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MODEL ROCKETRYAn Interesting Educational Tool
-by
Swapnil Kumar Sharma Former Child Scientist
Trained and Developed by:
Voluntary Institute for Community Applied Science VICAS, India
HD-86, ADA Colony, Naini, Allahabad, Uttar Pradesh PIN: 211008
e-mail: [email protected]
Visit us at: www.vicasindia.com OR http://vicasindia.blogspot.com
Cellphone No.: +91 94509 61953
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THINGS TO LEARN AND UNDERSTAND BY THE MODEL ROCKETRY ACTIVITY
Through this activity the participants will easily understand the concepts of Physics, Chemistry, and
Mathematics and will know some facts about space which are as follows:
PHYSICS:
How Thrust helps a rocket to fly?
Practical aspect of Center of Gravity and Center of Pressure i.e. how they are related with
each other, their role and importance in a flight of a rocket. How to find the Center of Gravity
and Center of Pressure of an object?
Factor affecting the flight of a Rocket and how? Brief discussion on the wind, gravitational pull,
air friction and thrust and their role.
How stability is attained and its role?
How to determine and check whether an object is balanced/stable by Center of Gravity and
Center of Pressure. Practical application of Newtons Third Law.
Practical application and realization of force, mass and acceleration with their relation
(Newtons Second Law).
Application of equations of motion i.e. v = u + at, S = u + at2 and v2 = u2 + 2aS
CHEMISTRY:
Learn and understand the types of fuels used in propellants.
Role of Oxidizer and fuel and basic difference between them.
MATHEMATICS:
Practical application of Trigonometry and Geometry in real life.
Calculating in determining the approximate height of a flying object in free space with the help
of geometry.
SPACE:
By the videos being displayed the students will get a feel of different situations and conditions
faced by the astronauts in space.
Biological effects on the body of the astronauts in space and detail discussion on their cause. Technical aspect of countdown before any launch.
Comparison between a Model Rocket & Real Rocket and Rocket & Airplane.
TEAM WORK:
Experience of working in a team/group while developing model rocket.
Experience of coordinating with different teams/groups while the launch of model rocket i.e. at
the time of launch there are two recording teams, two tracking teams, one launching team
monitored and instructed by the resource person.
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THRUST:
Thrust (T):
This is the instantaneous force or push produced at a
given time by the operating engine. It is measured in
Newtons.
The Thrust may or may not remain constant throughout
the operation of the engine but follows curve path.
Thrust of the Engine:
This is the reaction Produced due the action of the ho
gases coming out of the nozzle. This force acts in the
direction opposite of the gas coming out.
Maximum Thrust:
It is the highest amount of push produced by the engine
during the whole operation.
Burn out Time or Thrust Duration (tB):
It is the length of the time for which thrust is produced by
the engine i.e. the time from state of ignition to the time
when the ceases to produce thrust.
The instant when the engine ceases to produce thrust is
known as 'Burn Out'
Total Thrust:
It is the total impulse by the engine. If the thrust duration
is broken in small time intervals and thrust is found out for
every small interval, then sum of the products of the smal
time interval and thrust at that time upto all small time
intervals give the total thrust.
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Average Thrust (Tav):
It is the equivalent constant thrust which would act for the same total duration and produces same
amount of total thrust. It is the total thrust divided by the burn out time.
CENTER OF GRAVITY AND CENTER OF PRESSURE:
Center of Gravity/Balancing point:
The center of gravity is a geometric property of any object. The center of gravity is the average
location of the weight of an object. We can completely describe the motion of any object through
space in terms of the translation of the center of gravity of the object from one place to another and
the rotation of the object about its center of gravity if it is free to rotate. If the object is confined to
rotate about some other point, like a hinge, we can still describe its motion. In flight, both
airplanes and rockets rotate about their centers of gravity
The center of mass ormass center is the mean location of all the mass in a system. In the case of
a rigid body, the position of the center of mass is fixed in relation to the body. In the case of a loose
distribution of masses in free space, such as shot from a shotgun or the planets of the solar system,
the position of the center of mass is a point in space among them that may not correspond to the
position of any individual mass. The use of the mass center often allows the use of
simplified equations of motion, and it is a convenient reference point for many other calculationsin physics, such as angular momentum or the moment of inertia. In many applications, such as orbita
mechanics, objects can be replaced by masses located at their mass centers for the purposes of
analysis.
