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ContentsContents Jet engine Jet engine ComponentsComponents How does a jet engine work How does a jet engine work TypeType DescriptionDescription AdvantagesAdvantages DisadvantagesDisadvantages

Jet engineJet engine

A A jet enginejet engine is an engine that is an engine that accelerates and discharges a fast accelerates and discharges a fast moving jet of moving jet of fluid to generate thrust to generate thrust in accordance with in accordance with Newton's 's third law of motion. This broad . This broad definition of jet engines includes definition of jet engines includes turbojets, , turbofans, , rockets and and ramjets and and water jets, but in common , but in common usage, the term generally refers to a usage, the term generally refers to a gas turbine used to produce a jet of used to produce a jet of high speed exhaust gases for special high speed exhaust gases for special propulsive purposes.propulsive purposes.

ComponentsComponents

The basic components (parts) of a jet The basic components (parts) of a jet (air-breathing) engine are the : inlet, (air-breathing) engine are the : inlet, compressor or fan, combustor compressor or fan, combustor (burner), a turbine (engine) and an (burner), a turbine (engine) and an exit nozzle. Some engines may not exit nozzle. Some engines may not have all these parts. The figure below have all these parts. The figure below shows a jet engine with these shows a jet engine with these components components

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InletInlet:: The inlet is where the air goes into the The inlet is where the air goes into the

engine. The inlet slows the air flow into engine. The inlet slows the air flow into the compressor. The compressor works the compressor. The compressor works better when the air flow coming in is better when the air flow coming in is slow. The inlet is made differently if slow. The inlet is made differently if the airplane is going to fly faster than the airplane is going to fly faster than the speed of soundthe speed of sound

Compressor:Compressor: The compressor takes air in and squeezes The compressor takes air in and squeezes

it. The same thing happens to the air you it. The same thing happens to the air you blow into a balloon. When the air under blow into a balloon. When the air under pressure goes out of the balloon it causes pressure goes out of the balloon it causes thrust (push). When the air leaves the thrust (push). When the air leaves the compressor it is mixed with fuel (gas) and compressor it is mixed with fuel (gas) and burned. The gases move at high speed, burned. The gases move at high speed, just like coming out of a balloon. just like coming out of a balloon.

CombustorCombustor: : As the air comes into the combustor it is As the air comes into the combustor it is

mixed with fuel and burned. The right mixed with fuel and burned. The right amount of air and gas must be mixed and amount of air and gas must be mixed and burned to get the most thrust. The gases burned to get the most thrust. The gases become very, very hot. The engineer must become very, very hot. The engineer must make sure the combustor is made of metals make sure the combustor is made of metals that will not bend or break when there is that will not bend or break when there is high heat. The hot gases that come out go high heat. The hot gases that come out go to the turbine to increase thrust and to run to the turbine to increase thrust and to run the compressorthe compressor. .

Turbine: Turbine: A turbine is also called an engine. This A turbine is also called an engine. This

engine or turbine runs the compressor. The engine or turbine runs the compressor. The hot gases move very fast out of the hot gases move very fast out of the combustor through the turbine and out into combustor through the turbine and out into the air as thrust. the air as thrust.

Sometimes an "afterburner" is added after Sometimes an "afterburner" is added after the turbine. The afterburner heats up the the turbine. The afterburner heats up the gases again to get more thrust. The amount gases again to get more thrust. The amount of heat is important. Too much heat can of heat is important. Too much heat can bend or break parts in the turbine. bend or break parts in the turbine.

Nozzle:Nozzle: All the heated gases finally come out All the heated gases finally come out

the back end of the jet engine through the back end of the jet engine through a nozzle. The nozzle is designed a a nozzle. The nozzle is designed a special way to help the hot gases move special way to help the hot gases move even faster. This creates more thrust. even faster. This creates more thrust. The nozzle must be made of the kind of The nozzle must be made of the kind of metals that won't melt or bend under metals that won't melt or bend under high heat. high heat.

