Jet Propulsion

Jet Propulsion

Made by-

Bhawesh

Lesson Objectives
 After

to:






this lesson students should be able

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
 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 Law
F=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

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
 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

Types of Jet Engines

Lesson Objectives
 After

to:





this lesson students should be able

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  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)

Turbofan Cont’

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 (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
SR-71
 No moving parts

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
 V Ganeshan (Mc Graw Hill)-Third Edition
 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