The term center of mass is often used interchangeably with center of gravity, but they are
physically different concepts. They happen to coincide in a uniform gravitational field, but where
gravity is not uniform center of gravity refers to the mean location of the gravitational force acting on
a body. This results in small but measurable gravitational torque that must be accounted for in the
operation of artificial satellites.
The center of mass of a body does not generally coincide with its geometric center, and this property
can be exploited. Engineers try to design a sports cars center of mass as low as possible to make
the car handle better. When high jumpers perform a Fosbury Flop, they bend their body in such a
way that it is possible for the jumper to clear the bar while his or her center of mass does not.
The center of momentum frame is an inertial frame defined as the inertial frame in which the center of
mass of a system is at rest. A specific center of momentum frame in which the center of mass is not
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only at rest, but also at the origin of the coordinate system, is sometimes called the center of mass
frame, or center of mass coordinate system.
Center of Pressure:
As an object moves through a fluid, the velocity of the fluid varies around the surface of the object
The variation of velocity produces a variation of pressure on the surface of the object. Integrating the
pressure times the surface area around the body determines the aerodynamic force on the object
We can consider this single force to act through the average location of the pressure on the surface
of the object. We call the average location of the pressure variation the center of pressure in the
same way that we call the average location of the weight of an object the center of gravity. The
aerodynamic force can then be resolved into two components, lift and drag, which act through the
center of pressure in flight.
Determining the center of pressure is very important for any flying object. To trim an airplane, or to
provide stability for a model rocket or a kite, it is necessary to know the location of the center ofpressure of the entire aircraft.
Calculation of cp:
The figure shows a simplified version of the calculation procedure that you can use to calculate the cp
of a model rocket. We assume that we already know the projected area and location, relative to some
reference location, of each of the major parts of the rocket: the nose, body tube, and fins. The
projected area A of the rocket is the sum of the projected area a of the components.
A = a(nose) + a(tube) + a(fins)
Since the center of pressure is an average location of the projected area, we can say that the area of
the whole rocket times the location of the center of pressure cp is equal to the sum of the projected
area of each component times the distance dof that component from the reference location.
A * cp = [a * d](nose) + [a * d](tube) + [a * d](fins)
The location of each component is the distance of each components center of pressure from the
reference line. So you must calculate or determine the center of pressure of each of the components
For example, the projected area of the body tube is a rectangle. The center of pressure is on the axis,
half way between the end planes.
Mechanically determining of Center of Gravity and Center of Pressure:
For a model rocket, there is a simple mechanical way to determine the center of pressure for each
component or for the entire rocket. Make a two dimensional tracing of the shape of the component, or
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rocket, on a piece of cardboard and cut out the shape. Hang the cut out shape by a string, and
determine the point at which it balances. This is just like balancing a pencil with a string! The point at
which the component, or rocket, is balanced is the center of pressure. You obviously could not use
this procedure for a very large rocket like the Space Shuttle. But it works quite well for a model.
Relation between Center of Gravity and Center of Pressure:
As a model rocket flies through the air, aerodynamic forces act on al
parts of the rocket. In the same way that the weight of all the rocke
components acts through the center of gravity(cg), the aerodynamic
forces act through a single point called the center of pressure (cp).
The relation between center of gravity and center of pressure plays a
major role in the stability of the model rocket. Therefore it is said that
the difference of the distance of center of gravity from the bottom of the model rocket to the distance of the
center of pressure from the bottom of the rocket must be less than the diameter of the tube of the mode
rocket.
FACTORS AFFECTING THE FLIGHT OF A ROCKET:
Thrust:
It is the force which moves a rocket through the air. Thrust is used to overcome the drag of a rocket,
and to overcome the weight of a rocket. Thrust is generated by the engines of the rocket throughsome kind of propulsion system.
Thrust is a mechanical force, so the propulsion system must be in physical contact with a working
fluid to produce thrust. Thrust is generated most often through the reaction of accelerating a mass of
gas. Since thrust is a force, it is a vector quantity having both a magnitude and a direction. The
engine does work on the gas and accelerates the gas to the rear of the engine; the thrust is
generated in the opposite direction from the accelerated gas. The magnitude of the thrust depends
on the amount of gas that is accelerated and on the difference in velocity of the gas through the
engine.
