft li(e fighter planes or e$perimental high speed aircraft re#uire very highe$cess thrust to accelerate #uic(ly and to overcome the high drag associated with highspeeds. +or these airplanes engine efficiency is not as important as very high thrust.odern military aircraft typically employ afterburnerson a low bypass turbofan core.+uture hypersonic aircraft will employ some type of ramjetor roc(et propulsion.

The site was prepared at !A,A /lenn by the Learning Technologies 0roject &LT0' toprovide background informationon basic propulsion for secondary math and scienceteachers. The pages were originally prepared as teaching aidsto support1ngine,im aninteractive educational computer program that allows students to design and test jet

engines on a personal computer. Other slides were prepared to support LT0videoconferencing wor(shops&http:22www.grc.nasa.gov2WWW2345627o127oemain.html' forteachers and students. And other slides were prepared as part of0ower 0oint0resentationsfor the %igital Learning !etwor(.

There is a special section of the -eginner"s /uide which deals with compressible or highspeed aerodynamics. This section is intended for undergraduateswho arestudying shoc( wavesor isentropic flowsand contains severalcalculators andsimulatorsfor that flow regime.

Common types

#here are two types of 7et engine that are seen

co..only today: the turbofan which is used on al.ost

all co..ercial airliners: and roc;et engineswhich are

used for spaceflightand other terrestrial uses such as

e7ector seats: flares: firewor;s etc$

)
http://www.grc.nasa.gov/WWW/K-12/airplane/thrsteq.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/propeller.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/turbine.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/turbine.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/ramjet.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/ramjet.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/rocket.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/rocket.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/newton1a.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/newton1a.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/newton1a.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/cruise.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/cruise.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/smotion.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/smotion.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/exthrst.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/exthrst.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/sfc.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/propeller.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/propeller.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/propeller.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/turbfan.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/turbfan.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/turbab.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/ramjet.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/rocket.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/ngnsim.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/ngnsim.htmlhttp://www.grc.nasa.gov/WWW/K-12/CoE/Coemain.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/topics.htmhttp://www.grc.nasa.gov/WWW/K-12/airplane/topics.htmhttp://www.grc.nasa.gov/WWW/K-12/airplane/topics.htmhttp://www.grc.nasa.gov/WWW/K-12/airplane/topics.htmhttp://nasadln.nmsu.edu/dln/http://www.grc.nasa.gov/WWW/K-12/airplane/shortc.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/oblique.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/oblique.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/isentrop.htmlhttp://www.grc.nasa.gov/WWW/K-12/UndergradProgs/index.htmhttp://www.grc.nasa.gov/WWW/K-12/UndergradProgs/index.htmhttp://en.wikipedia.org/wiki/Rocket_enginehttp://en.wikipedia.org/wiki/Spaceflighthttp://www.grc.nasa.gov/WWW/K-12/airplane/thrsteq.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/propeller.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/turbine.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/ramjet.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/rocket.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/newton1a.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/cruise.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/smotion.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/exthrst.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/sfc.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/propeller.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/turbfan.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/turbab.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/ramjet.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/rocket.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/ngnsim.htmlhttp://www.grc.nasa.gov/WWW/K-12/CoE/Coemain.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/topics.htmhttp://www.grc.nasa.gov/WWW/K-12/airplane/topics.htmhttp://nasadln.nmsu.edu/dln/http://www.grc.nasa.gov/WWW/K-12/airplane/shortc.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/oblique.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/isentrop.htmlhttp://www.grc.nasa.gov/WWW/K-12/UndergradProgs/index.htmhttp://www.grc.nasa.gov/WWW/K-12/UndergradProgs/index.htmhttp://en.wikipedia.org/wiki/Rocket_enginehttp://en.wikipedia.org/wiki/Spaceflight


