To move an airplane
or a
model rocket
through the air,
we must use a propulsion system to generate
thrust. Different types of aircraft use
different types of propulsion devices, but all aircraft rely on some
type of engine to generate power. Rocket
engines, internal combustion, or piston engines, and jet
engines all depend on the burning of 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 aircraft and 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 can be a solid,
liquid, or gas, although for airplane propulsion the fuel is usually
a liquid. The oxidizer, likewise, could be a
solid, liquid, or gas,
but is usually a gas (air) for airplanes. For model rockets, a
solid
fuel and oxidizer is used.
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 the gasoline is burned in air,
which contains 21% oxygen and 78% nitrogen, the exhaust can also
include nitrous oxides (NOX, nitrogen + oxygen). The
temperature
of the exhaust is high because of the
heat
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 also necessary to start 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.
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