MISSILE PROPULSION Flashcards
What is Missile propulsion?
Propulsion is the means of providing power to accelerate the missile body and sustain, to reach the required target. The basis for the working of missile propulsion systems are the Newton’s laws of motion.
First Law
A body continues in its state of rest or in uniform motion in a straight line unless acted upon by an unbalanced force.
Second Law
The rate of change of momentum is proportional to the impressed force and takes place in the direction of the force.
Third Law
Action and reaction are equal and opposite. That is, if a body exerts a force on another body, the other body too exerts a force on the first body of the same
magnitude but in the opposite direction.
Newton’s law and missile propulsion
The propulsion of a missile is achieved with the help of a rocket engine. It produces thrust by ejecting very hot gaseous matter, called propellant. The hot
gases are produced in the combustion chamber of the rocket engine by chemical reactions. The propellant is exhausted through a nozzle at a high speed. This
exhaust causes the rocket to move in the opposite direction (Newton’s third law).
As per the second law, also called the law of momentum, the rate of’ change of momentum causes a force to be developed. The change in momentum
of the missile body including the rocket motor casing, the nozzle and other systems due to the ejected matter
creates a force leading to the propulsive action on the missile body.
The missiIe, propelled into air, would continue to move if there were no other forces acting on it. However, resistance to its forward movement due to air (commonly called the aerodynamic drag) and the force of gravity acting downwards towards the centre of the earth are to be taken into account. By using Newton’s first law, also called the law of inertia, compensative forces are imparted to the missile to overcome these negative forces.
WHAT ARE PARTS OF PROPULSION SYSTEM - Igniter, Nozzle,Chamber
Nozzle:-
All types of rocket propulsion engines contain a chamber, a nozzle, and an igniter. The chemical reaction of propellant chemicals (usually a fuel and an oxidiser) takes place in the chamber and produces
gases. The energy due to this high pressure reaction permits the heating of the product gases to a very high temperature (2000-3500 “C). These gases
subsequently are expanded in the nozzle and accelerated to high velocities (2000-4500 ds). The nozzle design, i.e., its shape and size are critical for the efficient function of the propulsion system. The
theoretical model of the thermodynamic processes inside a rocket furnish the analytical data necessary for this.
The nozzle is essentially a conduit of varying crpss-section from a maximum area to a section of minimum cross-section (called the throat of the nozzle)
and again enlarging to larger cross-section. The nozzle would be subsonic, sonic or supersonic depending
upon whether the exhaust velocity is below, equal to or greater than the speed of sound in air.
WHAT ARE PARTS OF PROPULSION SYSTEM - Igniter, Nozzle,Chamber
Igniter-The igniter, though a tiny element among the components of the rocket engine or rocket motor, has the function of initiating the proplusion system. Igniter is the device that helps to start the burning of the main propellant grain of the rocket
motor. The propellant ignition consists of a series of complex rapid events, commencing with the receipt of an electrical pulse and heat generation and heat transfer from the
ignition products (hot gases and particles) to the propellant grain surface. Flame spread is achieved to bum the entire surface area to fill the free volume of
the chamber. Ignitess can be categorized as pyrotechnic, pyrogen, etc. Conventional igniters are made of heat releasing compounds such as black powder, metal oxides and metal powder formulations
and initiated by electrical means by passing current through an element (wire) which is imbedded in the pyrotechnic mixture.
There are certain propellant combinations which do not need an igniter and they are called hypergolic. These propellants burn spontaneously when they come in contact in a certain proportion.
WHAT ARE TYPES OF TYPES OF PROPULSlON SYSTEMS
Missile propulsion will be are of the following two types:
Air breathing, and Non-air breathing.
Air breathing:- The air breathitlg rocket engines use the surronnding medlium of air for the support of their oxidiser. Thus. they can be used only used within the Earth’s atmosphere.
In this case the advantage is taken of the
atmospheric oxygen for burning the fuel thereby reducing the quantity of propellants to be carried by missile. This lowers the weight of the rocket greatly
as 75 per cent of the total propellant’s weight is due to oxidiser. This can be used either by using small turbojet engines to power the missile or ramjets.
Unlike turbojets which have extensive rotary machinery (and are therefore costly), there is no such system in ramjets. Here the speed of incoming air is utilized, i.e., when we slow it down using the geometry
to intake passage, its pressure rises. Then we add fuel to this and through a nozzle obtain the thrust force.
Here a conventional rocket motor (normally solid type) called booster is used to provide the velocity initially at which a ramjet engine can start operating
in a steady way. Ramjets cannot operate without atmosphere and also at extremely high speeds. They also have constraints of producing high thrust for a given size. They are highly suited for long range, low
manoeuvre, steady and level flying missiles. For such missions they result in a lighter missile.
Non-air breathing propulsion system
In the case of non-air breathing
engines the rocket engine itself’ carries its fuel and oxidiser on board and hence can be used in space above the Earth’s atmosphere also and is thus
independent of the air medium.
Depending on the physical state of matter of the propellant used, the rocket propulsion system is designated as a solid rocket rnotor, a liquid propulsion system or a hybrid propulsion system.
WHAT IS PROPELLANT GRAIN
Propellant grain. Solid propellants have fuel and oxidiser mixed together in a suitable proportion. Finished propellarlt body called grain have rigid shape and form as per design. This shape is obtained by casting or extrusion under pressure.
What are types of propellants.
On composition basis there are two types of propellants.
Homogeneous: They are so called because in these oxidiser and fuel are at molecular level. Famous example being ‘double base’ type which is a mixture
of nitrocellulose and nitroglycerine in a certain proportion. They are gelled into a semi-rigid body and extruded. They have a fairly long shelf life of more than twelve years.
Heterogeneous: In this, as the name suggests, the oxidiser and fuel are mixed mechanically in a mixer. They are also called composite propellants.
Oxidisers are inorganic crystalline salts like
perchlorates or nitrates of sodium, potassium or ammonia, while the fuel, which also acts as binder, is an organic resin. Famous resins used belong to the
polybutadiene family.
Homogeneous propellants (double base) give specific impulse of about 220 seconds maximum, while composite propellants give 260 seconds and have
higher densities but have smaller shelf life. In longer range missiles only composite propellants are used while in smaller tactical missiles, double base propellants are used. Most of the current ballistic missiles are based on solid propellants because they are storable and ready
for use and minimal logistic support is needed.
Propellant grains may vary in size depending on application. For example, the smallest grain for an anti-tank missile may be only a few kilograms while
the largest is 125 tonnes used in space shuttle boosters where two solid boosters are used.
