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ROCKETRY
Instructors Guide
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Table of Contents
Table of Contents........................................................................................... 2
1.
Introduction ....................................................................................... 3
2.
Applications of Rockets ................................................................... 8
3.
Principles of Rocket Motors ........................................................... 15
4.
Types of Rocket Motor .................................................................... 22
5.
Rocket Structures and Systems .................................................... 36
6.
Launch Vehicles .............................................................................. 43
7.
Model Rockets ................................................................................. 48
Acronyms ..................................................................................................... 56
Further Material for Instructors .................................................................. 57
Front Cover: A European Ariane 5 rocket takes another payload to space
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1.
Introduction
1.1
History
It is widely accepted that the first rockets were made in China in the ninth
century. They were propelled by black powder, a form of gunpowder, and
were used to extend the range of arrows. The potential of the rocket as a
weapon was quickly realised. Pictures from the twelfth century show rockets
very similar to a modern firework rocket only much bigger and with a warhead
designed to explode. The Chinese discovered that a rocket could be made
more accurate by having a long stick attached to the rocket body, similar to a
modern firework.
Chinese rocket technology spread westwards with the Mongols. Rockets
were first used in Europe at the siege of Constantinople in 1453. The
effectiveness of muskets and cannons, and the relative inaccuracy of rockets,
prevented rockets from being adopted in European warfare for over three
centuries.
The first serious use of rockets as a weapon came in the early nineteenth
century during the Napoleonic wars. Sir William Congreve developed an
iron rocket weighing over 14 kilogrammes which looked like a very large
firework. The stick was 5 metres long and mounted down the centre of the
rocket, allowing it to be launched from a tube.
The accuracy of Congreve’s rockets made them an effective weapon, and
they were used in many land and sea battles including the naval
bombardment of Copenhagen in 1807 where over 300 rockets were fired.
The most celebrated use of Congreve’s rockets was the battle of Baltimore
when the Royal Navy fired rockets into Fort McHenry. This is recorded in the
first verse of the American National Anthem in the lines:
“And the rockets' red glare, the bombs bursting in air,
Gave proof through the night that our flag was still there”
In 1844 another Englishman, William Hale, realised that both accuracy and
range could be improved by spinning the rocket. This allowed the stick to be
removed, making the rockets shorter and easier to handle. Hale’s rockets
were heavier than Congreve’s. They were used during the Crimean war, but
improvements in artillery prevented the widespread use of rockets.
1.2
Early 20th Century
Until the 20th century rockets were imprecise weapons, and had little scientific
use. The problem with rockets was that it was difficult to control their
direction and speed. Solid propellants such as gunpowder could be lit very
easily but could not be throttled back or extinguished; they would burn until
the propellant was exhausted. Rockets also lacked guidance, so they would
travel roughly in the direction in which they were pointed, but their trajectory
would be affected by gusts of wind.
The control and guidance of rockets became possible with the introduction of
two new technologies:

Liquid and gaseous propellants
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
Gyroscopic guidance.
The great technical advances that created modern rocketry occurred in the
first half of the 20th century. A Russian schoolteacher called Konstantin
Tsiolkovsky established the principles for high powered, multi staged,
rockets which could overcome earth’s gravity. An American scientist, Robert
Goddard, built the first liquid fuelled rockets and experimented with rocket
designs and principles. Goddard launched the first liquid fuelled rocket at a
farm in Massachusetts in 1926, and continued to develop rockets and rocket
motors until his death in 1945. During his life he solved many of the practical
problems of rocket flight.
Figure 1 - Goddard’s Liquid Fuelled Rocket
In France Robert Esnault-Pelterie, an aircraft designer best known as the
inventor of the joystick and the radial engine took an interest in rocketry.
Esnault-Pelterie never met Tsiolkovsky but independently reached the same
conclusions about rocket propulsion.
At the same time a German rocket enthusiast Hermann Oberth developed
the basic mathematics of rocket propulsion. Oberth tested his first liquid
fuelled rocket motor in 1929, assisted by his student Wernher von Braun.
By the Second World War the basic technology existed for building rockets
capable of carrying a ton of payload to over a hundred miles. Wernher von
Braun led a team which developed the first mass produced rocket, the V2.
The V2 was a German weapon which was used to attack London from the
safety of Northern Europe.
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Figure 2 - The Rocket Scientists
At the end of the Second World War the large stockpiles of V-2 were
captured, and Von Braun and his team emigrated to America to form the
nucleus of the American space programme. Von Braun and his team settled
in Huntsville, Alabama, where they developed rockets for both military and
peaceful purposes, promoting the scientific exploration of space.
1.3
The Space Race
The Second World War saw the rocket develop as a long-range weapon
capable of reaching the edge of space. The period after the war saw
increasing political and military tension between Russia and its allies and the
west, led by the United States of America. Russia and America both realised
that long range rockets could be used to deliver nuclear weapons into each
other’s territory. Both nations developed Intercontinental Ballistic Missiles
(ICBM) that could lift nuclear warheads into space, and drop them onto each
other’s counties.
A Russian scientist called Sergei Korolyov was responsible for the design of
their ICBM. Korolyov, like von Braun, realised that the exploration of space
could have many scientific benefits. In October 1957 Korolyov used one of
his R-7 ballistic missiles to launch the first satellite, Sputnik.
America was shocked that its rival, Russia, was so advanced in rocketry. The
American Government responded by forming the National Aeronautical and
Space Administration (NASA) and launching its own satellite, Explorer 1, on
top of a Juno rocket.
On 12th April 1961 a Russian pilot, Yuri Gagarin, became the first man to
travel in space. He was launched to orbit on a modified R-7 ICBM and
orbited the world once. Throughout the early 1960s most space flights used
modified ICBMs as there were no rockets designed specifically for manned
flight. The American Redstone missile and Atlas ICBM were used for their
early manned flights, while the Russians continued to develop the R-7 ICBM
to carry increasingly large payloads.
On 25th May 1961 the American president, John F Kennedy, set an ambitious
goal for NASA. He publicly committed America to land a man on the Moon
and return him safely to Earth before the end of the 1960s. A new and larger
rocket was needed, and Von Braun developed the largest rocket ever built:
the Saturn V. The Saturn V was a staggering engineering accomplishment.
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It was over 110 meters tall, weight over 3 million kilogrammes, and comprised
over 6 million components. It was so big that the world’s largest building had
to be built in order to assemble a Saturn V. The building was so large that
Wembley Stadium would fit on its roof.
Figure 3 - Saturn V Launch
The space race ended on 20th July 1969 when Neil Armstrong and Buzz
Aldrin walked on the moon. It would not have been possible without the
theories of Tsiolkovsky, the persistence of Goddard, the experimentation of
Oberth and the engineering brilliance of Von Braun.
1.4
The Exploration of Space
After the space race it was realised that the spaceflight had many scientific
and technical applications for example international communications, the
Global Positioning System, astronomy, better weather forecasting and earth
observation. Manned spaceflight continued to develop with space stations
such as Skylab, Salyut, Mir and the International Space Station (ISS).
Many of the Russian ICBMs have been converted into satellite launch
vehicles, providing a cheap and efficient way to launch small satellites to
altitudes of a few hundred km.
Launching larger payloads required a new generation of rockets. America
developed the Space Shuttle, Atlas, Delta and Titan families of rockets,
while Russia developed the Cosmos, Zenit and Proton. Other nations and
regions also became involved in the business of space launches. Europe
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developed the very successful Ariane rockets, while China developed the
Long March, Japan developed the H-2 and India developed the GSLV. The
number of spacefaring nations continues to increase.
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2.
Applications of Rockets
2.1
Uses of Rockets
Rockets have been used for many purposes since they were invented. We’ve
all been entertained with firework rockets, but rocket technology is often used
for more serious purposes. This chapter describes some of the many uses
for rockets.
Rockets are vehicles, and like all vehicles they exist to move things, for
example:
-
Move a scientific instrument from the ground into the upper
atmosphere
-
Take astronauts to space (and back)
-
Fire a rescue line between two ships
-
Place a weapon onto a target
-
Eject a pilot from a damaged aircraft
An object carried by a rocket for peaceful purposes is called the payload
whereas an object carried for military purposes is called a warhead.
2.2
Military Rockets and Missiles
A rocket-propelled object used for military purposes is either called a rocket or
a missile. Military rockets are unguided weapons as they have no ability to
steer themselves towards a target. Once fired, a military rocket will follow a
ballistic trajectory. Missiles are guided weapons as they have the ability to
change direction and follow instructions to help them to hit the target.
The German V2 was the first rocket to be employed as a long range weapon.
The rocket motor accelerated the V2 to about 20 miles altitude and a velocity
of 3,000 miles per hour before consuming all its propellant. During this phase
of flight an arrangement of gyroscopes and vanes kept the rocket on course.
After the motor burned out it coasted to an altitude of around 55 miles and
dropped its warhead onto the target.
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Figure 4 - V2 Flight Profile
Subsequent long-range rockets employed radio and radar guidance to help
them to hit the target. Like the V2, they fell ballistically onto the target.