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Turbofan

[edit]Turbofan enginesMain article: Turbofan

)ost .odern 7et engines are actually turbofans: where

the low pressure co.pressor acts as a fan: supplyingsupercharged air not only to the engine core: but to a

bypass duct$ #he bypass airflow either passes to a

separate


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Turbofan

[edit]Rocket enginesMain article: Rocket engine

A co..on for. of 7et engine is the roc;et engine$

oc;et engines are used for high altitude flights becausethey gi/e /ery high thrustand their lac; of reliance on

at.ospheric o6ygen allows the. to operate at arbitrary

altitudes$

#his is used for launching satellites: space

e6plorationand .anned access: and per.itted landing

on the .oonin 1,",$

owe/er: the high e6haust speed and the hea/ier:o6idiser!rich propellant results in .ore propellant use

than turbo7ets: and their use is largely restricted to /ery

high altitudes: /ery high speeds: or where /ery high

accelerations are needed as roc;et engines the.sel/es

ha/e a /ery high thrust!to!weight ratio$

An appro6i.ate e@uation for the net thrust of a roc;et

engine is

hereFis the thrust:'sp(acis the specific

i.pulse:g6is a standard gra/ity: is the propellant

flow in ;gBs:Aeis the area of the e6haust bell at the

e6it: and)is the at.ospheric pressure$

[edit]General physical principles

All 7et engines are reaction engines that generate

thrust by e.itting a7etof fluid rearwards at relati/ely

high speed$ #he forces on the inside of the engine

needed to create this 7et gi/e a strong thrust on the

engine which pushes the craft forwards$

Cet engines .a;e their 7et fro. propellant fro.

tan;age that is attached to the engine >as in a


28/47

Turbofan

fluid >/ery typically air? and e6pelling it at higher

speed$

[edit]Thrust

#he .otion i.pulse of the engine is e@ual to the fluid.ass .ultiplied by the speed at which the engine

e.its this .ass

I D . c

where . is the fluid .ass per second and c is the

e6haust speed$ In other words: a /ehicle gets the

sa.e thrust if it outputs a lot of e6haust /ery

slowly: or a little e6haust /ery @uic;ly$ >In practice

parts of the e6haust .ay be faster than others:

but it


29/47

Turbofan

% D . >c!/?

#his e@uation i.plies that as / approaches

c: a greater .ass of fluid .ust go through

the engine to continue to accelerate at thesa.e rate: but all engines ha/e a designed

li.it on this$ Additionally: the e@uation

i.plies that the /ehicle canF? upon the

/ehicle speedBe6haust speed ratio >/Bc? for air!

breathing 7et and roc;et engines

Energy efficiency >M? of 7et engines installedin /ehicles has two .ain

co.ponents: cycle efficiency>Mc?! how

efficiently the engine can accelerate the

7et: andpropulsive efficiency>Mp?!how .uch

of the energy of the 7et ends up in the

/ehicle body rather than being carried

away as ;inetic energy of the 7et$

E/en though o/erall energy efficiency Missi.ply

M N MpMc

or all 7et engines thepropulsive

efficiencyis highest when the engine

e.its an e6haust 7et at a speed that is

the sa.e as: or nearly the sa.e as: the

/ehicle /elocity as this gi/es the

s.allest residual ;inetic

)7
http://en.wikipedia.org/w/index.php?title=Jet_engine&action=edit&section=10http://en.wikipedia.org/wiki/Propulsive_efficiencyhttp://en.wikipedia.org/wiki/Propulsive_efficiencyhttp://en.wikipedia.org/wiki/File:Propulsive_efficiency.pnghttp://en.wikipedia.org/wiki/File:Propulsive_efficiency.pnghttp://en.wikipedia.org/w/index.php?title=Jet_engine&action=edit&section=10http://en.wikipedia.org/wiki/Propulsive_efficiencyhttp://en.wikipedia.org/wiki/Propulsive_efficiency


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Turbofan

energy$>9ote[2]? #he e6act for.ula for

air!breathing engines .o/ing at

speed (with an e6haust /elocity cis

gi/en in the literature as[21]is

And for a roc;et

[22]

