Rocket, general term for a jet propulsion device
propelled by the expulsion of gases generated in a combustion chamber.
Because the combustible propellants contain both fuel and an oxidizer, a
rocket develops thrust independent of its surroundings, unlike other types
of jet engines that utilize oxygen from the atmosphere to burn fuel carried
aboard. A rocket engine, therefore, is self-contained and is the only type
of device suitable for flight propulsion into outer space.
Thrust to propel a rocket is based on Isaac Newton's
third law of motion, which states that for every action, there is an equal
and opposite reaction. The principle of a rocket motor may be understood by
considering the example of a closed container filled with a compressed gas.
Within this container the gas exerts equal pressure on every point of its
walls. If a hole is punched in the bottom of the container, however, the gas
at the bottom escapes and the pressure against the top of the container is
no longer equalized. The internal gas pressure then pushes the container
upwards in reaction to the jet of air escaping downwards. The amount of
thrust developed by a rocket motor depends mainly on two factors, the
velocity with which the burning gases leave the combustion chamber, and the
mass of the burning gases.
Rockets may be divided into two principal
classes, solid-propellant rockets, such as intercontinental ballistic
missiles (ICBM), and liquid-propellant rockets, such as the Saturn 5 space
booster. In both types of rockets, the combustion chamber in which the
fuel burns is called the motor. In a liquid-propellant rocket, the
propellants are carried in separate tanks and delivered to the rocket motor
as required; in solid-propellant rockets, the propellant charge is stored
and burned in the motor.
The term rocket has frequently been used to describe both the
thrust-producing device and the whole rocket-powered vehicle. To avoid
confusion, especially in the case of large vehicles such as missiles and
space-launch vehicles, the propulsion device is now usually referred to as a
rocket engine.
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