Unlike the V2, these weapons had guidance to continuously correct their
trajectory and were classified as ballistic missiles.
Other rockets and missiles have been designed to act as weapons.
Unguided rockets have been launched from man-portable tubes, aircraft and
ships for attaching targets on the ground. Unguided weapons are useful
when the target is fixed or moving slowly, such as buildings or vehicles.
Another form of unguided weapon is the rocket torpedo.
This unusual
weapon is designed to travel underwater at speeds of 200 to 300 knots for up
to 5 miles. It was designed as a defensive weapon to destroy incoming
torpedoes as far away from the target as possible.
Unguided weapons have their limitations. When a target is moving, or greater
accuracy is needed, a weapon must employ guidance to ensure that it hits the
target. There are many ways of guiding a missile onto its target, for example:
-
using a TV camera in the nose of the missile.
-
using a small radar in the missile to track the position of the target
-
having a sensor on the missile to look for emissions from the target,
particularly heat and radio emissions
-
if the target can be seen by the missiles operator, instructions can be
sent to the missile by wire or radio
Missile guidance systems are classified as active homing or passive
homing. In an active homing system the missile uses its own radar
transmitter and receiver to find and track the target, sending commands to the
flight system to steer the missile towards the target. A missile with active
homing can steer itself onto the target without external assistance. In a
passive homing system the missile does not transmit, but looks for a signal to
indicate where the target is. This could be a radio or heat signal from the
target itself, radar reflections from another radar, or target illumination.
Figure 5 - Active and Passive Homing
Military rockets and missiles are generally classified in one of the following
groups:
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-
Surface to Air Missile (SAM). Launched from the ground or ships to
attack aircraft.
-
Air to Surface Missile (ASM). Launched from aircraft to attack targets
on the ground.
-
Surface to Surface Missile (SSM).
attack targets on the ground.
-
Air to Air Missile (AAM).
aircraft.
-
Anti Tank Missile.
Launched from the surface with a warhead
specifically designed to penetrate the armour of tanks.
-
Anti Shipping Missile. Launched from the surface, ships or aircraft to
attack ships.
-
Ballistic Missile. Launched from the surface to carry nuclear or
conventional warheads to surface targets. ballistic missiles capable of
travelling great distances are called Intercontinental Ballistic
Missiles (ICBM)
-
Anti Ballistic Missile. Surface launched missiles designed to destroy
incoming ballistic missiles.
Launched from the ground to
Launched from aircraft to attack other
2.3
Rocket Vehicles
Rocket propulsion is not only used for flight but has also been used for
ground vehicles.
The first recorded rocket car was the RAK-1 made by Fritz Opel in 1928 as a
publicity stunt. The car achieved a top speed of 47 mph. Opel’s second
attempt to build a rocket powered car was a bit more serious, achieving a
speed of 143 mph. They subsequently built a rocket powered train but this
was destroyed when it left the tracks on its second run.
There have been many rocket cars since Opel’s. The performance of rocket
powered cars has never matched that of jet powered cars, so most of the
land speed records have been set by jets. The fastest rocket car was the
Blue Flame which achieved a speed of 622.407 mph at the Bonneville Salt
Flats on 23 Oct 1970.
Rocket sleds are similar to rocket trains as they both run along a track. Sleds
have no wheels so they slide along the track with curved grips to hold the sled
on the track. Rocket sleds have a unique role as platforms for testing
experimental equipment on the ground, including ejector seats and rocket
systems. The fastest recorded speed by a rocket sled is 6416 mph on the
experimental track at Holloman Air Force base in 2003.
2.4
Rocket Planes
Rocket propelled planes have similar origin’s to rocket cars. In 1928 Fritz
Opel fitted two black powder rocket motors to a tailless glider made by
Alexander Lippisch. The aircraft, called the “Ente” (German for “Duck”) made
one successful flight before it caught fire on the second flight and was
destroyed.
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During the 1930s several experimental rocket planes were made by German
and Russian designers. The first and only rocket plane to be mass produced
was the Messerschmitt Me163. Made in 1944 this aircraft was designed to
be a rocket powered fighter aircraft. It saw service in the closing days of
World War 2.
As jet engines became more efficient the performance advantages of rocket
planes diminished, and they were mainly used for experimental purposes.
The first aircraft to break the sound barrier was the rocket propelled Bell X-1,
piloted by Chuck Yeager.
This heralded an era of highly experimental
rocket planes, culminating in the North American X-15. The X-15 was the first
aircraft to fly into space, and was used as a test vehicle for many of the ideas
which were incorporated into manned spacecraft.
Rocket planes lost their advantage when manned spaceflight became a
practical proposition using conical space capsules. The capsules were
launched by rockets and re-entered the atmosphere using heat shields and
parachutes. In recent years there has been a resurgence of interest in rocket
planes. Bert Rutan’s novel rocket plane “Spaceship One” has taken men
into space for a fraction of the cost of a conventional rocket and, unlike
rockets, can be re-used. This opens the possibility of rocket planes being
used for space tourism.
Figure 6 - The X-15 Experimental Rocket Plane
In the United Kingdom a company called Reaction Engines are developing
an aircraft with a very novel engine. Inside the atmosphere it acts as a jet
engine, and at high altitudes, where there is not enough air for the jet engine
to operate, it acts as a rocket engine. This will power the Skylon aircraft
which is designed to take passengers on very fast long distance flights, and
also take objects into space.
2.5
Flares
Rocket powered flares are a simple but very effective way of signalling. A red
flare launched from a ship or small boat is an internationally recognised
distress signal. The rocket propels the red flare to many hundreds of feet.
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The flare drifts slowly down under the parachute giving coastguards plenty of
time to notice the flare.
Rocket flares are also used by the military to provide early warning of enemy
approaches. Trip flares can be hidden in the undergrowth and fired by trip
wires across likely enemy lines of approach. When an enemy sets off the
flare it gives away his position. At night the flare illuminates the approaching
enemy, giving defenders a clear advantage.
2.6
Spaceflight
Space begins at the Karman Line at an altitude of 100 km (62.1 miles) above
sea level. At this altitude there is no useable atmosphere so air breathing
propulsion such as jets and internal combustion engines will not work.
Aircraft have no atmosphere to develop lift from their wings so they won’t
work. Balloons have no atmosphere to provide buoyancy, so they won’t work.
The only type of propulsion that will get objects into space is a rocket motor.
The exploration of space is thus linked strongly to the evolution of rocket
motors.
Rockets have been used for every type of space mission: launching
satellites, manned space flights, sending probes to other planets and the
exploration of the upper atmosphere. Rockets that are used to propel objects
to orbits outside the atmosphere are called launch vehicles.
2.7
Rescue
Rocket motors are very useful for rapid escape systems as they can provide a
lot of thrust very quickly. Rocket powered escape systems can be used to
rapidly remove people or equipment from hazardous situations.
Ejector seats are installed in many in military aircraft to rapidly remove the
pilot from danger. Early ejector seats used a small explosive charge to move
the seat, but it was found that the sudden and violent acceleration was
causing back injuries. While this was usually preferable to staying in the
aircraft, it was sometimes causing injuries that prevented the pilot from
resuming flying duties.
A significant improvement was the replacement of the explosive charge with
two rocket motors, one either side of the seat. The gradual acceleration of
rocket motors resulted in far fewer pilots receiving back injuries. Modern
ejector seats use rockets to fire the seat and pilot clear of the aircraft then
deploy a parachute to lower the pilot safely to the ground. Once fired, the
seat’s operation is automatic, giving an unconscious or injured pilot a good
chance of survival.
It is impractical to install individual ejector seats into space capsules, so an
alternative system is used for manned space missions. A large rocket motor
is clamped to the top of the capsule. In an emergency the motor is
automatically fired and the capsule is lifted clear of the rocket. Small
boosters at the top of the escape rocket are used to steer the capsule. To
prevent setting fire to the capsule the escape system has 3 nozzles so the hot
exhaust plume is deflected around the side of the capsule
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Figure 7 - Astronaut Escape System
Rockets are not only used to effect escapes from aircraft and spacecraft.
They have been used at sea for many years to allow escape from damaged
ships. A small rocket pulls a rope from one ship to the other. When the rope
is secured at both ends the crew sit in a special seat called the breeches
buoy, which resembles a floatation ring with a leg harness, and are pulled to
safety.
2.8
Entertainment
Rockets have been used for entertainment since Chinese fire masters
organised the first firework display during the thirteenth century. Rockets
have been a key part of firework displays since the start as they add height to
a display. It is widely believed that the fireworks rockets were adapted to form
the first military rockets.
Fireworks have continued to be key part of celebrations and entertainment
across many cultures. In the United Kingdom we celebrate bonfire night and
New Year with firework displays. Displays have become so complex that
many of the professional displays are computer controlled, with fireworks lit
by electrical igniters under the control of a central program. This allows
effects such as the launch of a sequence of rockets of different colours and
altitudes, creating complex patterns in the sky.
National and international fireworks competitions continue to push the limits
of firework rocket technology to create higher and more impressive displays.
