CHAPTER 2: BASIC ROCKET DESIGN

2.1 Classification of rocket propulsion systems

2.2 Working of solid rocket motor

2.3 Solid rocket engine

2.3.1 Motor Casing

2.3.2 Igniter

2.3. Grain
2.1 Classification of Rocket Propulsion systems

INTRODUCTION:
A rocket is a propulsion system , which generates propulsive force (thrust ) when the fuel burns
in the combustion chamber and produces gases of high temperature and pressure, which will
expand in the nozzle. It is one of the main subsystems of the missile. Thrust generated by the
rocket motor will accelerate the payload and gives supersonic velocity to the payload, which take
the payload to the required altitude and range.

Propulsion :

Propulsion means to push forward or drive an object forward. A propulsion system is a machine
that produces thrust to push an object forward. Thrust is usually generated through application of
Newton’s third law, which states that “For every action, there is an equal and opposite
reaction”. The engine accelerates a gas or working fluid and the reaction to this acceleration
produces a force on the engine. Different propulsion systems generate thrust in slightly different
ways. The internal energy of the gas is converted into kinetic energy of the exhaust flow and the
gas pressure on the surface exposed to the gas produces the thrust.

CLASSIFICATION OF PROPULSION SYSTEM

The four principle propulsion systems are :

 The Propeller
 The Gas Turbine or Jet Engine
 The Ramjet Engine
 The Rocket

The Air breathing systems, which depend for their action on variation of momentum of air. In the
case of direct reaction systems , the variation of the momentum depends directly on
thermodynamic process (turbojet, ramjet and pulse jet). In this case of indirect reaction systems
the variation in momentum is obtained with the aid of an engine and a propeller. Certain
machines involve both direct reaction (jet) and indirect reaction (propeller).

The Non air breathing systems, in which the propulsive effort or thrust is obtained by variation
of the momentum of the system itself. Rockets are systems of this type. They do not depend on
the atmospheric air , either as oxidizer or as a necessary element for the production of the
reaction effect. They offer the only possibility for propulsion in space.

Paradoxically, the most modern type of motive power the

rocket is at the same time the simplest and the oldest motor known to man. It has been in use for
seven centuries, perhaps even longer. Yet only during the past twenty years has man been able to
put it to effective use and to develop it toward its full potential. Its basic elements are
disarmingly simple. But to make these elements
work together efficiently, and to develop propellants which will make the rocket a practical and
useful vehicle has stretched the limits of man's technical resources and pushed the frontiers of his
knowledge of the basic sciences outward.
Development of the rocket to the gigantic dimensions of the missile we see
today capable of traveling distances of 5,000 to 6,000 miles over the earth's surface, and probing
many times this distance into outer space has been largely a matter of developing fuels, structural
materials and thrust chamber designs which can make more efficient use of the rather simple
principle of the rocket motor. No new scientific discoveries have been necessary in order to
achieve this. Rather, it has been a question of waiting until the arts of metallurgy, chemistry and
thermodynamics were better understood by man. This is typical of all technological
development. Great scientific discoveries of one age are frequently not exploited for many
generations—or even centuries—until mankind has acquired the companion knowledge and
developed the tools which are required to make effective use of them. The steam engine
conceived in principle by Isaac Newton—lay dormant for two hundred years until a Welsh miner
decided it was practical to lay steel rails on which a steam locomotive could run, and until
engineers developed methods of manufacturing boilers, pistons, rods and valves that were
sufficiently strong to withstand the pressures and stresses placed on them.
2.2 WORKING OF SOLID ROCKET MOTOR
The principle of rocket propulsion is based on Newton's third law of motion which
states that for every action there is an equal and opposite reaction produced normally in an
opposite direction. In simple terms this means that if you lift an object weighing fifty pounds a
distance of one foot you are also exerting fifty-foot pounds of pressure on the earth. And,
theoretically, the earth should move in the opposite direction a distance proportional to its weight
as compared with the weight of the object you lifted. Since the earth is so massive, there is no
measurable movement on its part in this instance; but there would be if you were able to catapult
an object weighing many billions of tons into the air. The classic examples of the reaction
principle, as it is called, are the firing of a gun, and the escaping of air from a balloon. When a
gun is fired it recoils, or kicks back, in a direction opposite to that which the bullet took. When
air escapes from a balloon, the balloon flies off in a direction opposite to the escaping air. Since
rocket motors work on the same principle they are often referred to as reaction motors.