In addition to propulsi/e

efficiency: another factor is cycle

efficiency essentially a 7et

engine is typically a for. of heat

engine$ eat engine efficiency is

deter.ined by the ratio of

te.peratures that are reached in

the engine to that they are

e6hausted at fro. the no==le:

which in turn is li.ited by

the o/erall pressure ratiothatcan be achie/ed$ -ycle efficiency

is highest in roc;et engines

>G"H?: as they can achie/e

e6tre.ely high co.bustion

te.peratures and can ha/e /ery

large: energy efficient no==les$

-ycle efficiency in turbo7et and

si.ilar is nearer to 3: thepractical co.bustion

te.peratures and no==le

efficiencies are .uch lower$

&6
http://en.wikipedia.org/wiki/Jet_propulsion#cite_note-19http://en.wikipedia.org/wiki/Jet_propulsion#cite_note-20http://en.wikipedia.org/wiki/Jet_propulsion#cite_note-RPE-21http://en.wikipedia.org/wiki/Cycle_efficiencyhttp://en.wikipedia.org/wiki/Cycle_efficiencyhttp://en.wikipedia.org/wiki/Overall_pressure_ratiohttp://en.wikipedia.org/wiki/File:Specific-impulse-kk-20090105.pnghttp://en.wikipedia.org/wiki/Jet_propulsion#cite_note-19http://en.wikipedia.org/wiki/Jet_propulsion#cite_note-20http://en.wikipedia.org/wiki/Jet_propulsion#cite_note-RPE-21http://en.wikipedia.org/wiki/Cycle_efficiencyhttp://en.wikipedia.org/wiki/Cycle_efficiencyhttp://en.wikipedia.org/wiki/Overall_pressure_ratio


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Turbofan

%pecific i.pulseas a function of speed

for different 7et types with ;erosene

fuel >hydrogen Ispwould be about twice

as high?$ Although efficiency plu..etswith speed: greater distances are

co/ered: it turns out that efficiency per

unit distance >per ;. or .ile? is

roughly independent of speed for 7et

engines as a group howe/er airfra.es

beco.e inefficient at supersonic

speeds

[edit]Fuelpropellantconsumption

A closely related >but different?

concept to energy efficiency is

the rate of consu.ption of

propellant .ass$ +ropellant

consu.ption in 7et engines is

.easured by !pecific Fuel

Consumption: !pecificimpulseor Effective exhaust

velocity$ #hey all .easure the

sa.e thing: specific i.pulse and

effecti/e e6haust /elocity are

strictly proportional: whereas

specific fuel consu.ption is

in/ersely proportional to the

others$

or airbreathing engines such as

turbo7ets energy efficiency and

propellant >fuel? efficiency are

.uch the sa.e thing: since the

propellant is a fuel and the

source of energy$ In roc;etry: the

propellant is also the e6haust:

and this .eans that a highenergy propellant gi/es better

&
http://en.wikipedia.org/wiki/Specific_impulsehttp://en.wikipedia.org/w/index.php?title=Jet_engine&action=edit&section=11http://en.wikipedia.org/wiki/Specific_Fuel_Consumptionhttp://en.wikipedia.org/wiki/Specific_Fuel_Consumptionhttp://en.wikipedia.org/wiki/Specific_impulsehttp://en.wikipedia.org/wiki/Specific_impulsehttp://en.wikipedia.org/wiki/Effective_exhaust_velocityhttp://en.wikipedia.org/wiki/Effective_exhaust_velocityhttp://en.wikipedia.org/wiki/File:Specific-impulse-kk-20090105.pnghttp://en.wikipedia.org/wiki/Specific_impulsehttp://en.wikipedia.org/w/index.php?title=Jet_engine&action=edit&section=11http://en.wikipedia.org/wiki/Specific_Fuel_Consumptionhttp://en.wikipedia.org/wiki/Specific_Fuel_Consumptionhttp://en.wikipedia.org/wiki/Specific_impulsehttp://en.wikipedia.org/wiki/Specific_impulsehttp://en.wikipedia.org/wiki/Effective_exhaust_velocityhttp://en.wikipedia.org/wiki/Effective_exhaust_velocity


32/47

Turbofan

propellant efficiency but can in

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[23]