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2.9
Hobby and Amateur Rocketry
In the 1950s a small group on enthusiast in the South Western USA started
to design and build their own model rockets. This hobby has grown into a
worldwide movement of amateur rocket enthusiasts. We’ll learn more about
this exciting hobby in Chapter 7.
2.10
Transportation
It’s always been man’s ambition to fly in a personal flying machine. Rocket
belts have been seen as one way to make this become reality.
Like many rocket technologies, the first experiments with rocket belts took
place during the Second World War, but they were seen as having little
military significance. They first came to public attention in the opening
sequence of the James Bond film “Thunderball” in 1965.
A rocket belt is strapped to the pilot’s back like a parachute. Two nozzles,
one on the left and the other on the right, propel gas downwards causing the
pilot to lift. The pilot can control the amount of thrust from each nozzle and
can use this to manoeuvre himself around. The limitation of rocket belts is
that the pilot can only carry a small amount of propellant, typically enough for
only 10 to 30 seconds flight. This makes them impractical as a method of
transport, but they remain an impressive sight when demonstrated.
Rocket belts have found a practical use in the weightlessness of space. The
rockets thrust is only needed to start and stop movement, not to support the
astronaut’s weight. NASA’s Man Manoeuvring Unit (MMU) is a practical
rocket belt that has seen extensive use on space shuttle missions. The MMU
uses bursts of compressed gas to provide propulsion.
Figure 8 - Man Manoeuvring Unit (MMU)
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3.
Principles of Rocket Motors
3.1
Newton’s Third Law
When a rifle is fired, it recoils and pushes itself back into the firer’s shoulder.
We’ve all seen this in films, and may cadets will have experienced it on the
range. This is described in Sir Isaac Newton’s third law of motion: “To
every action there is an equal and opposite reaction”. The action is the bullet
travelling at high speed towards the target, and the reaction is the recoil of the
rifle.
The bullet leaves the rifle at a very high velocity, and the rifle is pushed back
into the shoulder. Imagine that the shoulder wasn’t there to absorb the recoil.
The rifle would travel in the opposite direction to the bullet, but at a much
slower velocity because the rifle is much heavier than the bullet.
V
M
m
v
Figure 9 - Newton’s Third law
The rifle has a mass M kg and recoils at a velocity V m/s the bullet has a
mass m kg and leaves the rifle at a velocity v m/s. It can be shown from
Newton’s laws that:
MV  mv
The mass of the bullet multiplied by its velocity is called the momentum of the
bullet. This equation tells us that the momentum of the rifle is the same as
the momentum of the bullet, but in the opposite direction.
For example, if we fire a bullet weighing 0.01 kg at a velocity 200 m/s from a
rifle weighing 2 kg, we can calculate the velocity of the recoil. We know that
M = 2kg, m = 0.01kg and v = 250 m/s, so from the equation above:
V 
 mv  0.01  200

 1 m/s
M
2
If we now fire another bullet the rifle will increase its velocity by another 1 m/s
and will now be travelling at 2m/s.
Imagine the rifle is replaces with a machine gun. Every time the machine gun
fires a bullet the gun will increase its velocity, or accelerate, in the opposite
direction to the bullets. The momentum of the machine gun will increase by
an amount equal to the mass of the total number of bullets fired, multiplied by
the velocity of the bullets.
TIP
Momentum can be explained with skateboards. Put two cadets of different
weights on skateboards and ask them to push each other apart. The lighter
cadet should travel faster than the heavier cadet. Don’t forget the safety
gear.
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3.2
How Rocket Motors Work
A rocket motor doesn’t fire bullets out the back of the rocket, but forces gas
through a constriction called the throat. Because the throat is narrow the gas
is squeezed through it and accelerated to a higher speed. Like the bullets,
this gas is an “action”, which produces the “reaction” of pushing the motor in
the opposite direction.
When you inflate a balloon the skin tightens, increasing the air pressure
inside it. When you release the balloon the air rushes out through the neck,
which acts as a throat. This movement of air causes an “action”, and the
“reaction” causes the balloon to fly off on the opposite direction.
Figure 10 - Principles of a Rocket Motor
A balloon would not make a very practical rocket motor as the supply of air
would quickly expire. In a rocket motor a constant supply of new gas is
required to sustain thrust for longer periods. A more practical motor could be
made by having a cylinder of compressed air feeding through a small throat.
The air supply would last longer than a balloon, but it would be an inefficient
motor.
The reaction that pushes the balloon is called its thrust. Thrust is the amount
of “push” that the balloon receives from the escaping air. A balloon gives very
little thrust whereas rocket motors can be required to give much higher thrust
to move many tonnes of rocket. The amount of thrust produced by a balloon
or a rocket motor is measured in Newtons. One Newton is the amount of
force required to make a mass of 1 kg accelerate by 1 m/s/s.
3.3
Practical Rocket Motors
Science teaches us that heating any gas in a confined space produces an
increase in its pressure. If we use a hot gas in a rocket motor then its
pressure will be higher. Gas at higher pressure will move faster through the
throat, increasing the amount of thrust we can get from the rocket motor.
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A more effective rocket motor can be made if we create hot gas within the
motor. The easiest way to do this is to burn a fuel which generates a lot of
gas. In a real rocket motor the fuel, called propellant, is burned with an
oxidiser inside a combustion chamber to produce gas at very high
temperature and pressure. The hot gas that results from this combustion can
only escape through the throat, where it is squeezed to accelerate it to a very
high velocity.
Figure 11 - Parts of a Rocket Motor
A Swedish engineer called Gustav de Laval made an important discovery
about the design of rocket motors in 1897. He attached a bell-shaped nozzle
behind the throat. De Laval realised that, by accelerating the gas to exactly
the speed of sound (Mach 1) in the throat, the gas would further accelerate as
it expanded in the bell. As the diameter of the bell increased the gas would
travel faster.
The de Laval nozzle accelerated the gas to much higher exhaust velocity
than was possible by simply constricting the gas. Rocket motors using this
design gave the gas a higher velocity, and hence a higher momentum. The
de Laval nozzle greatly increased the thrust available from rocket motors, and
it is still the basic nozzle design in most modern rockets.
3.4
Calculating Thrust
The thrust produced by a rocket motor depends on the amount of hot gas
generated by the motor and the velocity at which it leaves the nozzle. We call
the mass of exhaust gas that leaves the motor in one second the mass flow
 (pronounced “m dot”). The velocity at
rate which is usually written as m
which the gas leaves the rocket motor is called the exhaust velocity, usually
written ve. When a rocket motor is operating at its maximum efficiency, the
thrust produced by the motor, denoted by the letter F, can be calculated from
the simple equation:
 ve
F m
The thrust produced by the of the exhaust gases is called the momentum
thrust because the thrust generated by the motor comes from the change of
momentum of the gas.
Example: A rocket motor burns 10kg of propellant every second and has an
exhaust velocity of 1500 m/s. Calculate the momentum thrust.
 = 10 kg
m
ve = 1500 m/s
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 ve = 10 × 1500 = 15,000 Newtons
F m
Measuring the mass of exhaust gas leaving the motor in one second is very
difficult. We know that the amount of gas leaving the motor must be the
same as the mount of propellant and oxidiser entering the motor, so we can
measure the mass flow rate by knowing the mass of propellant and oxidiser
that enters the combustion chamber in one second.
TIP
Try making some balloon powered drag racers. Four wheels, a straw to
direct the air from the balloon, and a lightweight frame can make a
simple drag racer. Race these over a 5 meter track, and experiment with
thick and thin straws to produce different mass flow rates.
3.5
Flow Expansion
As the exhaust gas flows through the nozzle it expands and its pressure
drops. At the exit of the nozzle the exhaust gas is trying to expand into the
atmosphere, while the surrounding atmosphere is trying to limit that
expansion. A rocket motor will operate at its maximum efficiency when the
pressure of the exhaust gases at the nozzle exit is the same as the pressure
of the surrounding atmosphere.
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Figure 12 - Ideal Exhaust Plume
Figure 7 shows a rocket motor operating in ideal conditions. The blue graph
shows the gas velocity in the throat as being Mach 1 (M=1). As the exhaust
gas expands in the nozzle its velocity increases and its pressure decreases.
Ideally, the pressure at the nozzle exit will be atmospheric pressure.
When the exhaust gasses leave the nozzle they become the exhaust plume.
This is the bright tail of high temperature gas that can be seen from every
rocket motor.
If the exhaust gases at the nozzle exit are at the same
pressure as the atmosphere then the rocket motor will be operating in ideal,
or ambient, conditions. The exhaust plume will be straight and have a
constant width.
If the pressure at the nozzle exit Pexit is not the same as the atmospheric
pressure Patm the motor will operate less efficiently. There are two possible
conditions:

If the atmospheric pressure is higher than the nozzle exhaust pressure
(Patm > Pexit) the exhaust is compressed by the atmosphere causing it
to separate from the walls of the nozzle. This condition is called an
over expanded flow.

If the atmospheric pressure is lower than the nozzle exhaust pressure
(Patm < Pexit) the exhaust will not expand as much as it should in the
nozzle.
The exhaust gases will continue to expand after they leave
the nozzle. This condition is called an under expanded flow.
