Jet Propulsion

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Jet Propulsion - PowerPoint PPT Presentation

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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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
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
• 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 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

Schematic of a rocket engine

Drawing Courtesy of Understanding Flight

Where are jet engines used?

Commercial Airliners – Boeing 757

Where are jet engines used?

Business and personal jets - Learjet

Where are jet engines used?

Military Bombers

B-52 “Stratofortress”

B-2 “Spirit”

Photo Courtesy of www.af.mil

Where are jet engines used?

Military Fighters

F-15 “EAGLE”

F-22 “Raptor”

Photo Courtesy of www.af.mil

Where are jet engines used?

Helicopters - Apache

Photo Courtesy of www.army.mil

Where are jet engines used?

M-1 Abrams Tank

Photo Courtesy of www.army.mil

Where are jet engines used?

Tractor Pulling

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

Where are jet engines used?

Speed boats

Photo Courtesy of gas-turbines.com

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 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 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

• High power to weight ratio
• No reciprocating parts
• Less parasitic power loss – no need to constantly accelerate and decelerate pistons
• Less required maintenance
• 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
• 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 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
Six different types of jet engines
• Turbojet
• Turbofan
• Turboshaft
• Turboprop
• Pulsejet
• Ramjet

X-15 with ramjet engine

Photo Courtesy of NASA

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
• 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’
• 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
• 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’
• 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
• 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
• 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”
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
• 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 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
• 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
• 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 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 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
• 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
• 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
• Fuel System
• Ignition System
• Flame Holder
• Lubrication System
Other Auxiliary Components
• Turbofan – Inlet fan
• Turboshaft – Gear reduction unit
• Turboprop - Gear reduction unit
• Pulsejet – reed valves
• Afterburners
• Thrust Vectoring Systems
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

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

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

Russian Su-37, which incorporates thrust vectoring

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?
Objectives
• After this lesson students should be able to:
• List possible applications for each type of jet engine
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
• 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
• 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
• 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
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
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
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
Pulsejet
• Relatively inexpensive
• Doesn’t have the ability to produce thrust at low speeds
• Simple construction
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 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
• 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