How does a jet engine How does a jet engine workwork

jet engine uses a gas turbine to drive jet engine uses a gas turbine to drive a fan that sucks in air, mixes it with a fan that sucks in air, mixes it with burning fuel, and then blasts the burning fuel, and then blasts the expanded air-gas mixture out the expanded air-gas mixture out the backback. . Because every action has an Because every action has an equal and opposite reaction, as the equal and opposite reaction, as the air streams backward the engine is air streams backward the engine is forced forwardforced forward.  .  

TypeType There are a large number of different There are a large number of different

types of jet engines, all of which get types of jet engines, all of which get propulsion from a high speed propulsion from a high speed exhaust jet.exhaust jet.

TypeType DescriptioDescriptionn

AdvantageAdvantagess

DisadvantageDisadvantagess

Water Water jetjet

Squirts Squirts water out water out the back the back of a boatof a boat

Can run in Can run in shallow shallow water, water, powerful, powerful, less less harmful to harmful to wildlifewildlife

Can be less Can be less efficient than efficient than a propeller, a propeller, more more vulnerable to vulnerable to debrisdebris

thermojetthermojetMost primitive Most primitive airbreathing airbreathing jet engine. jet engine. Essentially aEssentially a supercharged piston engine piston engine with a jet with a jet exhaustexhaust. .

Can run in Can run in shallow shallow water, water, powerful, powerful, less less harmful to harmful to wildlifewildlife

Heavy, Heavy, inefficient inefficient and and underpowunderpoweredered

turbojetturbojetGeneric Generic term for term for simple simple turbine turbine engineengine

Simplicity Simplicity of designof design

Basic Basic design, design, misses misses many many improvemeimprovements in nts in efficiency efficiency and powerand power

turbofanturbofanFirst stage First stage compressor compressor greatly greatly enlarged to enlarged to provide provide bypass bypass airflow airflow around around engine coreengine core

Quieter due to Quieter due to greatergreater mass flow and and lower total lower total exhaust speed, exhaust speed, more efficient more efficient for a useful for a useful range of range of subsonic subsonic airspeeds for airspeeds for same reason, same reason, cooler exhaust cooler exhaust temperaturetemperature

Greater complexity Greater complexity (additional ducting, (additional ducting, usually multiple usually multiple shafts), large shafts), large diameter engine, diameter engine, need to contain need to contain heavy blades. More heavy blades. More subject tosubject to FOD and and ice damage. Top ice damage. Top speed is limited due speed is limited due to the potential for to the potential for shockwaves to shockwaves to damage enginedamage engine

roketroketCarries all Carries all propellants propellants onboard, onboard, emits jet for emits jet for propulsionpropulsion

Very few moving Very few moving parts, Mach 0 to parts, Mach 0 to

Mach 25+, efficient Mach 25+, efficient at very high speed at very high speed

(> Mach 10.0 or so), (> Mach 10.0 or so), thrust/weight ratio thrust/weight ratio

over 100, no over 100, no complex air inlet, complex air inlet, high compression high compression

ratio, very high ratio, very high speed exhaust, good speed exhaust, good

cost/thrust ratio, cost/thrust ratio, works best works best

exoatmospheric exoatmospheric which is kinder on which is kinder on

vehicle structure at vehicle structure at high speedhigh speed . .

Needs lots of Needs lots of propellant- very lowpropellant- very low specific impulse- - typically 100-450 typically 100-450 seconds. Extreme seconds. Extreme thermal stresses of thermal stresses of combustion chamber combustion chamber can make reuse can make reuse harder. Typically harder. Typically requires carrying requires carrying oxidiser onboard which oxidiser onboard which increases risksincreases risks. .

TURBOPROPTURBOPROPStrictly not a jet Strictly not a jet at allat all——a gas a gas turbine engine turbine engine is used as is used as powerplant to powerplant to drive (propeller) drive (propeller) shaftshaft

High High efficiency at efficiency at lower lower subsonic subsonic airspeeds(300 airspeeds(300 knots plus), knots plus), high shaft high shaft power to power to weightweight

Limited top Limited top speed speed (aeroplanes), (aeroplanes), somewhat somewhat noisy, noisy, complex complex transmissiontransmission

PROPFANPROPFANTurboprop Turboprop engine drives engine drives one or more one or more propellers. propellers. Similar to a Similar to a turbofan turbofan without the without the fan cowlingfan cowling. .

