Rocket Theory
Appendix E
ROCKET THEORY
Rocketry encompasses a wide range of topics, each of which takes many years of study
to master. This chapter provides an initial foundation toward the study of rocket theory
by addressing the physical laws governing motion/propulsion, rocket performance
parameters, rocket propulsion techniques, reaction masses (propellants), chemical rockets
and advanced propulsion techniques.
PROPULSION BACKGROUND
matter, which depends on both how much
and how fast propellants are used (mass
Rockets are like other forms of
flow rate) and the propellant’s speed
propulsion in that they expend energy to
when it leaves the rocket (effective
produce a thrust force via an exchange of
exhaust velocity).
momentum with some reaction mass in
Like other forms of transportation,
accordance with Newton’s Third Law of
rockets consist of the same basic elements
Motion. But rockets differ from all other
such as a structure providing the vehicle
forms of propulsion since they carry the
framework, propulsion system providing
reaction mass with them (self contained)
the force for motion, energy source for
and are, therefore, independent of their
powering the vehicle systems, guidance
surrounding environment.
system for direction control
Other forms of
and last and most important
propulsion depend on their
(indeed the reason for
environment to provide the
having the vehicle at all), the
reaction mass. Cars use
payload. Examples of
the ground, airplanes use
payloads are passengers,
the air, boats use the water
scientific instruments or
and sailboats use the wind.
supplies. When a rocket is
The rockets we are most
used as a weapon for
familiar with are chemical
destructive purposes, we call
rockets in which the
it a missile; its payload is a
propellants (reaction mass)
warhead.
are the fuel and oxidizer.
With chemical rockets, the
ROCKET PHYSICS
propellants are also the
Fig. 5-1. Sir Isaac Newton
energy source. A conventional chemical
Sir Isaac Newton (Fig. 5-1) set forth
rocket is a type of internal combustion
the basic laws of motion; the means by
engine burning fuel and oxidizer in a
which we analyze the rocket principle.
combustion chamber producing hot, high
Newton’s three laws of motion apply to
pressure gases and accelerating them
all rocket-propelled vehicles. They apply
through a nozzle. In electric and nuclear
to gas jets used for attitude control, small
rockets, the propellant is essentially an
rockets used for stage separations or for
inert mass.
trajectory corrections and to large rockets
According to Newton’s Second Law,
the thrust force is equal to the rate of
change of momentum of the ejected
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used to launch a vehicle from the surface
gravity is acting opposite to the direction
of the Earth. They apply to nuclear,
of the thrust of the engine.
electric and other advanced types of
As the rocket operates, the forces
rockets as well as to chemical rockets.
acting on it change. The force of gravity
Newton’s laws of motion are stated
decreases as the vehicle’s mass decreases,
briefly as follows:
and it also decreases with altitude. As the
rocket passes through the atmosphere,
Newton’s 1st Law
drag increases with increasing velocity
(Inertia)
and decreases with altitude (lower
atmospheric density).1 As long as the
Every body continues in a state of
thrust remains constant, the acceleration
uniform motion in a straight line,
profile changes with the changing forces
unless it is compelled to change that
on the vehicle. The predominate effect is
state by a force imposed upon it.
that the acceleration increases at an
increasing rate as the vehicle’s mass
Newton’s 2nd Law
decreases.2
(Momentum)
Figure 5-2 shows the general
When a force is applied to a body,
acceleration and velocity profiles during
the time rate of change of
powered flight. The acceleration and
momentum is proportional to, and
velocity are low at launch due to the small
in the direction of, the applied
net force and high vehicle mass at that
force.
time. Both acceleration and velocity
Newton’s 3rd Law
(Action—Reaction)
For every action there is a reaction
that is equal in magnitude but
opposite in direction to the action.
In relating these laws to rocket theory
and propulsion, we can paraphrase and
simplify them. For example, the first law
says, in effect, that the engines must
develop enough thrust force to overcome
the force of gravitational attraction
between the Earth and the launch vehicle.
The engines must be able to start the
Fig. 5-2. Acceleration and Velocity
vehicle moving and accelerate it to the
desired velocity. Another way of
expressing this for a vertical launch is to
increase rapidly as the engine burns
say that the engines must develop more
propellants (reducing vehicle mass and
pounds of thrust than the vehicle weighs.
increasing the net force).
When applying the second law, we
At first stage burnout, the acceleration
must consider the summation of all the
drops (the acceleration at this point is due
forces acting on the body; the
to the environment: gravity and drag) and
accelerating force is the net force acting
is generally opposite the direction of
on the vehicle. This means if we launch a
200,000-lbf vehicle vertically from the
1The term “Max Q” refers to the highest struc-
Earth with a 250,000-lbf thrust engine,
tural pressure due to atmospheric drag.
there is a net force at launch of 50,000-
2As the net force on the vehicle increases and the
lbfthe difference between engine thrust
mass decreases, the acceleration increases at an
and vehicle weight. Here the force of
increasing rate.
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motion. With second stage ignition,
It is his Third Law of Motion that
acceleration and velocity will increase
explains the working principle of all
again. As the upper stage rocket
propulsion systems.
engine(s) burn more propellants, rapid
A rocket engine is basically a device
increases in acceleration and velocity
for expelling small particles of matter at
occur. When the vehicle reaches the
high speeds producing thrust through the
correct velocity (speed and direction) and
exchange of momentum. When liquid or
altitude for the mission, it terminates
solid chemicals are used as propellants,
thrust. Acceleration drops as the net
the exhaust consists of gas molecules.
force on the vehicle is due to the
Recent scientific advances have involved
environment, mainly gravity, after thrust
experimental and theoretical work on
termination, or burnout, and the vehicle
rocket engines using ions (charged atomic
begins free flight. For vehicles with three,
particles), nuclear particles and even
four or more stages, similar changes
beams of light (photons) as “propellants.”
appear in both the acceleration and
Two items are necessary for
velocity each time staging occurs.
propulsion: matter and energy. Matter is
Staging a vehicle increases the velocity in
the reaction mass and is the source of
steps to the high values required for space
momentum exchange. The reaction mass
missions.
begins with the same momentum as the
Once a vehicle is in orbit, we say it is in
rocket vehicle, but as the rocket expels
a “weightless” condition. In fact, the
this mass, the rocket and all remaining
vehicle is continually in free-fall, always
propellants receive an equal increase in
accelerating toward the center of the
momentum in the opposite direction.
Earth. The acceleration still depends on
It takes energy to accelerate the
the summation of the forces acting on the
reaction mass (impart momentum). The
vehicle (or the net force).
faster propellants are accelerated, the
In a free-fall condition, we don’t have
more propulsive force achieved; however,
to continually counter act the force of
it also takes more energy.
gravity, the vehicle’s momentum
accomplishes this task.3 In this
ROCKET PERFORMANCE
“weightless” condition, even a very small
thrust (0.1 pound) operating over a long
There are several rocket performance
period of time can accelerate a vehicle to
parameters that, when taken together,
great speeds, escape velocity and more
describe a rocket’s overall performance:
for interplanetary missions.
