ROCKET PROPELLANTS:These are the chemical substances which on ignition provide thrust for the rocket to move forwarded.
A propellant is a combination of an oxidiser and a full which when ignited under goes combustion to release large
quantities of hot gases .The passage of hot gases through the nozzle of the rocket motor provides the necessary
thrust for the rocket to move forward according to Newton’s third law of motion i.e. "to every action there is an equal
and opposite reaction".
TYPES OF ROCKET PROPELLANTS :1. Solid Propellants
2. Liquid Propellants
3. Hybrid Propellants
1. SOLID PROPELLANTS
These are the mixtures of a solid fuel and a solid oxidizer.
These are of two types:
(1) Composite propellants
(2) Double base propellants
(i) COMPOSITE PROPELLANTS
These are solid propellants which use polymeric binder such as polyurethane or poly butadiene as a fuel and solid
oxides such as ammonium per chlorate, nitrate or chlorate.
Performance of these propellants can be increased by using some additives such as finely divided magnesium or
aluminum metal along with fuel.
(ii) DOUBLE BASE PROPELLANTS
These are solid propellants which mainly use bitingly cerise and nitro cellulose. The nitro cellulose gels in
nitroglycerine set in as a solid mass.
The main disadvantage of solid propellants is that these propellants once ignited will routine burning with
predetermined rate. These cannot be regulated.
2. LIQUID PROPELLANTS
These are called liquid propel abuts because these are the mixture of liquid oxidizer and liquid fuel. Liquid oxidizers
are liquid origin, nitrogen peroxide or nitric acid and liquid fuels are kerosene, alcohol, hydrazine or liquid hydrogen.
These are of two types
(1) Mono propellants
(2) Bipropellants
(i) MONO PROPELLANTS
The propellants in which a single chemical compound act as a fuel oxidizer care called monopropellants, e.g.
hydrazine, nitro methane methyl nitrate, hydrogen peroxide, etc.
Except hydrazine, the other compounds contain both the oxidizer and the fuel elements in the same molecule.
(ii) BI-PROPELLANTS
These are propellants in which the fuel and oxidizer are stored separately but are allowed to combine at the time of
combustion. E.g. kerosene and liquid oxygen. Etc.
ADVANTAGE OF LIQUID PROPELLANTS OVER SOLID PROPELLANTS
(i) The liquid propellants give higher thrust than solid propellants.
(ii) The thrust generated by liquid propellants can be controlled by switching on and off the flow of propellants. On the
other hand, the thrust cannot be controlled in solid propellants.
3. HYBRID PROPELLANTS
These are the propellants which consist of solid fuel and a liquid oxidize. For example, liquid N 2O4 (liquid oxidize) and
acrylic robber (solid fuel)
Examples of propellants used in different rockets
(I) SATURN BOOSTER ROCKETS of American space programmed used a Minturn of kerosene and liquid oxygen as
the propellant in the initial stage whereas liquid oxygen and liquid hydrogen are used as propellant in high altitudes
(ii) Russian rockets such as piton use a liquid propellant consisting of kerosene and liquid oxygen.
(iii) The Indian satellites SLV-3 and ASLV used composite solid propellants.
(iv) The rockets PLSV will use solid propellant in the first and third stages and liquid propellant in second and fourth
stages. The liquid propellant will consist of N2O4 and unsymmetrical diethyl hydrazine (UDMH) and N2 O4 and mono
methyl hydrazine (M M H) respectively.
Rocket Engines
A typical rocket engine consists of the
nozzle, the combustion chamber, and the
injector, as shown in Figure 1.4. The
combustion chamber is where the
burning of propellants takes place at high
pressure. The chamber must be strong
enough to contain the high pressure
generated by, and the high temperature
resulting from, the combustion process.
Because of the high temperature and
heat transfer, the chamber and nozzle
are usually cooled. The chamber must
also be of sufficient length to ensure
complete combustion before the gases
enter the nozzle
Propellant efficiency
Typical temperature (T), pressure (p), and velocity (v) profiles in a de Laval Nozzle
For a rocket engine to be propellant efficient, it is important that the maximum pressures possible be created on the walls of the
chamber and nozzle by a specific amount of propellant; as this is the source of the thrust. This can be achieved by all of:



heating the propellant to as high a temperature as possible (using a high energy fuel, containing hydrogen and carbon and
sometimes metals such as aluminium, or even using nuclear energy)
using a low specific density gas (as hydrogen rich as possible)
using propellants which are, or decompose to, simple molecules with few degrees of freedom to maximise translational velocity
Since all of these things minimise the mass of the propellant used, and since pressure is proportional to the mass of propellant
present to be accelerated as it pushes on the engine, and since from Newton's third law the pressure that acts on the engine also
reciprocally acts on the propellant, it turns out that for any given engine, the speed that the propellant leaves the chamber is
unaffected by the chamber pressure (although the thrust is proportional). However, speed is significantly affected by all three of the
above factors and the exhaust speed is an excellent measure of the engine propellant efficiency. This is termed exhaust velocity,
and after allowance is made for factors that can reduce it, the effective exhaust velocity is one of the most important parameters
of a rocket engine (although weight, cost, ease of manufacture etc. are usually also very important).