Higher fuel Higher fuel efficiency, efficiency, potentially potentially less noisy less noisy than than turbofans, turbofans, could lead to could lead to higher-speed higher-speed commercial commercial aircraft, aircraft, popular in the popular in the 1980s during 1980s during fuel shortagesfuel shortages

Development Development of propfan of propfan engines has engines has been very been very limited, limited, typically more typically more noisy than noisy than turbofans, turbofans, complexitycomplexity

PULSJETPULSJETAir is Air is compressed compressed and and combusted combusted intermittently intermittently instead of instead of continuously. continuously. Some designs Some designs use valvesuse valves. .

Very simple Very simple design, design, commonly commonly used on used on model aircraftmodel aircraft

Noisy, Noisy, inefficient (low inefficient (low compression compression ratio), works ratio), works poorly on a poorly on a large scale, large scale, valves on valves on valved valved designs wear designs wear out quicklyout quickly

PULS PULS DETONATIODETONATION ENGINEN ENGINE

Similar to a Similar to a pulsejet, but pulsejet, but combustion combustion occurs as aoccurs as a detonation instead of ainstead of a deflagration, , may or may may or may not need not need valvesvalves

Maximum Maximum theoretical theoretical engine engine efficiencyefficiency

Extremely Extremely noisy, parts noisy, parts subject to subject to extreme extreme mechanical mechanical fatigue, hard fatigue, hard to start to start detonation, detonation, not practical not practical for current for current useuse

SCRAMJETSCRAMJETSimilar to a Similar to a ramjet ramjet without a without a diffuser; diffuser; airflow airflow through the through the entire entire engine engine remains remains supersonicsupersonic

Few Few mechanical mechanical parts, can parts, can operate at operate at very highvery high Mach Mach numbersnumbers ((Mach 8 to 15) Mach 8 to 15) with good with good efficienciesefficiencies[1][1]

Still in Still in development development stages, must stages, must have a very have a very high initial high initial speed to speed to function (Mach function (Mach >6), cooling >6), cooling difficulties, very difficulties, very poor poor thrust/weight thrust/weight ratio (~2), ratio (~2), extreme extreme aerodynamic aerodynamic complexity, complexity, airframe airframe difficulties, difficulties, testing testing difficulties/expedifficulties/expensense

TURBOROKETTURBOROKETA turbojet A turbojet where an where an additionaladditional oxidizeroxidizer such such asas oxygenoxygen is is added to the added to the airstream to airstream to increase max increase max altitudealtitude

Very close to Very close to existing existing designs, designs, operates in operates in very high very high altitude, wide altitude, wide range of range of altitude and altitude and airspeedairspeed

Airspeed Airspeed limited to limited to same range same range as turbojet as turbojet engine, engine, carrying carrying oxidizer likeoxidizer like LOXLOX can be can be dangerousdangerous

PRECOOLEDJETPRECOOLEDJETIntake air is Intake air is chilled to very chilled to very low low temperatures temperatures at inlet before at inlet before passing passing through a through a ramjet or ramjet or turbojet turbojet engineengine

Easily tested on Easily tested on ground. Very high ground. Very high thrust/weight ratios thrust/weight ratios are possible (~14) are possible (~14) together with good together with good fuel efficiency over a fuel efficiency over a wide range of wide range of airspeeds, mach 0-airspeeds, mach 0-5.5+; this 5.5+; this combination of combination of efficiencies may efficiencies may permit launching to permit launching to orbit, single stageorbit, single stage,,

Exists only at Exists only at the lab the lab protoyping protoyping stage. stage. Examples Examples includeinclude RB545RB545, , SABRESABRE, , ATREXATREX