&)
http://en.wikipedia.org/wiki/Specific_fuel_consumption_(thrust)http://en.wikipedia.org/wiki/Specific_impulsehttp://en.wikipedia.org/wiki/Effective_exhaust_velocityhttp://en.wikipedia.org/wiki/Effective_exhaust_velocityhttp://en.wikipedia.org/wiki/Effective_exhaust_velocityhttp://en.wikipedia.org/wiki/NK-33http://en.wikipedia.org/wiki/SSMEhttp://en.wikipedia.org/wiki/Ramjethttp://en.wikipedia.org/wiki/J-58http://en.wikipedia.org/wiki/J-58http://en.wikipedia.org/wiki/Rolls-Royce/Snecma_Olympus_593http://en.wikipedia.org/wiki/Rolls-Royce/Snecma_Olympus_593http://en.wikipedia.org/wiki/Rolls-Royce/Snecma_Olympus_593http://en.wikipedia.org/wiki/Rolls-Royce/Snecma_Olympus_593http://en.wikipedia.org/wiki/Rolls-Royce/Snecma_Olympus_593http://en.wikipedia.org/wiki/Jet_propulsion#cite_note-LARGE-22http://en.wikipedia.org/wiki/Jet_propulsion#cite_note-LARGE-22http://en.wikipedia.org/wiki/Jet_propulsion#cite_note-LARGE-22http://en.wikipedia.org/wiki/Jet_propulsion#cite_note-LARGE-22http://en.wikipedia.org/wiki/General_Electric_CF6http://en.wikipedia.org/wiki/General_Electric_CF6http://en.wikipedia.org/wiki/General_Electric_CF6http://en.wikipedia.org/wiki/Jet_propulsion#cite_note-LARGE-22http://en.wikipedia.org/wiki/Specific_fuel_consumption_(thrust)http://en.wikipedia.org/wiki/Specific_impulsehttp://en.wikipedia.org/wiki/Effective_exhaust_velocityhttp://en.wikipedia.org/wiki/Effective_exhaust_velocityhttp://en.wikipedia.org/wiki/Effective_exhaust_velocityhttp://en.wikipedia.org/wiki/NK-33http://en.wikipedia.org/wiki/SSMEhttp://en.wikipedia.org/wiki/Ramjethttp://en.wikipedia.org/wiki/J-58http://en.wikipedia.org/wiki/Rolls-Royce/Snecma_Olympus_593http://en.wikipedia.org/wiki/Rolls-Royce/Snecma_Olympus_593http://en.wikipedia.org/wiki/Rolls-Royce/Snecma_Olympus_593http://en.wikipedia.org/wiki/Jet_propulsion#cite_note-LARGE-22http://en.wikipedia.org/wiki/Jet_propulsion#cite_note-LARGE-22http://en.wikipedia.org/wiki/Jet_propulsion#cite_note-LARGE-22http://en.wikipedia.org/wiki/Jet_propulsion#cite_note-LARGE-22http://en.wikipedia.org/wiki/General_Electric_CF6http://en.wikipedia.org/wiki/General_Electric_CF6http://en.wikipedia.org/wiki/General_Electric_CF6http://en.wikipedia.org/wiki/Jet_propulsion#cite_note-LARGE-22


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Turbofan

[edit]Thrust"to"#eightratioMain article: Thrust-to-weight

ratio#he thrust to weight ratio of 7et

engines of si.ilar principles

/aries so.ewhat with scale: but

.ostly is a function of engine

construction technology$ -learly

for a gi/en engine: the lighter the

engine: the better the thrust to

weight is: the less fuel is used toco.pensate for drag due to the

lift needed to carry the engine

weight: or to accelerate the .ass

of the engine$

As can be seen in the following

table: roc;et engines generally

achie/e /ery .uch higher thrust

to weight ratios than ductenginessuch as turbo7et and

turbofan engines$ #his is

pri.arily because roc;ets al.ost

uni/ersally use dense li@uid or

solid reaction .ass which gi/es a

.uch s.aller /olu.e and hence

the pressurisation syste. that

supplies the no==le is .uchs.aller and lighter for the sa.e

perfor.ance$ 0uct engines ha/e

to deal with air which is 2!3

orders of .agnitude less dense

and this gi/es pressures o/er

.uch larger areas: and which in

turn results in .ore engineering

.aterials being needed to hold

&&
http://en.wikipedia.org/w/index.php?title=Jet_engine&action=edit&section=12http://en.wikipedia.org/wiki/Thrust-to-weight_ratiohttp://en.wikipedia.org/wiki/Thrust-to-weight_ratiohttp://en.wiktionary.org/wiki/duct_enginehttp://en.wiktionary.org/wiki/duct_enginehttp://en.wikipedia.org/w/index.php?title=Jet_engine&action=edit&section=12http://en.wikipedia.org/wiki/Thrust-to-weight_ratiohttp://en.wikipedia.org/wiki/Thrust-to-weight_ratiohttp://en.wiktionary.org/wiki/duct_enginehttp://en.wiktionary.org/wiki/duct_engine


