Guided Missiles - PART 1

Hav you ever wondered how militaries in world target 

their enemies who are hundereds of miles away from 

them? Use of modern weapon systerns has revolutionised

the concept of warfare. Technology as a force

multiplier, provides the competitive and cutting edge.

The technology of guided missiles ellcompasses the

r-tiultiple streams of engineering. technology and

applied sciences. A number of factors are responsible

for the successful launcti of missiles. 'These involve

coordination of a variety of subsystems.

In this series I have attempted to give a bird's-eye

view of the interaction of rnany

specialisations-aeronautics; mechanical, chemical

arid n.letallurgical engineering; electronics;

computers; chemistry, physics and mathematics. An

effort has been made to familiarize the reader with

some of the oft-used terminologies connected with

missiles.

By describing in simple terms, the underlying

principles in the building and launching of missile

like propulsion, guidance and control, I have

provided an introduction to the vast subject. The latest

advances in these areas and also the salient features

of the Indian missiles c~~r rent luyn der development

have been covered.

                 WHAT IS A MISSILE

Basically any object thrown at a target with the

aim of hitting it is a missile. Thus, a stone thrown at

a bird is a missile. The bird, by using its power of

reasoning may evade the missile (the stone) by moving

either to the Left, right, top or bottom with respect to

the flight path (trajectory) of the missile. Thus, the

missile in this case has been ineffective in its objective

of hitting the bird (the target) . Now, if the stone too

is imparted with some intelligence and quick response

to move with respect to the bird, to overcome aiming

errors and the bird's evasive actions and hit it

accurately, the stone now.becornes a guided missile.

The incorporation of energy source in a missile

to provide the required force for its movement

(propulsion), intelligence to go in the correct direction

(guidance) and effective manoeuvring (control) are

mainly the technologies of guided missiles. They help

in making a missile specific to a target, that is, they

determine the size, range and state of motion of a

missile.

  HISTORY OF GUIDED MISSILES

Looking back into the history of rockets and

guided missiles, we find that rockets were used in

China and India around 1000 AD for fireworks as

well as for war purposes. During the 18th century,

unguided rocket propelled missiles were used by

Hyder Ali and his son Tipu Sultan against the British.

There is a reference that two rockets belonging to

Tipu's forces were captured during the fourth Mysore

war in the siege of Seringapatnam in 1799 by

companies of the Bengal and Bombay Artillery of the

East India Company.

The current phase in the history of missiles began

during the World War I1 with the use of V1 and V2

missiles by Germany. Since then there has been a

tremendous and rapid global advancement in this

field. It spawned the growth and pushed the frontiers

of many new technologies in the areas of materials

science, aeronautics, communications, radars and

computers. Huge amounts of prime resources have

been channelised into this field resulting in the

development of sophisticated missiles. The readers

would no doubt be aware of the importaw role missiles

played in the recently concluded Gulf war.

       TYPES OF GUIDED MISSILES


Presently, there are many types of guided missiles.
They can be broadly classified on the basis of their
features such as type of target; range; mode of
launching; system adopted for control, propulsion or
guidance; aerodynamics; etc. They are also termed in
a broad sense as strategic or tactical, defensive or
offensive.
On the basis of target they could be called :-

• Anti-tanwanti-armour,
• Anti-personnel,
• Anti-aircraftthelicopter,
• Anti-ship/anti-submarine,
• Anti-satellite, or
• Anti-missile.

The missile Milan manufactured in India is an
anti-tank missile. Roland, Rapier, Crotale, etc., are
examples of anti-aircraft missiles and the much
talked-about Patriot missile belongs to the anti-missile
class.
Another classification of missiles which is very
popular is based on the method of launching. The
following list will clarify this further as also Fig 1.
Surface-to-surface-missiles (SSM),
Surface-to-air missiles (SAM),
Air-to-air missiles (AAM), and
Air-to-surface missiles (ASM).

