22-11-2012, 04:08 PM
ROCKET THEORY
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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
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.
ROCKET PHYSICS
Sir Isaac Newton (Fig. 5-1) set forth
the basic laws of motion; the means by
which we analyze the rocket principle.
Newton’s three laws of motion apply to
all rocket-propelled vehicles. They apply
to gas jets used for attitude control, small
rockets used for stage separations or for
trajectory corrections and to large rockets
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.
PROPULSION TECHNIQUES
From our previous discussion of rocket
performance parameters, we see that we
would like to be as efficient as possible in
developing thrust. To develop thrust, we
have to exchange momentum with some
reaction mass (propellant). Any way that
we can do this is a valid propulsion
option. We would like to choose the
option that decreased the overall mission
cost while still providing for mission
success.
We are most familiar with chemical
rocket systems, however, there are other
ways we can produce rocket propulsion.
The two main ways of accelerating a
propellant to provide thrust are:
thermodynamic expansion and electrostatic/
magnetic acceleration. The
methods for providing the thermal energy
for thermodynamic expansion, or
electricity for electrostatic acceleration,
can come from chemical, nuclear, or solar
sources.
Chemical Rockets
Chemical rockets are unique in that the
energy required to accelerate the
propellant comes from the propellant
itself, and in this sense, are considered
energy limited. Thus, the attainable
kinetic energy per unit mass of propellant
is limited primarily by the energy released
in chemical reaction; the attainment of
high exhaust velocity requires the use of
high-energy propellant combinations that
produce low molecular weight exhaust
products. Currently, propellants with the
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.