gas dynamics-rocket propulsion

GDJP Anna University

Rocket

PROPULSION

Rocket Propulsion-Introduction


Aircraft engine application is limited to altitudes of about

100,000 ft or less. But Rocket can function outside the

atmosphere

Rocket can operate in vacuum and achieve any altitude

The thrust of the rocket engines is independent of the flight

speed

Rocket works based on Newton’s law of motion

Rockets carry own oxidizer and fuel tank

Rocket Propulsion-Introduction Cont..


Newton’s First law - Applied to Rocket Liftoff



Rocket Engine Thrust



Forces on a Model Rocket



Flight of a Model Rocket



Forces at Liftoff



Acceleration at Liftoff



Forces in Powered Flight



Forces in Coasting Flight

The rocket uses up all its fuel, the engine goes out and the thrust goes to zero



Forces during Recovery



Terminal Velocity

Rocket Propulsion- Types


Rockets can be classified by

A.Based on source of energy1. Chemical Rockets 2. Solar Rockets 3. Electrical Rockets

4. Nuclear Rockets

B.Based on propellant1. Liquid propellant 2. Solid propellant 3. Hybrid propellant

C.Based on application field1. Space rocket 2. Military rocket 3. Aircraft rocket 4. Booster rocket

D.Based on number stage 1. Single stage rocket 2. Multi stage rocket

E.Based on size and range1. Short range and small rocket 2. Long range and large rocket

Rocket Propulsion- Chemical Rocket Propellants


Solid Propellant

Liquid Propellant

Propellant is a chemical mixture burned to produce thrust in rockets andconsists of a fuel and oxidizer

In a liquid system the fuel and oxidizer are separately stored and are sprayed under high pressure (20 to 60 bar) into the combustion chamber

In solid system, both fuel and oxidizer are contained in the propellant grain and the burning takes place on the surface of the propellant

Rocket Propulsion- Solid Propellant grains


Well mixed fuel and oxidizer called as Propellant grain. Several grain configurations are employed to obtain burning at the desired rate.

Propellant cross section

Rocket Propulsion- Solid Propellants


Exit velocity range1500 m/s to 3000 m/s

Star-grained solid rocket motor

Fuel : Plastic, Resin material

Oxidizer : Nitrates, Perchlorates

Rocket Propulsion- Hybrid Propellants


Regulator

-

Liquidoxidizer

Oxidizer injector

Solid Fuel

Nozzle

Fuel : Solid type Oxidizer : Liquid type

Beryllium hydride(Be-H2 ) Fluorine(F2 )Lithium hydride (Li H) Chlorine Trifluoride(ClF3)Lithium hydride (Li H) Nitrogen tetroxide(N2O4 )Hydrocarbon(CH2)n Nitrogen tetroxide(N2O4 )

Special Type: Hypergolic propellants

Rocket Propulsion- Liquid Propellant Types


Liquid Propellants

Monopropellant Bipropellant

Fuel and Oxidizer in a single chemical – MonopropellantsHydrogen peroxide (H2O2), Hydrazine (N2H4), Nitro methane (CH3NO2)

Fuel and Oxidizers are different in chemical - Bipropellants

Oxidizer Fuel mox/mf Combustion temperature (K)

Liquid Oxygen Gasoline, Hydrazine, UDMH, Ethanol

2.5, 0.92, 1.65, 1.8

3294, 34003600, 3422

Hydrogen Peroxide Hydrazine, UDMH

1.844.54

28172922

Nitric acid (RFNA) Aniline, Hydrazine

3.001.47

30453083

Rocket Propulsion- Propellant Properties


Liquid Propellants1) Energy released during combustion should be high2) High density propellants are preferred3) Low freezing point propellants are preferred 4) Non-corrosive, chemically stable and should not absorb moisture5) Low values of vapor pressure and viscosity are preferred6) They should not be poisonous and hazardous7) It should be cheap and abundantly available

Solid Propellants1) Propellants should be easily available and safe to handle2) Physical and chemical properties should not change considerably during

processing as well as during time3) It should release large amount of heat energy during combustion4) They should be chemically inert before ignition5) Low molecular weight and high density of propellants are preferred6) Exhaust gases should be smokeless and colorless7) Propellants should not react with atmospheric air and moisture

Rocket Propulsion- Liquid Propellant feed system


Liquid Propellant feed system - Cont..


Liquid Propellant feed system - Cont..


Pressure Fed system

Rocket Thrust


Solid Motor

Liquid Motor

eAeceemm ..

Rocket Nozzle


Rocket Nozzle- Function


The function of the nozzle is to convert the chemical-thermal energy generated in the combustion chamber into kinetic energy.