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Turbofan

the engine together and for the

air co.pressor$

Engine

Thrust-to-

weight ratio

=oncordes ?olls#?oyce>(necma

*lympus 7&turbojet

8.6 with

reheat[)8!

left to

right? turboshaft: low

bypassandturbo7etto fly at 1 ;.

altitude in /arious speeds$ ori=ontal

a6is ! speed: .Bs$ Jertical a6is displays

engine efficiency$

#urbopropsobtain little thrust

fro. 7et effect: but are useful forco.parison$ #hey are gas turbine

&8
http://en.wikipedia.org/wiki/Thrust-to-weight_ratiohttp://en.wikipedia.org/wiki/Thrust-to-weight_ratiohttp://en.wikipedia.org/wiki/Concordehttp://en.wikipedia.org/wiki/Concordehttp://en.wikipedia.org/wiki/Rolls-Royce/Snecma_Olympus_593http://en.wikipedia.org/wiki/Rolls-Royce/Snecma_Olympus_593http://en.wikipedia.org/wiki/Turbojethttp://en.wikipedia.org/wiki/Jet_propulsion#cite_note-23http://en.wikipedia.org/wiki/J-58http://en.wikipedia.org/wiki/J-58http://en.wikipedia.org/wiki/SR-71http://en.wikipedia.org/wiki/Jet_propulsion#cite_note-24http://en.wikipedia.org/wiki/Space_shuttlehttp://en.wikipedia.org/wiki/Space_shuttlehttp://en.wikipedia.org/wiki/SSMEhttp://en.wikipedia.org/wiki/Rocket_enginehttp://en.wikipedia.org/wiki/Rocket_enginehttp://en.wikipedia.org/wiki/Rocket_enginehttp://en.wikipedia.org/wiki/Jet_propulsion#cite_note-25http://en.wikipedia.org/wiki/RD-180http://en.wikipedia.org/wiki/RD-180http://en.wikipedia.org/wiki/NK-33http://en.wikipedia.org/wiki/Jet_propulsion#cite_note-26http://en.wikipedia.org/w/index.php?title=Jet_engine&action=edit&section=13http://en.wikipedia.org/wiki/Turboshafthttp://en.wikipedia.org/wiki/Turbofan#Low-bypass_turbofanshttp://en.wikipedia.org/wiki/Turbofan#Low-bypass_turbofanshttp://en.wikipedia.org/wiki/Turbojethttp://en.wikipedia.org/wiki/Turboprophttp://en.wikipedia.org/wiki/File:JetSuitabilityEn.pnghttp://en.wikipedia.org/wiki/File:JetSuitabilityEn.pnghttp://en.wikipedia.org/wiki/Thrust-to-weight_ratiohttp://en.wikipedia.org/wiki/Thrust-to-weight_ratiohttp://en.wikipedia.org/wiki/Concordehttp://en.wikipedia.org/wiki/Rolls-Royce/Snecma_Olympus_593http://en.wikipedia.org/wiki/Rolls-Royce/Snecma_Olympus_593http://en.wikipedia.org/wiki/Turbojethttp://en.wikipedia.org/wiki/Jet_propulsion#cite_note-23http://en.wikipedia.org/wiki/J-58http://en.wikipedia.org/wiki/SR-71http://en.wikipedia.org/wiki/Jet_propulsion#cite_note-24http://en.wikipedia.org/wiki/Space_shuttlehttp://en.wikipedia.org/wiki/SSMEhttp://en.wikipedia.org/wiki/Rocket_enginehttp://en.wikipedia.org/wiki/Rocket_enginehttp://en.wikipedia.org/wiki/Jet_propulsion#cite_note-25http://en.wikipedia.org/wiki/RD-180http://en.wikipedia.org/wiki/NK-33http://en.wikipedia.org/wiki/Jet_propulsion#cite_note-26http://en.wikipedia.org/w/index.php?title=Jet_engine&action=edit&section=13http://en.wikipedia.org/wiki/Turboshafthttp://en.wikipedia.org/wiki/Turbofan#Low-bypass_turbofanshttp://en.wikipedia.org/wiki/Turbofan#Low-bypass_turbofanshttp://en.wikipedia.org/wiki/Turbojethttp://en.wikipedia.org/wiki/Turboprop


