Jet Propulsion. Lesson Objectives After this lesson students should be able to: – Define what a...

Jet PropulsionJet Propulsion

Lesson ObjectivesLesson Objectives

After this lesson students should be able to:– Define what a jet engine is– Describe how Newton’s laws apply to jet or

rocket engines– List examples of jet engine applications– List some key points in the history of jet

propulsion– List advantages and disadvantages of jet

engines

Definition of a Jet EngineDefinition of a Jet Engine

An engine that burns fuel and uses the expanding exhaust gases to turn a turbine and/or produce thrust

The concept of thrust is based on the principle of Newton’s Third Law

Newton’s Third LawNewton’s Third Law

For every action there is an equal and opposite reaction

An example of this is a spray nozzle on a garden hose

Newton’s Second LawNewton’s Second LawF=M x AF=M x A

Newton’s second law states - The force of an object is equal to its mass times its acceleration

The force of the spray nozzle is equal to the mass of the water multiplied by the acceleration of the water when it comes through the nozzle

This is the same principle used in rocket and jet engines

Newton in PracticeNewton in Practice

Schematic of a rocket engine

Drawing Courtesy of Understanding Flight

Where are jet engines used?Where are jet engines used?Commercial Airliners – Boeing 757

Where are jet engines used?Where are jet engines used?Business and personal jets - Learjet

Where are jet engines used?Where are jet engines used?Military Bombers

B-52 “Stratofortress”

B-2 “Spirit”

Photo Courtesy of www.af.mil

Where are jet engines used?Where are jet engines used?Military Fighters

F-15 “EAGLE”F-22 “Raptor”

Photo Courtesy of www.af.mil

Where are jet engines used?Where are jet engines used?Helicopters - Apache

Photo Courtesy of www.army.mil

Where are jet engines used?Where are jet engines used?M-1 Abrams Tank

Photo Courtesy of www.army.mil

Where are jet engines used?Where are jet engines used?Tractor Pulling

Photo Courtesy of gasturbine.pwp.blueyonder.co.uk

Where are jet engines used?Where are jet engines used?Speed boats

Photo Courtesy of gas-turbines.com

History of Jet EnginesHistory of Jet Engines

Invented in the 1930’sCo-invented by Dr. Hans von Ohain

(German) and Sir Frank Whittle (British)Developed their ideas separately and at the

time knew nothing of the other’s work

History of Jet EnginesHistory of Jet Engines Germans were the first to utilize the jet engine as a

military tool The jet powered ME-262 was the first jet powered

airplane to see combat– It had a top speed of 540 mph

Photo Courtesy of Stormbirds.com

History of Jet EnginesHistory of Jet Engines

The SR-71 “Blackbird” set the current speed and altitude record for a jet powered aircraft in 1961

– Its top speed is still classified but is in excess of 2,200 mph

Photo Courtesy of NASA

Advantages of Jet EnginesAdvantages of Jet Engines

High power to weight ratioNo reciprocating parts

– Less parasitic power loss – no need to constantly accelerate and decelerate pistons

– Less required maintenance

Disadvantages of Jet EnginesDisadvantages of Jet Engines

The high speeds and high operating temperatures make designing and manufacturing gas turbines complex from both the engineering and materials standpoint

These complexities lead to a higher price Jet engines do not produce high torque levels,

which is why they aren’t used in automobiles

Review QuestionsReview Questions

Describe how a rocket or jet engine produces thrust

How do Newton’s laws relate to jet engine operation

Give some examples of jet engine applications When and where were jet engines developed What are some advantages of jet engines What are some disadvantages of jet engines

Types of Jet EnginesTypes of Jet Engines

Lesson ObjectivesLesson Objectives

After this lesson students should be able to:– List the six different types of jet engines– Describe how each type of engine propels the

vehicle it is used in– List advantages and disadvantages of each type

Six different types of jet Six different types of jet enginesengines

TurbojetTurbofanTurboshaftTurbopropPulsejetRamjet

X-15 with ramjet engine

Photo Courtesy of NASA

Turbojet EngineTurbojet Engine Thrust produced by gasses expelled from the exhaust

nozzle Very noisy Used on high speed aircraft due to its small size


TurbofanTurbofan Some of the thrust is produced by gasses expelled from

the exhaust nozzle just like a turbojet engine Most of the thrust is produced from the large inlet fan The Bypass ratio of a turbofan is typically 8:1 (eight

times more air is bypassed than passes through the compressor and combustion chamber)