The physics involved in the generation of thrust is introduced in middle school and studied in some
detail in high school and college. To accelerate the gas, we have to expend energy. The energy is
generated as heat by the combustion of some fuel. The thrust equation describes how the
acceleration of the gas produces a force. The type of propulsion system used on an aircraft may vary
from airplane to airplane and each device produces thrust in a slightly different way.
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It is a reaction force described quantitatively by Newtons second and third laws. When a system
expels or accelerates mass in one direction the accelerated mass will cause a proportional but
opposite force on that system.
A rocket is propelled forward by a thrust force equal in magnitude, but opposite in direction, to the
time-rate of momentum change of the exhaust gas accelerated from the combustion chamber through
the rocket engine nozzle. This is the exhaust velocity with respect to the rocket, times the time-rate atwhich the mass is expelled, or in mathematical terms:
T = (dm/dt)v
where:
T is the thrust generated (force)
dm/dt is the rate of change of mass with respect to time (mass flow rate of exhaust);
v is the speed of the exhaust gases measured relative to the rocket.
For vertical launch of a rocket the initial thrust must be more than the weight.
Thrust at zero speed is zero power. Power requires work to be done, so zero velocity indicates zero
work and zero power. Therefore the power of a rocket or aircraft engine is thrust times forward speed.
Power (watts) = thrust (newtons) x speed (metres/second)
Gravitational Pull:
The attraction that one object has for another object due to the invisible force of gravity. The mass ofan object affects its gravitational pull. In context of a rocket it is very important factor as the rocket has
to move above in the space working against the gravitational pull of the earth and to make it to the
space it has to overcome the gravitational pull of it.
Air Friction/Resistance (Drag Force):
Astrodynamic drag force is sometimes called air resistance orair friction refers to forces tha
oppose the relative motion of an object through air. Drag forces act in a direction opposite to the
oncoming moving velocity.
Unlike other resistive forces such as dry friction, which is nearlyindependent of velocity, drag forces depend on velocity.
For a solid object moving through air, the drag is the component of the net aerodynamic force acting
opposite to the direction of the movement. The component perpendicular to this direction is
considered lift. Therefore drag opposes the motion of the object, and in a powered vehicle it is
overcome by thrust.
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In astrodynamics, and depending on the situation, atmospheric drag can be regarded as an
inefficiency requiring expense of additional energy during launch of the space object or as a bonus
simplifying return from orbit.
In order to reduce the drag force we have to make the consideration in designing the shape of the
nose and fins of a rocket.
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Wind:
In order to maintain the stability of a rocket to move in the desired path it is best that not to make a
launch of a rocket where the flow of wind is not favorable to us i.e. if the wind is blowing at very high
speed then the launching of the rocket must not be done. It is also safe not to launch rocket when its
cloudy and raining. The rocket must be launched in an open space i.e. ground for the security
reasons.
STABILITY:
In order to fly a straight and predictable trajectory, A Model Rocket must be stable. Basic Rule of
Stability
A Model Rocket will be Stable if its center of Pressure is behind its center of gravity. Rockets Motion
is a simultaneous combination of Translational and Rotational Motion.
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S.NO. TRANSLATIONAL MOTION ROTATIONAL MOTION
01. Always points same direction Always points different direction
02. Three forces give us this motion
Weight, Drag, Engines thrust
One force gives this force Wind Force
03. Act through center of gravity of Rocket Act perpendicular to the Rockets
center line
04. Gives how high & how Far away lands Gives Stability of the Model Rocket
Stability depends on following factors:
1. Static Margin Difference between Center of Pressure & Center of Gravity.
2. Speed
Determine whether the rocket is balance/stable or not:
To determine this just tie the thread at the center of the gravity point determined by balancing thehanging model rocket through thread. Now try to swing the model rocket above your head and try to
notice the direction of the movement of the model rocket that whether it is moving at which of the
following conditions:
a) with nose at the front
b) fins at the front
c) at some angle with nose pointing upwards
d) at some angle with fins pointing upwards
If the model rocket is moving in the pattern a and c then the model rocket is balance or else there is
some error while the development of the model rocket.
Before coming to the conclusion regarding the stability of the rocket try to repeat the swing test for at
least 5 times and make a note of the pattern of the movement. For a stable rocket at least 4 out of 5
times must satisfy the conditions of stability.