In the rocket motor, hot gases are produced by means of

rapidly burning some chemical substance (liquid or solid) which is called a propellant. A
propellant generally consists of a fuel (or reducer) and an oxidizer (or oxidant). Neither will burn
satisfactorily by itself, but in combination they create a chemical reaction which produces heat,
and gas products which have mass and weight. The heat generated by this reaction creates a
pressure within whatever vessel is being used. This pressure acts to drive the gas products out of
the vessel, through any opening that is provided. In a pistol, the chamber in which the burning of
the gun powder takes place is the cartridge casing. The opening provided for the escape of the
gas products is the muzzle of the barrel. In a rocket it is very much the same. The main body of
the rocket motor is called the combustion chamber. The escape port is generally called the
nozzle. A combustion chamber and a nozzle together constitute a complete rocket motor—in its
simplest form. But even for simple rockets two other things are required in order to give the
rocket direction and stability in flight. These are a nose cone, and fins. In Illustration 1 these four
basic elements of a simple rocket are shown. The only items which must be added to this
assembly to make it fly are the propellant (fuel, plus oxidizer) and a means of igniting it.
Illustration 2 shows all the essential parts, with dimensions, of a basic rocket which you can
build as a beginning project.
 ASSEMBLED AMATEUR SOLID MOTOR ROCKET

A rocket achieves its forward motion by expelling, or exhausting, hot gas particles from a nozzle.
This motion of gases toward the rear causes the rocket body to move forward, as a reaction. The
gases do not push against anything. It is simply their velocity as they move rearward multiplied
by their mass, that determines how much force, or thrust, is imparted to the rocket. Naturally, it
is desirable to have as much gas as possible (mass) ejected from the nozzle as rapidly as possible
(velocity) in order to get a high degree of thrust. To achieve this, it is necessary to burn a
substance in the rocket chamber which will create a large volume of gas rapidly and produce
enough pressure to drive the gases out of the chamber at a high rate of speed. The substance must
not burn so rapidly that it amounts to an explosion, however; and neither must it be allowed to
create so high a pressure in the chamber that it will crack the chamber casing before the gases
can be expelled. Preventing this from happening, of course, is a Matter of engineering design;
and this design must achieve a balance among a great variety of factors, any one of which can
cause a failure of the system if it is not properly controlled. The most important of these factors
are: the burning temperature of the propellant substance, the burning rate (i.e., how much of it is
burned in one second), the size of the combustion chamber, the composition and thickness of the
chamber wall, the diameter of the nozzle throat, and the length of the nozzle. The propellant
substance burned in the rocket motor chamber may be either solid or liquid; and if solid, it may
either be in the form of a cast grain or a tightly packed powder. Nearly all amateur rockets
employ a solid propellant, and 95% of these are in powder form (usually a mixture of a zinc dust
and powdered sulfur). Powders, of course, are readily mixed and relatively easy to load, which
accounts for their widespread use. They have serious disadvantages, however, in that it is very
difficult to control the density to which they are packed, and virtually impossible to predict with
any accuracy how rapidly they will burn. Generally speaking they tend to burn unevenly
(frequently due to excess air being trapped among the particles) and this uneven burning may
disturb the aerodynamic balance of the rocket, causing it to travel erratically and zoom off in
unpredictable directions. Also, powders tend to burn all at once, rather than gradually, creating a
very high pressure for a very short period of time. If this condition occurs, the rocket is likely to
explode.

2.3 SOLID ROCKET ENGINE

A Simple solid rocket motor consists of

 Motor casing
 Igniter
 Grain propellant
 Nozzle

2.3.1 Motor Casing

The case not only contains the propellant grain, but also serves as a highly loaded pressure
vessel. Case design is usually governed by a combination of motor and vehicle requirements.
The case frequently serves also as the primary structure of the missile or launch vehicles.
Typically, the following conditions are considered at the beginning of a case design .

1) Temperature (internal heating, aerodynamic heating, temperature cyclic during storage,

or thermal stresses and strains)
2) Corrosion (moisture/chemical, galvanic, stress corrosion, or hydrogen embrittlement)
3) Space conditions ( vacuum or radiation)

2.3.2 IGNITER

Motors are electrically ignited using a device called an electric match called igniters. The
igniter propellant mass is small and burns mostly at low chamber pressure, it contributes very
little to the motor overall total impulse, just enough to assure ignition in the motor under all
operating conditions.