1) Thrust, 2) Specific Impulse, and 3)
To relate Newton’s third law, or
Mass Ratio.
“action-reaction law” to rocket theory
and propulsion, consider what happens in
Thrust (T)
the rocket motor. All rockets develop
thrust by expelling particles (mass) at high
The thrust is the amount of force an
velocity from their nozzles. The effect of
engine produces on the rocket (and on the
the ejected exhaust appears as a reaction
exhaust stream leaving the rocket,
force, called thrust, acting in a direction
conservation of momentum). The amount
opposite to the direction of the exhaust.
of thrust, along with the rocket mass,
The rocket is exchanging momentum with
determines the acceleration. The mission
the exhaust.
profile will determine the required and
acceptable accelerations and thus, the
required thrust. Launching from the
Earth typically requires a thrust to weight
ratio of at least 1.5 to 1.75. Once the
3When the vehicle’s orbit doesn’t intersect the
vehicle is in orbit and the vehicle’s
Earth’s surface, we say the gravitational force is
momentum balances the gravitational
balanced by the inertial force.
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force, smaller thrust forces are usually
The more propellant the vehicle can
sufficient for any maneuvering.
carry with respect to its “dry” weight, or
weight without propellant aboard, the
Specific Impulse (Isp )
faster it will be able to go. Mass ratio is
an expression relating the propellant mass
Specific impulse is a measure of
to vehicle mass; the higher the mass ratio,
propellant efficiency, and numerically is
the higher the final speed of the rocket.
the thrust produced divided by the weight
Therefore, a rocket vehicle is made to
of propellant consumed per second
weigh as little as possible in its “dry”
(ending up with units of seconds).
state. Increasing the weight of the vehicle
So, Isp is really another measure of a
payload results in decreasing the mass
rocket’s exhaust velocity. Specific
ratio, and therefore cutting down the
impulse is the common measure of
maximum altitude or range. For example,
propellant and propulsion system
the addition of one pound of payload to a
performance, and is somewhat analogous
high-altitude sounding rocket may reduce
to the reciprocal of the specific fuel
its peak altitude by as much as 10,000
consumption used with conventional
feet.
automobile or aircraft engines. The larger
the value of specific impulse, the better a
PROPULSION TECHNIQUES
rocket’s performance.
We can improve specific impulse by
From our previous discussion of rocket
imparting more energy to the propellants
performance parameters, we see that we
(increasing the exhaust velocity), which
would like to be as efficient as possible in
means that more thrust will be obtained
developing thrust. To develop thrust, we
for each pound of propellant consumed.
have to exchange momentum with some
We can think of specific impulse as the
reaction mass (propellant). Any way that
number of seconds for which one pound
we can do this is a valid propulsion
of propellant will produce one pound of
option. We would like to choose the
thrust. Or, we can think of it as the
option that decreased the overall mission
amount of thrust one pound of propellant
cost while still providing for mission
will produce for one second.
success.
We are most familiar with chemical
Mass Ratio (MR)
rocket systems, however, there are other
ways we can produce rocket propulsion.
Since the rocket engine is continually
The two main ways of accelerating a
consuming propellants, the rocket’s mass
propellant to provide thrust are:
is decreasing with time. If the thrust
thermodynamic expansion and electro-
remains constant, the vehicle’s
static/ magnetic acceleration. The
acceleration increases reaching its highest
methods for providing the thermal energy
value at engine cut-off; for example, the
for thermodynamic expansion, or
space shuttle reaches 3 Gs just before
electricity for electrostatic acceleration,
main engine cut-off.
can come from chemical, nuclear, or solar
The purpose of a rocket is to place a
sources.
payload at specified position with a
specific velocity. This position and
Thermodynamic Expansion
velocity depends on the mission. We can
equate the energy needed to do this to the
Thermodynamic expansion is the
change in velocity (or delta-v, ∆v) the
mechanism we are most familiar with. All
rocket imparts to the satellite. For a
of our chemical systems use this method
rocket, the ideal ∆v gain depends on the
to accelerate the propellants. However,
Isp (exhaust velocity, ve ) and the mass
we can also use nuclear or electrical
ratio.
energy to heat the propellant.
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In thermodynamic expansion, we heat
going into the radial velocity is wasted.
the propellant to turn it into a high
The contoured or bell-shaped nozzle
pressure, high temperature gas. We then
provides for rapid early expansion
allow that gas to expand in a controlled
producing shorter (less massive) nozzles,
way to turn the thermal potential energy
and redirects the exhaust toward the axial
into directed kinetic energy, which
direction near the nozzle exit. The plug
produces thrust. The basic device used to
and expansion-deflection type nozzles are
create these large volumes of gas and to
much shorter than a conventional conical
harness their heat energy is extremely
nozzle with the same expansion ratio.
simple and often contains no moving
These nozzles have a center body and
parts.
an annular chamber. The plug changes
The rocket engine using
the direction of the gas flow from the
thermodynamic expansion creates a
throat during expansion from radial to an
pressure difference between the thrust
axial direction. The expansion of exhaust
chamber (combustion chamber) and the
gas is determined by ambient pressure. A
surrounding environment. It is this
variation of the plug nozzle is the
pressure difference that accelerates the
aerospike, which uses radial auxiliary
gases.
combustion chambers around the exit to
A rocket engine usually operates at
the main combustion chamber. The
what the gas dynamist calls supercritical
exhaust plumes from the auxiliary
conditionshigh chamber pressure
chambers expand to form a "nozzle" for
exhausting to low external pressure. The
the gases escaping from the engine. Over
Swedish engineer Carl G.P. De Laval
expansion and under expansion can be
showed that for supercritical conditions
largely compensated for by increasing or
gases should be ducted through a nozzle
decreasing the thrust of the auxiliary
that converges to a throat (section of
chambers.
smallest area) and then diverges to
transform as much of the gases’ thermal
Chemical Rockets
energy into kinetic energy.
Chemical rockets are unique in that the
Nozzles
energy required to accelerate the
propellant comes from the propellant
There are a number of nozzle types;
itself, and in this sense, are considered
Figure 5-3 depicts four of them. The
energy limited. Thus, the attainable
conical nozzle is simple and easy to
kinetic energy per unit mass of propellant
fabricate and provides adequate
is limited primarily by the energy released
performance for most applications;
in chemical reaction; the attainment of
however. it also has off axis exhaust
high exhaust velocity requires the use of
velocity components which reduces the
high-energy propellant combinations that
efficiency. The radial velocity
produce low molecular weight exhaust
components cancel and don’t contribute
products. Currently, propellants with the
to the overall thrust, therefore the energy
best combinations of high energy content
and low molecular weight seem capable
of producing specific impulses in the
range of 400 to 500 seconds or exhaust
velocities of 13,000 to 14,500 ft/sec.