Figure 13 - Over and Under Expanded Exhaust Plumes
An over expanded flow often contains “Mach diamonds”. These are formed
as the exhaust plume expands and contracts, setting up shock waves inside
the gas. They can be very beautiful, but are a sign of an inefficient motor.
Figure 14shows Mach diamonds in a static test of a hydrogen/oxygen motor.
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Figure 14 - Mach Diamonds in an Over Expanded Exhaust
The detail of a Mach diamond can be seen clearly in Figure 15.
diamonds are created by shock waves in the plume.
The
Figure 15 - Details of a Mach Diamond
For the maximum efficiency the pressure in the exhaust plume must be the
same as the atmospheric pressure. Because atmospheric pressure changes
with altitude, it is clear that a rocket motor will only operate at its maximum
efficiency at one altitude. This creates a real problem for manufacturers of
launch vehicles as the motor need to operate from sea level to several tens of
miles altitude.
It is normal practice on launch vehicles to tune the motor for an altitude
around the middle of the range of pressures and accept some loss of
efficiency at the start and end of the burn.
3.6
Rocket Motor Efficiency
We judge the performance of cars by considering how many miles they can
travel on a gallon of fuel. This is a poor measure of rocket performance as
rockets consume very large amounts of fuel. The Saturn 5, built for the
manned missions to the moon, burned 15 tonnes of fuel every second!
The performance of a rocket motor is judged by determining how much thrust
can be obtained in 1 second by burning 1 kg of propellant. We call this
measure of performance the specific impulse of the propellant, and define it
Specific Impulse =
Thrust obtained x Duration of burn
Weight of propellant burned
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as:
Example: If we burn 20 kg of propellant for 5 seconds, getting 800 Newtons
of thrust, the Specific Impulse is 800 x 5 / 20 = 200 seconds. We could arrive
at the same value of specific impulse by burning the same amount of
propellant at half the thrust (400 Newtons) for twice the duration (10
seconds). This would give 400 x 10 / 20 = 200 seconds.
The specific impulse of a fuel is independent of how fast it is used. Specific
impulse is thus a good measurement for comparing fuels. The specific
impulse of some common propellants can be seen in the table below.
Propellant
Specific
Impulse
Comments
Black Powder (BP)
40 s
Used in model rocket motors and
fireworks
Used in solid rocket boosters and
high power model rocket motors
Ammonium
150 s
percholorate composite
propellant (APCP)
Hydrazine
220 s
(monopropellant)
RP-1 and Liquid Oxygen 370 s
(LOX)
Very widely used in launch
vehicles. RP-1 is a form of pure
kerosene.
Figure 16 - Popular Propellants
The table shows that liquid hydrogen and liquid oxygen give the best specific
impulse. RP-1 and liquid oxygen are also widely used as RP-1 is easier to
store and handle than liquid hydrogen.
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4.
Types of Rocket Motor
4.1
Classification of Motors and Engines
By convention any rocket propulsion systems that have moving parts, for
example pumps and turbines, are called engines. Propulsion systems without
moving parts are called motors. This convention will be followed throughout
this chapter.
This chapter considers several types of propulsion system. The majority of
motors and engines derive thrust from exothermic chemical reactions, which
means that they generate heat. The heating of the gases generated by the
chemical reaction causes the propulsion effects. Such motors are known as
chemical motors.
Chemical motors have a very high thrust to weight ratio but a relatively low
specific impulse. They are ideal for overcoming the forces of gravity and
aerodynamic drag, and are very useful inside the atmosphere. They are,
however, less useful for performing accurate manoeuvres in the vacuum of
space. Other classes of motor based on electrical or nuclear forces offer
much better performance in space. These motors have low thrust to weigh
ratio but very high specific impulse.
Figure 17 - Types of Motor and Engine
4.2
Cold Gas Motors
The cold gas motor is the simplest form of rocket propulsion. It is very similar
to the balloon “motor” that we considered in chapter 3, but with a little more
technology to improve the efficiency and duration of the motor.
Figure 18 - Cold Gas Propulsion
Nitrogen is the most commonly used gas in cold gas systems as it is cheap,
safe and readily available. Compressed Nitrogen is stored in a metal tank.
When thrust is needed a tap is released, and the gas escapes through a
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nozzle where it is accelerated. The advantages of cold gas motors are that
they are cheap, simple and safe. The disadvantages of cold gas propulsion
systems are that they do not provide much thrust and are very inefficient.
Cold gas propulsion is used where small amounts of thrust are needed for
short periods of time.
Two common applications are NASA’s Man
Manoeuvring Unit (MMU) for astronaut spacewalks, and on small satellites.
4.3
Solid Propellant Motors
Solid propellant motors are the most common type of rocket motor. The
advantages of solid motors are that they are easy to manufacture and simple
to use. The disadvantage of solid rocket motors is that, once ignited, it
cannot be controlled or extinguished. Solid propellant rocket motors are ideal
for applications which require a predictable thrust for a fixed period of time
including missiles, launch vehicles, flares, model rockets, safety systems,
ejector seats and flares.
The picture shows a cross section of a solid rocket motor. In this motor the
propellant has been packed into the case with a hollow core in the centre.
The igniter is at the front of the motor.
Figure 19 - Cross Section of a Solid Propellant Motor
This motor is called a core burning motor as, after ignition, the hollow core of
the motor burns. It can be seen that the propellant fills most of the case,
except for the throat and nozzle.
Figure 20 - Cutaway of a Solid Propellant Motor
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When the igniter fires it sets fire to the surface of the propellant. The hot gas
from this combustion fills the hollow core of the motor and rushes out through
the throat and nozzle. The propellant burns outwards from the core to the
case until it has all been consumed, often referred to as burn out.
Figure 21 - Solid Propellant Motor Burn Sequence
Figure 21 shows a sequence of diagrams from ignition to burn out. As the
propellant is burned away the radius of the hollow core increases, and thus
the area of the burning surface increases. The amount of exhaust products
produced every second is continuously increasing because more gas is being
produced, thus the mass flow rate of this motor increases throughout the
burn. If the mass flow rate increases then the thrust must also increase, so
this core burning motor starts with a low thrust and generates more thrust
over time.
It can be concluded that the thrust of a solid rocket motor at any time during
its burn depends on the burning surface area of the propellant at that time.
This is a very important property of solid rocket motors. Rocket motor
designers take advantage of this property to manufacture motors with specific
thrust profiles. By changing the shape of the propellant inside the case it is
possible to create motors with thrust profiles that increase with time
(progressive), remaining constant with time (neutral) or decrease with time
(regressive). Some common propellant shapes are shown in Figure 22
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Figure 22 - Solid Propellant Motor Propellant Shapes
Solid propellant motors are normally ignited electrically using an igniter wire
coated with a pyrogen. When a current is passed trough the wire it gets hot
and ignites the pyrogen. The pyrogen produces a burst of heat at very high
temperature which ignites the surface of the propellant grain.
4.4
Liquid Propellant Engines
Liquid propellants are more easily controlled than sold propellants as the
amount of liquid flowing into the engine can be controlled. Engines using
liquid propellants are generally more efficient than motors using solid
propellants. The need for pumps, valves and control systems makes liquid
propellant engines more complex than solid propellant motors.
The propellants used in liquid propellant rocket engines can be either liquids
such as RP-1 or liquefied gasses such as liquid hydrogen (LH2). These are
burned with an oxidising agent, normally liquid oxygen (LOX).
4.4.1
Main Components of a Liquid Propellant Engine
The diagram shows the main components of a liquid propellant engine.
Propellant and LOX, stored in tanks elsewhere in the rocket, are pumped
under pressure into the combustion chamber and ignited. The hot exhaust
gasses pass through a de Laval nozzle to provide thrust. The pump which
pressurises the gases is often driven by a turbine which taps off a small
amount of propellant and LOX from the low pressure system.
Figure 23 - Liquid Fuelled Rocket Engine
The efficiency of the motor depends on how well the propellant and LOX are
mixed within the combustion chamber. The liquids are injected into the
chamber as a fine spray to ensure that they mix as completely as possible.
Perfect mixing ensures that all the propellant and LOX are consumed, and
the maximum thrust can be obtained from the engine.
The design of the injectors is thus very important to the efficiency of the
engine. Poorly designed injectors will cause a reduction in thrust as all the
fuel will not be burned. In some circumstances a poorly mixed combustion
can become unstable causing the engine to explode.
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Figure 24 - Vulcain-2 Engine
The picture shows the Vulcain-2 engine which powers Europe’s Ariane 5
launch vehicle. The two detailed pictures on the right show the injector
assembly A and the combustion chamber B.
4.4.2
Cooling Systems
The temperature of the combustion can be very high, typically over 2000ºC.
This can be hot enough to melt the steel components in the motor,
particularly around the combustion chamber and the nozzle. It is important to
cool these components to prevent damage and increase the operating life of
the engine.
One commonly used technique is called regenerative cooling. This
technique pumps the cold propellant through fine tubes around hot
components. Heat from the engine is transferred to the propellant, cooling
the engine and pre-heating the propellant.
