1. Jet Propulsion

2. 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

3. 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

4. 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

5. 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

6. Schematic of a rocket engine
Newton in Practice
Schematic of a rocket engine
Drawing Courtesy of Understanding Flight

7. Where are jet engines used?
Commercial Airliners – Boeing 757

8. Where are jet engines used?
Business and personal jets - Learjet

9. Where are jet engines used?
Military Bombers
B-52 “Stratofortress”
B-2 “Spirit”
Photo Courtesy of

10. Where are jet engines used?
Military Fighters
F-15 “EAGLE”
F-22 “Raptor”
Photo Courtesy of

11. Where are jet engines used?
Helicopters - Apache
Photo Courtesy of

12. Where are jet engines used?
M-1 Abrams Tank
Photo Courtesy of

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

14. Where are jet engines used?
Speed boats
Photo Courtesy of gas-turbines.com

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. Types of Jet Engines

22. 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

23. Six different types of jet engines
Turbojet
Turbofan
Turboshaft
Turboprop
Pulsejet
Ramjet
X-15 with ramjet engine
Photo Courtesy of NASA

24. 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

25. 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

26. 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)

27. Turbofan Cont’

28. 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

29. 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

30. 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

31. 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”

32. V-1 Flying Bomb

33. 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

34. 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

35. 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?

36. Components of a Turbine Jet Engine
Turbine Blade

37. 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

38. 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

39. Basic Components of a Turbine Jet Engine

40. 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

41. Basic Components of a Turbine Jet Engine

42. 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

43. Basic Components of a Turbine Jet Engine

44. 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

45. 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

46. Other Essential Systems
Fuel System
Ignition System
Flame Holder
Lubrication System

47. Other Auxiliary Components
Turbofan – Inlet fan
Turboshaft – Gear reduction unit
Turboprop - Gear reduction unit
Pulsejet – reed valves
Afterburners
Thrust Vectoring Systems

48. Turbofan Inlet Fan
Most of the thrust is produced from the large inlet fan
The Bypass ratio of a turbofan is typically 8:1

49. Turboshaft Gear Reduction Unit

50. 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

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

52. 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

53. Afterburners

54. 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

55. Thrust Vectoring Maneuverability
Russian Su-37, which incorporates thrust vectoring

56. Thrust Vectoring Systems

57. 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?

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

59. 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

60. 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

61. 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)

62. 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)

63. 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

64. 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

65. 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

66. 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

67. Pulsejet Relatively inexpensive
Doesn’t have the ability to produce thrust at low speeds
Simple construction

68. Ramjet
Only used in extremely high speed applications (mostly military / NASA)
Only produces thrust at high speeds
No moving parts
SR-71
X-15

69. 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?

70. References
Books
Understanding Flight by David Andreson and Scott Eberhardt
Websites
How Stuff Works –
NASA –
Factors Affecting Fuel Consumption -
US Army –
Pratt and Whitney – www. pwc.com
US Air Force –