Internal combustion engines are the propulsion systems that most of us can relate to as they power our cars on a daily basis. Most vehicles utilize a reciprocating, piston-driven internal combustion engine that uses gasoline as the combustion agent. The combustion occurs in a chamber where the reciprocating motion of the pistons turns the drive shaft of the engine which turns the wheels of the vehicle. There are also rotary internal combustion engines where the combustion directly generates the rotation of the drive shaft instead of with pistons. Another type of internal combustion engine is the propeller type which is used to generate thrust in airplanes, ships and boats. In this type, the engine turns a propeller that generates thrust through the air or water.
A gas turbine engine, sometimes called a jet engine, is a system that takes in surrounding air and converts it into hot exhaust gases that flow through a nozzle and accelerate across a turbine, creating thrust. An internal compressor is used to compress the air where it is mixed with fuel and burned, creating the hot air exhaust. Gas turbines are used to propel not only aircraft but watercraft as well. Although used quite a bit on military vessels, gas turbines also render military units vulnerable to heat-seeking missiles because of the high temperature exhaust that is a trademark of these engines.
Ramjet propulsion can be described as gas turbine propulsion without the internal compressor. Thus, the ramjet has no moving parts and relies on the movement of the vessel to "ram" air into a scoop-like device where it slows down and compresses as it approaches the nozzle. As with the gas turbine model, this is where fuel is mixed with the air and burned to create the hot exhaust air across the nozzle, creating thrust.
Rocket propulsion is based on the same principle as the gas turbine and ramjet propulsion models. The big difference is that rockets travel in areas where oxygen is not available to be scooped up and compressed. Therefore, rocket propulsion systems have to carry their own source of oxygen, known as oxidizers. There are two types of rocket engines. There is the liquid rocket engine, where the fuel and oxidizers are stored separately in liquid form and mixed together in a combustion chamber. The second type is the solid rocket engine where the fuel and oxidizers are stored together in a solid form. An igniter is required in a solid rocket engine to initiate the combustion process.