Chemical rockets may use liquid or
solid propellants or, in some schemes,
combinations of both. Liquid rockets
may use one (monopropellant), two
(bipropellant) or more propellants.
Bipropellants consist of a combination of
Fig. 5-3. Nozzle Types
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a fuel (kerosene, alcohol, hydrogen) and
an oxidizer (oxygen, nitric acid, fluorine).
The nuclear rocket is an attempt to
The liquids are held in tanks and fed into
increase specific impulse by using nuclear
the combustion chamber where they react
reaction to replace chemical reaction as
and then expand through the nozzle.
the energy source. The nuclear reactor
In contrast, solid propellants are an
generates thermal energy and heats the
intimate mixture containing all the
propellant which is then expanded
material necessary for reaction. The
through a conventional nozzle.
entire block of solid propellant, called the
Compared to the chemical rocket, the
grain, is stored within the combustion
nuclear rocket has some advantages. The
chamber. Combustion proceeds from the
energy released in a nuclear reaction is
surface of the propellant.
very much larger than that of a chemical
A chemical rocket engine is little more
reaction (on the order of a million times
than a gas generator. The rapid
larger), and since the energy source is
combination (combustion) of certain
separate from the propellant, we have a
chemicals results in the release of energy
larger latitude for propellant choice.
and large volumes of gaseous products.
Thus, hydrogen would be a good
The gas molecules generated have
propellant because it has the lowest
considerable energy in the form of heat.
atomic weight, and would provide the
In ordinary chemical rocket engines, the
highest exhaust velocities for a given
temperature of the resulting gases can rise
chamber pressure and temperature.
higher than 5,500 degrees Fahrenheit.
We might think that the abundant
For chemical systems in general, liquid
energy in nuclear rockets would mean
propellants provide higher specific
that we could employ indefinitely high
impulses than solid propellants. We call
chamber temperatures. This is definitely
liquid Hydrogen (LH) and liquid Oxygen
not the case, however, since the heat is
(LOX) high energy propellants because of
transferred from a solid reactor to the
the large energy release during
propellant. Thus the structural
combustion and the high transfer of
components within the nuclear rocket,
thermal energy into directed kinetic
unlike those in a chemical rocket, must be
energy of the exhaust stream.
hotter than the propellant, and the
An efficient LH/LOX burning engine
temperature cannot exceed the limiting
produces around Isp = 390-430 sec. on
temperature of the structure or the
average.4 Solid propellant motors
reactor material. The attainable
produce around Isp = 265-295 sec.
temperatures in nuclear rockets to date
The total impulse of a rocket is the
are considerably below the temperatures
product of thrust and the effective firing
attained in some chemical rockets, but the
duration. A typical shoulder launched
use of hydrogen as the propellant more
short-range rocket may have an average
than offsets this temperature
thrust of 660 pounds for an effective
disadvantage. Thus, as far as specific
duration of 0.2 seconds, giving a total
impulse is concerned, the increased
impulse of 132 lbf-sec. In contrast, the
performance of nuclear rockets is entirely
Saturn rocket had a total impulse of 1.14
due to the use of a propellant with a low
billion lbf-sec.
atomic weight. The nuclear fission rocket
offers roughly twice the specific impulse
of the best chemical rocket (about 800-
Nuclear Rockets
1,000 seconds), while delivering fairly
high thrusts for long periods of time.
One theoretical improvement is a high-
4The Isp of any particular engine depends upon its
density reactor using fast neutrons. This
design altitude. The Space Shuttle Main Engines
type of reactor is expected to produce
(SSME) produce Isp = 363.2 @ sea level, and
higher performance levels in a smaller
455.2 @ vacuum.
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package than the thermal (or slow)
the chamber and the propellant is heated
reactors. Another improvement is a gas
to high temperatures as it interacts with
core reactor in which the operating
the arc. After the heating, the propellant
temperature could be much higher. This
is expanded through a conventional
increase in temperature would occur
nozzle (Fig. 5-5).
because of the elimination of the solid
This type of propulsion takes
core of fuel elements used in slow and
advantage of using hydrogen as a
fast reactors. These structural elements
propellant, and, like nuclear rockets,
are temperature limited.
experiences a similar performance gain in
NASA’s Lewis Research Center is
pursuing a concept for a reusable vehicle
propelled by a nuclear thermal rocket
(NTR) to take astronauts to the Moon
and back (Fig. 5-4). With the addition of
modular hardware elements, the lunar
transit vehicle would become the core of
a spacecraft to land astronauts on Mars
early in the 21st century.
Specific impulse has reached about 850
seconds in nuclear engines, while the best
Fig. 5-5. Conventional Nozzle
specific impulse (up to 1,200 seconds).
Unlike nuclear rockets, arcjets are small,
producing little more than several pounds
of thrust.
Electrical Propulsion
Electrical and electromagnetic rockets
fundamentally differ from chemical
Fig. 5-4. Reusable Rocket
rockets with respect to their performance
liquid oxygen/liquid hydrogen combustion
limitations. Chemical rockets are energy-
engines only approach 475 seconds (in a
limited, since the quantity of energy is
vacuum). Such a system could decrease
limited by the chemical behavior of the
transit times to Mars from 9-15 months
propellants. If a separate energy source is
down to 4-6 months, leaving more time
used, much higher propellant energy is
for exploration. Of course nuclear rockets
possible. Further, if the temperature
have drawbacks. Nuclear reactors are not
limitations of solid walls could be made
only heavy, but while in operation,
unimportant by direct electrostatic or
produce large amounts of radiation. The
electromagnetic propellant acceleration
mass and radiation hazard prohibit its use
without necessarily raising the fluid
as a launch vehicle. However, once in
temperature, there would be no limit to
space the benefits on long range missions
the kinetic energy we could add to the
would more than offset the extra mass.
propellant. However, the rate of
conversion from nuclear or solar to
Electrothermal Rockets
electrical energy and then to propellant
kinetic energy is limited by the mass of
Another method using thermodynamic
the conversion equipment. Since this
expansion is the arcjet. The arcjet is an
mass is likely to be a large portion of the
electrothermal rocket because it uses
total mass of the vehicle, the electrical
electrical energy to heat a propellant. In
rocket (including electrothermal/static/
this method, an annular arc is created in
magnetic) is essentially power-limited.
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Electrostatic/magnetic rockets convert
that an ion rocket employing cesium
electrical energy directly to propellant
propellant would require over 2,000 kW
kinetic energy without necessarily raising
of electrical power per pound of thrust.
the temperature of the working fluid. For
The propellant for ion engines may be
this reason the specific impulse is not
any substance that ionizes easily. Unlike
limited by the temperature limitations of
thermodynamic expansion, the size of the
the wall materials, and it is possible to
molecules is not a primary factor. The
achieve very high exhaust velocities,
most efficient elements are mercury,
although at the cost of high power
cesium or the noble gases.
consumption.
