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Rocket Propulsion

Origins of Rocket Propulsion
Although the precise history of the development of rocket propulsion is obscure, we know that the first rockets were developed in ancient China. Modern rocket engines that lift spacecraft into orbit operate on the very same principles as the first Chinese rockets. Both ancient and modern rockets are reaction devices.

Key to the development of the early Chinese rocket was the invention of something similar to black powder, a mixture of saltpeter (the oxidizer), charcoal and sulfur that burns rapidly and furiously when ignited. The first crude firecracker probably was a sealed bamboo tube filled with black powder. When ignited, the black powder exploded violently. If only one end of the tube was sealed, the burning black powder forced a mass of flames, gas and particles out of the unsealed open end. In reaction, the bamboo tube would move rapidly in the opposite direction. This device was a crude rocket. 

Rockets are Reaction Devices
A rocket works in accordance with Newton's Third Law of Motion: For every action there is an equal and opposite reaction. This law can be illustrated by the firing of a gun. The action is the expulsion of the bullet by the pressure of the rapidly burning gunpowder (propellant). The reaction is the "kick" or recoil of the gun against the shooter's shoulder. In a rocket, the action is the expulsion of gases from a nozzle at the open end of the rocket body. The reaction is the movement of the rocket body in a direction opposite that of the expelled gases.

The rocket body moves not because the burning gases push against the air, but because the gases exert pressure against the closed end of the rocket body. Since rocket propellants contain their own oxygen supply in the form of an oxidizer, they need no outside air in order to burn. For that reason rockets operate most efficiently in a complete vacuum because there is no air resistance to slow them down. 

Liquid Propellant Rocket Engines
The first rocket engines to be used in space flight applications were not solid propellant motors. Instead, liquid fuels and oxidizers were used. Although more complex than solid rockets, liquid propellant engines were more easily controlled. For example, it became possible to vary the thrust of (or throttle) the engine and even shut it down at will. The ability to stop and restart certain types of liquid fuel engines was developed and became particularly important as the space program evolved. In time, solid rockets were integrated with liquid rockets into space flight systems as illustrated by the use of both types of engines in the Dyna-Soar and the Space Shuttle.

Solid Propellant Rocket Motors
The black powder used by the Chinese was a form of solid propellant (even though it was a loose powder). In holiday fireworks, the spectacular skyrocket displays are launched by solid propellants. In recent times, solid propellant rocket motors have been used in anti-tank weapons, air-to-air and air-to-ground rockets, and intercontinental ballistic missiles, as well as launch vehicles for spacecraft. In these motors the propellant is not a powder but a semi-liquid substance that is cast in the rocket body. The propellant then hardens into a rubber-like substance that will burn in a controlled way when ignited.

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