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Turbofan

engines that ha/e a rotating fan

that ta;es and accelerates the

large .ass of air but by a

relati/ely s.all change in speed$

#his low speed li.its the speed of

any propeller dri/en airplane$

hen the plane speed e6ceeds

this li.it: propellers no longer

pro/ide any thrust >c!/ K ?$

owe/er: because they

accelerate a large .ass of air:

turboprops are /ery efficient$

turbo7etsand other si.ilar

engines accelerate a .uch

s.aller .ass of the air and

burned fuel: but they e.it it at

the .uch higher speeds possible

with a de La/al no==le$ #his is

why they are suitable for

supersonic and higher speeds$

Low bypass turbofansha/e the

.i6ed e6haust of the two air

flows: running at different speeds

>c1 and c2?$ #he thrust of such

engine is

% D .1 >c1 ! /? H .2 >c2 ! /?

where .1 and .2 are the air

.asses: being blown fro. theboth e6hausts$ %uch engines

are effecti/e at lower speeds:

than the pure 7ets: but at

higher speeds than the

turboshafts and propellers in

general$ or instance: at the

1 ;. altitude: turboshafts

are .ost effecti/e atabout )ach$4 >$4 ti.es the

&
http://en.wikipedia.org/wiki/Turbojethttp://en.wikipedia.org/wiki/De_Laval_nozzlehttp://en.wikipedia.org/wiki/Turbofan#Low_bypass_turbofanshttp://en.wikipedia.org/wiki/Mach_numberhttp://en.wikipedia.org/wiki/Turbojethttp://en.wikipedia.org/wiki/De_Laval_nozzlehttp://en.wikipedia.org/wiki/Turbofan#Low_bypass_turbofanshttp://en.wikipedia.org/wiki/Mach_number


36/47

Turbofan

speed of sound?: low bypass

turbofans beco.e .ore

effecti/e at about )ach $(5

and turbo7ets beco.e .ore

effecti/e than .i6ed e6haust

engines when the speed

approaches )ach 2!3$

oc;et enginesha/e

e6tre.ely high e6haust

/elocity and thus are best

suited for high speeds

>hypersonic? and great

altitudes$ At any gi/en

throttle: the thrust and

efficiency of a roc;et .otor

i.pro/es slightly with

increasing altitude >because

the bac;!pressure falls thus

increasing net thrust at the

no==le e6it plane?: whereas

with a turbo7et >or turbofan?the falling density of the air

entering the inta;e >and the

hot gases lea/ing the no==le?

causes the net thrust to

decrease with increasing

altitude$ oc;et engines are

.ore efficient than e/en

scra.7ets abo/e roughly )ach15$[2*]

[edit]$ltitude andspeed

ith the e6ception

of scra.7ets: 7et engines:

depri/ed of their inlet syste.s

can only accept air at aroundhalf the speed of sound$ #he

&%
http://en.wikipedia.org/wiki/Rocket_enginehttp://en.wikipedia.org/wiki/Hypersonichttp://en.wikipedia.org/wiki/Jet_propulsion#cite_note-27http://en.wikipedia.org/w/index.php?title=Jet_engine&action=edit&section=14http://en.wikipedia.org/wiki/Scramjethttp://en.wikipedia.org/wiki/Rocket_enginehttp://en.wikipedia.org/wiki/Hypersonichttp://en.wikipedia.org/wiki/Jet_propulsion#cite_note-27http://en.wikipedia.org/w/index.php?title=Jet_engine&action=edit&section=14http://en.wikipedia.org/wiki/Scramjet


37/47

Turbofan

inlet syste.