SSMs are common ground-to-ground ones
though these may also be launched from a ship to
another ship. Underwater weapons which are
launched from a submarine also come under this class
of missiles. Some examples of SSMs with their
respective size and range are shown in Fig. 2.
SAMs are essential complement of modern air
defence systems along with anti-aircraft guns which
are used against hostile aircraft.
AAMs are for airbarne battle among fighter/
bomber aircraft. These are usually mounted under

the wings or fuselage of the aircraft and are fired at
enemy airborne targets by the pilot through the press
of a button. In his decision to launch a missile at a
particular moment, the pilot is aided by a computer
and radar network onboard as well as from groundbased
data link. The missiles in certain types are
ignited before release while in others ignition takes
place after release.
On the basis of range, missiles can be broadly
classified as
Short-range missiles;
Medium-range ballistic missiles (MRBM);
Intermediatelrange ballistic missiles(1RBM);
Intercontinental or long-range ballistic
missiles(1CBM).
This classscation is mainly used in the context
of SSMs. Missiles which travel a distance of about 50
to 100 km are designated as short-range missiles.
Those with a range of 100 to 1500 km are called
medium-range missiles and missiles having a range
upto 5000 km are said to be intermediate-range
missiles. ICBMs belong to the class of long-range
missiles which can travel a distance of 12000 km. The
Indian technology demonstrator Agni, is in IRBM
class.
On the basis of launch platform, missiles can be
termed as :-

• Shoulder firedltripod launched,
• Landmobile (wheeled vehicle or tracked
• vehicle),
• Aircraft/helicopter-borne,
• Space-based (Star Wars concept).

Based on guidance, missiles are broadly classified
as :-

• Command guidance,
• Homing guidance,
• Beam rider guidance, and
• Inertial navigation guidance.
• Depending on the aerodynamic control adopted,
• a missile is called
• Wing controlled.
• Tail mntrolled, or
• Canard controlled.

One more classification is based on the type of
trajectory and a missile is called a ballistic missile or a
cruise missile.
By definition a ballistic missile is the one which
covers a major part of its range outside the atmosphere
where the only external force acting on the missile is
the gravitational force of Earth, while the cruise missile
is the one which travels its entire range in the
atmosphere at a nearly constant height and speed.
However, a missile could have a combination of the
two also where a missile could cover part of the flight
in ballistic mode and later a terminal portion in cruise
mode.
Yet another classification is based on the
propulsion system provided in the missile. In rocket
propulsion, we have:

• Solid propulsion,
• Liquid propulsion, and
• Hybrid propulsion.
• In air-breathing propulsion, we have:
• Gas turbine engine jet or propeller
• Ramjets or ram-rockets

Currently, other types of propulsion like ionic,
nuclear, plasma, etc. are under research and
development but no known missile uses these.


              Missile Propulsion


Propulsion is the means of providing power to
accelerate the missile body and sustain, if necessary,
to reach the required target. The basis for the working
of missile propulsion systems are the well-known
Newton's laws of motion. In order to aid a quick
retrospect, these are stated here again.
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.
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.

   PARTS OF PROPULSION SYSTEM


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. Thus the
common shapes of nozzles are convergent type,
divergent type, or of the converging-diverging type.
There are also conical and bell-shaped nozzles. Bell
shaped nozzle or contoured nozzle is also named after
its inventor as GVR Rao's nozzle.
The igniter, though a tiny element among the
components of the rocket engine or rocket motor, has
the function of initiating the proplusion system. 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.

  PARAMETERS OF PROPULSION
             PERFORMANCE


The terms relevant for all types of rocket engines
and some of which are used as standards for gauging
the performance levels of different rocket motors are:
thrust, specific impulse, exhaust velocity, specific
propellant consumption, mass ratio, factor of safety,
etc. The relevant mathematica1 equations are given in
Appendix A.
The success of a rocket design is also governed
by a term called burning rate. The burning surface
of a propellant recedes as combustion proceeds. The
rate of regression is called burning rate (r) and is
expressed in cmls. It (r) is a function of propellant
composition itself and is manipulated by variation of'
catalysts, particle size, pcrcerltage of oxidiser, heat of
combustion of binder and other rrleans. ?'he basic
burning laws are shown in Appendix B.
Th e merit of rocke: propul s io~d~es ign is govr.rned
by the impulse delivered per kilogram mass. If this
figure is high, it means that we have obtained a hettel.
desigr~i,. e., we are delivering the required thrust force
to the missile with lesser weight of propulsion systeln.
Since a major portion of the weight of most of the
missiles is due to propulsion systcnl and more so for-
Ioliger range systellls (For I(:Rhls i r is >9(i0/b of total
weight) this paranletel. is very ijl~portsnt.'l 'herc arc
multiple stages of' propulsio~li ll largel. li~issilesb ased
o11 the velocity req~tircniel~tIs(.: Hhls gerierall!. I~ave
three r o four stages and long range surfice t o air
lliissiles are of two stages.

