ROCKET PROPELLANTS | Page 1 |
The gauge for rating the efficiency of rocket propellants is specific impulse, stated
in seconds. Specific impulse indicates how many pounds (or kilograms) of thrust are obtained by
the consumption of one pound (or kilogram) of propellant in one second. Specific impulse is
characteristic of the type of propellant, however, its exact value will vary to some extent with
the operating conditions and design of the rocket engine.
Liquid Propellants
In a liquid propellant rocket, the fuel and oxidizer are stored in separate tanks, and are
fed through a system of pipes, valves, and turbopumps to a combustion chamber where they are
combined and burned to produce thrust. Liquid propellant engines are more complex then their
solid propellant counterparts, however, they offer several advantages. By controlling the flow
of propellant to the combustion chamber, the engine can be throttled, stopped, or restarted.
A good liquid propellant is one with a high specific impulse or, stated another way, one with
a high speed of exhaust gas ejection. This implies a high combustion temperature and exhaust
gases with small molecular weights. However, there is another important factor which must be
taken into consideration: the density of the propellant. Using low density propellants means
that larger storage tanks will be required, thus increasing the mass of the launch vehicle.
Storage temperature is also important. A propellant with a low storage temperature, i.e. a
cryogenic, will require thermal insulation, thus further increasing the mass of the launcher.
The toxicity of the propellant is likewise important. Safety hazards exist when handling,
transporting, and storing highly toxic compounds. Also, some propellants are very corrosive,
however, materials that are resistant to certain propellants have been identified for use in
rocket construction.
Liquid propellants used by NASA and in commercial launch vehicles can be classified into
three types: petroleum, cryogenics, and hypergolics.
Petroleum fuels are those refined from crude oil and are a mixture of complex
hydrocarbons, i.e. organic compounds containing only carbon and hydrogen. The petroleum used as
rocket fuel is kerosene, or a type of highly refined kerosene called RP-1 (refined petroleum).
It is used in combination with liquid oxygen as the oxidizer.
RP-1 and liquid oxygen are used as the propellant in the first-stage boosters of the
Atlas/Centaur and Delta launch vehicles. It also powered the first-stages of the Saturn 1B
and Saturn V rockets. RP-1 delivers a specific impulse considerably less than cryogenic fuels.
Cryogenic propellants are liquefied gases stored at very low temperatures,
namely liquid hydrogen (LH2) as the fuel and liquid oxygen (LO2) as the
oxidizer. LH2 remains liquid at temperatures of -423 degrees F (-253 degrees C) and
LO2 remains in a liquid state at temperatures of -298 degrees F (-183 degrees C).
Because of the low temperatures of cryogenic propellants, they are difficult to store over
long periods of time. For this reason, they are less desirable for use in military rockets
which must be kept launch ready for months at a time. Also, liquid hydrogen has a very low
density (0.59 pounds per gallon) and, therefore, requires a storage volume many times greater
than other fuels. Despite these drawbacks, the high efficiency of liquid hydrogen/liquid oxygen
makes these problems worth coping with when reaction time and storability are not too critical.
Liquid hydrogen delivers a specific impulse about 40% higher than other rocket fuels.
Liquid hydrogen and liquid oxygen are used as the propellant in the high efficiency main
engines of the space shuttle. LH2/LO2 also powered the upper stages of the
Saturn V and Saturn lB rockets as well as the second stage of the Atlas/Centaur launch vehicle,
the United States' first LH2/LO2 rocket (1962).
Hypergolic propellants are fuels and oxidizers which ignite spontaneously on
contact with each other and require no ignition source. The easy start and restart capability of
hypergolics make them ideal for spacecraft maneuvering systems. Also, since hypergolics remain
liquid at normal temperatures, they do not pose the storage problems of cryogenic propellants.
Hypergolics are highly toxic and must be handled with extreme care.