Turbofan Cont’Turbofan Cont’ If one wanted to increase thrust you would either have to

increase the speed of the air being moved or increase the mass of the air being moved (Thrust = Mass x Acceleration) ... However…

It is more efficient to accelerate a larger mass of air to a lower velocity

Due to this principle the turbofan is more efficient than the turbojet

Due to the lower velocity the turbofan is also significantly quieter than a turbojet

Almost all modern commercial aircraft use turbofan engines (excluding the Concord)

Turbofan Cont’Turbofan Cont’

TurboshaftTurboshaftExhaust gas is used to turn turbine shaft

which is then used to propel the vehicleExhausted gas produces little thrust because

most of the energy is used up by the turbine

Drawing Courtesy of www.aircraftenginedesign.com

Turboshaft Cont’Turboshaft Cont’ Because of the high speed (RPM) of a turboshaft

engine gear reduction must be used to obtain a usable shaft speed – much like the transmission in your car

This gear reduction also produces torque multiplication


TurbopropTurbopropA turboprop is essentially a turboshaft

engine that is attached to a propellerA propeller is more efficient at low speeds

than a turbofan or turbojet


PulsejetPulsejet

Doesn’t Use a compressor or turbineDoesn’t have the ability to produce thrust at

low speed (<100 mph)Germans used this design during WWII in

their V-1 “Flying Bomb”

V-1 Flying BombV-1 Flying Bomb

PulsejetPulsejetUses one-way reed valves in the front of the

engine to force exhaust gasses out the rear of the engine and allow fresh air in the front

RamjetRamjet

Used for extremely high speeds (minimum 400 mph)

Doesn’t contain any moving parts (I.e.compressor, turbine, reed valves)

Relies on the inertia of the incoming air for compression

Used in the SR-71 Blackbird at supersonic speeds


What are the six types of jet enginesWhat is the difference between a turbojet

and a turbofan engine?Which type of jet engine could be used to

run a stationary electrical generator?Why aren’t turbojets used in commercial

aircraft anymore? Why not ramjets?

Components of a Turbine Jet Components of a Turbine Jet EngineEngine

Turbine Blade

ObjectivesObjectives

After this lesson students should be able to:– List the basic parts required to construct a jet

turbine engine– Describe the difference between an axial flow

jet engine and a radial flow jet engine– List the auxiliary systems needed for various jet

engines– Explain how an afterburner works

Basic Components of a Basic Components of a Turbine Jet EngineTurbine Jet Engine

Housing – The rigid frame that supports and contains the parts needed for operation as well as the combustion event

Air inlet and diffuser – The area of the jet where fresh air comes in, the design of the diffuser straightens and alters the speed of the incoming airs


Compressor – Compresses the incoming air at a ratio of approximately 30:1

Burner or combustion chamber – The area of the engine where fuel is ignited


Exhaust Nozzle – accelerates the engine exhaust to the most efficient and effective speed for producing thrust

Turbine – Converts the energy from the heated and expanding exhaust gasses to a rotating shaft which is used to turn the compressors, or in the case of a turboshaft engine, power the vehicle

Radial vs. Axial FlowRadial vs. Axial Flow

Axial flow compressors – the air travels along the axis of the engine

Radial flow engines use a centrifugal compressor – they push the air out radially rather than along the axis of the engine

Radial vs. Axial FlowRadial vs. Axial Flow

Axial flow compressors are more efficient Radial flow compressors are less expensiveMost large and high-performance jet

engines use an axial flow configuration

Other Essential SystemsOther Essential Systems

Fuel SystemIgnition SystemFlame HolderLubrication System

Other Auxiliary ComponentsOther Auxiliary Components

Turbofan – Inlet fanTurboshaft – Gear reduction unitTurboprop - Gear reduction unitPulsejet – reed valvesAfterburnersThrust Vectoring Systems

Turbofan Inlet FanTurbofan Inlet Fan

The Bypass ratio of a turbofan is typically 8:1

Most of the thrust is produced from the large inlet fan

Turboshaft Gear Reduction Turboshaft Gear Reduction UnitUnit

Pulsejet Reed ValvesPulsejet Reed Valves

The reed valves force the expanding exhaust gasses out the rear of the engine and allow fresh air to enter the front

TurbopropTurboprop

A turboprop is essentially a turboshaft engine that is attached to a propeller

AfterburnersAfterburners

An afterburner injects fuel directly into the exhaust stream and burns it using the remaining oxygen.

