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Propulsion for the TLV7
The recent ½ scale rocket was powered by a solid rocket motor
built by Aerotech Rocketry, a consumer rocket motor manufacturer. We chose to use a solid rocket motor for
this flight due to the ease of use, safety record, and the cost vs.
performance of this type of motor.
For the full-scale rocket we will use a liquid rocket motor (the
reasons for this decision will appear in later editions of this
presentation), but for flights dedicated to relatively low-altitude flights
it is simpler, safer, and much more inexpensive to use this type of
motor.
The heart of a solid rocket motors is a chemical that burns at
a very rapid rate. This rapid
combustion produces gas that is accelerated through a nozzle to supersonic
speeds, creating thrust in the direction of flight (conservation of
momentum & “Equal and opposite reaction”).
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The rocket motor is shown in this cutaway view of the back
end of the TLV7. In this picture,
the orange propellant burns to create hot gases which build up in the
thrust chamber, and are expelled through the gray nozzle at the
bottom. Note that the nozzle is
far away from the rest of the rocket, so that no hot gases touch the
aerodynamic surfaces.
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A basic solid rocket motor is comprised of three parts: A casing or “thrust chamber”,
the propellant, and a nozzle.
A rocket motor casing serves two functions. It is the package that holds the
propellant and nozzle together, and it is a pressure vessel that contains
the propellant as it is burning.
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In this photograph, taken just 100 feet from the launch pad,
you can see that the motor is ejecting fire but has not yet lifted off
the launch pad. This picture was
taken in the few milliseconds of time between ignition and liftoff
– when the motor is building up pressure in the combustion chamber.
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Rocket motor nozzles are typically
convergent-divergent nozzles. This
means that the nozzle is shaped such that high-pressure low-velocity gas
enters the nozzle and is compressed as it approaches smallest diameter section,
where the gas velocity increases to exactly the speed of sound. Next, if the combustion chamber pressure
is high enough, the final section of the nozzle accelerates the gas to
several times the speed of sound.
The flow entering the nozzle has
high-pressure, high-temperature, and low velocity. The gas flow exiting the nozzle has
low-pressure, low-temperature, and high velocity. Nozzles exchange one physical state for a
different, more desired state—in this case; temperature and pressure
are exchanged for high-velocity.
When the propellant is burning, it
produces gas. By burning the same
propellant under high pressure, it burns faster and produces more gas. It is this type of controlled
“run-away” reaction that makes solid propellant rocket motors
work. By placing large quantities of
propellant in a small space, and burning the propellant with only a small
hole for the gas to escape (the nozzle) the combustion rate speeds up to an
equilibrium point set by the size of the nozzle. This high-rate of reaction results in
high-pressure, which causes the escaping gas to accelerate to the speed of
sound in the nozzle throat, as described above. The final result of the high-pressure gas
flow is the high-velocity exhaust products that exit the nozzle, which
create thrust.
The size of the nozzle throat is a
key parameter for the safe operation of the rocket motor. If the nozzle is sized too large, the
pressure inside the motor will not get high enough to cause rapid
combustion. If the nozzle is sized
too small, however, the combustion pressure will build up too quickly,
causing the motor to rupture. This
is a catastrophic condition that will cause the rest of the rocket to be
destroyed. Fortunately, much is
known about typical rocket propellants, and one can use standard equations
to determine the proper nozzle size for a given propellant and chamber
pressure.
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These are the simple principles that allow solid rocket motors
to create thrust. Every part of a
sold rocket motor works together to continue the chain reaction, and the designer
must anticipate the forces and reaction rates of the components so as to
design the propellant, thrust chamber, and nozzle to produce the desired
results.
The rocket motor that ASA used in the TLV 7 was classified as
an “N2000” rocket motor.
Each successive letter in the rocket motor lettering scheme
indicates a motor that has twice the energy of the previous letter. For example, our motor had twice the
energy of an “M” class motor.
The number after the letter indicates the average thrust of the
motor, measured in Newtons. The
N2000’s average thrust is 2000 Newtons, or about 450 lbs-thrust. The actual thrust of a solid motor
starts a little bit higher than this, and decays below this value during
operation, due to the erosion of the propellant itself. This erosion steadily creates a
larger-cavity thrust chamber, and the propellant cannot produce gas fast
enough to fill the growing void, so the pressure, and hence thrust, drops
off. This is shown in the figure
below, which is a plot of thrust vs. time.
This plot was obtained from the Aerotech website (the motor
manufacturer).
This page summarized the basic concepts and design problems of
solid-fuel rocket motors. ASA will
continue to use these rocket motors for the purposes of test flights, while
continuing the development of our liquid-fueled rocket motor. Please check back with this web page in
a week or so for the next propulsion topic, and contact Rob Morehead at rmorehead@asa-houston.org if
you have questions relative to the ASA propulsion department.
Future topics for this page
include:
·
A presentation on rocket motor efficiency (Specific
Impulse)
·
Liquid rocket
motor fundamentals
·
Liquid rocket
motor components / basic design
·
The ASA
liquid-rocket motor
·
Efficiency
comparisons of various types of rocket motors (Liquid, Solid, and Hybrid
systems)
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