Newtons Second Law of Motion:
The change of momentum of a body is proportional to the impulse impressed on the body, andhappens along the straight line on which that impulse is impressed.
Force = Mass * Acceleration
2nd Law in simple terms:
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The change is the direction of the acceleration is proportional to the force applied to the mass
Basically, i see a ball rolling towards me. I push it really hard; it will roll away really fast. Or i can push
it really lightly and it will roll away really slowly.
Newton's Second Law provides a relationship between the unbalanced force on the object, the mass
of the object and the acceleration that is produced: unbalanced force = mass x acceleration or F =
ma.
The unbalanced force F is measured in newtons (N), the mass m is measured in kilograms (kg) and
acceleration a is measured in metres per second per second (m/s2).
Newton's Second Law of Motion is concerned with the effect that unbalanced forces have on motion
Unbalanced force acting on an object causes it to accelerate. There are two points to note about the
acceleration of an object when an unbalanced force acts on it:
The bigger the unbalanced force acting on the object the bigger the acceleration of the object. The
more mass the object has the more inclined it is to resist any change in its motion. For example, if
you apply the same unbalanced force to a mass of 1000 kg and a mass of 1 kg, the acceleration
(change in motion) of the 1000 kg mass will be much less than that of the 1 kg mass.
Newtons Third Law of Motion:
For every action there is an equal and opposite re-action. What does this mean? This means that forevery force there is a reaction force that is equal in size, but opposite in direction. That is to say that
whenever an object pushes another object it gets pushed back in the opposite direction equally hard.
Let's study how a rocket works on Newton's Third Law.
The rocket's action is to push down on the
ground with the force of its powerfu
engines, and the reaction is that the
ground pushes the rocket upwards with an
equal force.
Three Equations of Motion:
Rockets are propelled by the momentum reaction of the exhaust gases expelled from the tail. Since
these gases arise from the reaction of the fuels carried in the rocket, the mass of the rocket is not
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constant, but decreases as the fuel is expended. The equation of motion for a rocket projected
vertically upward in a uniform gravitational field, neglecting atmospheric friction, is:
Where ,
m is the mass of the rocket and
v is the velocity of the escaping gases relative to the rocket.
The variable quantities in a uniformly accelerated rectilinear motion are time, speed, distance covered
and acceleration. Simple relations exist between these quantities. These relations are expressed in
terms of equations called equations of motion
The equations of motion are:
(1) v = u + at
(3) v2 - u2 = 2aS
Derivation of the First Equation of Motion
Consider a particle moving along a straight line with uniform acceleration 'a'. At t = 0, let the particlebe at A and u be its initial velocity and when t = t, v be its final velocity.
v = u + at I equation of motion
Second Equation of Motion
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From equations (1) and (2)
The first equation of motion is v = u + at.
Substituting the value of v in equation (3), we get
Third Equation of Motion
The first equation of motion is v = u + at.
v - u = at ... (1)
From equation (2) and equation (3) we get,
Multiplying equation (1) and equation (4) we get,
(v - u) (v + u) = 2aS
[We make use of the identity a2 - b2 = (a + b) (a - b)]
v2 - u2 = 2aS III equation of motion
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DERIVATIONS OF EQUATIONS OF MOTION (GRAPHICALLY)
First Equation of Motion
Graphical Derivation of First Equation
Consider an object moving with a uniform velocity u in a
straight line. Let it be given a uniform acceleration a at time t =0 when its initial velocity is u. As a result of the acceleration,
its velocity increases to v (final velocity) in time t and S is the
distance covered by the object in time t.
The figure shows the velocity-time graph of the motion of the
object.
Slope of the v - t graph gives the acceleration of the moving
object.
Thus, acceleration = slope = AB =
v - u = at
v = u + at I equation of motion
Second Equation of Motion
Let u be the initial velocity of an object and 'a' the acceleration produced in the body. The distance
travelled S in time t is given by the area enclosed by the velocity-time graph for the time interval 0 to t
Graphical Derivation of Second Equation
Distance travelled S = area of the trapezium ABDO
= area of rectangle ACDO + area of DABC
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(v = u + at I eqn of motion; v - u = at)
Third Equation of Motion
Let 'u' be the initial velocity of an object and a be the acceleration produced in the body. The distance
travelled 'S' in time 't' is given by the area enclosed by the v - t graph.