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Figure 25 - Regeneratively Cooled Liquid Fuelled Rocket Engine
Regenerative cooling is very efficient, but is not without risks. The slightest
leak in the propellant pipes would expose heated propellant to the motor
plume, with potentially catastrophic results. The manufacture of regenerative
cooling systems requires the very highest quality of workmanship and
materials. The pictures below show the construction of the nozzle for the HM7 engine from rectangular tubes, each of which must be perfectly aligned and
sealed. The task requires very high accuracy so the nozzle is assembled and
laser welded by robots.
Figure 26 - Regenerative Cooling in the HM-7 Engine
A technique for preventing damage to the nozzle is called curtain cooling (in
some books it is called film cooling). The exhaust gases from the turbine are
at a much lower temperature than the combustion products. Curtain cooling
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takes the turbine exhaust products and injects them into the nozzle. This
forms a thin but cooler gas layer in contact with the inner surface of the
nozzle, and this thin film of cooler gas insulates the nozzle from the main
combustion products.
Figure 27 - Curtain Cooled Liquid Fuelled Rocket Engine
Sometimes designers will use a combination of techniques to cool the engine.
The F-1 rocket engines, used in the Saturn V launch vehicle, used
regenerative cooling for the upper nozzle and curtain cooling for the lower
nozzle.
Figure 28 – Regenerative and Curtain Cooled F-1 Engine
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4.4.3
Fuel Management System
It is possible to control the thrust generated by a liquid propellant engine by
controlling the amount of propellant it burns. This is because the mass flow
rate of the motor changes with the amount of propellant being burned. This is
very similar to the throttle on an aircraft.
The propellant can be pumped into the engine, as seen in previous diagrams,
or it can be forced in by pressurised gas using a blowdown system. A
blowdown system uses gas to pressurise the propellant and oxidiser tanks
and force the liquids out of the tanks and into the engine. An inert gas which
will not react with the propellant or oxidiser is used. Helium is the most
common choice.
Figure 29 shows a typical blowdown system. The pressure to each tank is
controlled by two valves. Increasing the pressure will increase the flow rate of
the propellant from each tank, allowing the thrust to be controlled.
Figure 29 - Blowdown System
Blowdown systems are very common in smaller rockets where the weight of a
small pressurized gas tank is less than the pumps and pipes required to drive
a larger motor. They are also used in larger rockets to maintain tank pressure
in engines that are driven by pumps.
The fuel tanks occupy most of the volume of a rocket. The tanks are usually
stacked vertically in the rocket body. The pipes for the forward tank have to
be routed past the aft tank. The simplest system is to route the pipes around
the outside of the aft tank, usually outside the skin of the rocket.
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A better design is to run the pipes through the aft tank. Thus can be
combined with the joining of the two tanks by a common bulkhead, which
reducing weight and complexity. The used of a common bulkhead and
internal pipe requires very high quality workmanship as there is only one layer
of metal separating propellant and oxidiser. The slightest leak could be
catastrophic.
Figure 30 shows two common fuel tank arrangements.
Figure 30 - Fuel Tank Arrangements
Other, less common, fuel tank arrangements include concentric tanks, where
one tank is located inside the other, and the use of multiple tanks.
4.5
Bi-Propellant Engines
Bi-propellant engines are a form of liquid propellant engine which requires no
ignition system. They use two chemicals which spontaneously ignite as soon
as they come into contact with each other and release a lot of heat. Chemical
reactions which spontaneously ignite are called hypergolic.
The advantage of bi-propellant engines is their simplicity; they only require
two tanks, valves and a combustion chamber. The disadvantage of bi-
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propellant engines is that the chemicals can be very dangerous to
manufacture, transport and store.
Figure 31 - Bi-Propellant Engine
Nitrogen tetroxide and hydrazine are useful reagents in bi-propellant engines.
They can be stored for long periods, are relatively light liquids, and react
strongly. Bi-propellant engines based on these two chemicals are commonly
used in satellites and deep space probes.
4.6
Monopropellant Engines
Monopropellant engines use a single chemical that spontaneously ignites in
the presence of a catalyst. The catalyst is not consumed by the reaction but
acts a trigger that forces the propellant to decompose into other chemicals.
Some monopropellants can reach full operating temperature and pressure in
1/100 of a second or less, which makes them useful in applications where a
short burst of thrust is required. They are often used to manoeuvre satellites.
Figure 32 - Monopropellant Engine
The advantages of monopropellant engines are their simplicity, relatively high
specific impulse, and quick response. As with bi-propellant engines, the
disadvantages of are that the chemical can be very dangerous to
manufacture, transport and store.
Hydrogen peroxide and hydrazine are two commonly used
monopropellants. Very pure hydrogen peroxide, sometimes referred to as
high test peroxide (HTP), is very unstable. When it comes into contact with
a platinum catalyst it spontaneously decomposes to water and oxygen,
releasing a lot of heat and creating superheated steam and oxygen.
Hydrazine decomposes into ammonia, nitrogen and hydrogen at about 800ºC
when it comes into contact with a platinum catalyst.
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4.7
Hybrid Motors
The previous sections considered propulsion systems which used solid and
liquid propellants. These two types of system dominated rocketry throughout
the twentieth century. In the twenty-first century a new type of motor, the
hybrid motor, has started to establish itself as a credible alternative.
A hybrid motor uses a pressurised oxidiser in liquid or gaseous form and
forces it through a solid propellant. The propellant can be a conventional
solid propellant, or any material that can burn at a high temperature and
produce gas. This opens up the possibility of using materials such as plastics
as the solid propellant, and LOX or Nitrous Oxide as the oxidiser.
The advantages of hybrid motors are their relatively simple construction,
relatively low cost, safety during construction and transportation, and higher
specific impulse than solid propellant motors. The disadvantage of hybrid
motors is that some residual propellant is usually left in the combustion
chamber to prevent damage to the chamber walls, thus not all the propellant
is consumed.
Figure 33 - Hybrid Motor
Hybrid motors have stated to gain popularity. The first privately funded
manned spaceflight, Spaceship One, was propelled to space using a hybrid
motor. Amateurs and universities have found hybrid motors to be
inexpensive, simple and safe alternatives to liquid and solid propulsion. Solid
propellants such as recycled tyres and foodstuffs have been successfully
burned in hybrid motors.
4.8
Electrical Propulsion
Chemical motors use a chemical reaction to generate hot exhaust products.
Electric propulsion systems accelerate small amounts of ionised gas with
strong electrical fields. The mass flow rates of ion thrusters are very low but
the exhaust velocity is very high
One common type of type of electrical propulsion is the ion thruster. The
basic arrangement of an ion thruster can be seen in Figure 34.
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Figure 34 - Ion Thruster
An ion thruster comprises an ionisation chamber with a hot cathode at one
end and a grid anode at the other. Atoms of Xenon gas in this chamber
collide with the electrons flowing between the anode and cathode and
become positively charged. The Xenon ions accelerate to very high velocity,
typically 30 km per second. As the ions pass through the grid they are
bombarded with electrons to neutralise the positive charge.
An intense magnetic field is generated in the ionisation chamber by current
passing through a coil around the chamber. This magnetic field forces the
electrons to follow a spiral path through the chamber, increasing the
probability of their colliding with a Xenon atom. This greatly improves the
efficiency of the thruster as it increases the number of Xenon ions generated,
however it requires a lot of electrical power from the host spacecraft to
generate the field.
The advantages of ion thrusters are their simplicity, very high specific
impulse, and light weight. The disadvantages of ion thrusters are their low
thrust and need for a lot of electrical power from other sources, normally solar
panels. They are ideal as manoeuvring thrusters for satellites where low
thrust and high efficiency are important.
4.9
Nuclear Thermal
Nuclear thermal engines have the potential to offer both high thrust and a
specific impulse of over 1000. A light gas, ideally hydrogen, is pumped from
a storage tank through a nuclear reactor. The reactor heats the gas to a very
high temperature, and it expands through a de-Laval nozzle.
There have been experiments to make nuclear thermal engines, starting with
the NERVA programme in 1963, but these engines have yet to be used in a
rocket. The difficulties are not technical but political; no nation wants nuclear
reactors flying over its territory.
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Figure 35 - Nuclear Thermal Engine
The advantages of nuclear thermal motors are their high thrust and high
specific impulse. The disadvantages are that they are unproven technology
and the political issues of launching nuclear reactors.
4.10
Solar Thermal
Solar thermal propulsion is a novel proposal for generating small thrusts for
long durations with high efficiency. It is suitable to gradually accelerate small
objects in space.
The principles of solar thermal propulsion are quite simple. The sun’s rays
can be focussed with a mirror or lens to produce enough heat to ignite some
materials. Solar thermal propulsion focuses the sun’s rays on a small
chamber through which gas is being pumped. The gas is heated to a
temperature of 2000ºC, which causes it to expand through a nozzle.
Figure 36 - Solar Thermal Engine
The advantages of solar thermal propulsion are its simplicity, and the
availability of limitless energy from the sun. The disadvantages are that
power decreases as the motor gets further from the sun, the need for
accurate pointing of the mirror, and that this technology has yet to be tried in
space.