    TYPES OF PROPULSlON SYSTEMS


Missile propulsio~i will be nlainl!. of the following
two types:

• Air breathing, and
• Non-air breathing.

I'he air breathitlg rocket engines use the
sl~rronndingm eclium ol' air for. the support of their
oxidiser. Thus. t h e \ c a ~ble used onl!. \r.ithin the Earth's
at mosphere 1%-her-eains the case of nori-air breathing
engines the rocket engine itself' carries its fuel and
osiciiser on board and hence can be used in space
above the Earth's ntuiosphere also and is thus
independent of the air meclihni.
Depend,ing on the physical state of matter of the
pppellant used, the rocket propulsion system is
designated as a solid rocket rnotor, a liquid propulsion
system or a hyl>rid propulsion system.
Solid Rocket Motor (SRM)
In a soiid propellant rocket, the propellant to be
burnt is contained within the combustion chanlber or
case. Figure 3 shows a typical solid rocket motor. 'I'he
propellant charge or the grain contains the chemical
elements for complete burning. Once ignited, it burns
at a designed ratc till the pr.opellent is completely
consumed. Solid rot kets are relatively simple as
compared to the other systems.

Following are some of the main components of
a solid rocket motor.

Casing.
 In all solid rocket motors, the casing is a
pressure vessel designed and fabricated to withsand
upto certain internal pressures. It can be made of a
composite material such as fibre reinforced plastic
(FRP). Such a casing has low weight and high strength.
Casings are also rnade of metallic alloys. For
appIications in smaller rockets, titanium alloys and
aluminium alloys are used and for bigger rockets
nickel alloy steels are used. They are fabricated to give
cylindrical shells with ends flared for joints. Complex
welding and heat treatment fixtures and processes
have been specially evolved for specific casing. The
casings are subjected to a number of quality assurance
tests for strength, toughness, soundness of weld and
hydraulic pressure. The casings are provided with
thermal insulation on their inner surface to protect
them from hot gases. The casing has provisions for
end covers, nozzle and handling, etc.

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. 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. l'hey 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, like PBAN-Polybutadiene
Anyiic Nitrile; HTPB-Hydroxy Terminated Poly
Butadiene; CTPB-Carboxy Terminated Poly
Butadiene; etc. HTPB has become the single most
widely used resin. In addition, fine metallic powders
(aluminium) are also added to increase the energetic
quality of the composite propellants along with small
quantities of catalysts for various properties.
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 oh
solid prqxllants 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. The Indian satellite
launch vehicle SLV-3 was also based on solid rocket
motors. The processing costs are a large portion of
the total cost of a rocket motor.
Igniter. Igniter is the device that helps to start the
burning of the main propellant grain of the rocket
motor. Its function is for short interval (0.1-2 seconds
depending upon size) only but vital. 'The igniter for
small motors will be a few grams of grains whiIe it wiIL
be a few hundred kilograms for large boosters. The
initiation is done using electrical power by heating a
resistance wire and initiating a primary composition.
Adequate safety provisions are made through electromechanical
devices to prevent accidental initiation.
Nozzle. The nozzle is the component through which
the hot gaseous mass in the motor case is expelled
out. This has to be designed to withstand high
temperatures and flow of gases at high velocities. The
dimensions of the nozzle are critical for the
performance and efficiency of the rocket motor.
Nozzles are also used for producing control force for
the missile. Such a technique is called Thrust Vector
Control. It is done by deflecting the flow out of the
nozzle or by gimballing of a portion of the nozzle.
This nozzle gimballing requires flexible bearing
nozzle.
Liquid Propulsion System
Most of the liquid propulsion rockets are used
where long duration of operation is required. Here
the oxidiser and fuel (both liquid) propellants are
stored in separate tanks in the missile. There are
basically two types of liquid propellants deployed:
cryogenic (with boiling temperature below 120 K like
liquid hydrogen, liquid oxygen, etc.) and noncryogenic
or storable type (like kerosene, hydrazine, nitrogen
tetraoxide, hydrogen peroxide, etc.). In space missions
usually both propellants (oxidiser and fuel) used are
cryogenic, whereas in missiles the propellants used
are storable or non-cryogenic. Sometimes in space
missions a combination is used where one propellant
is cryogenic while the other is storable.