Because of the massive energy
conversion equipment, electrical rockets
have low thrust, perhaps only one-
thousandth of vehicle weight in the
Earth’s gravitational field. For this
reason, they are mainly restricted to space
missions during which the gravitational
forces are very nearly balanced by inertial
forces. Low accelerations are quite
acceptable, since the journeys are of long
duration.
The propellant of an electrical rocket
Fig. 5-6. Ion Acceleration
consists of either discrete charged
particles accelerated by electrostatic
Electromagnetic Rockets
forces, or a stream of electrically
conducting fluid (plasma) accelerated by
There are three major types of
electromagnetic forces.
electromagnetic rockets: magnetogas-
dynamic, pulsed-plasma and traveling-
Electrostatic Rockets
wave. All methods use a plasma with
crossed electric and magnetic fields to
These are commonly called ion
accelerate the plasma.
rockets. Neutral propellant is converted
A plasma is an electrically conducting
to ions and electrons and withdrawn in
gas. It consists of a collection of neutral
separate streams. The ions pass through a
atoms, molecules, ions, and electrons.
strong electrostatic field produced
The number of ions and the number of
between acceleration electrodes. The
electrons are equal so that, on the whole,
ions accelerate to high speeds, and the
the plasma is electrically neutral. Because
thrust of the rocket is in reaction to the
of its ability to conduct electrons, the
ion acceleration (Fig. 5-6).
plasma can be subjected to
It is also necessary to expel the
electromagnetic forces in much the same
electrons in order to prevent the vehicle
way as solid conductors in electric
from acquiring a net negative charge.
motors.
Otherwise, ions would be attracted back
to the vehicle and the thrust would vanish.
Magneto-gas-dynamic Drive.
They remove these excess electrons by re-
Strong external electric and magnetic
injecting them back into the exhaust ion
fields direct and accelerate the plasma
beam.
stream, imparting high exhaust velocity.
Ion rockets offer very high specific
The performance is limited due to non-
impulses (a typical figure being 10,000
perpendicular currents flowing in the
seconds with values ranging up to 20,000
plasma at high field strengths. The
seconds), but very low thrust, one-half
specific impulse is lower than ion rockets
pound being high. It has been estimated
but still very high (around 10,000
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seconds). The mass flow rate is restricted
The inward radial force on the plasma
so the thrusts remain low.
in this accelerator appears to offer an
advantage in keeping the high
Pulsed-Plasma Accelerators.
temperature plasma away from the solid
One of the disadvantages of the
walls of the tube. The fact that no
steady crossed-field accelerators is that
electrodes are needed is also an attractive
they require a substantial external field
feature.
and therefore, a massive electromagnet.
It is possible to make an accelerator for
STAGING
which an electromagnet is unnecessary by
using the plasma current itself to generate
Currently, the only practical method
the magnetic field, which gives rise to the
we have for launching satellites is with
accelerating force. Whereas the crossed-
chemical systems. As we found out in the
field accelerator is analogous to a shunt
rocket performance section, specific
motor (which has separate current circuits
impulse and mass ratio limit our chemical
for the electric and magnetic fields), the
systems’ performance.
analog of this type of accelerator is the
What does this mean in terms of
series motor in which the magnetic field is
satellites and space probes? A rocket has
established by the same current which
to provide enough energy, essentially
interacts to establish the crossed field
25,000 ft/sec (17,500 mph), to orbit the
force.
Earth as a satellite and 36,700 ft/sec
(25,000 mph) to escape the Earth’s
Traveling-Wave
gravitational field and become a planetoid
A third type of plasma accelerator,
circling the Sun.
sometimes called the magnetic-induction
A body must attain a velocity of nearly
plasma motor, offers potential advantages
35,000 ft/sec to hit the Moon. No
over both the foregoing accelerators. It
practical rocket of one stage can reach the
requires neither magnets or electrodes,
critical velocities for satellites or space
and relies on currents being induced in the
probes.
plasma by a traveling magnetic wave.
A solution to this problem is to mount
If the current in a conductor
one or more rockets on top of one
surrounding a tube containing a plasma
another and to fire them in succession at
increases, the magnetic field strength in
the moment the previous stage burns out.
the plane of the conductor will increase.
For example, if each stage provides about
Then an electromotive force will be
9,000 ft/sec in velocity when fired as
induced in any loop in this plane. If the
above, it would take three stages to put a
conductor current increases rapidly
satellite in orbit, or four stages to reach
enough, the induced electric field will
the moon or go beyond it into space as a
establish a substantial plasma current.
deep space probe orbiting the sun.
The induced magnetic field and plasma
Staging reduces the launch size and
current then interact to cause a body force
weight of the vehicle required for a
normal to both, which tends to compress
specific mission and aids in achieving the
the plasma toward the axis of the tube and
high velocities necessary for specific
expel it axially.
missions.
A traveling-wave accelerator makes
Multistage rockets allow improved
use of a number of sequentially energized
payload capability for vehicles with a high
external conductors along the tube. As
∆v requirement, such as launch vehicles
the switches are fired in turn, the
or interplanetary spacecraft. In a
magnetic field lines move axially along the
multistage rocket, propellant is stored in
tube, interacting with induced currents
smaller, separate tanks rather than a larger
and imparting axial motion to the plasma.
single tank as in a single-stage rocket.
Since each tank is discarded when empty,
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energy is not expended to accelerate the
corrosiveness, availability and cost; size
empty tanks, thereby achieving a higher
and structural weight of the vehicle; and
total ∆v. Alternatively, a larger payload
payload weight.
mass can be accelerated to the same total
∆v. The separate tanks are usually
Liquid Propellants
bundled with their own engines, with each
discardable unit called a stage.
The term “liquid propellant” refers to
The same rocket equation describes
any of the liquid working fluids used in a
multistage and single-stage rocket
rocket engine. Normally, they are an
performance, but it must be applied on a
oxidizer and a fuel, but may include
stage-by-stage basis. It is important to
catalysts or additives that improve
realize that the payload mass for any stage
burning or thrust. Generally, liquid
consists of the mass of all subsequent
propellants permit longer burning time
stages plus the ultimate payload itself.
than solid propellants. In some cases, they
The velocity of the multistage vehicle at
permit intermittent operations. That is,
the end of powered flight is the sum of
combustion can be stopped and started by
velocity increases produced by each of the
controlling propellant flow.
various stages. We add the increases
Many combinations of liquid
because the upper stages start with
propellants have been investigated.
velocities imparted to them by the lower
However, no combination has all these
stages.
desirable characteristics:
A multistage vehicle with identical
specific impulse, payload fraction and
• Large availability of raw materials
structure fraction for each stage is said to
and ease of manufacture
have similar stages. For such a vehicle,
• High heat of combustion per unit of
the payload fraction is maximized by
propellant mixture
having each stage provide the same
• Low freezing point (wide range of
velocity increment. For a multistage
operation)
vehicle with dissimilar stages, the overall
• High density before combustion
vehicle payload fraction depends on how
(smaller tanks)
the ∆v requirement is partitioned among
• Low density after combustion
stages. Payload fractions will be reduced
(higher γ)
if the ∆v is partitioned suboptimally.
