The word is derived from two Latin words: pro meaning before or forwards and pellere meaning to drive. Propulsion means to push forward or drive an object forward. A propulsion system produces thrust to push an object forward. It is the study of how to design an engine that will provide the thrust that is needed for a plane to take off and fly through the air. Most spacecraft today are propelled by forcing a gas from the back/rear of the vehicle at very high speed through a supersonic nozzle. This sort of engine is called a rocket engine.
Liquid fuel propulsion systems use a fuel, such as liquid hydrogen, kerosene or alcohol and liquids such as oxygen liquids. The oxygen liquids provides the oxygen necessary to ignite and burn the fuel, which in turn enables a spacecraft’s propulsion system to work in the oxygen-less environment of space. The first propulsion system used for any craft was solid fuel. Propulsion systems under development use electrical energy from the Sun to ionize and electro statically Accelerate a propellant to produce thrust.
Propulsion works by throwing stuff, usually hot gases, out of the back of something like a rocket or jet airplane that you are trying to speed up. The thrust, or how much something is pushing you, is equal to the amount of stuff you are throwing out the back multiplied by how fast you are throwing those things. This is known as: Further research into improving the efficiency of different propulsion systems has produced other interesting advances. One of them being put into use is the ion engine. Rather than use combusted fuel to create thrust, the ion engine use accelerated ionized gas.
This system is more efficient in its use of propellant and is good for long voyages. Whatever new propulsion system is eventually adopted it will draw from the combined knowledge and research that has given us our modern space program. Different propulsion systems generate thrust in slightly different ways. That is: the propeller, the turbine engine, the ramjet, and the rocket. Each of these propulsion systems is applied everyday. Aero breaking allows a spacecraft to reduce the high point of an elliptical orbit by repeated brushes with the atmosphere at the low point of the orbit. This can save a considerable amount of fuel .