38/47

Turbofan

@uietest: whereas the fastest

7ets are the loudest$

Although so.e /ariation in 7et

speed can often be arrangedfro. a 7et engine >such as by

throttling bac; and ad7usting

the no==le? it is difficult to

/ary the 7et speed fro. an

engine o/er a /ery wide

range$ #herefore since

engines for supersonic

/ehicles such as -oncorde:

.ilitary 7ets and roc;ets

inherently need to ha/e

supersonic e6haust at top

speed: so these /ehicles are

especially noisy e/en at low

speeds$

[edit]$dvanced

designs[edit]&"'( combinedram)etturbo)et

#he %!(1 &lac;birdabo/e

)ach 2$4?: the engine used

/ariable geo.etry /anes to

direct e6cess air through "

bypass pipes fro.

downstrea. of the fourth

co.pressor stage into the

&@
http://en.wikipedia.org/w/index.php?title=Jet_engine&action=edit&section=16http://en.wikipedia.org/w/index.php?title=Jet_engine&action=edit&section=17http://en.wikipedia.org/wiki/SR-71_Blackbirdhttp://en.wikipedia.org/wiki/Pratt_%26_Whitney_J58http://en.wikipedia.org/wiki/Pratt_%26_Whitney_J58http://en.wikipedia.org/w/index.php?title=Jet_engine&action=edit&section=16http://en.wikipedia.org/w/index.php?title=Jet_engine&action=edit&section=17http://en.wikipedia.org/wiki/SR-71_Blackbirdhttp://en.wikipedia.org/wiki/Pratt_%26_Whitney_J58http://en.wikipedia.org/wiki/Pratt_%26_Whitney_J58


39/47

Turbofan

afterburner$[3]* of the %!

(1


40/47

Turbofan

in the for. of li@uid

hydrogenit has a density one

fourteenth that of water$ It is

also deeply cryogenic and

re@uires /ery significant

insulation that precludes it

being stored in wings$ #he

o/erall /ehicle would end up

being /ery large: and difficult

for .ost airports to

acco..odate$ inally: pure

hydrogen is not found in

nature: and .ust be.anufactured either /ia

stea. refor.ing or e6pensi/e

electrolysis$ 9e/ertheless:

research is ongoing and

hydrogen!fueled aircraft

designs do e6ist that .ay be

feasible$

[edit]+recooled )etenginesMain article: Precooled jetengine

An idea originated by obert

+$ -ar.ichael in 1,55[31]is

that hydrogen!fueled engines

could theoretically ha/e .uchhigher perfor.ance than

hydrocarbon!fueled engines if

a heat e6changer were used

to cool the inco.ing air$ #he

low te.perature allows lighter

.aterials to be used: a higher

.ass!flow through the

engines: and per.its

co.bustors to in7ect .ore fuel

86
http://en.wikipedia.org/wiki/Liquid_hydrogenhttp://en.wikipedia.org/wiki/Liquid_hydrogenhttp://en.wikipedia.org/w/index.php?title=Jet_engine&action=edit&section=19http://en.wikipedia.org/wiki/Precooled_jet_enginehttp://en.wikipedia.org/wiki/Precooled_jet_enginehttp://en.wikipedia.org/wiki/Jet_propulsion#cite_note-30http://en.wikipedia.org/wiki/Liquid_hydrogenhttp://en.wikipedia.org/wiki/Liquid_hydrogenhttp://en.wikipedia.org/w/index.php?title=Jet_engine&action=edit&section=19http://en.wikipedia.org/wiki/Precooled_jet_enginehttp://en.wikipedia.org/wiki/Precooled_jet_enginehttp://en.wikipedia.org/wiki/Jet_propulsion#cite_note-30


41/47

Turbofan

without o/erheating the

engine$

#his idea leads to plausible

designs li;e eaction Engines%A&E: that .ight

per.it single!stage!to!orbit

launch /ehicles:[32]and A#EM:

which could per.it 7et

engines to be used up to

hypersoni

Documents

turbofan engine