The nozzle converts the slow moving, high pressure, high temperature gas in the combustion chamber into high velocity gas of lower pressure and temperature. Since thrust is the product of mass and velocity, a very high gas velocity is desirable.

The nozzle is usually made long enough (or the exit area is great enough) such that the pressure in the combustion chamber is reduced at the nozzle exit to the pressure existing outside the nozzle.

It is under this condition, Pe=Po, where Pe is the pressure at the nozzle exit and Po is the outside ambient pressure, that thrust is maximum and thenozzle is said to be adapted, also called optimum or correct expansion.

When Pe is greater than Po, the nozzle is under-extended. When the opposite is true, it is over-extended.



The most efficient nozzle (1) is contoured to the exhaust stream, allowing the escaping gas to expand just enough to fill the nozzle.

A nozzle that lets the gas expand too much (2), or too little (3), wastes the energy and thrust potential of the exhaust system.

The most efficient nozzle (1) is contoured to the exhaust stream, allowing the escaping gas to expand just enough to fill the nozzle.

A nozzle that lets the gas expand too much (2), or too little (3), wastes the energy and thrust potential of the exhaust system.

C-D Nozzle

Rocket Propulsion – Combustion-Liquid Propellant


Combustion process involves: injection, atomization, mixing, vaporization, ignition andexothermic chemical reaction between fuel and oxidizer

Propellant Injectors

Rocket Propulsion – Solid Fuel Geometry


A solid fuel's geometry determines the area and contours of its exposed surfaces, and thus its burn pattern. There are two main types of solid fuel blocks used in the space industry. These are cylindrical blocks, with combustion at a front, or surface, and cylindrical blocks with internal combustion.

In the first case, the front of the flame travels in layers from the nozzle end of the block towards the top of the casing. This so-called end burner produces constant thrust throughout the burn.

In the second, more usual case, the combustion surface develops along the length of a central channel. Sometimes the channel has a star shaped, or other, geometry to moderate the growth of this surface.

Rocket Propulsion – Solid Fuel Geometry- Thrust profile


Cylindrical Channel – Progressive burning

Cylindrical Channel with central cylinder- Neutral burning

Star Profile Cruciform Profile – Regressive burning

Rocket Propulsion – Solid Fuel Geometry- Thrust profile


Double anchor profile

Cog profile

Rocket Propulsion- Solid motor- Combustion


Linear burning rateThe burning rate of the propellant grain depends on the initial temperature of the grain before combustion, equilibrium combustion pressure and the ratio of grain surface area and the nozzle throat area.

The burning surface of a rocket propellant grain recedes in a direction perpendicular to this burning surface. The rate of regression, typically measured in millimeters per second (or inches per second), is termed burn rate.

The linear burning rate is given by, Saint-Roberts law (regression law)where r- burn rate ; a-burn rate coefficient ; Pc - combustor pressure

n-pressure exponent (range 0.2 to 0.8)a =f (chemical composition, initial temperature of the propellant grain )n =f( combustor pressure)

ncaPr

Combustion limitSolid propellant grain requires certain minimum value of the combustion pressure for stable combustion. This minimum value of the pressure is known as the combustion limit ( it depends on type of propellant employed)

Usually it lies between the range of 5 and 55 bar

Rocket Propulsion- Solid motor- Combustion


Propellant consumption rate, cAncPacrAcm

.

.

Propellant flow rate, (1))(cm Vdt

dpm

. .

mass of gas in bore

mp

.

Equilibrium Combustion Pressure

Solid motor- Combustion Cont..


Propellant area ratio

The ratio of the surface area available for burning of a propellant grain and the throat area of the exhaust nozzle is known as the propellant area ratio ( Kp)

*A

cApK r and Kp increase with the combustion pressure



The function of the nozzle is to convert the chemical-thermal energy generated in the combustion chamber into kinetic energy.

The nozzle converts the slow moving, high pressure, high temperature gas in the combustion chamber into high velocity gas of lower pressure and temperature. Since thrust is the product of mass and velocity, a very high gas velocity is desirable.

The nozzle is usually made long enough (or the exit area is great enough) such that the pressure in the combustion chamber is reduced at the nozzle exit to the pressure existing outside the nozzle.

It is under this condition, Pe=Po, where Pe is the pressure at the nozzle exit and Po is the outside ambient pressure, that thrust is maximum and thenozzle is said to be adapted, also called optimum or correct expansion.

When Pe is greater than Po, the nozzle is under-extended. When the opposite is true, it is over-extended.

gas dynamics-rocket propulsion

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