Hypergolic fuels commonly include hydrazine, monomethyl hydrazine (MMH) and unsymmetrical
dimethyl hydrazine (UDMH). The oxidizer is typically nitrogen tetroxide (N2O4)
or nitric acid (HNO3). UDMH is used in many Russian, European, and Chinese rockets
while MMH is used in the orbital maneuvering system (OMS) and reaction control system (RCS) of
the Space Shuttle orbiter. The Titan family of launch vehicles and the second stage of the Delta
use a fuel called Aerozine 50, a mixture of 50% UDMH and 50% hydrazine.
Hydrazine is also frequently used as a mono-propellant in catalytic decomposition engines
. In these engines, a liquid fuel decomposes into hot gas in the presence of a catalyst. The
decomposition of hydrazine produces temperatures of about 1700 degrees F and a specific impulse
of about 230 or 240 seconds.
Solid Propellants
Solid propellant motors are the simplest of all rocket designs. They consist of a casing,
usually steel, filled with a mixture of solid compounds (fuel and oxidizer) which burn at a
rapid rate, expelling hot gases from a nozzle to produce thrust. When ignited, a solid
propellant burns from the center out towards the sides of the casing. The shape of the center
channel determines the rate and pattern of the burn, thus providing a means to control thrust.
Unlike liquid propellant engines, solid propellant motors can not be shut down. Once ignited,
they will burn until all the propellant is exhausted.
There are two families of solids propellants: homogeneous and composite. Both types are dense,
stable at ordinary temperatures, and easily storable.
Homogeneous propellants are either simple base or double base. A simple base propellant
consists of a single compound, usually nitrocellulose, which has both an oxidation capacity and
a reduction capacity. Double base propellants usually consist of nitrocellulose and
nitroglycerine, to which a plasticiser is added. Homogeneous propellants do not usually have
specific impulses greater than about 210 seconds under normal conditions. Their main asset is
that they do not produce traceable fumes and are, therefore, commonly used in tactical weapons.
They are also often used to perform subsidiary functions such as jettisoning spent parts or
separating one stage from another.
Modern composite propellants are heterogeneous powders (mixtures) which use a crystallized
or finely ground mineral salt as an oxidizer, often ammonium perchlorate, which constitutes
between 60% and 90% of the mass of the propellant. The fuel itself is aluminum. The propellant
is held together by a polymeric binder, usually polyurethane or polybutadienes. Additional
compounds are sometimes included, such as a catalyst to help increase the burning rate, or other
agents to make the powder easier to manufacture. The final product is rubberlike substance with
the consistency of a hard rubber eraser.
Solid propellant motors have a variety of uses. Small solids often power the final stage of
a launch vehicle, or attach to payloads to boost them to higher orbits. Medium solids such as
the Payload Assist Module (PAM) and the Inertial Upper Stage (IUS) provide the added boost to
place satellites into geosynchronous orbit or on planetary trajectories.
The Titan, Delta, and Space Shuttle launch vehicles use strap-on solid propellant rockets to
provide added thrust at liftoff. The Space Shuttle uses the largest solid rocket motors ever
built and flown. Each booster contains 1,100,000 pounds (499,000 kg) of propellant and can
produce up to 3,300,000 pounds (14,680,000 Newtons) of thrust.
Hybrid Propellants
Hybrid propellant engines represent an intermediate group between solid and liquid propellant
engines. One of the substances is solid, usually the fuel, while the other, usually the oxidizer,
is liquid. The liquid is injected into the solid, whose fuel reservoir also serves as the
combustion chamber. The main advantage of such engines is that they have high performance,
similar to that of solid propellants, but the combustion can be moderated, stopped, or even
restarted. It is difficult to make use of this concept for vary large thrusts, and thus, hybrid
propellant engines are rarely built.
Compiled and edited by Robert A. Braeunig, 1996.
Propellant is the chemical mixture burned to produce thrust in rockets and consists
of a fuel and an oxidizer. A fuel is a substance which burns when combined with oxygen
producing gas for propulsion. An oxidizer is an agent that releases oxygen for
combination with a fuel. Propellants are classified according to their state - liquid, solid,
or hybrid.
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