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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.

In a chemical rocket, the propellant is the reaction product of the oxidizer and the fuel. In a nuclear thermal rocket, it is normally hydrogen, since the specific impulse is dependant on the exhaust velocity of the rocket, which is higher for propellants with low molar mass. A laser ablation rocket will tend to use a combination of metal and plastic (due to their optical properties).