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The
first rockets ever built, the fire-arrows
of the Chinese, were not very reliable. Many just exploded on launching.
Others flew on erratic courses and landed in the wrong place. Being a rocketeer
in the days of the fire-arrows must have been an exciting, but also a highly
dangerous activity.
Today, rockets are much more reliable.
They fly on precise courses and are capable of going fast enough to escape
the gravitational pull of Earth. Modern rockets are also more efficient
today because we have an understanding of the scientific principles behind
rocketry. Our understanding has led us to develop a wide variety of advanced
rocket hardware and devise new propellants that can be used for longer
trips and more powerful takeoffs.
Rocket Engines
and Their Propellants
Most rockets today operate with either
solid or liquid propellants. The word propellant does not mean simply fuel,
as you might think; it means both fuel and oxidizer. The fuel is the chemical
rockets burn but, for burning to take place, an oxidizer (oxygen) must
be present. Jet engines draw oxygen into their engines from the surrounding
air. Rockets do not have the luxury that jet planes have; they must carry
oxygen with them into space, where there is no air.
Solid
rocket propellants, which are dry to the touch, contain both the fuel
and oxidizer combined together in the chemical itself. Usually the fuel
is a mixture of hydrogen compounds and carbon and the oxidizer is made
up of oxygen compounds. Liquid propellants, which are often gases that
have been chilled until they turn into liquids, are kept in separate containers,
one for the fuel and the other for the oxidizer. Then, when the engine
fires, the fuel and oxidizer are mixed together in the engine.
A solid-propellant rocket has the
simplest form of engine. It has a nozzle, a case, insulation, propellant,
and an igniter. The case of the engine is usually a relatively thin metal
that is lined with insulation to keep the propellant from burning through.
The propellant itself is packed inside the insulation layer.
Many solid-propellant rocket engines
feature a hollow core that runs through the propellant. Rockets that do
not have the hollow core must be ignited at the lower end of the propellants
and burning proceeds gradually from one end of the rocket to the other.
In all cases, only the surface of the propellant burns. However, to get
higher thrust, the hollow core is used. This increases the surface of the
propellants available for burning. The propellants burn from the inside
out at a much higher rate, and the gases produced escape the engine at
much higher speeds. This gives a greater thrust. Some propellant cores
are star shaped to increase the burning surface even more.
To fire solid propellants, many kinds
of igniters can be used. Fire-arrows were ignited by fuses, but sometimes
these ignited too quickly and burned the rocketeer. A far safer and more
reliable form of ignition used today is one that employs electricity. An
electric current, coming through wires from some distance away, heats up
a special wire inside the rocket. The wire raises the temperature of the
propellant it is in contact with to the combustion point.
Other igniters are more advanced
than the hot wire device. Some are encased in a chemical that ignites first,
which then ignites the propellants. Still other igniters, especially those
for large rockets, are rocket engines themselves. The small engine inside
the hollow core blasts a stream of flames and hot gas down from the top
of the core and ignites the entire surface area of the propellants in a
fraction of a second.
The nozzle in a solid-propellant
engine is an opening at the back of the rocket that permits the hot expanding
gases to escape. The narrow part of the nozzle is the throat. Just beyond
the throat is the exit cone.
The purpose of the nozzle is to increase
the acceleration of the gases as they leave the rocket and thereby maximize
the thrust. It does this by cutting down the opening through which the
gases can escape. To see how this works, you can experiment with a garden
hose that has a spray nozzle attachment. This kind of nozzle does not have
an exit cone, but that does not matter in the experiment. The important
point about the nozzle is that the size of the opening can be varied.
Start with the opening at its widest
point. Watch how far the water squirts and feel the thrust produced by
the departing water. Now reduce the diameter of the opening, and again
note the distance the water squirts and feel the thrust. Rocket nozzles
work the same way.
As with the inside of the rocket
case, insulation is needed to protect the nozzle from the hot gases. The
usual insulation is one that gradually erodes as the gas passes through.
Small pieces of the insulation get very hot and break away from the nozzle.
As they are blown away, heat is carried away with them.
The other main kind of rocket engine
is one that uses liquid propellants. This is a much more complicated engine,
as is evidenced by the fact that solid rocket engines were used for at
least seven hundred years before the first successful liquid engine was
tested. Liquid propellants have separate storage tanks - one for the fuel
and one for the oxidizer. They also have pumps, a combustion chamber, and
a nozzle.
The fuel of a liquid-propellant
rocket is usually kerosene or liquid hydrogen; the oxidizer is usually
liquid oxygen. They are combined inside a cavity called the combustion
chamber. Here the propellants burn and build up high temperatures and pressures,
and the expanding gas escapes through the nozzle at the lower end. To get
the most power from the propellants, they must be mixed as completely as
possible. Small injectors (nozzles) on the roof of the chamber spray and
mix the propellants at the same time. Because the chamber operates under
high pressures, the propellants need to be forced inside. Powerful, lightweight
turbine pumps between the propellant tanks and combustion chambers take
care of this job.
With any rocket, and especially with
liquid-propellant rockets, weight is an important factor. In general, the
heavier the design, the more the thrust needed to get it off the ground.
Because of the pumps and fuel lines, liquid engines are much heavier than
solid engines.
One especially good method of reducing
the weight of liquid engines is to make the exit cone of the nozzle out
of very lightweight metals. However, the extremely hot, fast-moving gases
that pass through the cone would quickly melt thin metal. Therefore, a
cooling system is needed. A highly effective though complex cooling system
that is used with some liquid engines takes advantage of the low temperature
of liquid hydrogen. Hydrogen becomes a liquid when it is chilled to -2530C.
Before injecting the hydrogen into the combustion chamber, it is first
circulated through small tubes that lace the walls of the exit cone. In
a cutaway view, the exit cone wall looks like the edge of corrugated cardboard.
The hydrogen in the tubes absorbs the excess heat entering the cone walls
and prevents it from melting the walls away. It also makes the hydrogen
more energetic because of the heat it picks up. We call this kind of cooling
system regenerative cooling.
Next
page > Thrust and Control
Information and Images Provided by
NASA
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