Jet Propulsion and its working principle.pdf

What is jet propulsion, air breathing and non-airbreathing engines, components of turboprop, turbojet and turbofan engines

1. Prepared by
Ankur Sachdeva
Assistant Professor, ME
Jet
Propulsion

2. History

3. What is Jet Propulsion
•Jetpropulsionreferstotheimpartingofforwardmotiontotheobjectasareactiontothe
exitofahigh-velocitygas/liquidstreamfromtherearendoftheobject.
•Jetpropulsiondevicesarepopularlyusedinhigh-speed,high-altitude
aircraft/missiles/spacecraft,etc.
•Jet propulsion is based on the principle of Newton’s
second law and third law of motion.
•In a jet propulsion engine, the objective is to get the
propelling thrust for the engine and for getting it the
momentum change occurs in fluid stream such that the
reaction to the impulse created by momentum change
gives propelling thrust.
•The change of momentum of the fluid stream flowing
across engine and the reaction to the impulse due to
momentum change are responsible for jet propulsion

4. What is Thrust and how it is generated?
•Thrust is theforcethat moves an aircraft through the air.
•Thrust is generated by the engines of the airplane.
•Thrust is a mechanical force.
•It is generated through thereactionof accelerating a mass of gas.
•The gas is accelerated to the rear and the engine (and aircraft) is
accelerated in the opposite direction.
•To accelerate the gas, we need some kind ofpropulsion system

6. Air Breathing and
Non-Air Breathing Engines
•Air Breathing Engines
•An air-breathing engine is an engine that takes in air from its
surroundings in order to burn fuel.
Non-Air Breathing Engines
•Non-air-breathing engines carry an oxygen supply.
•They can be used both in the atmosphere and in outer space.
•They are commonly called rockets and are of two kinds liquid-
propellant and solid-propellant.

7. Types of Jet Engines
(Gas Turbine Engines)
Turboprop Engines
Turbofan Engines
Turbojet Engines
Turboshaft Engines

8. Types of Jet Engines
Turboshaft Engines

10. Components of Turbojet Engine
Turbojet Engine consists of the following main parts:
•Diffuser
•Compressor
•Pump
•Fuel Injector
•Combustion chamber
•Shafts
•Turbine
•Nozzle

11. Working of Turbojet Engine

12. Working of Turbojet Engine
•It has a diffuser section at the inlet for realizing some compression of air passing through this
section.
•Due to this air reaching the compressor section has pressure more than ambient pressure. This
action of partly compressing air by passing it through the diffuser section is called “ramming
action” or “ram effect”.
•Subsequently, the compressor section compresses air which is fed to the combustion chamber and
fuel is added to it to cause combustion.
•Combustion products available at high pressure and temperature are then passed through the
turbine and expanded there.
•Thus, the turbine yields positive work which is used for driving the compressor.
•Expanding gases leaving the turbine are passed through the exit nozzle where it is further expanded
and results in the high-velocity jet at the exit.
•This high-velocity jet leaving the nozzle is responsible for getting the desired thrust for propulsion.
•Theseenginesareusedincommercialandmilitaryaircrafts.

13. Turbojet Engine with Afterburner
•Anafterburner(orareheat)isanadditional
componentpresentonsomejetengines,mostly
militarysupersonicaircraft.
•Itspurposeistoprovideanincreaseinthrust,
usuallyforsupersonicflight,takeoffandfor
combatsituations.
•Afterburningisachievedbyinjectingadditional
fuelintothejetpipedownstreamof(i.e.after)the
turbine.
•Theadvantageofafterburningissignificantly
increasedthrust;thedisadvantageisitsveryhigh
fuelconsumptionandinefficiency,thoughthisis
oftenregardedasacceptablefortheshortperiods
duringwhichitisusuallyused.

14. Thrust of Turbojet Engine

15. Turbo-Prop Engine

16. Working of a Turboprop Engine
•The turboprop uses a gas turbine core to turn a propeller.
Propeller engines develop thrust by moving a large mass
of air through a small change in velocity,
•Inaturbopropengine,theturbinesareconnectedtoa
propellershaftandthusalmostalltheenergyextractedby
theturbinesisusedtorotatethepropeller.
•Thereductiongearboxislocatedinbetweenthepropeller
andthepropellerturbineshaft.Thegearboxreducesthe
RPMofthepropellersothatitrotatesatanacceptable
rate.
•Mostturboproppropellersoperateatabout2000to3000
RPM.Thisisasignificantreductionastheengineturbine
rotatesatabout20,000to30,000RPM.
•Turboprop engines are used in small passenger planes,
cargo planes, etc.

