jet propulsion. lesson objectives after this lesson students should be able to: – define what a...
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Jet PropulsionJet Propulsion
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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
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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
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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
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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
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Newton in PracticeNewton in Practice
Schematic of a rocket engine
Drawing Courtesy of Understanding Flight
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Where are jet engines used?Where are jet engines used?Commercial Airliners – Boeing 757
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Where are jet engines used?Where are jet engines used?Business and personal jets - Learjet
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Where are jet engines used?Where are jet engines used?Military Bombers
B-52 “Stratofortress”
B-2 “Spirit”
Photo Courtesy of www.af.mil
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Where are jet engines used?Where are jet engines used?Military Fighters
F-15 “EAGLE”F-22 “Raptor”
Photo Courtesy of www.af.mil
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Where are jet engines used?Where are jet engines used?Helicopters - Apache
Photo Courtesy of www.army.mil
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Where are jet engines used?Where are jet engines used?M-1 Abrams Tank
Photo Courtesy of www.army.mil
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Where are jet engines used?Where are jet engines used?Tractor Pulling
Photo Courtesy of gasturbine.pwp.blueyonder.co.uk
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Where are jet engines used?Where are jet engines used?Speed boats
Photo Courtesy of gas-turbines.com
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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
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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
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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
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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
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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
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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
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Types of Jet EnginesTypes of Jet Engines
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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
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Six different types of jet Six different types of jet enginesengines
TurbojetTurbofanTurboshaftTurbopropPulsejetRamjet
X-15 with ramjet engine
Photo Courtesy of NASA
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Turbojet EngineTurbojet Engine Thrust produced by gasses expelled from the exhaust
nozzle Very noisy Used on high speed aircraft due to its small size
Drawing Courtesy of Understanding Flight
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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)
Drawing Courtesy of Understanding Flight
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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)
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Turbofan Cont’Turbofan Cont’
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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
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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
Drawing Courtesy of www.aircraftenginedesign.com
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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
Drawing Courtesy of www.aircraftenginedesign.com
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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”
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V-1 Flying BombV-1 Flying Bomb
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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
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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
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Review QuestionsReview Questions
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?
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Components of a Turbine Jet Components of a Turbine Jet EngineEngine
Turbine Blade
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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
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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
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Basic Components of a Basic Components of a Turbine Jet EngineTurbine Jet Engine
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Basic Components of a Basic Components of a Turbine Jet EngineTurbine Jet Engine
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
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Basic Components of a Basic Components of a Turbine Jet EngineTurbine Jet Engine
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Basic Components of a Basic Components of a Turbine Jet EngineTurbine Jet Engine
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
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Basic Components of a Basic Components of a Turbine Jet EngineTurbine Jet Engine
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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
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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
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Other Essential SystemsOther Essential Systems
Fuel SystemIgnition SystemFlame HolderLubrication System
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Other Auxiliary ComponentsOther Auxiliary Components
Turbofan – Inlet fanTurboshaft – Gear reduction unitTurboprop - Gear reduction unitPulsejet – reed valvesAfterburnersThrust Vectoring Systems
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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
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Turboshaft Gear Reduction Turboshaft Gear Reduction UnitUnit
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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
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TurbopropTurboprop
A turboprop is essentially a turboshaft engine that is attached to a propeller
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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
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AfterburnersAfterburners
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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
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Thrust Vectoring Thrust Vectoring ManeuverabilityManeuverability
Russian Su-37, which incorporates thrust vectoring
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Thrust Vectoring SystemsThrust Vectoring Systems
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Review QuestionsReview Questions
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?
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-Aircraft Engine Selection--Aircraft Engine Selection-Which Engine is the Best For Which Engine is the Best For
the Job?the Job?
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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
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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
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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)
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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)
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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
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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
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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
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TurboshaftTurboshaft
Used in turboprop, helicopter, and land based applications
Must use a gearbox to reduce rpm’sM-1 Abrams tank – 1500 hp turboshaft
engine
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PulsejetPulsejet
Relatively inexpensiveDoesn’t have the ability to produce thrust at
low speedsSimple construction
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RamjetRamjet
Only used in extremely high speed applications (mostly military / NASA)
Only produces thrust at high speeds No moving parts
X-15
SR-71
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Review QuestionsReview Questions
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?
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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