The ignition of amateur rockets is usually accomplished by means of passing an electrical current
through a short section of a heating element (nichrome wire) which causes loose propellant, or an
igniter charge, packed around the wire to burn. The heat and pressure caused by this burning
eventually ignites the entire propellant charge. To ensure sufficient heat and pressure being
generated, a diaphragm, or plug, of some type is ordinarily used to seal off the combustion
chamber until the pressure has risen high enough to ignite the entire propellant charge, at which
time the diaphragm, or plug, is blown out of the nozzle end of the rocket and the rocket takes off.
Other methods of ignition are possible, such as the use of fuses or ignition by radio impulse, but
they are foolhardy in the extreme and constitute the greatest single cause of accidents and
injuries in amateur rocketry. An electrical circuit makes possible ignition by remote control from
a safe distance (preferably 150 feet), and this is the one most important safety precaution that
must be observed in amateur rocket experimentation.

The igniter usually consists of a small piece of nichrome wire

(from any heating element), or any other high resistance conductor which will produce heat. It is
inserted into the nozzle end of the rocket and embedded in loose propellant or a small charge of
black powder. The latter is used if the propellant is a solid grain. It is also possible to insert the
igniter in the forward end of the combustion chamber. While this requires a little more
complicated arrangement, it frequently results in better and more rapid burning of the propellant,
since the hot gases produced by the original ignition must pass through the hollow core and
around the outer surfaces of the propellant charge on their way to the nozzle. This tends to raise
the temperature of the rest of the charge, promoting more rapid burning and more complete
consumption of the propellant. Here we will deal only with nozzle-end ignition, however. The
igniter is most frequently combined with the diaphragm into one assembly called the igniter
assembly. Diaphragms, or seals, are of two general types: the burst diaphragm, which is a thin
metal or plastic disc placed forward of the nozzle in the combustion chamber; and the cork or
plug type, which is shaped to fit the diverging contour of the nozzle exit and is inserted into the
exit end of the nozzle. Both types serve the same purpose: namely, to seal off the combustion
chamber until sufficient heat and pressure have built up to give the rocket real thrust. Without
some such seal, the propellant charge will frequently smolder slowly until it has been completely
consumed without having ever burned rapidly enough to develop the gas pressure necessary for
lift-off.
 An example of the burst diaphragm igniter assembly is shown in
FIGURE __, together with a diagram of a complete rocket motor with the diaphragm in place.
An example of the plug type seal is shown in Illustration 29 with another type of igniter. Burst
type diaphragms should be made of some brittle material, such as plastic or brass, which will
shatter into small pieces capable of being ejected through the nozzle. Thin sheets of copper are
not suitable for this purpose, as they tend to melt and form into a hard ball which blocks the
nozzle aperture. The plug type may be made of cork, rubber or balsa wood. In the case of both
types, the igniter leads passing through them should be tightly sealed to prevent the premature
escape of combustion gases. The igniter leads protruding from the nozzle exit are connected to
wires leading to the power source, which can be either a generator or a 12-volt battery. A 6-volt
battery will not usually provide sufficient current to give reliable ignition.
2.3.3 GRAIN

Grain is an extruded length into which solid propellant are forward. It is the shaped mass of
processed solid propellant inside the rocket motor.

The grain behaves like a solid mass, burning in a predictable fashion and
producing exhaust gases. The nozzle dimensions are calculated to maintain a design chamber
pressure, while producing thrust from the exhaust gases.

The propellant material and geometrical configuration of the grain determined the motor
performance characteristics. The propellant grain is a cast, molded or extruded body and its
appearance and feel is similar to that of hard rubber or plastic. Once ignited, it will burn on all its
exposed surfaces to form hot gases that are then exhausted through a nozzle. Few rocket motors
have more than one grain inside a single case or chamber and very few grain segments made of
different propellant composition.

There are two methods of holding the grain in the case,

 Cartridge – bonded grain and

 Freestanding grains

Free standing grains are manufactured separately from the case and then loaded into the casing.
In case-bonded grains the case is used as a mold and the propellant is cast directly into the case
and is bonded to the case or case insulation. Today almost all large motors use case bonding.

The thrust profile over time can be controlled by grain geometry. For example, a star
shaped core will have greater initial thrust because of the addition surface area.

Advantages of a SOLID MOTOR ROCKET

 Simple design
 No preparation time
 Few or no moving parts
 Ready to operate quickly
 Less overall weight
 Can provide high thrust for short duration
 Long storage life
 Thrust vector can be possible

Disadvantages of a SOLID MOTOR ROCKET

 Low specific impulse
 Once ignited, cannot change pre-determined thrust or duration