This heats and expands the exhaust gases further, and can increase the thrust of a jet engine by 50% or more.

The advantage of an afterburner is that you can significantly increase the thrust of the engine without adding much weight or complexity to the engine

AfterburnersAfterburners

Thrust Vectoring SystemsThrust Vectoring Systems

Thrust Vectoring redirects exhaust gasses to create thrust on a vector other than the centerline of the aircraft

Thrust Vectoring is used in aircraft such as the Harrier, F-22 Raptor, and Joint Strike fighter

Thrust Vectoring can be used to increase maneuverability or allow a plane to takeoff / land vertically

Thrust Vectoring Thrust Vectoring ManeuverabilityManeuverability

Russian Su-37, which incorporates thrust vectoring

Thrust Vectoring SystemsThrust Vectoring Systems


Which part of the jet engine converts the energy of the expanding exhaust gasses to mechanical (rotating) energy?

List 3 additional systems needed for operation of a jet turbine engine.

What is the difference between a radial flow and an axial flow jet engine?

How does an afterburner work?

-Aircraft Engine Selection--Aircraft Engine Selection-Which Engine is the Best For Which Engine is the Best For

the Job?the Job?

ObjectivesObjectives

After this lesson students should be able to:– List possible applications for each type of jet

engine– List advantages and disadvantages of each type

of jet engine

Selection CriteriaSelection Criteria

When selecting an engine for a particular vehicle the following criteria must be evaluated– Price– Designed speed of vehicle operation– Designed altitude of vehicle operation– Range – Fuel efficiency – Maintenance and Durability

Naturally Aspirated Piston Naturally Aspirated Piston EngineEngine

Relatively inexpensiveLimited power at high

altitudes due to the lower air density

Speed is limited due to propeller inefficiencies at high speeds (>500 mph)

Supercharged or Turbocharged Supercharged or Turbocharged Piston EnginePiston Engine

Able to operate at higher altitudes than a naturally aspirated engine– Turbocharging or Supercharging increases the

density of the air entering the engine (the engine thinks it is at a lower altitude)

Still somewhat limited by altitudeSpeed is still limited due to propeller inefficiencies at high speeds (>500 mph)

TurbojetTurbojet

No reciprocating partsThrust is not greatly affected by altitudeRelatively small frontal area is desirable for

high speed (supersonic) useRelatively high-speed, low-mass of exhaust

gasses make the turbojet somewhat inefficient

High speed exhaust is extremely noisy

TurbofanTurbofan

Because the large inlet fan moves a larger volume of air at a lower velocity, the turbofan is more efficient that the turbojet

Because of the lower exhaust speeds the noise level is greatly reduced

The large inlet fan creates a large frontal area which negatively affects drag at high speeds (especially supersonic)

Most effective at speeds below supersonic (Mach .5 – Mach .9) However modern fighters are now using state of the art

turbofans for supersonic flight

TurbopropTurboprop

Propellers are most efficient at low speeds Produce greater power than a comparable

piston engine with less weight, noise, and maintenance

More expensive than a piston engineMust use a gearbox to reduce the high

turboshaft rpm’s down to prop rpm’s

TurboshaftTurboshaft

Used in turboprop, helicopter, and land based applications

Must use a gearbox to reduce rpm’sM-1 Abrams tank – 1500 hp turboshaft

engine

PulsejetPulsejet

Relatively inexpensiveDoesn’t have the ability to produce thrust at

low speedsSimple construction

RamjetRamjet

Only used in extremely high speed applications (mostly military / NASA)

Only produces thrust at high speeds No moving parts

X-15

SR-71


Which types of engines are least practical at high rpms?

Which types of engines are least practical at supersonic speeds?

Which type of engine could be used to power an electrical generator?

Why is a turbofan more efficient than a turbojet engine?

ReferencesReferences

Books– Understanding Flight by David Andreson and Scott Eberhardt

Websites– How Stuff Works – www.howstuffworks.com– NASA – www.grc.nasa.com– Factors Affecting Fuel Consumption -

http://www.jal-foundation.or.jp/ – US Army – www.army.mil– Pratt and Whitney – www. pwc.com– US Air Force – www.af.mil

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