• Low toxicity and corrosiveness
(easier handling and storage)
ROCKET PROPELLANTS
• Low vapor pressure, good chemical
stability (simplified storage)
The type of rocket engine determines
the corresponding type of propellant
Liquid-propellant units can be
storage and delivery systems. In the case
classified as monopropellant, bipropellant
of chemical rocket engines, the
or tripropellant in nature (Fig. 5-7). A
propellants may be either liquid or solid.
monopropellant is a single liquid
Rocket engines can operate on
possessing the qualities of both an
common fuels such as gasoline, alcohol,
oxidizer and a fuel. It may be a single
kerosene, asphalt or synthetic rubber, plus
chemical compound, such as
a suitable oxidizer. Engine designers
nitromethane, or a mixture of several
consider fuel and oxidizer combinations
chemical compounds, such as hydrogen
having the energy release and the physical
peroxide and alcohol. The compounds
and handling properties needed for
are stable at ordinary temperatures and
desired performance. Selecting
pressures, but decompose when heated
propellants for a given mission requires a
and pressurized, or when a catalyst starts
complete analysis of mission, propellant
the reaction.
performance, density, storability, toxicity,
AU Space Reference Guide
Second Edition, 8/99
5 - 10
unsymmetrical dimethyldrazine (UDMH)
at 146° F and hydrazine at 236° F.
However, the term storage refers to
storing propellants on Earth. It does not
consider the problem of storage in space.
As described earlier, in order to store
the liquid propellants within the rocket
vehicle until such time as they are
introduced into the combustion chamber
of the rocket engine, large tanks are
required. Once combustion starts and
pressure is built up inside the combustion
chamber, the propellants will not flow
into the combustion chamber of their own
accord. A method of forcing the
propellants into the combustion chamber
Fig. 5-7: Liquid Propellants
against the combustion pressure is
required. Two methods presently used to
Monopropellant rockets are simple,
accomplish this are shown in Figure 5-8.
since they only need one propellant tank
The simplest of these provides a gas
and the associated equipment. The most
pressure, usually helium, in the propellant
common monopropellant systems use
tanks sufficient to force the propellants
hydrazine. Bipropellant units carry fuel
out of the tanks through the delivery
and oxidizer in separate tanks and bring
piping and into the combustion chamber.
them together in the combustion chamber.
The pressurization method requires
At present, most liquid rockets use
propellant tanks that are strong enough to
bipropellants. In addition to a fuel and
withstand the pressure and this, in turn,
oxidizer, a liquid bipropellant may include
means thick tank walls and increased
a catalyst to increase the speed of
tankage weight. This decreases the mass
reaction, or other additives to improve the
ratio. Therefore, there is a definite limit
physical, handling or storage properties.
A tripropellant has three compounds.
The third compound improves the specific
impulse of the basic propellant.
Liquid propellants are commonly
classified as either cryogenic or storable
propellants. A cryogenic propellant is
one that has a very low boiling point and
must be kept very cold. For example,
liquid oxygen boils at -297° F, liquid
fluorine at -306° F and liquid hydrogen at
-423° F. Personnel at the launch site load
these propellants into a rocket as near
Fig. 5-8. Propellant Feed Types
launch time as possible to reduce losses
from vaporization and to minimize
problems caused by their low
to the size of the rocket vehicle that can
temperatures.
use the pressurization method.
A storable propellant is one that is
The second method, as previously
liquid at normal temperatures and
described, utilizes pumps to drain the
pressures and may be left in a rocket for
propellants from the tanks and force them
days, months, or even years. For
into the combustion chamber. This
example, nitrogen tetroxide boils at 70° F,
requires a pump for each propellant as
well as some method of driving the
AU Space Reference Guide
Second Edition, 8/99
5 - 11
pumps. These pumps are usually the
hypergolic combinations are aniline and
centrifugal type. They are generally
nitric acid, fluorine/hydrazine, fluorine
driven by a turbine mounted on the same
and hydrogen, hydrazine/hydrogen
drive shaft. The turbine, in turn, is
peroxide, and aniline and nitrogen
powered by a small gas generator that
tetroxide.
may use the decomposition of high-
Monopropellants are chemicals which
strength (highly concentrated) hydrogen
decompose in the presence of a suitable
peroxide to produce steam. Other
catalyst or at a suitable temperature
sources of turbine power may be the two
releasing energy in the process.
rocket propellants, burned in a small
Hydrogen peroxide (75 percent pure or
auxiliary combustion chamber, or a small
better) is a common monopropellant used
solid-propellant grain burned to produce
in many vehicles for small adjustment or
driving gas. A novel method involves
vernier rockets. Such strong peroxide
bleeding some of the combustion gas
mixtures, however, must be handled with
from the rocket engine back to the
great care because they decompose with
turbine. This is a system which essentially
explosive suddenness in the presence of
“bootstraps” itself into operation. Pump
impurities. Other monopropellants are
delivery systems allow the use of
nitro-methane (CH3NO2), ethylene oxide
extremely thin-walled propellant tanks,
(C2H4O) and hydrazine (N2H4). Many of
which increases the possible mass ratio.
these propellants are highly unstable,
With liquid propellants, the combustion
many are highly toxic and some are both.
process starts when the propellants are
Liquid propellant engines are extremely
injected into the rocket engine. The
versatile, can be throttled, and can be
propellants are driven into the combustion
used again by simply reprovisioning the
chamber through an “injector,” which
propellant tanks. They provide high
often looks like an overgrown shower
specific impulses, but are more complex
head. The injector serves to break up the
and therefore, less reliable than a solid
propellants into atomized spray, thus
motor.
promoting mixing and complete
While it is possible to argue endlessly
combustion. Injectors are extremely
over the merits of both types, it is safe to
difficult to design, as there are no
say that both solid-propellant motors and
definitive mathematical equations that
liquid-propellant engines will continue to
analyze their operation. Modern injectors
be used in the future for specific
are built as a single unit that forms the
applications where their respective
forward end of the combustion chamber.
advantages outweigh their disadvantages.
They are perforated with hundreds of tiny
holes, the number, size, and angle of
which are critical.
Propellants may be chosen so that they
react spontaneously upon contact with
each other. Such propellants are known
as hypergolic and do not require a means
of ignition in order to get combustion
started. Ignition for non-hypergolic
propellants requires an igniter. Igniters
are usually pyrotechnic in nature,
although some engines have used spark
plugs.
