Propellant

Propellant is mass which is ejected from a jet engine (of which a rocket is one possible type) to produce thrust through Newton's third law of motion.

Chemical propulsion

In a chemical rocket, the propellant is the reaction product of the oxidizer and the fuel. Typical liquid combinations are hydrogen and oxygen, methane and oxygen or jet fuel and oxygen, but many mixes are available. Monopropellants are also possible combining both oxidizer and fuel in a single mixtures. Solids can also be used.

Nuclear thermal propulsion

In a nuclear thermal rocket, it is normally hydrogen, since the specific impulse is dependent on the exhaust velocity of the rocket, which is higher for propellants with low molar mass

Nuclear or Solar Electric propulsion

For Ion thrusters, an easily ionized gas is preferred. Vaporized liquid metals are good candidates. As for nuclear rockets, the lower the molar mass the higher the exhaust velocity. However, as electric engines work on ionized gases, the ionization energy of the propellant adds complexity to the engine and favors elements that are easy to ionize. The nobles gases, such as Argon, Neon Krypton and Xenon are all usable, with Xenon achieving the best propulsion properties for many missions.

Other types of propulsion

A laser ablation rocket will tend to use a combination of metal and plastic (due to their optical properties).

A cold gas thruster will use compressed gas to feed small thrusters used for maneuvering. The propellant is usually nitrogen.

A steam powered rocket uses the expansion of water into steam to provide thrust. Water is the only propellant involved. The energy can come from a nuclear reactor or solar concentrators.

Mass drivers can use magnetic fields to projects magnetically suspended propellant masses to propel a vehicle, or alternatively use some form of mass driver to capture high velocity mass sent from another mass driver. The second form of mass driver can sidestep the rocket equation and can be seen as a form of externally powered propulsion.