Graphical Derivation of Third Equation
S = area of the trapezium OABD.
Substituting the value of t in equation (1) we get,
2aS = (v + u) (v - u)
(v + u)(v - u) = 2aS [using the identity a2 - b2 = (a+b) (a-b)]
v2 - u2 = 2aS III Equation of Motion
FUELS USED AS PROPELLANT
Rocket propellant is mass that is stored in some form of propellant tank, prior to being used as the
propulsive mass that is ejected from a rocket engine in the form of a fluid jet to produce thrust. A fuelpropellant is often burned with an oxidizer propellant to produce large volumes of very hot gas. These
gases expand and push on a nozzle, which accelerates them until they rush out of the back of
the rocket at extremely high speed, making thrust. Sometimes the propellant is not burned, but can
be externally heated for more performance. For smaller attitude control thrusters, a compressed gas
escapes the spacecraft through a propelling nozzle.
Chemical rocket propellants are most commonly used, which undergo exothermic chemica
reactions which produce hot gas which is used by a rocket for propulsive purposes.
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Rockets create thrust by expelling mass backwards in a high speed jet (see Newton's Third Law).
Chemical rockets, the subject of this article, create thrust by reacting propellants within a combustion
chamber into a very hot gas at high pressure, which is then expanded and accelerated by passage
through a nozzle at the rear of the rocket. The amount of the resulting forward force, known as thrust,
that is produced is the mass flow rate of the propellants multiplied by their exhaust velocity (relative to
the rocket), as specified by Newton's third law of motion. Thrust is therefore the equal and opposite
reaction that moves the rocket, and not by interaction of the exhaust stream with air around the
rocket. Equivalently, one can think of a rocket being accelerated upwards by the pressure of the
combusting gases against the combustion chamber and nozzle. This operational principle stands in
contrast to the commonly-held assumption that a rocket "pushes" against the air behind or below it.
Rockets in fact perform better in outer space (where there is nothing behind or beneath them to push
against), because there is a reduction in air pressure on the outside of the engine, and because it is
possible to fit a longer nozzle without suffering from flow separation.
The maximum velocity that a rocket can attain in the absence of any external forces is primarily a
function of its mass ratio and its exhaust velocity. The relationship is described by the rocketequation: Vf= Veln(M0 / Mf). The mass ratio is just a way to express what proportion of the rocket is
propellant (fuel/oxidizer combination) prior to engine ignition. Typically, a single-stage rocket might
have a mass fraction of 90% propellant, 10% structure, and hence a mass ratio of 10:1 . The impulse
delivered by the motor to the rocket vehicle per weight of fuel consumed is often reported as the
rocket propellant's specific impulse. A propellant with a higher specific impulse is said to be more
efficient because more thrust is produced while consuming a given amount of propellant.
Lower stages will usually use high-density (low volume) propellants because of their lighter tankage
to propellant weight ratios and because higher performance propellants require higher expansionratios for maximum performance than can be attained in atmosphere. Thus, the Apollo-Saturn V first
stage used kerosene-liquid oxygen rather than the liquid hydrogen-liquid oxygen used on its upper
stages Similarly, the Space Shuttle uses high-thrust, high-density solid rocket boosters for its lift-off
with the liquid hydrogen-liquid oxygen SSMEs used partly for lift-off but primarily for orbital insertion.
CHEMICAL PROPELLANTS
There are three main types of propellants: solid, liquid, and hybrid.
Solid propellants
Solid propellants (and almost all rocket propellants) consist of an oxidizer and a fuel. In the case of
gunpowder, the fuel is charcoal, the oxidizer is potassium nitrate, and sulfur serves as a catalyst.
(Note: sulfur is not a true catalyst in gunpowder as it is consumed to a great extent into a variety of
reaction products such as K2S. The sulfur acts mainly as a sensitizer lowering threshold of ignition.)
During the 1950s and 60s researchers in the United States developed what is now the standard high-
energy solid rocket fuel, Ammonium Perchlorate Composite Propellant (APCP). This mixture is
primarily ammonium perchlorate powder (an oxidizer), combined with fine aluminum powder (a fuel)
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held together in a base of PBAN or HTPB (rubber-like fuels). The mixture is formed as a liquid, and
then cast into the correct shape and cured into a rubbery solid.