17. Thrust of Turboprop Engine

18. Turbofan Engine

19. Working of Turbofan Engine
•Turbofan engine is a modified turbojet engine in which additional thrust is realized by putting a fan
at the entry of the engine casing.
•Fan blades propel bypass air around the engine core between the inner and outer engine casing.
This air does not participate in combustion but provides additional thrust while leaving through the
exit nozzle.
•The propeller fan put at the inlet to the engine sucks air and it passes through the bypass passage as
shown up to the exit nozzle end.
•Thus there are two streams of air flowing, one air stream gets rammed, compressed, burnt,
expanded in the turbine and finally passes through the exit nozzle and the other air stream passes
through the passage between outer and inner casings from inlet to nozzle exit.
•Total thrust created will be due to two jet streams one due to cold air or fan air and the other due to
burnt gases leaving the turbine.

20. Thrust in Turbofan Engine

21. Performance Parameters of Jet Engines
•Thrust power (TP),
•Propulsive power (PP),
•Propulsive efficiency (η
prop)
•Thermal efficiency (η
th) and
•Overall efficiency (η
overall)

22. Performance of Jet Engines
1. Thrust Power, (TP):
•Thrust power indicates the actual power available for propulsion.
•It refers to the work done per unit time by the engine.
•This thrust power can be expressed by the product of thrust and velocity with which the engine
moves (flight velocity).

23. Performance of Jet Engines
2. Propulsive Power, (PP):
•Propulsive power indicates the total energy available for propulsion.
•It can be estimated by the difference between the rate of kinetic energy entering with air and
leaving with jet of exit gases.

24. Performance of Jet Engines
3. Propulsive Efficiency, (η
prop):
•Propulsive efficiency is a measure of effectiveness by which propulsive power is transformed into
thrust power i.e., how efficiently the propelling duct can propel the engine.
•It can be defined as the ratio of thrust power (TP) to propulsive power (PP).
•Propulsive efficiency is also called Froude efficiency.

25. Performance of Jet Engines
4. Thermal Efficiency (η
th):
•Thermal efficiency can be given by the ratio of kinetic energy available and total heat supplied.

26. Performance of Jet Engines
5. Overall Efficiency, (η
overall):
•Overall efficiency can be given by the ratio of useful work done to the energy supplied.

27. Rocket
Propulsion

28. Introduction to Rocket Propulsion
•Propulsion is a method by which an object is propelled
in a particular direction.
•The word “propulsion” stems from the Latin word
propellere, where pro means forward or backward and
pellere means drive or push.
•Spacecraft must produce thrust which must be equal to
the drag force caused due to the fluid motion over the
body of this spacecraft and the gravitational force.
•For accelerating the spacecraft, one needs to supply
higher thrust than that of drag forces and gravitational
force acting on it.
Ankur Sachdeva, ME, KIET Group of Institutions

29. History of Rocket Propulsion
•The real rocket was invented by the Chinese around the tenth century AD while experimenting
with gunpowder and bamboo.
•The gunpowder was discovered in the ninth century AD by a Taoist alchemist.
•Subsequently, Feng Jishen managed to fire a rocket using gunpowder and bamboo.
•A Chinese scholar, Wan Hu, had developed a rocket sled that comprised of a series of rockets
attached to the seat.
Ankur Sachdeva, ME, KIET Group of Institutions

30. Principle of Rocket Propulsion
•Rocket engines are non-air-breathing engines and carry their own
oxidizer for burning of fuel.
•Rocket propulsion is realized by the thrust produced by combustion
products leaving the exit nozzle.
•It has an injection system for fuel and oxidizer followed by a
combustion chamber and exit nozzle.
•In rocket engines the combustion products get discharged from the
exit nozzle with supersonic velocity and thus have very high kinetic
energy.
•Rocket gets the desired thrust by the reaction available from the
nozzle stream.
•Thrust is available due to the change of momentum and pressure
with which the jet comes out.

