ROCKET THEORY

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					                                        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
                                Fig. 5-1. Sir Isaac Newton
propellants are also the
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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                                            5-1
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             1
                                                      The 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-           2
                                                      As the net force on the vehicle increases and the
lbfthe difference between engine thrust            mass decreases, the acceleration increases at an
and vehicle weight. Here the force of               increasing rate.


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                                              5-2
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
3
                                                           ratio of at least 1.5 to 1.75. Once the
 When 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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                                                     5-3
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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                                              5-4
   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
conditionshigh       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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                                              5-5
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.
4
                                                              One theoretical improvement is a high-
 The 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
            Fig. 5-4. Reusable Rocket
                                                   fundamentally differ from chemical
                                                   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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                                             5-7
   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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                                              5-8
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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                                              5-9
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,


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                                               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


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                                              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


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                                              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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                                               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.


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                                               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
                                                             Fig. 5-11. Hybrid Rocket System
the turbines, exhaust gases are either
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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                                               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.


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                                              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


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                                                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
                                                                 Fig. 5-14. Vernier Rocket
each other to produce a cost effective, yet
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



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                                               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


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                                                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 INFO
Description: here types of rockets and hoe it works newton laws ROCKET PROPELLANTS and about total systematic diagrams of rockets are shown here.