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Principle
The principle that explains how a rocket works is Sir Isaac Newton's Third Law of Motion: "To every action there is an equal and opposite reaction.". Combustion provides the action for a rocket. The equal and opposite reaction is the rocket lifting up and leaving the ground. The more combustion, or action, the more opposite reaction, or thrust. Thrust is the force that moves the rocket through the air.
Chamber
Combustion occurs in a chamber. The combustion chamber holds the fuel and oxidizer that combine to produce thrust. The chamber is a closed system. It doesn't take in air from the outside, allowing it to function in outer space. This makes a rocket engine different from an automobile engine or airplane propeller engine which relies on outside air. Those engines wouldn't work in outer space.
Liquid
In a liquid rocket, the fuel and oxidizer, or source of oxygen, remain stored separately in a liquid state. Mix the two together and you have combustion. At any time, you can control the combustion and the resulting thrust by adjusting the flow of the fuel and oxidizer, known as propellants. At one end of the combustion chamber is an opening known as the nozzle. It allows gas to escape, producing thrust. The nozzle is narrow at the opening and then widens to produce greater thrust. Hot exhaust, or accelerated gas, passes through the nozzle as a result of combustion.
Solid
A solid rocket differs from a liquid rocket. The fuel and oxygen is pre-mixed and contained in a cylinder. Expose the fuel and oxygen to heat and the propellants ignite. Once ignited, a solid rocket can't be shut off like a liquid rocket. Combustion continues until no more propellant remains.