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 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

Schematic of a rocket engine 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 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 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 (<100 mph) Germans used this design during WWII in their V-1 “Flying Bomb”

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?

Components of a Turbine Jet Engine Turbine Blade

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

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

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

Basic Components of a Turbine Jet Engine

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 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 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

Turboshaft Gear Reduction Unit

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

Afterburners

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

Thrust Vectoring Systems

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?

-Aircraft Engine Selection- Which Engine is the Best For the Job?

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 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