Jet Propulsion

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Jet Propulsion
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Lesson Objectives

• After this lesson students should be able to
• Define what a jet engine is
• Describe how Newtons 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 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 Newtons Third Law

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Newtons 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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Newtons Second LawFM x A

• Newtons 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 Practice
Schematic of a rocket engine
Drawing Courtesy of Understanding Flight
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Where are jet engines used?

• Commercial Airliners Boeing 757

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Where are jet engines used?

• Business and personal jets - Learjet

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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?

• Military Fighters

F-15 EAGLE
F-22 Raptor
Photo Courtesy of www.af.mil
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Where are jet engines used?

• Helicopters - Apache

Photo Courtesy of www.army.mil
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Where are jet engines used?

• M-1 Abrams Tank

Photo Courtesy of www.army.mil
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Where are jet engines used?

• Tractor Pulling

Photo Courtesy of gasturbine.pwp.blueyonder.co.uk
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Where are jet engines used?

• Speed boats

Photo Courtesy of gas-turbines.com
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History of Jet Engines

• Invented in the 1930s
• Co-invented by Dr. Hans von Ohain (German) and
  Sir Frank Whittle (British)
• Developed their ideas separately and at the time
  knew nothing of the others work

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History 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 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 Engines

• High power to weight ratio
• No reciprocating parts
• Less parasitic power loss no need to constantly
  accelerate and decelerate pistons
• Less required maintenance

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Disadvantages 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 arent used in automobiles

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Review Questions

• Describe how a rocket or jet engine produces
  thrust
• How do Newtons 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 Engines
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Lesson 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 engines

• Turbojet
• Turbofan
• Turboshaft
• Turboprop
• Pulsejet
• Ramjet

X-15 with ramjet engine
Photo Courtesy of NASA
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Turbojet 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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Turbofan

• 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 81
  (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

• 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
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Turboshaft

• Exhaust gas is used to turn turbine shaft which
  is then used to propel the vehicle
• Exhausted 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

• 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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Turboprop

• A turboprop is essentially a turboshaft engine
  that is attached to a propeller
• A propeller is more efficient at low speeds than
  a turbofan or turbojet

Drawing Courtesy of www.aircraftenginedesign.com
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Pulsejet

• Doesnt Use a compressor or turbine
• Doesnt have the ability to produce thrust at low
  speed (lt100 mph)
• Germans used this design during WWII in their V-1
  Flying Bomb

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V-1 Flying Bomb
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Pulsejet

• Uses 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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Ramjet

• Used for extremely high speeds
• (minimum 400 mph)
• Doesnt 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 Questions

• What are the six types of jet engines
• What 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 arent turbojets used in commercial aircraft
  anymore? Why not ramjets?

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Components of a Turbine Jet Engine
Turbine Blade
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Objectives

• 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 Turbine 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 Turbine Jet Engine
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Basic Components of a Turbine Jet Engine

• Compressor Compresses the incoming air at a
  ratio of approximately 301
• Burner or combustion chamber The area of the
  engine where fuel is ignited

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Basic Components of a Turbine Jet Engine
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Basic Components of a Turbine 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 Turbine Jet Engine
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Radial 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 Flow

• Axial flow compressors are more efficient
• Radial flow compressors are less expensive
• Most large and high-performance jet engines use
  an axial flow configuration

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Other Essential Systems

• Fuel System
• Ignition System
• Flame Holder
• Lubrication System

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Other Auxiliary Components

• Turbofan Inlet fan
• Turboshaft Gear reduction unit
• Turboprop - Gear reduction unit
• Pulsejet reed valves
• Afterburners
• Thrust Vectoring Systems

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Turbofan Inlet Fan

• Most of the thrust is produced from the large
  inlet fan

• The Bypass ratio of a turbofan is typically 81

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Turboshaft Gear Reduction Unit
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Pulsejet 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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Turboprop

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

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Afterburners

• 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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Afterburners
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Thrust 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 Maneuverability
Russian Su-37, which incorporates thrust vectoring
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Thrust Vectoring Systems
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Review 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-Which Engine is the
Best For the Job?
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Objectives

• 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 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 Engine

• Relatively inexpensive
• Limited power at high altitudes due to the lower
  air density
• Speed is limited due to propeller inefficiencies
  at high speeds (gt500 mph)

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Supercharged or Turbocharged Piston 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 altitude
• Speed is still limited due to propeller
  inefficiencies at high speeds (gt500 mph)

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Turbojet

• No reciprocating parts
• Thrust is not greatly affected by altitude
• Relatively small frontal area is desirable for
  high speed (supersonic) use
• Relatively high-speed, low-mass of exhaust gasses
  make the turbojet somewhat inefficient
• High speed exhaust is extremely noisy

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Turbofan

• 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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Turboprop

• 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 engine
• Must use a gearbox to reduce the high turboshaft
  rpms down to prop rpms

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Turboshaft

• Used in turboprop, helicopter, and land based
  applications
• Must use a gearbox to reduce rpms
• M-1 Abrams tank 1500 hp turboshaft engine

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Pulsejet

• Relatively inexpensive
• Doesnt have the ability to produce thrust at low
  speeds
• Simple construction

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Ramjet

• Only used in extremely high speed applications
  (mostly military / NASA)
• Only produces thrust at high speeds
• No moving parts

SR-71
X-15
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Review 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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References

• 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