Typical non-hypergolic combinations
are alcohol/LOX, gasoline/LOX, liquid
hydrogen/LOX, alcohol and nitric acid,
and kerosene (RP-1)/LOX. Typical
AU Space Reference Guide
Second Edition, 8/99
5 - 12
In addition, a grain may be designed to
Solid Propellants
burn with increasing area and thrust
(progressive) or with decreasing area and
The solid-propellant motor (Fig. 5-9)
thrust (regressive). Choice of grain style
is the oldest of all types and is by far the
depends on the motor’s use.
simplest in construction. Since the
There are many chemical combinations
propellants are in solid form, usually
that make good solid propellants. Aside
from gunpowder and metal-powder
mixtures (such as zinc and sulfur) which
have erratic burning rates and poor
physical properties, there are two classes
of solid propellants which were originally
developed for rockets during and after
World War II and are in wide use today:
double-base (homogeneous) and
composite (heterogeneous) propellants.
Double-based propellants consist chiefly
Fig. 5-9. Solid Propellant Motor
of a blend of nitrocellulose and
nitroglycerin with small quantities of salts,
mixed together, and since a solid-
wax, coloring and organic compounds to
propellant charge undergoes combustion
control burning rates and physical
only on its surface, there is no need to
properties. The double-based propellants
inject it continuously into the combustion
may be regarded as complex colloids with
chamber from storage tanks. Solid
unstable molecular structure.
propellants are therefore, placed right in
Homogeneous propellants have oxidizer
the combustion chamber itself. A solid
and fuel in a single molecule. The blast
propellant rocket motor combines both
from a small chemical igniter easily starts
the combustion chamber and the
the rapid recombination of this structure
propellant storage facilities in one unit. A
in the process of burning. Aging allows a
solid-propellant charge, or “grain,” is
slower rearrangement of the molecules,
ignited and burns until it is exhausted,
and thus often significantly changes the
changing the effective size and shape
during its operation.
Since a solid-propellant grain burns
only on its surface, the shape of the grain
may be designed to regulate the amount
of grain area undergoing combustion.
Since the thrust is dependent upon the
mass flow rate, which is in turn dependent
upon the amount of propellant being
consumed per second, the thrust output
of a solid-propellant rocket motor can be
determined in advance, or “programmed.”
A grain that burns with constant area
during the thrust period yields constant
thrust and is known as a restricted or
neutral-burning grain (It might, for
Fig. 5-10. Grain Configurations
example, burn from the aft end to the
forward end in the manner of a cigarette)
burning properties of the propellant.
(Fig. 5-10).
Double-based propellants can be formed
efficiently in many shapes by either
casting or extrusion through dies.
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Second Edition, 8/99
5 - 13
Composite propellants, as the name
Propellant Tanks
implies, are mixtures of an oxidizer,
usually an inorganic salt such as
The function of the propellant tanks is
ammonium perchlorate, in a hydrocarbon
simply the storage of one or two
fuel matrix, such as an asphalt like
propellants until needed in the combustion
material. The fuel contains small particles
chamber. Depending upon the kind of
of oxidizer dispersed throughout. The
propellants used, the tank may be nothing
fuel is called a binder because the oxidizer
more than a low pressure envelope or it
has no mechanical strength. Usually in
may be a pressure vessel for containing
crystalline form, finely ground oxidizer is
high pressure propellants. In the case of
approximately 70 to 80 percent of the
cryogenic propellants (described later),
total propellant weight. Composites are
the tank has to be an exceptionally well
usually cast to shape. Current work with
insulated structure to keep propellants
composites and double-based propellants
from boiling away.
incorporates light metals (such as boron,
As with all rocket parts, weight of the
aluminum, and lithium), which yield very
propellant tanks is an important factor in
high energies.
their design. Many liquid propellant tanks
Although less energetic than good
are made out of very thin metal or are thin
liquid propellants (lower specific
metal sheaths wrapped with high-strength
impulse), solids have the advantages of
fibers. These tanks are stabilized by the
fast ignition (0.025 seconds is common)
internal pressure of their contents, much
and good storability in the rocket.
the same way balloon walls gain strength
Making them, however, is costly,
from the gas inside. Very large tanks and
complex and dangerous.
tanks that contain cryogenic propellants
require additional strengthening or layers.
An ideal solid propellant would possess
Structural rings and ribs are used to
these characteristics:
strengthen tank walls, giving the tanks the
• High release of chemical energy
appearance of an aircraft frame. With
• Low molecular weight of
cryogenic propellants, extensive insulation
combustion products
is needed to keep the propellants in their
• High density before combustion
liquefied form. Even with the best
• Readily manufactured from easily
insulation, cryogenic propellants are
obtainable substances by simple
difficult to keep for long periods of time
processes
and will boil away. For this reason,
• Insensitive to shock and
cryogenic propellants are usually not used
temperature changes and no
with military rockets/ missiles.
chemical or physical deterioration
The propellant tanks of the shuttle can
while in storage
be used as an example of the complexities
• Safe and easy to handle.
involved in propellant tank design. The
• Ability to ignite and burn uniformly
external tank (ET) consists of two smaller
over a wide range of operating
tanks and an intertank. The ET is the
temperatures
structural back bone of the shuttle and
• Nonhygroscopic (nonabsorbent of
during launch it must bear the entire
moisture)
thrust produced by the solid rocket
• Smokeless and flashless
boosters and the Orbiter main engine.
The forward or nose tank contains
It is improbable that any propellant will
LOX. Antislosh and antivortex baffles
have all of these characteristics.
are installed inside the LOX tank as well
Propellants used today possess some of
as inside the other tank to prevent gas
these features at the expense of others,
bubbles inside the tank from being
depending upon the application and the
pumped to the engines along with the
desired performance.
AU Space Reference Guide
Second Edition, 8/99
5 - 14
propellants. Many rings and ribs
strengthen this tank.
HYBRID ROCKETS
The second tank contains LH. This
tank is two and a half times the size of the
Another rocket engine should be
LOX tank. However, the LH tank weighs
mentioned. Composite (hybrid) engines
only one third as much as the LOX tank
are combinations of solid and liquid
because LOX is 16 times denser than LH.
propellant engines. In a composite
Between the two tanks is an intertank
engine, the fuel may be in solid form
structure. The intertank is not actually a
inside the combustion chamber with the
tank but a mechanical connection between
oxidizer in a liquid form that is injected
the LOX and LH tanks. Its primary
into the chamber.
function is to join the two tanks together
Though not in widespread use, they do
and distribute thrust loads from the solid
offer some advantages in rocket
rocket boosters. The intertank also
propulsion. Figure 5-11 depicts a
houses a variety of instruments.
simplified structure of the hybrid system.
