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Rocket propellant is mass ejected as exhaust to create thrust for a rocket. There are several types of rocket propellant currently in use. Along with the most commonly used propellants, scientists are continuing to look for and develop new and more efficient kinds.
First there are the propellants for solid rockets. Most of us are already familiar with gun powder. However there are others that are commonly used in space flight. The booster rockets used in space shuttle launches are solid rockets. In their case, the solid propellant is mostly ammonium perchlorate, mixed with aluminum, iron oxide, and a polymer as a binding agent to hold the mixture together. The solid fuel in hybrid rockets is paraffin, a substance that is being highly touted because its exhaust – carbon dioxide and water – is environmentally friendly. Other types include “candy” propellant, which uses a sugar-based fuel and composite which is normally a mixture like the one mentioned above. The mixtures vary to get different types of thrust. Project Orion, a proposed nuclear pulse rocket used plastic as its propellant as it would break down under nuclear fission into lighter hydrogen atoms.
Liquid fuel rockets use a different type of propellant; the various components are in their liquid state. The main ones used are liquid oxygen and hydrogen. However, what a rocket uses depends on its mission. The Saturn V rockets used kerosene and the space shuttle uses a compound called hydrazine. The interesting thing about liquid propellants is that in most rockets they have to function as fuel and oxidizer. To this purpose one tank can hold hydrogen as the fuel source and liquid oxygen as the oxidizer aiding combustion.
Scientist have now found new more exotic sources of propellants. For example, scientists experimented using ions or plasma as a propellant. A charged gas like xenon is bombarded with electrons becoming ionized. The charged gas is then ejected using magnetic or electrostatic forces. Another unique option proposed is to have a ramjet engine for a spaceplane that uses the ambient charged particles in space as a propellant.
As you can see, the propellants for a rocket is anything that can be ejected as exhaust to create an equal and opposite reaction propelling the rocket.
If you enjoyed this article there are others on Universe Today you might like. One is about chemical propulsion and the other is on ion engines.
Also make sure to check Astronomy Cast. Episode 100 about Rockets also has some great information.