ME2351 GAS DYNAMICS AND JET PROPULSION

Unit- IV

Jet Propulsion

Jet Propulsion

Principle of jet propulsion is obtained from the application of “Newton’s third law”

“For every action there is an equal

and opposite reaction”.

Thrust

When the fluid is to be accelerated, a force is required to produce this acceleration in this fluid.

At the same time, there is an equal and opposite reaction force acting on this fluid.

This opposite reaction force of the fluid on the engine is known as “Thrust”.

Classification of Jet Propulsion

1. Air breathing enginesCombustion takes place by using

atmospheric air.2. Rocket engines

Combustion takes place by using its own oxygen supply.

Classification of Air breathing engines

1. Ram jet engine2. Pulse jet engine3. Turbo jet engine4. Turbo prop engine5. Turbo fan engine

Ramjet Engine

Headings:1. Diagram2. Construction3. Working4. Advantages5. Disadvantages6. Applications

Diagram

Main parts

Supersonic diffuserSubsonic diffuserCombustion chamberDischarge nozzle section

Ram effect Ram Pressure:The function of supersonic and

subsonic diffusers are to convert the kinetic energy of the entering air into pressure energy. This energy transformation is called ram effect and the pressure rise is called the ram pressure.

AdvantagesVery simple and does not have any

moving parts.Low cost.Less maintenance.Better specific fuel consumption.No upper limit to the flight speed.Light weight when compared with

turbojet engine.

Dis Advantages

Take off thrust is zero, it is not possible to start a ram jet engine without an external launching device.

Very difficult to design a diffuser.Low thermal efficiency.

Applications

High speed air crafts and missiles due to its high thrust and high operational speed.

Subsonic ramjets are used in target weapons.

Pulse Jet Engine (or) Flying Bomb

Main Parts

DiffuserValve grid which contains springs that close on

their own spring pressureCombustion chamberSpark plugTail pipe or discharge nozzle.

Advantages

Very simple device next to ramjet engine.

Less maintenanceLow costLight weight when compared with

turbojet engine.

Dis AdvantagesHigh rates of fuel consumption.Maximum flight speed is limited to

750 km/h.Low propulsive efficiency than

turbojet engines.High degree of vibration leads to

noise pollution.

Applications

Subsonic flightsGerman V-I missilesTarget aircraft missilesPilotless air craft

Turbojet EngineMost common type of air breathing

engine is the turbojet engine.Main parts

1. Diffuser2. Rotary compressor3. Combustion chamber4. Turbine5. Exhaust nozzle

Diagram

Advantages

• Simple construction• Less wear and tear• Less maintenance cost• It runs smoothly• High flight speed• Low grade fuels like kerosene, paraffin can be

used. This reduces fuel cost.

Dis advantages

• Low take-off thrust and poor starting characteristics.

• High fuel consumption.• Costly materials are used.• Sudden decrease of speed is difficult.• Propulsive efficiency and thrust are lower at

lower speeds.

Applications

• Piloted air-crafts.• Military air-crafts.

Turbo-Prop Engine (or) Turbo-Propeller Engine

• It is very similar to turbojet engine. In this type, the turbine drives the compressor and propeller.

• Main parts:DiffuserCompressorCombustion chamberTurbineExhaust nozzleReduction gear and Propeller

Diagram

Advantages

• High take-off thrust• Good propeller efficiency at a speed below 800km/h.• Reduced vibration and noise.• Better fuel economy.• Easy maintenance.• Wide range of speeds.• Power output not limited.• Sudden decrease of speed is possible by thrust

reversal.

Disadvantages

• Propeller efficiency is rapidly decreases at high speeds due to shocks and flow separation.

• It requires a reduction gear which increases the cost of the engine.

• More space needed than turbo jet engine.• Engine construction is more complicated.

Applications

• The turbo prop engine is best suited for commercial and military air-craft operation due to its high flexibility of operation and good fuel economy.

Turbofan Engine

• The turbofan engine is a combination of the turbo prop and the turbojet engines combining the advantages of both.

• Main parts:– Fan– Compressor– Combustion chamber– Turbine– Fan nozzle

Diagram

By Pass Ratio:The ratio of the mass flow rates of cold air and the hot air is known as By Pass Ratio.

Advantages

• Thrust is developed is higher than turbo jet engine.

• Weight per unit thrust is lower than turbo prop engine.

• Less noise• High take-off thrust.

Disadvantages

• Increased frontal area.• Fuel consumption is high compared to turbo

prop engine.• Construction is complicated compared to

turbo jet engine.• Lower speed limit than turbojet engine.

Unit-V

Rocket Propulsion

The thrust required for the propulsion of

rocket is produced by the high velocity of

gases leaving from the nozzle which is similar

to jet propulsion.

In jet propulsion, the oxygen required for

combustion is taken from the atmosphere. But in

the case of rocket propulsion, since the altitude

of rocket engine is very high, enough oxygen is

not available in the atmosphere. So the oxygen is

filled in a tank in the rocket engine itself and

used for combustion purposes.