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4.11
Summary of Motor Types
This chapter has considered many different types of rocket propulsion. The
table below compares the typical range of thrust and specific impulse
available from each type of propulsion.
Type
Thrust range
(Newtons)
Specific
Impulse
Comments
(seconds)
Cold gas
0.1 N to 250 N
70 s
Solid propellant
1 N to 12,500,000 N
80 s to 250 s
Liquid propellant
100 N to 6,600,000 200 s to 450 s
N
Bi-propellant
5 N to 400 N
250 s to 400 s
Monopropellant
0.5 N to 500 N
150 s to 300 s
Hybrid
10 N to 60,000 N
200 s to 400 s
Ion thruster
0.02 N to 10 N
2500-10000 s
Nuclear Thermal
1,000,000 N
1000 s
Unproven technology
Solar Thermal
<20N
800 s
Unproven technology
Higher thrust hybrids
are being developed.
Figure 37 - Characteristics of Propulsion Systems
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5.
Rocket Structures and Systems
5.1
Forces on a Rocket
When a rocket lifts off there are three main forces acting on it. The three
forces and their directions, shown in Figure 38, are:

Thrust from the motor, at the bottom of the rocket trying to push it
upwards.

The weight of the rocket, propellant and payload, trying to hold the
rocket on the ground.

Aerodynamic drag, pushing on the nose and trying to slow the rocket
down.
Figure 38 -Forces on a Rocket
As we saw in chapter 3, thrust depends on the type of propellant, the mass
flow rate and the exhaust velocity of the gas. It is possible to control thrust by
controlling the amount of propellant and oxidiser that is being burned in the
combustion chamber. The weight of the rocket is on the weight of the empty
rocket, known as its dry mass, added to the weight of the payload and fuels.
Drag is a force that depends on the aerodynamic shape of the rocket and its
velocity. As velocity increase the amount of drag increases.
At the moment the rocket starts to move, called lift off, the rocket is
stationary. As it has no velocity the drag force is zero. For the rocket to lift off
the thrust must be greater than the weight. Rocket scientists refer to a
rocket’s thrust/weight ratio, which is similar to a car’s power/weight ratio. If
the thrust is greater than the weight, the ratio is greater than one and the
rocket will fly. It is normal for a launch vehicle to have a thrust/weight ratio of
about 1.2 to 1.6.
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Once the rocket is moving it must continue to accelerate, and for this to
happen the thrust must overcome both weight and drag. As the flight
progresses the weight decreases as propellant is burned, whereas drag
increases as the rocket gains speed.
The thrust and drag forces act at opposite ends of the rocket and are trying to
crush the rocket. There is a time in the rocket’s flight where the forces trying
to crush the rocket are at a maximum, and this is called “max Q”. It is
common practice to reduce the rockets thrust before max Q and then
increase thrust again after max Q. This reduces the maximum forces on the
rocket, and allows a reduction in the designed strength of the rocket and
hence some saving in weight.
The payload, mounted at the top of the rocket, is protected from the wind
pressure and turbulence by a fairing. The fairing is not needed after the
rocket leaves the atmosphere and is jettisoned.
5.2
Structures
Rockets are very complex structures. They need to be as light as possible to
increase the power to weight ratio, yet they need to be retain their strength
and stiffness throughout launch and flight. An easy option would be for the
rocket manufacturer to strengthen the rocket by thickening the outer skin and
adding struts and other structural parts. For every kg of metal that is added to
the rocket a kg of payload has to be removed. It’s clearly very desirable for
the rocket designer to maximise the payload that can be launched; that’s how
rocket manufacturers make their money.
Figure 39 - Rocket Structure
Two common techniques are used for making strong but lightweight rocket
structures. The first is to make a tubular frame from struts, which run the
length of the rocket body, and ribs which run around the body. This frame is
inherently strong, so the major components of the rocket, such as the motor,
tanks and guidance systems, can be attached to the frame using support
structures. A lightweight skin is then attached to the frame, with access
panels cut into the frame to allow maintenance and testing of the rocket.
This technique does waste some space, particularly between the walls of the
tanks and the skin of the rocket. The frame can be quite heavy, although this
is partially offset by the thinness of the skin.
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Figure 40 – Monocoque Rocket Structure
An alternative method for making a strong but lightweight rocket is to use a
thicker skin, but to pressurise the whole rocket. This is known as a
monocoque structure. Pressurising the rocket makes the skin stretch and
stiffens the rocket. The propellant tanks use the skin of the rocket as their
outer wall, and are separated internally by a bulkhead.
The tanks need to be kept at a constant pressure to keep the rocket stiff
throughout its flight. As propellant and oxidiser are consumed the pressure is
maintained by pumping helium into the tank. This has the effect of
pressurising the rocket, so to maintain stiffness throughout the flight, and also
acting as a blowdown system.
It is very desirable for a rocket to be light, strong and to carry as much fuel
and payload as possible. There are two standard ratios that are used to
assess the strength and weight of a rocket design. The first is called the
mass ratio. This is defined as the ratio of the rocket with propellant (the wet
mass) to the ratio of the rocket without propellant (the dry mass). The
second ratio is called the propellant mass fraction. This is defined as the
ratio of the mass of propellant to the wet mass of the rocket. The propellant
mass fraction shows how much of the lift-off mass is propellant.
If m0 is the initial total mass, including propellant, mf is the final total mass,
and mp is the mass of propellant, then:
Mass ratio =
mf
m0
Propellant mass fraction =
mp
m0
The mass ratio for several different rockets is shown in the table at Figure 41.
The rockets are presented in historic order with the newest at the top,
illustrating how the steady improvement in rocket technology. Modern rockets
have a mass ratio ten times better than the V2.
Vehicle
Mass ratio
Ariane 5
39.9
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Space Shuttle
15.4
Saturn V
23.1
X-15
2.3
V2
3.8
Figure 41 - Mass Ratios
A propellant mass fraction above 0.8 is regarded as good, and some modern
rockets have propellant mass fractions of over 0.85. The propellant mass
fraction of the Space Shuttle is about 0.82.
5.3
Guidance
Both rockets and missiles need to be guided during flight. This keeps the
rocket on the desired course, and corrects any minor changes in direction
caused by wind, gravity or other external forces.
Like aircraft, rockets can spin or roll, the nose can pitch up and down, and
the rocket can yaw from side to side. To make calculations easier,
aeronautical engineers define roll, pitch and yaw as rotations around
imaginary lines which pass through the centre of gravity of a rocket. These
lines are called the roll axis, pitch axis and yaw axes. They are illustrated for
a rocket, and an aircraft, in Figure 42.
Figure 42 - Roll, Pitch and Yaw
In an aircraft there are clear ideas of the direction of these axes. The roll axis
is in the direction of travel, the pitch axis runs parallel with the horizon and at
90 to the direction of travel, and the yaw axis runs vertically through the
aircraft. In a rocket the roll axis is about the direction of travel, just like an
aircraft, but it is difficult to define pitch and yaw axes in a round bodied rocket
or missile. Rocket and missile engineers choose these directions arbitrarily,
usually aligning the axes with the direction of fins or the orientation of any
gyroscopes inside the rocket.
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The control surfaces of an aircraft interact with the air to generate forces, and
these forces are used to turn the aircraft. Missiles usually operate inside the
atmosphere so they often use control surfaces to “fly” in a similar manner to
an aircraft. These control surfaces, called fins, are used to align with the yaw
and pitch axes, and are deflected to produce aerodynamic forces.
Figure 43 - Fins on a Missile
The AIM-9 Sidewinder used two sets of fins to steer the missile. The forward
fins controlled pitch and yaw, and worked in pairs to steer the missile. The aft
fins controlled roll using an ingenious gyroscope system powered by the
airflow over the fins.
Figure 44 – Sidewinder Missile
Changing the direction of a rocket is often more complex than an aircraft. In
the upper atmosphere or in space there is not enough air for fins and control
surfaces to have an effect, so engineers have created other techniques.
Many of Robert Goddard’s more successful rockets guided the rocket by
inserting vanes into the exhaust plume. Inserting a vane deflected the flow
changed the direction of the thrust. A set of four vanes allowed the rocket to
be steered in both pitch and yaw. Rigid fins controlled the roll of the rocket.
Figure 45 shows the vanes on the back of a Robert Goddard rocket from
1935, and how vanes can be used to steer the rocket.
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Figure 45 - Vanes
Goddard made his vanes from metal. Because his rockets were experimental
the engine only burned for a few seconds. The metal vanes did not have time
to melt in the hot exhaust plume during such short flights. Vanes were
subsequently used on the V2 rocket. These were made of graphite to prevent
them melting on the longer engine burns of the V2.
As rocket engines became more powerful, and motor burns of 10-25 minutes
became normal, vanes became an impractical technique. It became too
difficult to create materials that could survive extreme heat of the exhaust
plumes for long periods. Engineers realised that it was simpler to mount the
engine on a gimbal and move the whole motor using hydraulics. This
technique is still commonly used in modern rockets and missiles.
Figure 46 shows the Vulcain engine, made by EADS and used on the Ariane
5 launch vehicle. The engine pivots around the gimbal at the top the
combustion chamber.
