To launch a
or to move a rocket through space,
we must use a propulsion system to generate
thrust. Thrust is generated through an
application of Newton's
of motion; a working gas is accelerated to the rear of the rocket engine,
and the re-action is a thrust force applied to the engine in the
foward direction. In
fueled rocket engines, the working gas is produced through
the burning of a fuel to produce power.
Burning a fuel is called combustion, a chemical process that
we study in middle or high school.
Because combustion is so important for rocket propulsion, we
will review the fundamentals. Combustion is a chemical process
in which a substance reacts rapidly with oxygen and gives off heat.
The original substance is called the fuel, and the source of
oxygen is called the oxidizer. The fuel and oxidizer can be a
solid, liquid, or gas.
For rocket propulsion the fuel and oxidizer are usually stored
as either a liquid or a solid.
During combustion, new chemical substances are created from the
fuel and the oxidizer. These substances are called exhaust.
Most of the exhaust comes from chemical combinations of the fuel and
oxygen. When a hydrogen-carbon-based fuel (like gasoline) burns, the
exhaust includes water (hydrogen + oxygen) and carbon dioxide (carbon
+ oxygen). But the exhaust can also include chemical combinations
from the oxidizer alone. If gasoline is burned in air,
which contains 21% oxygen and 78% nitrogen, the exhaust can also
include nitrous oxides (NOX, nitrogen + oxygen). The
of the exhaust is high because of the
that is transferred to the exhaust during combustion.
Because of the high temperatures, exhaust usually occurs as a gas,
but there can be liquid or solid exhaust products as well.
Soot, for example, is a form of solid
exhaust that occurs in some combustion processes.
During the combustion process, as the fuel and oxidizer are turned
into exhaust products, heat is generated. Interestingly, some source
of heat is usually necessary to initiate combustion. Gasoline and air are
both present in your automobile fuel tank; but combustion does not
occur because there is no source of heat. Since heat is both
required to start combustion and is itself a product of combustion,
we can see why combustion takes place very rapidly. Also, once
combustion gets started, we don't have to provide the heat source
because the heat of combustion will keep things going.
We don't have to keep lighting a campfire, it just keep burning.
To summarize, for combustion to occur three things must be
present: a fuel to be burned, a source of oxygen, and a source of
heat. As a result of combustion, exhausts are created and heat is
released. You can control or stop the combustion process by
controlling the amount of the fuel available, the amount of oxygen
available, or the source of heat.
There are some unique liquid rocket propellants,
called hypergolic propellants,
that do not require an external heat source to begin combustion.
Hypergolic propellants burn on contact with one another.
The advantage of using hypergolic propellants is that the
engine design is simplified because no igniter is required, and
the engine "fires" every time. The disadvantage is that the propellants
are highly toxic and must be kept separate at all times to prevent
unwanted combustion. Hypergolic propellants have been used on
missiles like the Titan series and for thrusters on some spacecraft.
Match Rocket: Grade 10-12
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