For aerodynamic reasons the flow goes sonic ("chokes") at the narrowest part of the nozzle, the 'throat'. Since the speed of sound in
gases increases with the square root of temperature, the use of hot exhaust gas greatly improves performance. By comparison, at
room temperature the speed of sound in air is about 340 m/s while the speed of sound in the hot gas of a rocket engine can be over
1700 m/s; much of this performance is due to the higher temperature, but additionally rocket propellants are chosen to be of low
molecular mass, and this also gives a higher velocity compared to air.
Expansion in the rocket nozzle then further multiplies the speed, typically between 1.5 and 2 times, giving a
highly collimated hypersonic exhaust jet. The speed increase of a rocket nozzle is mostly determined by its area expansion ratio—
the ratio of the area of the throat to the area at the exit, but detailed properties of the gas are also important. Larger ratio nozzles are
more massive but are able to extract more heat from the combustion gases, increasing the exhaust velocity.
Testing
Rocket engines are usually statically tested at a test facility before being put into production. For high altitude engines, either a
shorter nozzle must be used, or the rocket must be tested in a large vacuum chamber.
Safety
Rocket vehicles have a reputation for unreliability and danger; especially catastrophic failures. Contrary to this reputation, carefully
designed rockets can be made arbitrarily reliable] In military use, rockets are not unreliable. However, one of the main non-military
uses of rockets is for orbital launch. In this application, the premium has typically been placed on minimum weight, and it is difficult
to achieve high reliability and low weight simultaneously. In addition, if the number of flights launched is low, there is a very high
chance of a design, operations or manufacturing error causing destruction of the vehicle
Ignition
With liquid and hybrid rockets, immediate ignition of the propellant(s) as they first enter the combustion chamber is essential.
With liquid propellants (but not gaseous), failure to ignite within milliseconds usually causes too much liquid propellant to be inside
the chamber, and if/when ignition occurs the amount of hot gas created can exceed the maximum design pressure of the chamber,
causing a catastrophic failure of the pressure vessel. This is sometimes called a hard start or a rapid unscheduled
disassembly (RUD). Ignition can be achieved by a number of different methods; a pyrotechnic charge can be used, a plasma torch
can be used or electric spark ignition may be employed. Some fuel/oxidiser combinations ignite on contact (hypergolic), and nonhypergolic fuels can be "chemically ignited" by priming the fuel lines with hypergolic propellants (popular in Russian engines).
Gaseous propellants generally will not cause hard starts, with rockets the total injector area is less than the throat thus the chamber
pressure tends to ambient prior to ignition and high pressures cannot form even if the entire chamber is full of flammable gas at
ignition.
Solid propellants are usually ignited with one-shot pyrotechnic devices.Once ignited, rocket chambers are self-sustaining and
igniters are not needed. Indeed, chambers often spontaneously reignite if they are restarted after being shut down for a few
seconds. However, when cooled, many rockets cannot be restarted without at least minor maintenance, such as replacement of the
pyrotechnic igniter.
Cooling
For efficiency reasons, and because they physically can, rockets run with combustion temperatures that can reach ~3,500 K
(~3,200 °C or ~5,800 °F).
In rockets the coolant methods include:
1.
2.
3.
4.
5.
6.
7.
uncooled (used for short runs mainly during testing)
ablative walls (walls are lined with a material that is continuously vaporised and carried away).
radiative cooling (the chamber becomes almost white hot and radiates the heat away)
dump cooling (a propellant, usually hydrogen, is passed around the chamber and dumped)
regenerative cooling (liquid rockets use the fuel, or occasionally the oxidiser, to cool the chamber via a cooling jacket
before being injected)
curtain cooling (propellant injection is arranged so the temperature of the gases is cooler at the walls)
film cooling (surfaces are wetted with liquid propellant, which cools as it evaporates)