Classification of Rocket Engines(1)

Chemical rocket engines1. Liquid propellant rocket engine2. Solid propellant rocket engine3.Hybrid propellant rocket engine

Nuclear rocket enginesElectrical rocket enginesSolar rocket engines

Classification of Rocket Engines(2)

Based on Application1. Space rockets2. Military rockets3. Weather forecasting rockets4. Booster rockets

Based on Number of stages1. Single stage rockets2. Multistage rockets

Based on Size and Range1. Short range small rocket2.Long range large rocket

Liquid Propellant Rocket Engine

Advantages• It can be reused after recovery. So it is

economical.• Combustion process is controllable.• Speed regulation.• High specific impulse.• More economical for long range operation.• Malfunctions and accidents can be rectified at

any stage.

Disadvantages• More complicated.• High manufacturing cost.• Propellants are poisonous and corrosive.• Size and weight of the engine is more

compared to solid propellant rocket engine.• High vibration.• Proper insulation is needed.

Liquid Propellants

Types:1. Monopropellants2. Bipropellants

Monopropellants

A liquid propellant which contains both the fuel and oxidizer in a single chemical is known as monopropellant.

It is stable at normal ambient conditions and liberates thermo-chemical energy on heating.

Examples:1. Nitroglycerine2. Nitro methane3. Hydrogen Peroxide4. Hydrazine

Bipropellants

If the fuel and oxidizer are different from

each other in its chemical nature, then the

propellant is called bipropellant.

Widely used in liquid propellant rocket

systems.

Oxidizer Fuel

Liquid oxygen Gasoline, Methane, Ethanol, Hydrazine

Red fuming nitric acid Aniline, Ethanol

White fuming nitric acid

Alcohol

Hydrogen peroxide Hydrazine, Ethanol, Methanol

Properties of Liquid Propellants

• High calorific value.

• High density.

• Low vapour pressure and density.

• High specific heat and thermal conductivity.

• Products of combustion produces high jet velocity.

• Non-corrosive and non-reactive with components.

• Should not be poisonous and hazardous

• Cheap and easily available.

• Easily ignitable.

Liquid Propellant Feed System

Types:

1. Gas pressure feed system.2. Pump feed system.

Gas Pressure Feed system

Pump Feed System

Combustion

Basic Processes:InjectionAtomizationMixingVaporizationIgnitionChemical reaction between fuel and oxidizer

Various methods to atomize and mix the fuel and oxidizer

Parallel stream typeImpinging stream typeSpray injection type

Solid Propellant Rocket Engine

Advantages

• Simple in design and construction• No need for feed system (No pump & valves)• Less vibration due to absence of moving parts.• Less maintenance.• Suitable for short range applications.• Problem arising from the sudden emptying of

propellant tanks are absent.

Disadvantages• In case of emergency it is difficult to stop the

engine in the mid way.• Decrease of speed is not possible.• Low specific impulse.• Nozzle cooling is not possible.• Transportation and handling of these rockets

before firing require greater care due to the presence of propellants throughout.

Solid Propellants

Types:

1. Heterogeneous (or) Composite propellants.

2. Homogeneous propellants.

Heterogeneous propellants

Fuels:Plastics, Polymers and PVC

Oxidizers:Nitrates and Per chlorates

Homogeneous propellants

Nitroglycerine and nitrocellulose

It combines the properties of fuel and

oxidizers.

Properties of Solid Propellants

• It should release large amount of heat during combustion.

• Physical and chemical properties should not change during processing.

• High density.• It should not be poisonous and hazardous.• Cheap and easily available.• Non-corrosive and non-reactive with components of

the engine.• Storage and handling should be easy.

Hybrid propellant rocket engine

Advantages

• Speed regulation is possible by regulating the supply of oxidizer.

• High load capacity.• Hybrid rockets are lighter when compared to

the liquid propellant type rockets.• Higher fuel density.• In case of accident or crash the explosion is less

destructive compared to the liquid propellant rocket engines.

Disadvantages

• Nozzle erosion cannot be avoided.

Hybrid propellants

Fuel OxidizerBeryllium hydride FluorineLithium hydride Chlorine trifluorideLithium hydride Nitrogen tetroxide

Hydrocarbon Nitrogen tetroxide

Nuclear Rocket Engine

Electrical Rocket Engine

The fundamental requirement for rocket

propulsion is the generation of very high exhaust

velocities in order to minimize the propellant

consumption.

Electrical propulsion engines generate exhaust

velocities from about 4 to 100 times those of

chemical rocket engines.

Types of Electrical rocket engines

1. Arc plasma rocket engine

2. Ion rocket engine

3. Magneto-plasma rocket engine

Arc plasma rocket engine

Ion Rocket Engine

Magneto-Plasma Rocket Engine(or) Magneto Hydrodynamics Rocket Engine (MHD)