Advantages
Solid fueled rockets are much easier to store and handle than liquid fueled rockets, which makes
them ideal for military applications. In the 1970s and 1980s the U.S. switched entirely to solid-fuelled
ICBMs: the LGM-30 Minuteman and LG-118A Peacekeeper (MX). In the 1980s and 1990s, theUSSR/Russia also deployed solid-fuelled ICBMs (RT-23, RT-2PM, and RT-2UTTH), but retains two
liquid-fuelled ICBMs (R-36 and UR-100N). All solid-fuelled ICBMs on both sides have three initial
solid stages and a precision maneuverable liquid-fuelled bus used to fine tune the trajectory of the
reentry vehicle.
Their simplicity also makes solid rockets a good choice whenever large amounts of thrust are needed
and cost is an issue. The Space Shuttle and many other orbital launch vehicles use solid fuelled
rockets in their first stages (solid rocket boosters) for this reason.
Disadvantages
Relative to liquid fuel rockets, solid rockets have a number of disadvantages. Solid rockets have a
lower specific impulse than liquid fueled rockets. It is also difficult to build a large mass ratio solid
rocket because almost the entire rocket is the combustion chamber, and must be built to withstand
the high combustion pressures. If a solid rocket is used to go all the way to orbit, the payload fraction
is very small. (For example, the Orbital Sciences Pegasus rocket is an air-launched three-stage solid
rocket orbital booster. Launch mass is 23,130 kg, low earth orbit payload is 443 kg, for a payload
fraction of 1.9%. Compare to a Delta IV Medium, 249,500 kg, payload 8600 kg, payload fraction 3.4%
without air-launch assistance.)A drawback to solid rockets is that they cannot be throttled in real time, although a predesigned thrus
schedule can be created by altering the interior propellant geometry.
Solid rockets can often be shut down before they run out of fuel. Essentially, the rocket is vented or
an extinguishant injected so as to terminate the combustion process. In some cases termination
destroys the rocket, and then this is typically only done by a Range Safety Officer if the rocket goes
awry. The third stages of the Minuteman and MX rockets have precision shutdown ports which, when
opened, reduce the chamber pressure so abruptly that the interior flame is blown out. This allows a
more precise trajectory which improves targeting accuracy.
Finally, casting very large single-grain rocket motors has proved to be a very tricky business. Defects
in the grain can cause explosions during the burn, and these explosions can increase the burning
propellant surface enough to cause a runaway pressure increase, until the case fails.
Liquid propellants
Current Types
The most common liquid propellants in use today:
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LOX and kerosene (RP-1). Used for the lower stages of most Russian and Chinese boosters, the
first stages of the Saturn V and Atlas V, and all stages of the developmental Falcon 1and Falcon
9. Very similar to Robert Goddard's first rocket. This combination is widely regarded as the most
practical for boosters that lift off at ground level and therefore must operate at full atmospheric
pressure.
LOX and liquid hydrogen, used in the Space Shuttle orbiter, the Centaur upper stage of the Atlas
V, Saturn V upper stages, the newer Delta IV rocket, the H-IIA rocket, and most stages of the
European Ariane rockets.
Nitrogen tetroxide (N2O4) and hydrazine (N2H4), MMH, or UDMH. Used in military, orbital and
deep space rockets, because both liquids are storable for long periods at reasonable
temperatures and pressures. N2O4/UDMH is the main fuel for the Proton rocket. This combination
is hypergolic, making for attractively simple ignition sequences. The major inconvenience is that
these propellants are highly toxic, hence they require careful handling.
Monopropellants such as hydrogen peroxide, hydrazine and nitrous oxide are primarily used
for attitude control and spacecraft station-keeping where their long-term storability, simplicity of
use and ability to provide the tiny impulses needed, outweighs their lower specific impulse as
compared to bipropellants. Hydrogen peroxide is also used to drive the turbopumps on the first
stage of the Soyuz launch vehicle.
Advantages
Liquid fueled rockets have better specific impulse than solid rockets and are capable of being
throttled, shut down, and restarted. Only the combustion chamber of a liquid fueled rocket needs to
withstand combustion pressures and temperatures and they can be regeneratively cooled by the
liquid propellant. On vehicles employing turbopumps, the propellant tanks are at very much less
pressure than the combustion chamber, and thus can be built far more lightly than a solid propellant
rocket case, permitting a higher mass ratio. For these reasons, most orbital launch vehicles use liquid
propellants.
