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Jet Propulsion. Lesson 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

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lesson objectives
Lesson 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 engine
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 Newton’s Third Law
newton s third law
Newton’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 law f m x a
Newton’s Second LawF=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 practice
Newton 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 used8
Where are jet engines used?

Business and personal jets - Learjet

where are jet engines used9
Where are jet engines used?

Military Bombers

B-52 “Stratofortress”

B-2 “Spirit”

Photo Courtesy of www.af.mil

where are jet engines used10
Where are jet engines used?

Military Fighters

F-15 “EAGLE”

F-22 “Raptor”

Photo Courtesy of www.af.mil

where are jet engines used11
Where are jet engines used?

Helicopters - Apache

Photo Courtesy of www.army.mil

where are jet engines used12
Where are jet engines used?

M-1 Abrams Tank

Photo Courtesy of www.army.mil

where are jet engines used13
Where are jet engines used?

Tractor Pulling

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

where are jet engines used14
Where are jet engines used?

Speed boats

Photo Courtesy of gas-turbines.com

history of jet engines
History of Jet Engines
  • Invented in the 1930’s
  • 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 other’s work
history of jet engines16
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

history of jet engines17
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

advantages of jet engines
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
disadvantages of jet engines
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 aren’t used in automobiles
review questions
Review 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
lesson objectives22
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
six different types of jet engines
Six different types of jet engines
  • Turbojet
  • Turbofan
  • Turboshaft
  • Turboprop
  • Pulsejet
  • Ramjet

X-15 with ramjet engine

Photo Courtesy of NASA

turbojet engine
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

turbofan
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 8:1 (eight times more air is bypassed than passes through the compressor and combustion chamber)

Drawing Courtesy of Understanding Flight

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)
turboshaft
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

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

turboprop
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

pulsejet
Pulsejet
  • Doesn’t Use a compressor or turbine
  • Doesn’t have the ability to produce thrust at low speed (<100 mph)
  • Germans used this design during WWII in their V-1 “Flying Bomb”
pulsejet33
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
ramjet
Ramjet
  • 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
review questions35
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 aren’t turbojets used in commercial aircraft anymore? Why not ramjets?
objectives
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
basic components of a turbine jet engine
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
basic components of a turbine jet engine40
Basic Components of a Turbine 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
basic components of a turbine jet engine42
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
radial vs axial flow
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
radial vs axial flow45
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
other essential systems
Other Essential Systems
  • Fuel System
  • Ignition System
  • Flame Holder
  • Lubrication System
other auxiliary components
Other Auxiliary Components
  • Turbofan – Inlet fan
  • Turboshaft – Gear reduction unit
  • Turboprop - Gear reduction unit
  • Pulsejet – reed valves
  • Afterburners
  • Thrust Vectoring Systems
turbofan inlet fan
Turbofan Inlet Fan
  • Most of the thrust is produced from the large inlet fan
  • The Bypass ratio of a turbofan is typically 8:1
pulsejet reed valves
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

turboprop51
Turboprop
  • A turboprop is essentially a turboshaft engine that is attached to a propeller
afterburners
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
thrust vectoring systems
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
thrust vectoring maneuverability
Thrust Vectoring Maneuverability

Russian Su-37, which incorporates thrust vectoring

review questions57
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?
objectives59
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
selection criteria
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
naturally aspirated piston engine
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 (>500 mph)
supercharged or turbocharged piston engine
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 (>500 mph)
turbojet
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
turbofan64
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
turboprop65
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 rpm’s down to prop rpm’s
turboshaft66
Turboshaft
  • Used in turboprop, helicopter, and land based applications
  • Must use a gearbox to reduce rpm’s
  • M-1 Abrams tank – 1500 hp turboshaft engine
pulsejet67
Pulsejet
  • Relatively inexpensive
  • Doesn’t have the ability to produce thrust at low speeds
  • Simple construction
ramjet68
Ramjet
  • Only used in extremely high speed applications (mostly military / NASA)
  • Only produces thrust at high speeds
  • No moving parts

SR-71

X-15

review questions69
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?
references
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