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Figure 46 - Vectored Thrust
Another technique, used on larger rockets and in space, is to have small
thrusters on the rocket. These are mounted to give a sideways thrust. When
it is necessary to steer the rocket one or more of these thrusters is fired,
nudging the rocket in pitch or yaw.
Figure 47 – Thrusters
Thrusters have been used to provide roll control on finned rockets. They
were mounted inside the fins and facing sideways. By firing pairs of thrusters
the rocket can be forced to roll.
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6.
Launch Vehicles
Since the launch of the first spacecraft, Sputnik, satellites have revolutionised
our lives. They bring television to our homes, carry news from the most
inaccessible places, allow telephone and internet access from any point on
the earth, allow military and emergency services to communicate, tracked
weather systems and monitored the health of the planet.
Many of these satellites need to maintain a constant position above the earth.
If a satellite is launched to a point 36000 km above the equator it will orbit the
earth once every 24 hours and appear to be stationary to an observer on the
ground. This orbit is known as a geostationary orbit. It is sometimes known
as the Clarke orbit after Arthur C Clarke who first realised the potential of
satellites in this unique orbit.
Figure 48 – Geostationary Orbit
In 1945 Clarke suggested that three satellites, equally spaced around the
equator in geostationary orbits, will cover most of the earth’s surface, and all
the populated areas. The idea was ahead of its time as no-one built a rocket
powerful enough to place a satellite in that orbit for another 18 years.
Rockets designed for the purpose of launching satellites to geostationary or
other orbits are called launch vehicles. The purpose of a launch vehicle is to
accelerate a satellite (usually called the payload) to the required final velocity
of 10.3 km/s. At this velocity the satellite will coast up to 36,000 km, which is
the same altitude as the geostationary orbit. Rocket motors onboard the
satellite are used to accurately place the satellite in that orbit.
6.1
Staging
The performance of a launch vehicle can be improved by getting rid of bits of
the structure that are no longer needed during the flight. This technique,
called staging, is used in all current launch vehicles.
The easiest way of illustrating the advantages of staging is with an example.
Imagine we have a rocket with a launch weight of 100 tonnes, which
comprises:

80 tonnes of propellant

2 tonnes of satellite payload

18 tonnes of structure.
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The 80 tonnes of propellant is used to lift the 18 tonnes of structure and 2
tonnes of satellite, 20 tonnes total. Half way through this flight there is 40
tonnes of propellant left to lift the 20 tonnes of structure and satellite.
Let’s divide 18 tonnes of the rocket structure into 2 stages of 9 tonnes, and
divide the 80 tonnes of propellant equally between them. For the first half of
the flight we burn 40 tonnes of propellant to lift the 20 tonnes of rocket. At
this point we drop the first stage, which comprises 9 tonnes of redundant
rocket structure. In the second half of the flight we have 40 tonnes of
propellant to lift 9 tonnes of structure and 2 tonnes of satellite.
When we compare the single stage rocket to the staged rocket we find that,
and the middle of the flight, they have the same amount of remaining fuel but
the staged rocket is much lighter. The satellite will thus be accelerated to a
much higher velocity by the second stage, and will thus finish in a higher orbit.
Rockets can be divided into three or more stages. This technique is very
widely used, and is called “multi staging”.
There are two standard forms of staging: serial staging and parallel
staging. In serial staging the stages are stacked vertically in the rocket and
are fired one after another. Parallel staging uses stages strapped alongside
each other which are fired simultaneously. This gives a much higher thrust,
allowing high velocities to be reached at a lower altitude, but the propellant is
burned a higher rate so the rocket “burns out” sooner.
Figure 49 - Serial and Parallel Staging
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Nowadays most rockets use a combination of serial and parallel staging.
Solid rocket boosters (SRB) are attached to the first stage, increasing the
initial thrust. When the SRBs burn out, the first stage carries on burning until
its propellant is exhausted. The first stage is then discarded, and a
conventional serial staged rocket is left to complete the launch. The
advantage of this is that the first part of the flight is through the dense
atmosphere, so the additional thrust obtained from the SRB is useful for
overcoming atmospheric drag.
By the time the SRBs burn out the
atmosphere is sufficiently thin that drag is no longer a significant force.
6.2
Launching a Satellite to GEO
If a satellite is to reach GOE it needs to be accelerated to a velocity of 10.3
km/s. It is normal for GEO launch vehicles to have 3 stages, and for each
stage to be dropped as its propellant is consumed. Figure 50 shows the
stages of a launch from ignition to deployment of the satellite.
Figure 50 – Launch phases
The satellite’s own propulsion system takes over when it is clear of the rocket.
It needs to reach a velocity of 10.3 km/s and is only travelling at 9.5 km/s, so
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the satellite fires its apogee boost motor (ABM) to accelerate it to the
required velocity.
This puts the satellite into a Geostationary Transfer Orbit (GTO) with its
highest point (apogee) at 36,000 km and lowest point (perigee) at about 500
km. The period of this orbit is about 10 hours. As the satellite coast to
apogee it is slowed down by gravity until is has a forward velocity of just 1.6
km/s.
Figure 51 - Geostationary Transfer Orbit
The ground controllers monitor the satellite for seven orbits, using this data to
calculate the exact time for the next manoeuvre. When the satellite reaches
apogee on its seventh orbit it has an altitude of 36,000 km and a velocity of
1.6 km/s.
Figure 52 – Apogee Boost Motor “Burn”
To transfer from GTO to the geostationary orbit it needs to accelerate from
1.6 km/s to 3.1 km/s, a velocity change of 1.5 km/s. The satellite is
accelerated by firing the ABM for a precisely calculated time.
This
manoeuvre is called the ABM burn and it places the satellite in geostationary
orbit.
Finally the satellite does a controlled drift to its final location on the equator.
This is done using manoeuvring thrusters in a series of small manoeuvres.
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6.3
Launch Vehicles
Launch vehicles are specified in terms of the largest weight of satellite which
they can accelerate into a GTO from their normal launch site. The table
below shows the main launch system in current use.
Operator
Launcher
name
Payload
to Launch sites
GTO (tonnes)
USA
Atlas
3.6
Cape
USA
Canaveral,
USA
Delta
3.8
Cape
USA
Canaveral,
Russia
Proton
4.5
Baikonur, Russia
Europe
Ariane 5
5.9
Kourou,
Guiana
Japan
H2
4.0
Tanegashima, Japan
China
Long March
4.5
Xichang, China
USA/Russia Sea Launch
5.9
Offshore platform
French
Figure 53 - Current launch vehicles
The Space Shuttle is not in this list as it is no longer used to launch satellites.
Nearly all Shuttle missions are for supporting the International Space Station.
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7.
Model Rockets
You can have your own rocketry programme through model rocketry. The Air
Training Corps has included model rocketry on the list of approved activities.
The policy for Squadrons that want to fly model rockets is in ACP 20 ACTI 75.
The specialist body for all models, medium and high power rocketry in the UK
is the United Kingdom Rocketry Association (UKRA). UKRA offers specialist
advice to new rocketeers, and provides insurance through the British Model
Flying Association (BMFA). UKRA has affiliated clubs all over the UK who
can help you to get started safely in this fascinating activity. Clubs can be
contacted through the UKRA website www.ukra.org.uk, which is also a source
of useful advice and material for getting started with model rocketry.
Model rockets are not toys but are real rockets in miniature, capable of flying
to several hundred miles and hour and hundreds of feet in the air. They are
launched from a launch pad, which supports the rocket while it accelerates.
The motors are ignited electrically from a safe distance. The rocket deploys a
parachute which ensures a soft landing, allowing the rocket to be flown many
times.
7.1
Phases of Flight
Each model rocket flight comprises several phases, as shown in Figure 54.
These phases are called boost, coast, ejection, descent and landing.
Figure 54 – Phases of Flight
Boost: An electrical igniter is used to ignite the propellant and launch the
rocket. The rocket is guided along a launch rod for the first one or two meters
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of flight. At the end of the rod the rocket is travelling between 30 and 50 mph,
and has enough airflow over the fins to fly in a straight line.
Coast: When the motor burns out the rocket is travelling very fast, typically
between 200 and 600 mph. The loss of thrust from the motor means that the
drag and weight of the rocket will cause it to progressively decelerate. During
this phase the rocket coasts to the highest point of its flight, called its apogee,
emitting tracking smoke to improve its visibility.
Ejection: At apogee the motor fires a small charge which ejects the nosecone
and parachute. The parachute deploys and the rocket starts its descent.
Descent: The rocket descends quite slowly under parachute, typically
between 5 to 15 feet/sec. The rate of descent depends on the weight of the
rocket and the size of the parachute.
Landing: The rocket will drift while descending and will land downwind from
the launch site.
7.2
Model Rockets and Motors
A model rocket has many of the properties of a full-sized rocket. The
structure must be strong and light, it is powered by a solid rocket motor with a
de Laval nozzle and it is capable of carrying small payloads.
The structure of the rocket is built around a body tube made from light but
strong materials such as resin impregnated cardboard or plastic. Internal
structural components, fins and nosecones are made from balsa wood,
cardboard and plastic.