The primary performance advantage of liquid propellants is due to the oxidizer. Several practical
liquid oxidizers (liquid oxygen, nitrogen tetroxide, and hydrogen peroxide) are available which have
much better specific impulse than the ammonium perchlorate used in most solid rockets, when paired
with comparable fuels. These facts have led to the use of hybrid propellants: a storable oxidizer used
with a solid fuel, which retain most virtues of both liquids (high ISP) and solids (simplicity).
While liquid propellants are cheaper than solid propellants, for orbital launchers, the cost savings do
not, and historically have not mattered; the cost of propellant is a very small portion of the overall cost
of the rocket.
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Disadvantages
The main difficulties with liquid propellants are also with the oxidizers. These are generally at least
moderately difficult to store and handle due to their high reactivity with common materials, may have
extreme toxicity (nitric acids), moderately cryogenic (liquid oxygen), or both (liquid fluorine, FLOX- a
fluorine/LOX mix). Several exotic oxidizers have been proposed: liquid ozone (O3), ClF3, and ClF5, al
of which are unstable, energetic, and toxic.
Liquid fuelled rockets also require potentially troublesome valves and seals and thermally stressed
combustion chambers, which increase the cost of the rocket. Many employ specially designed
turbopumps which raise the cost enormously due to difficult fluid flow patterns that exist within the
casings.
Gas propellants
A gas propellant usually involves some sort of compressed gas. However, due to the low density and
high weight of the pressure vessel, gases see little current use, but are sometimes used for vernier
engines, particularly with inert propellants.
GOX was used as one of the propellant for the Buran program for the orbital manoeuvring system.
Hybrid propellants
A hybrid rocket usually has a solid fuel and a liquid or gas oxidizer. The fluid oxidizer can make it
possible to throttle and restart the motor just like a liquid fuelled rocket. Hybrid rockets are also
cleaner than solid rockets because practical high-performance solid-phase oxidizers all contain
chlorine, versus the more benign liquid oxygen or nitrous oxide used in hybrids. Because just one
propellant is a fluid, hybrids are simpler than liquid rockets.
Hybrid motors suffer two major drawbacks. The first, shared with solid rocket motors, is that the
casing around the fuel grain must be built to withstand full combustion pressure and often extreme
temperatures as well. However, modern composite structures handle this problem well, and when
used with nitrous oxide and a solid rubber propellent ( HTPB ) , relatively small percentage of fuel is
needed anyway, so the combustion chamber is not especially large.
The primary remaining difficulty with hybrids is with mixing the propellants during the combustion
process. In solid propellants, the oxidizer and fuel are mixed in a factory in carefully controlled
conditions. Liquid propellants are generally mixed by the injector at the top of the combustion
chamber, which directs many small swift-moving streams of fuel and oxidizer into one another. Liquid
fuelled rocket injector design has been studied at great length and still resists reliable performance
prediction. In a hybrid motor, the mixing happens at the melting or evaporating surface of the fuel.
The mixing is not a well-controlled process and generally quite a lot of propellant is left unburned [3]
which limits the efficiency and thus the exhaust velocity of the motor. Additionally, as the burn
continues, the hole down the center of the grain (the 'port') widens and the mixture ratio tends to
become more oxidiser rich.
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There has been much less development of hybrid motors than solid and liquid motors. For military
use, ease of handling and maintenance have driven the use of solid rockets. For orbital work, liquid
fuels are more efficient than hybrids and most development has concentrated there. There has
recently been an increase in hybrid motor development for nonmilitary suborbital work:
The Reaction Research Society, although known primarily for their work with liquid rocket
propulsion, has a long history of research and development with hybrid rocket propulsion.
Several universities have recently experimented with hybrid rockets. Brigham Young University,
the University of Utah and Utah State University launched a student-designed rocket called Unity
IV in 1995 which burned the solid fuel hydroxy-terminated polybutadiene (HTPB) with an oxidizer
of gaseous oxygen, and in 2003 launched a larger version which burned HTPB with nitrous
oxide. Stanford University researches nitrous-oxide/paraffin hybrid motors.