31. Performance Parameters of
Rocket Engines
•For maximizing thrust exit velocity should be maximized,
and pressure difference at exit (p
e – p
a) should be maximized.
•Thus rocket would get maximum thrust when atmospheric
pressure is not there i.e., p
a = 0, which means maximum
thrust would be available in a vacuum.
•Thrust could also be given in terms of rocket performance
parameters called effective jet velocity.

32. Performance Parameters of
Rocket Engines

33. Types of Rocket Engines
•On the basis of application:
•Space Rockets
•Military Rockets
•Weather Rockets
•Aircraft propulsion
•On the basis of no. of stages:
•Single stage
•Multi stage
•On the basis of size and range:
•Small-range small rockets
• Large-range large rockets
Ankur Sachdeva, ME, KIET Group of Institutions

34. Chemical Rocket Engines
•In the case of chemical rocket engines, chemical energy released during the burning of fuel and
oxidizer is used to raise the temperature and pressure of the gas which is expanded in a CD nozzle
to produce thrust.
•Generally, the hot gases at high pressure are accelerated to high supersonic velocities in the range
of 1500–4000 m/s for producing thrust.
•Both fuel and oxidizer are being carried along with the engine unlike in air-breathing engines.
•Based on the physical state of the propellant (fuel and oxidizer), chemical rocket engines can be
broadly divided into three categories:
(1) solid propellant,
(2) liquid propellant,
(3) hybrid propellant.
Ankur Sachdeva, ME, KIET Group of Institutions

35. Solid Propellant Rocket Engines
•Solid-propellant rocket engine (SPRE) is one of the oldest non-air-breathing
engines.
•The solid propellant composition, which was initially black powder, underwent
a series of changes with time.
•Propellant, which mainly consists of fuel, oxidizers, and various additives, is
entirely stored within the combustion chamber in the form of blocks of definite
shape called grain and is supported by the walls.
•Grain contributes to around 80%– 95% of the total mass of an SPRE.
•The igniter initiates the combustion process on the surface of the propellant
when actuated with the help of an electrical switch.
•As a result, the propellant grains will start burning and filling the empty
combustion chamber, hence building up the chamber pressure.
•Subsequently, the high-temperature and high-pressure gases are expanded in the
supersonic nozzle to produce the requisite thrust.
• A solid rocket engine is considered to be a non-air-breathing vehicle without
any moving parts
Ankur Sachdeva, ME, KIET Group of Institutions

36. Solid Propellant Rocket Engines
Advantages
•It is simple to design and develop.
•It is easier to handle and store unlike liquid
propellant.
•Detonation hazards of many modern SPREs are
negligible.
•Better reliability than Liquid Propellant Rocket
Engine (LPRE) (>99%).
•Development and production cost of SPREs is
much smaller than that of LPREs, especially in the
high-thrust bracket
Disadvantages
•It has lower specific impulse compared to LPREs
and hybrid propellant rocket engines (HPREs).
•It is difficult to turn off its operation unlike in an
LPRE.
•Transport and handling of solid propellants are
quite cumbersome.
•The cracks on the propellant can cause an
explosion.
Ankur Sachdeva, ME, KIET Group of Institutions

37. Liquid Propellant Rocket Engines
•Around 1927, an American professor, Robert Goddard, had designed and developed an
LPRE.
•In addition to having a liquid form, this propellant can be stored in a separate tank and can
be controlled easily, and hence thrust can be varied easily unlike in an SPRE.
•As LPREs are stored in separate tanks unlike SPRE, one can achieve a higher level of
thrust and is thus considered to be more powerful than an SPRE. Therefore, it is preferred
for large spacecraft and ballistic missiles.
•Both fuel and oxidizer propellants are stored separately in special tanks at high pressure.
•The pressurized liquid propellants are converted into spray consisting of arrays of droplets
with the help of atomizers.
•An igniter is used to initiate the combustion process on the surface of the propellant.
•As a result, the propellant will start burning and fill up the empty thrust chamber, thereby
building up pressure in the chamber
•High-temperature and high-pressure gases are expanded in a CD nozzle to produce the
requisite thrust.
Ankur Sachdeva, ME, KIET Group of Institutions