Figure 55 – Parts of a Model Rocket
The main parts of a model rocket can be seen in Figure 55. A small motor
hook holds the motor in place in the motor tube, and allows new motors to be
installed quickly. At launch a thin tube called the launch lug is used to hold
the rocket to the launch rod. During descent the nosecone, parachute and
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main body are held together by a shock cord, usually made from elastic or
Kevlar.
Motors for the smallest model rockets have a rolled paper combustion
chamber, usually referred to as the motor case. A ceramic de Laval nozzle is
moulded into one end of the motor case. The motor is packed with a black
powder propellant, above which is a sulphur rich black powder delay grain,
and topped with a loose black powder ejection charge. The igniter comprises
a pyrogen coated wire which is inserted through the nozzle and held in place
by a coloured plastic plug.
Figure 56 – Motors and Igniters
The burn sequence of a motor can be seen in Figure 57. When a current
passes through the igniter its sets off the pyrogen, creating a small pulse of
heat which ignites the propellant (1). The pressure from the burning
propellant pushes the plastic plug and igniter out of the motor (2). The solid
propellant burns from one end, creating the thrust during the “boost” phase
(3). When the propellant has been consumed it ignites the delay grain, which
emits tracking smoke while the rocket coasts to apogee (4). When the delay
grain has been consumed it ignites a black powder ejection charge, which
pushes the nosecone off the rocket and deploys the parachute (5)
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Figure 57 – Motor Burn Sequence
Model rocket motors are classified by three properties: their impulse, the
thrust which they provide during boost, and the delay time during the coast
phase.
Impulse is defined as the average thrust multiplied by the duration of the burn
in seconds. For example, a motor with a thrust of 6 Newtons which burns for
2 seconds has an impulse of 12 Newton-seconds (Ns). A different motor
providing 4 Newtons of thrust for 3 seconds would have the same impulse,
but a lower thrust and a longer burn. Model rocket motors are grouped by
their total impulse. Every class has double the impulse of the previous class,
and is designated by a letter.
Impulse
Class
Impulse
Propellant
A
1.26-2.50 Ns
BP
B
2.51-5.00 Ns
BP
C
5.01-10.00 Ns
BP
D
10.01-20.00
Ns
BP or AP
E
20.01-40.00
BP or AP
F
40.01-80.00
AP
G
80.01-160.00
AP
Figure 58 – Model Rocket Motor Classes
The thrust of the motor is given in Newtons, and the delay time is measured
in seconds. The properties of a motor are stamped onto the motor case as a
simple code, as shown in Figure 59.
Figure 59 – Model Rocket Motor
One of the skills of flying model rockets is selecting the correct motor to
successfully and safely complete the flight so it’s useful to be able to correctly
interpret the motor code. A C6-5 motor will have a higher impulse than a B64 motor because C impulse is higher than B impulse. It will thus will boost a
rocket to a higher altitude. A C6-3 motor will have a shorter delay (3 seconds)
than a C6-5 motor (5 seconds). A D12-3 motor has a higher impulse and
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thrust than a C6-3 motor and, with the larger thrust, could be a better choice
for heavier rockets.
7.3
Preparation for Launch
Having selected the right motor the next step is to prepare the rocket for flight.
The first step is to loosely place a few sheets of flameproof wadding into the
motor tube (1). This acts as a barrier between the hot cases of the ejection
charge and the parachute, preventing the parachute from being melted by the
hot gas. The Parachute is then packed loosely and slipped into the motor
tube (2). The nosecone is then placed on the rocket (3).
Figure 60 – Preparing a Rocket for Launch
The final stage of preparation is to insert the motor, ensuring that it is held
firmly in place by the motor hook (4). Do not insert the igniter at this time, but
wait until the rocket is on the launch rod.
7.4
Designing Model Rockets
Most people start by buying a simple model rocket kit, and progress to
building more complex kits such as scale models and gliders. Eventually they
start to have their own ideas about rocket designs, but need to know how well
their design will fly, whether it is aerodynamically stable, and which motors it
would fly on.
There are two excellent computer programs for designing model rockets.
They are called Rocksim and SpaceCad, and they take all the guesswork out
of rocket design. Both programs let you create rocket designs from a range
of components and materials, examine the rockets appearance, and produce
graphs and reports about its performance with different motors.
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Figure 61 – Screen Shot from Rocksim
The advantage of using these programs is that they can perform all the
complex calculations for checking the aerodynamic stability of the rocket.
Once the rocket design is complete the programs allow you to load different
motors and perform simulated flights, from which you can select the ideal
motors to use with the rocket.
7.5
Safety
Model rockets travel at very high speeds and, if not treated with respect, can
cause injury or damage to property. UKRA has established a safety
framework to promote good practice and manage the risks of model rocketry.
At the heart of this safety framework is the Safety and Technical Committee.
Their roles are to maintain and publish a Safety Code for the launching of
rockets, liaise with government departments on safety and regulatory matters,
and appoint Range Safety Officers (RSO) from UKRA’s most experienced
rocketeers.
The RSO’s role is to supervise every launch and ensure that it can proceed
safely. This includes the technical assessment of the planned rocket flight,
monitoring airspace around the launch site, and ensuring that legal and
insurance requirements are met.
Some basic safety issues in the safety code are:
-
A summary of the legal and regulatory requirements for launching
rockets
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-
Definitions of minimum safe distances and range dimensions for the
different impulse classes of motor
-
A certification scheme for high powered rocketry
-
Role and authority of the RSO
-
Registration details for launch sites
Additional safety material is provided on UKRA’s website. This includes
generic risk assessments and information on legal issues such as motor
storage.
7.6
Next Steps
Most model rocket fliers progress from model rockets to medium powered
rockets. These use larger and more powerful motors, allowing the rockets to
travel higher and faster, and also to carry heavier payloads such as cameras
and altimeters. A common technical challenge is to launch and recover a raw
egg; it’s harder than it sounds. Medium powered rockets may have two or
more motors, either arranged to allow staging or fired simultaneously as a
cluster of motors.
Having mastered the techniques of medium powered rocketry the ultimate
challenge is to progress to High Power Rocketry. The motors are larger and
much more powerful than models. High power rockets can carry electronics
to activate parachutes, operate cameras, and transmits data and video over
radio links. They travel to much higher altitudes than model rockets, and can
break the sound barrier.
Figure 62 – High Power Rocketry
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The specialist body for all model, medium and high power rocketry in the UK
is the United Kingdom Rocketry Association (UKRA). In addition to offering
specialist advice and insurance, UKRA has affiliated clubs all over the UK
who can help you to get started safely in this fascinating activity. Clubs can
be contacted through the UKRA website www.ukra.org.uk, which is also a
source of useful advice and material for getting started with model rocketry.
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Acronyms
AAM
ABM
AP
APCP
ASM
BP
GEO
GTO
HTP
ICBM
ISS
LH2
LOX
MMU
NASA
RSO
SAM
SRB
SSM
UKRA
Air to Air Missile
Apogee Boost Motor
Ammonium Perchlorate
Ammonium Perchlorate Composite Propellant
Air to Surface Missile
Black Powder
Geostationary Earth Orbit
Geostationary Transfer Orbit
High Test Peroxide
Intercontinental Ballistic Missile
International Space Station
Liquid Hydrogen
Liquid Oxygen
Man Manoeuvring Unit
National Aeronautical and Space Administration
Range Safety Officer
Surface to Air Missile
Solid rocket boosters
Surface to Surface Missile
United Kingdom Rocketry Association
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Further Material for Instructors
NASA has some very good material online in their education area.
includes a lot of PowerPoint, video and other materials.
This
If, as instructors, you’re interested in reading further into this subject the
following texts are recommended:
Rotate Anticlockwise
Move Left
Move Right This is an (almost) non“It’s Rotate
OnlyClockwise
Rocket Science”
By Lucy Rogers.
mathematical introduction to spaceflight
“Rocket Propulsion Elements” by G.P.Sutton and O.Biblarz. This is the
standard university text on all aspects of rocket propulsion. It contains some
degree level maths and physics but this should not deter readers. The
descriptive text is excellent.
“Understanding Space” by J.J.Sellers is a good introduction to spaceflight
aimed at undergraduates. Chapter 14 is a very good description of rocket
motors and introduces the basic equations in greater detail than this ACP. It’s
full of pictures and describes many aspects of spaceflight very clearly.
“Saturn” by Alan Lawrie contains a system-by-system description of the
largest rocket ever built: the Saturn V. Its good supplementary reading for the
syllabus as it takes you through a case study of how rocket technology is
used. The free CD contains some great video and pictures of how the rocket
was manufactured and assembled.
The “Handbook of Model Rocketry” by G Harry Stine is a great introduction to
model rocketry. If your Squadron wants to try model rocketry buy a copy of
this book.
“A method of reaching Extreme Altitudes” by Robert H Goddard, published in
1919. Available online. This remarkable paper is a good introduction to basic
rocket science, made all the more interesting by simplicity of his experiments.
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2. Applications of Rockets - No.2473 (Highlands & Southgate