The Rochester Institute of Technology is currently creating a HTPB hybrid rocket to launch smal
payloads into space and to several near Earth objects. Its first launch is scheduled for Summer2007.
Scaled Composites SpaceShipOne, the first private manned spacecraft, is powered by a hybrid
rocket burning HTPB with nitrous oxide. The hybrid rocket engine was manufactured
by SpaceDev. SpaceDev partially based its motors on experimental data collected from the testing
of AMROC's (American Rocket Company) motors at NASA's Stennis Space Center's E1 test
stand. Motors ranging from as small as 1000 lbf (4.4 kN) to as large as 250,000 lbf (1.1 MN) thrust
were successfully tested. SpaceDev purchased AMROCs assets after the company was shut
down for lack of funding.
Inert propellants
Some rocket designs have their propellants obtain their energy from non chemical or even external
sources. For example water rockets use the compressed gas, typically air, to force the water out of
the rocket.
Solar thermal rockets and Nuclear thermal rockets typically propose to use liquid hydrogen for
an Isp(Specific Impulse) of around 600-900 seconds, or in some cases water that is exhausted as
steam for an Isp of about 190 seconds.Additionally for low performance requirements such as attitude jets, inert gases such as nitrogen have
been employed.
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Determining the Height of an object in free space using Trigonometry and Geometry:
The height of an object in the free space can be determined using two methods one is the
Trigonometric and another is the Geometry with the help of the tracking instrument.
Trigonometric method:
Just stand at the distance of 100m apart from the launching point of the model rocket. The tracking
team must be ready with their tracking instrument which has protector attached with it to make the
record of the angle. As soon as the model rocket is launched the tracking team must move their
hands in line of the height attained by the rocket. The angle recorded in the tracking instrument must
be kept in the equation:
tan = Pependicular /Base
where
= the angle recorded in the tracking instrument
Base = 100m
Perpendicular = the height of the object from the groundMake sure that the model rocket launched was whether stable or unstable at the time of flight
because we have to take the record of the model rockets which goes straight in the sky.
Geometric Method:
This method is more accurate then the Trigonometric method but requires 4 teams i.e. 2 tracking
teams and 2 recording teams. In this method the two tracking teams takes their position 100m apart
from the launching point lying diagonal with
each other and the other two corners are
taken by the recording team which make
sure that the model rocket launched was
whether stable or unstable at the time of
flight. This method is based on the theory of
similar triangles. Just scale the 100m+100m
distances apart on the graph paper taking
launching point as the center on the paper
The angle recorded by the tracking teams of
the maximum height attained by the rocket isthen plotted on the graph paper on the either
sides and is extended. The point at which the
two lines intersect is the required point. The
length the perpendicular dropped from the
point to the base is the height attained by the
model rocket as shown in the figure.
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Biological Effects on the body of the Astronauts:
Due to the absent of the gravitational pull in the space it is found that there is increase in the height of
the body of Astronauts present in the space up to 4-6 inches in size. In order to incorporate this
serious threat they are suppose to do the exercises in the space shuttle.
Technological aspect of COUNTDOWN:
The fact is that each and every major and minor components installed in the rocket are assigned
some numbers ranging from 1 to total number of components like 1,2,3,4,5,274 and while
countdown every component is tested and checked and in case if found that any of the component is
not functioning in the desired manner then the launch is stopped at once and is postpone for some
time till the problem is sort out. When the countdown reaches 1 successfully then finally the launch
takes place.
Difference between the Model Rocket and the Real Rocket:
S.NO. MODEL ROCKET REAL ROCKET01. 4 forces throughout flight 4 forces during atmospheric flight
02. All of flight in atmosphere Short time in atmosphere
03. Aerodynamics very important Aerodynamics less important
04. Very short powered flight Long powered flight
05. Solid rocket engine Liquid or solid rocket engine
06. No control Active control
07. Low speed High speed
08. Inexpensive materials Expensive materials
09. Card sheet and paper Aluminum, titanium, nickel alloy
Difference between the Rocket and the Aircraft:
S.NO. AIRCRAFT ROCKET
01. Designed and developed to fly in
atmosphere
Designed and developed to fly in
space.
02. All of flight in atmosphere Short time in atmosphere
03. Aerodynamics very important Aerodynamics less important
04. Liquid fuel engine Liquid or solid rocket engine