Rocket Theory

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  • Topic: Spacecraft propulsion, Rocket, Specific impulse
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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.

matter, which depends on both how much
and how fast propellants are used (mass
flow rate) and the propellant’s speed
when it leaves the rocket (effective
exhaust velocity).
Like other forms of transportation,
rockets consist of the same basic elements
such as a structure providing the vehicle
framework, propulsion system providing
the force for motion, energy source for
powering the vehicle systems, guidance
system for direction control
and last and most important
(indeed the reason for
having the vehicle at all), the
payload.
Examples of
payloads are passengers,
scientific instruments or
supplies. When a rocket is
used as a weapon for
destructive purposes, we call
it a missile; its payload is a
warhead.

PROPULSION BACKGROUND

Rockets are like other forms of
propulsion in that they expend energy to
produce a thrust force via an exchange of
momentum with some reaction mass in
accordance with Newton’s Third Law of
Motion. But rockets differ from all other
forms of propulsion since they carry the
reaction mass with them (self contained)
and are, therefore, independent of their
surrounding environment.
Other
forms
of
propulsion depend on their
environment to provide the
reaction mass. Cars use
the ground, airplanes use
the air, boats use the water
and sailboats use the wind.
The rockets we are most
familiar with are chemical
rockets in which the
propellants (reaction mass)
are the fuel and oxidizer.
ROCKET PHYSICS
With chemical rockets, the
Fig. 5-1. Sir Isaac Newton
propellants are also the
Sir Isaac Newton (Fig. 5-1) set forth
energy source. A conventional chemical
the basic laws of motion; the means by
rocket is a type of internal combustion
which we analyze the rocket principle.
engine burning fuel and oxidizer in a
Newton’s three laws of motion apply to
combustion chamber producing hot, high
all rocket-propelled vehicles. They apply
pressure gases and accelerating them
to gas jets used for attitude control, small
through a nozzle. In electric and nuclear
rockets used for stage separations or for
rockets, the propellant is essentially an
trajectory corrections and to large rockets
inert mass.
According to Newton’s Second Law,
the thrust force is equal to the rate of
change of momentum of the ejected
AU Space Reference Guide

Second Edition, 8/99

5-1

used to launch a vehicle from the surface
of the Earth. They apply to nuclear,
electric and other advanced types of
rockets as well as to chemical rockets.
Newton’s laws of motion are stated
briefly as follows:

gravity is acting opposite to the direction
of the thrust of the engine.
As the rocket operates, the forces
acting on it change. The force of gravity
decreases as the vehicle’s mass decreases,
and it also decreases with altitude. As the
rocket passes through the atmosphere,
drag increases with increasing velocity
and decreases with altitude (lower
atmospheric density).1 As long as the
thrust remains constant, the acceleration
profile changes with the changing forces
on the vehicle. The predominate effect is
that the acceleration increases at an
increasing rate as the vehicle’s mass
decreases.2
Figure 5-2 shows the general
acceleration and velocity profiles during
powered flight. The acceleration and
velocity are low at launch due to the small
net force and high vehicle mass at that
time. Both acceleration and velocity

Newton’s 1st Law
(Inertia)
Every body continues in a state of
uniform motion in a straight line,
unless it is compelled to change that...
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