Hybrid Propulsion
In this system one of the propellants is solid while
the other is liquid. Usually the oxidiser is in liquid
state. This system is very rarely used though it has
certain advantages. It has not found much favour with
missile designers the world over. Figure 5 shows a
simple hybrid motor.


Airbreathing Propulsion
In this case the advantage is taken of the
atmospheric oxygen for burning the fuel thereby
&&g the quantity of propellants to be carried by
*unissile. This lowers the weight of the rocket greatly
75 per cent of the total propellant's weight is due
&e oxidiser. This can be used either by using small
t u h j e t engines to power the missile or ramjets.
Udke 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.
However, advances in ramjets are being made to
meet hypersonic propulsion requirements.
Theoretical studies have shown promise in providing
airbreathing propulsion even at near orbital speeds
like twenty to twenty five times the speed of sound.
These engines are called scramjets and National
Aeronautics and Space Administration (NASA) of
USA has evolved a project in which scramjets are used
to develop an aerospace vehicle called NASP which
will replace the space shuttle eventually in delivering
payloads into orbits effeciently. NASP stands for
National Aero Space Plane. In scran!jet engines the
very high airflow is slowed down and combustion is
carried out when the speeds are supersonic say
between Mach 2 and Mach 5. Liquid hydrogen fuel
is the suitable fuel for such engines. These craft are
winged and can take off from large conventional
runways horizontally unlike rockets. They also land
back and can be used many times. However a large
effort has to go in to make this realisable.
State-of-art propulsion systems use chemical
combustion as energy source though nuclear, solar
radiation, electrical, anti-matter, anti-gravity and the
like are under varying stages of feasibility studies and
research. It would not be surprising if superconductivity
which is creating iaves in the world of
science currently, too is considered as a prospective
candidate for missile propulsion systems in the coming
decades.

  TESTING OF PROPULSION SYSTEM
Before a rocket engine can be put to use, it has
to be tested. This is true whether it is in the case of
the quality assurance of a rocket engine, R&D of a
new or modified rocket engine or evaluation of the
suitability of a new or a modified rocket motor to a
specific application. Some of the tests are as follows.
Manufacturing, inspection and fabrication
tests (pressure tests, bursts tests, leak tests,
electro-mechanical checks).
Component tests (functional and operational
tests on igniters, valves, injectors, structures,
etc.)
Static rocket systems tests (with complete
rocket engine on test stand) :
 (a) simuIated
rocket operation (for proper function,
calibration, ignition, operation-usually
withou~ establishing full combustion or
nuclear reactivity);
 (b) complete engine tests
(under rated conditions, off design conditions
with intentional variations in environment or
calibration).
Static vehicle tests (when rocket engine is
installed in a restrained non-flying vehicle).
Flight tests:
 (a) on a specially instrumented
flight test range with special flight test vehicle
(b) with production vehicle.
Above all, flight testing of the integrated system
is the ultimate in such tests. This is clone in cot~junction
with tests of vehicles and other systems such as
guidance, control, ground systems, structures, the
details of which are enu~nerated in the succeeding
chapters. These tests are usually conducted at missile
or space launch ranges over the oceans. Data from
most missile and space flight tests is telernetered to a
ground receiving station as the test measurements are
made. Some flight tests rely on salvaging sorne sections
or pieces or data capsules. Some form a part of re-entry
?ethnology and recove? sj7st ems.

    USES OF PROPULSION SYSTEMS
As stated earlier, the rocket engines are used in
all kinds of missiles, satellite launch vehicles, etc. The
technology of warhead gtridance accuracy determines
the lethal capacity of a missile. These technologies are
kept a closely guarded secret by all countries.
However, rockets with satellite payloads are used in
civil applications. They are particularly used in
meteorology, weather forecasting data, survey for
minerals, satellite communication, mapping, etc.