Spacecraft Propulsion - Chemical

Chemical is the type of propulsion that most people are familiar with and is the most common. This type of propulsion includes the white solid rocket motors on the space shuttle and the Saturn V rocket. The main types of chemical propulsion systems are solid fueled, liquid fueled, and cold gas. 

Solid Fuel Propulsion

Solid fuel propulsion systems provide large amounts of thrust, but cannot be shut off once ignited. They are safe and stable until ignited, which is why solid fuel engines are used to power model rockets as well as to give the Shuttle Orbiter its initial boost. The fuel and the catalyst combine to generate high temperatures and pressures. The gaseous fuel then passes through a nozzle and is expelled. The release of the hot gas creates an equal and opposite force pushing the engine forward, as represented by Newton's third law. Higher combustion temperatures result in greater thrust. A solid fuel engine burns from the inside out, which means that the cavity inside the engine is getting larger. The temperature of the combustion is fixed and therefore by the ideal gas law:

Pressure * Volume = Amount of gas molecules * Gas constant * Temperature

If the volume inside increases, then the pressure must decrease. A decrease in pressure means that the nozzle doesn't work as efficiently. 

Liquid Fuel Propulsion

Liquid fuel propulsion systems, such as the main engine on the Shuttle Orbiter, typically provide less thrust per kilogram of fuel than solid fuel propulsion systems and are much more volatile. Liquid fueled engines do have advantages. For instance, they can be turned off and on whenever they are needed and the efficiency reduction over the lifetime of the engine is smaller. The basic concept is similar to the solid fuel engines. Two liquids are combined inside of a combustion chamber and ignited. The resulting gas is expelled through a nozzle. In order to shut the engine off you stop the flow of the liquids.

Cold Gas Propulsion

Cold gas is the simplest of all of the types of chemical propulsion. The propulsion system consists of a single gas and a nozzle. Whenever thrust is needed the nozzle is opened and some of the gas is expelled. This is the safest of all of the chemical propulsion systems, but due to the lack of combustion cold gas systems do not produce as much thrust per kilogram of fuel as liquid or solid fueled systems. The thrust and the efficiency rely solely on the pressure in the containment tank. As the cold gas system is used the pressure goes down and so does thrust and efficiency. This concept is best visualized with a balloon. When an inflated balloon is released the air inside is expelled. As the volume of air reduces the balloon slows down and eventually lands.

So in summary...

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