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Fuel and oxidizers are the propellants. Fuel is the substance that burns to create the expanding gasses that power a rocket engine. In order for anything to burn, you need oxygen or something chemically similar: an oxidizer. Oxidizers become necessary in the vacuum of space, where there is no oxygen to help ignite the fuel. The simplest type of fuel and oxidizer would be liquid hydrogen and liquid oxygen. These were used on the Saturn V which sent astronauts to the Moon and in the Space Shuttle Main Engines. When they burned, they created water vapor!