Theoretical work on hybrid propulsion
Turbopumps
Turbopumps provide the required flow
of propellants from the low-pressure
propellant tanks to the high-pressure
rocket chamber. Power for the pumps is
produced by combusting a fraction of the
propellants in a preburner. Expanding
gases from the burning propellants drive
one or more turbines which, in turn, drive
the turbopumps. After passing through
the turbines, exhaust gases are either
Fig. 5-11. Hybrid Rocket System
directed out of the rocket through a
dates back to the 1930s in both the U.S.
nozzle or are injected, along with liquid
and Germany. In the 1940s, a hybrid
oxygen into the chamber for more
motor was built that burned Douglas Fir
complete burning.
wood loaded with carbon black and wax
in 10% liquid oxygen. Germany’s
Combustion Chamber and Nozzle
wartime experiments tried powdered and
re-formed coal fuel cores, but even clean
The combustion chamber of a liquid
coal contained too many impurities to be
propellant rocket is a bottle-shaped
a good rocket fuel. Work continued into
container with openings at opposite ends.
the 1960s with both the Navy and Air
The openings at the top inject the
Force funding research.
propellants into the chamber. Each
The hybrid fuel burns only on contact
opening consists of a small nozzle that
with the oxidizer, and cracks in the fuel
injects either fuel or oxidizer. The main
grain do not admit enough oxidizer to
purpose of the injectors is to mix the
support catastrophic failures common to
propellants to ensure smooth and complete
solids. Also, unlike conventional solids,
combustion with no detonations.
the flow of oxidizer makes the hybrid
Combustion chamber injectors come in
throttleable and restartable. Even though
many designs.
hybrids cannot match the density-impulse
The purpose of the nozzle is to provide
of solid rocket motors loaded with
for gas expansion to achieve the
aluminum, motors with thrusts ranging
maximum transfer of thermal energy into
from 60,000 to 75,000 pounds have been
directed kinetic energy.
tested. Future tests expect thrusts
reaching 225,000 pounds.
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Second Edition, 8/99
5 - 15
Safety is an inherent advantage, claim
enough to absorb a lot of heat. Cooling
makers of hybrid systems. As noted
occurs by heat loss through radiation into
above, cracks in the fuel, because they are
the exhaust plume. Radiation cooling can
not exposed to the oxidizer, do not cause
set an upper limit on the temperature
an explosion. Hybrid propulsion makes
attained by the walls of the thrust
launch vehicles safer in flight. Engine
chamber. The rate of heat loss varies
thrust can be verified on the pad before
with the fourth power of the absolute
releasing the vehicle for flight. And,
temperature and becomes more significant
unlike solids, hybrids can be shut down on
as the temperature rises.
the pad if something goes wrong.
Environmental concerns are lessened
Ceramic Linings
using hybrid systems specially designed to
minimize pollution effects. The hydrogen
In relatively small (low temperature)
chloride in solid fuel exhaust has already
rockets, the interior walls of the
become an environmental concern for the
combustion chamber and nozzle may be
acid it dumps on the surface of the Earth,
lined with a heat-resistant (refractory)
and the damage it does to the protective
ceramic material. The ceramic gets hot,
ozone. Aluminum oxide, an exhaust
but because it is a poor conductor of heat,
component of traditional solid rockets, is
it prevents the metal walls of the
also environmentally suspect. A hybrid
motor/engine from becoming overheated
launch vehicle using polybutadiene fuel
during the short operating period. This
and liquid oxygen produces an exhaust of
method is not adequate for large rockets
carbon dioxide, carbon monoxide and
in which the more intense heat must be
water vapor similar to that of
transferred rapidly from the walls of the
kerosene/liquid oxygen engines.
thrust chamber. Ceramic linings are also
too heavy for use in large rockets.
COOLING TECHNIQUES
Ablation Cooling
The very high temperatures generated
in the combustion chamber transfer a
As mentioned earlier, in the ablation
great deal of heat energy to the
cooling method, the interior of the thrust
combustion chamber and nozzle walls.
chamber is lined with an ablative material,
This heat, if not dissipated, will cause
usually some form of fabric reinforced
most materials to lose strength. Without
plastic. This material chars, melts and
cooling the chamber and nozzle walls, the
vaporizes in the intense heat of the
combustion chamber pressures will cause
nozzle. In this type of “heat sink cooling,”
structural failure. There are many
the heat absorbed in the melting and
methods of cooling, all with the objective
burning (the energy alters the chemical
of removing heat from the highly stressed
form instead of raising its temperature) of
combustion chamber and nozzle.
the ablative material prevents the
temperature from becoming excessively
Radiation Cooling
high. The charred material also serves as
an insulator and protects the rocket case
This is probably the simplest method of
from overheating. The gas produced by
cooling a rocket engine or motor. The
burning the ablative material provides an
method is usually used for
area of “cooler” gas next to the nozzle
monopropellant thrusters, gas generators,
walls. The synthetic organic plastic
and lower nozzle sections. The interior of
binder material is reinforced with glass
the combustion chamber is covered with a
fiber or a synthetic substance. Solid
refractory material (graphite,
rocket motors use ablative cooling
pyrographite, tungsten, tantalum or
almost exclusively, as there are no other
molybdenum) or is simply made thick
fluids to use to cool the nozzle throat.
AU Space Reference Guide
Second Edition, 8/99
5 - 16
of the propellant rises, causing it to
vaporize faster upon injection. This
Film Cooling
cooling method is often used with gas
generator systems as a way to drive
With this method of cooling, liquid
turbopumps (Fig. 5-12).
propellant is forced through small holes at
the periphery of the injector forming a
film of liquid on the interior surface of the
Solid Rocket Motor Cooling
combustion chamber. The film has a low
thermal (or heat) conductivity since it
In solid propellant motors, the nozzle
readily vaporizes and protects the wall
material from the hot combustion gases.
Cooling results from the vaporization of
the liquid which absorbs considerable
heat. Film cooling is especially useful in
regions where the walls become
exceptionally hot, e.g., the nozzle throat
area.
Transpirational Cooling
This technique is very similar to film
cooling. The combustion chamber has a
double-walled construction in which the
Fig. 5-12. Regenerative Cooling
inside wall is made of a porous material.
Propellant is circulated through the space
serves the same purpose as in the liquid
between the walls and seeps continuously
engine. Because there is no super-cooled
through inner wall pores into the
propellant available to provide cooling,
combustion chamber. There it forms a
we use other methods for thermal
film which rapidly vaporizes. The cooling
protection. If not properly constructed,
action is much the same as film cooling,
the walls of the combustion chamber will
but has the additional advantage of
become excessively hot. This could cause
allowing considerable heat to be absorbed
case failure under the high operating
by the propellant within the walls of the
pressures existing in the interior. To
chamber. This method is also referred to
prevent this, the inner wall of the motor
as evaporative or sweat cooling. Major
case is coated with a liner or inhibitor.
drawbacks to transpirational cooling are
This liner provides a bond between the
that it is difficult to manufacture this type
propellant grain and the case preventing
of chamber, and also difficult to maintain
combustion from spreading along the
a steady liquid flow through the pores.
walls, and acts as a thermal insulator,
protecting the case from heat in areas
Regenerative Cooling
where there is no propellant. The
unburned propellant provides additional
This is the most common method of
thermal protection as it must be vaporized
cooling for cryogenic propellant rockets.
before it will burn.