38. Liquid Propellant Rocket Engines
Advantages
•An LPRE can be reused.
•It provides greater control over thrust.
•It can have higher values of specific impulse.
•It can be used for long-duration applications.
•It is easy to control this engine as one can vary the
propellant flow rate easily.
•The heat loss from the combustion gas can be
utilized for heating the incoming propellant.
Disadvantages
•This engine is quite complex compared to the
SPRE.
•It is less reliable as there is a possibility of
malfunctioning of the turbopump injectors and
valves.
•Certain liquid propellants require additional safety
precaution.
•It takes much longer to design and develop.
•It becomes heavy, particularly for short-range
application.
Ankur Sachdeva, ME, KIET Group of Institutions

39. Hybrid Propellant Rocket Engines
•This engine can use both solid and liquid types of propellants.
•Most widely used propellant combination is a liquid oxidizer along with a solid
propellant.
•Only the oxidizer propellant in the present example is stored in a special tank under
high pressure.
•The pressurized propellants are converted into spray consisting of arrays of droplets
with the help of atomizers.
•It consists of major components, namely, a propellant feed system, a combustion
chamber, a solid fuel grain, an igniter system, and a nozzle.
•Some of the propellant evaporates due to the recirculation of hot gases and comes into
contact with the gaseous fuel that emanates from the solid fuel grains due to pyrolysis
•The combustion products start burning and fill the empty thrust chamber, thereby
building up pressure inside the chamber.
Ankur Sachdeva, ME, KIET Group of Institutions

40. Hybrid Propellant Rocket Engines
Advantages
•An HPRE can be reused.
•It provides greater control over thrust.
•It has relatively lower system cost compared
to the LPRE.
• It can have higher values of average specific
impulse compared to the SPRE.
Disadvantages
•This engine is quite complex compared to the
LPRE.
•It takes much longer to design and develop.
•It becomes heavy, particularly for short-range
applications.
•Certain liquid propellants require additional
safety precautions.
Ankur Sachdeva, ME, KIET Group of Institutions

41. What is a Propellant
•A propellant consists of all the chemical materials, including fuel and oxidizer,
along with certain additives necessary for sustaining the combustion process to
produce high-pressure hot gases, that which are expanded in a nozzle to produce
thrust.
•Principal ingredients of a propellant are the fuel and the oxidizer.
•Fuel is a chemical substance that reacts with an oxidizer while releasing thermal
energy
Ankur Sachdeva, ME, KIET Group of Institutions

42. Classification of Propellants
Ankur Sachdeva, ME, KIET Group of Institutions

43. Classification of Propellants
•Homogeneous propellant:
•fuel and oxidizer are contained in the same molecule of the propellant.
•Heterogenous propellant:
•solid fuel and oxidizer retain their respective physical identities.
•Monopropellants:
•A liquid propellant that contains both the fuel and the oxidizer in a single chemical is called a monopropellant.
•Example:
•Hydrogen Peroxide, Hydrazine, Nitroglycerine, and Nitromethane
•Bipropellants:
•A liquid propellant in which an oxidizer and a fuel are stored separately in the tanks and mixed in the combustion
chamber.
•Example:
•Liquid oxygen and liquid hydrogen, Liquid oxygen and kerosene.
•Hypergolic propellants:
•Liquid fuel and oxidizer react spontaneously without external ignition energy
•Nonhypergolic propellants:
• A suitable amount of ignition energy is provided to ignite the liquid fuel and oxidizer for combustion to take
place
Ankur Sachdeva, ME, KIET Group of Institutions

44. Numerical Problem (AKTU Exams 2021-22)
•A turbojet flying at 800 km/h has an air flow rate of 50 kg/s.
The enthalpy drop across the nozzle is 200 kJ/kg. The air-fuel
ratio is 80 and the calorific value of fuel is 41 MJ/kg.
Estimate propulsive power, Thrust power, propulsive
efficiency, thermal efficiency, and overall efficiency of the
unit.

45. Numerical Problem Numerical Problem
(AKTU Exams 2018-19)
•Q. A turbo jet engine consumes air at the rate of 60.2 kg/sec
when flying at a speed of 1000 km/hr. Calculate: (a) the exit
velocity of jet when the enthalpy change in the nozzle is 230
kJ/kg and velocity coefficient is 0.96, (b) fuel flow rate in
kg/sec when air fuel ratio is 70:1, (c) thrust specific fuel
consumption, (d) propulsive power, (e) propulsive efficiency,
and (f) the overall efficiency