It involves circulating one of the super-
In solid-propellant rockets, the
cooled propellants through a cooling
nozzle’s form is often achieved with a
jacket around the combustion chamber
shaped insert which keeps the nozzle
and nozzle before it enters the injector.
throat cool to prevent significant damage
The propellant removes heat from the
during the operation of the motor.
walls, keeping temperatures at acceptable
Common insert materials include both
levels. At the same time, the temperature
refractory substances, like pyrographite
AU Space Reference Guide
Second Edition, 8/99
5 - 17
and tungsten or ablative substances. The
for small deflection angles. This method
ablative materials are fabric reinforced
is relatively common (Fig. 5-13).
high temperature plastics as previously
discussed. There is usually no significant
change in motor performance due to
deterioration of nozzle throat ablatives.
Another method of keeping the nozzle
throat cool is the use of a cooler burning
propellant located near the throat area
which will burn and form a thin layer of
cooler gas next to the nozzle walls. This
thin film of gas protects the nozzle from
the high temperature gas created by the
main propellant.
Fig. 5-13. Gimbaled Engine
Vernier Rockets
THRUST VECTOR CONTROL
Vernier rockets are small auxiliary
In a rocket, the rocket engine or motor
rocket engines. These engines can
not only provides the propulsive force but
provide all attitude control, or just roll
also the means of controlling its flight
control for single engine stages during the
path by redirecting the thrust vector to
main engine burn, and a means of
provide directional control for the
controlling the rocket after the main
vehicle’s flight path. This is known as
engine has shut off (Fig. 5-14).
thrust vector control (TVC). TVC can be
divided into those systems for use with
liquid engines and those for solid motors.
When choosing a TVC method, we
need to consider the characteristics of the
engine/motor and its flight application and
duration. Also, the maximum angular
accelerations required or acceptable, the
environment, the number of
engines/motors on the rocket, available
actuating power, and the weight and
space limitations are all weighed against
each other to produce a cost effective, yet
Fig. 5-14. Vernier Rocket
appropriate, system of control. The
Jet Vanes
effective loss of engine performance due
Jet vanes are small airfoils located in
to the use of a particular TVC method
the exhaust flow behind the nozzle exit
and the maximum thrust vector deflection
plane. They act like ailerons or elevators
required are major design considerations
on an aircraft and cause the
vehicle to change direction by
Liquid Rocket TVC Methods
redirecting the rocket. Jet vanes
are made of heat-resistant
Gimbaled Engines
materials like carbon-carbon and
other refractory substances.
Some liquid propellant rockets use an
Unfortunately, this control
engine swivel or gimbal arrangement to
system causes a two to three
point the entire engine assembly. This
percent loss of thrust, and
arrangement requires flexible propellant
erosion of the vanes is also a
lines, but produces negligible thrust losses
major problem (Fig. 5-15).
Fig. 5-15.
Jet Vanes
AU Space Reference Guide
Second Edition, 8/99
5 - 18
Solid Rocket TVC Methods
surface which allows the under expanded
region to be moved 360 degrees around
Rotating Nozzle
the rocket nozzle to produce pitch and
yaw control. This system was developed
The rotating nozzle has no throat
for the Polaris SLBM.
movement. These nozzles work in pairs
and are slant-cut to create an area of
Secondary Fluid Injection
under expansion of exhaust gases on one
side of the nozzle. This creates an
A secondary fluid is injected into the
unbalanced side load and the inner wall of
exhaust stream to deflect it, thereby
the longer side of the nozzle. Rotation of
changing the thrust vector (Fig. 5-17).
the nozzles moves this side load to any
Fluid injection creates unbalanced shock
point desired and provides roll, yaw and
waves in the exhaust nozzle which
pitch control. This system is simple but
deflects the exhaust stream. There are
produces slow changes in the velocity
two types of fluid injection systems.
vector. Rotating nozzles are usually
The Liquid Injection TVC uses both
supplemented with some other form of
inert (water) and reactive fluids (rocket
TVC.
propellants) for the TVC. Reactive fluid
combustion in the exhaust plume creates
Swiveled Nozzle
the greater effect. Hydrazine, water,
nitrogen tetroxide, bromine, hydrogen
The swiveled nozzle changes the
peroxide, and Freon have all been used.
direction of the throat and nozzle. It is
similar to gambaling in liquid propellant
engines. The main drawback in using this
method is the difficulty in fabricating the
seal joint of the swivel since this joint is
exposed to extremely high pressures and
temperatures (Fig. 5-16).
Movable Control
Surfaces
Fig. 5-17. Thrust Vectoring
Movable Control
Surfaces physically
The Hot Gas Injection TVC uses gas
deflect the exhaust or
either vented from the main combustion
create voids in the
chamber, or from an auxiliary gas
exhaust plume to
generator. These gases are “dumped”
divert the thrust
into the nozzle to cause the unbalanced
vector. This method
shock wave.
includes jet vanes, jet
Fig. 5-16.
tabs, and mechanical
Swiveled Nozzle
probes. These TVC
approaches are all based on proven
technology with low actuator power
required. They suffer from erosion and
cause thrust loss with any deflection.
A similar system is the jetavator, a slip-
ring or collar at the nozzle exit which
creates an under expansion region (as
discussed in conjunction with rotating
nozzles). The jetavator is a movable
AU Space Reference Guide
Second Edition, 8/99
5 - 19
SUMMARY
Table 5-1 summarizes the capabilities of the different types of rocket engines and
propellants. Each has its own advantages and disadvantages. Specific use of a particular
type depends upon the mission.
Type
Thrust
Isp
Missions
(1000 lbs)
Chemical
Manned missions near Earth and
Liquid
1500
260-455
Moon. Instrumented probes to Venus
200-300
and Mars.
Solid
2000-3000
Nuclear 250 600-1000 Heavy payload manned missions to
Moon, Venus and Mars.
Arc-Jet .01 400-2500 Very heavy payloads from Earth orbit.
Plasma .005 2000-10,000 To other planets and stationkeeping
Ion .001 7500- 30,000 For deep space missions
Table 5-1. Rocket Engines and Propellants
TOC
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REFERENCES
Asker, James R., “Moon/Mars Prospects May Hinge on Nuclear Propulsion,” Aviation
Week & Space Technology, December 2, 1991, pp. 38-44.
Hill, Philip G., Peterson, Carl R., Mechanics and Thermodynamics of Propulsion.
Addison-Wesley Publishing Company, MA, 1970.
Jane’s Spaceflight Directory, Jane’s, London, 1987.
Space Handbook, Air University Press, Maxwell Air Force Base, AL, January 1985.
Sutton, George P., Rocket Propulsion Elements, John Wiley & Sons, New York, 1986.
Wertz, James R., and Wiley J. Larson, ed., Space Mission Analysis and Design, Kluwer
Academic Publishers, Boston, MA, 1991.
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Document Outline
- PROPULSION BACKGROUND
- ROCKET PHYSICS
- ROCKET PERFORMANCE
- PROPULSION TECHNIQUES
- STAGING
- ROCKET PROPELLANTS
- HYBRID ROCKETS
- COOLING TECHNIQUES
- THRUST VECTOR CONTROL
- SUMMARY
