Rockets can be broadly classified into one of two categories: those that use a chemical reaction as their energy source, and those that use some other kind of energy source. An example of the former are rockets that are powered by the chemical reaction between liquid oxygen and liquid hydrogen. When these two chemicals react with each other, they produce very hot steam (water vapor). The escape of steam from the back of the rocket provides the propulsive force that drives the jet engine forward.
Chemical rockets make use of either liquid fuels, such as the rocket described above, or of solid fuels. An example of the latter are the solid rockets used to lift a space shuttle into orbit. These rockets contain a fuel that consists of a mixture of aluminum metal (the fuel), ammonium perchlorate (the oxidizer), and a plastic resin (the binder).
Nuclear and electric rockets are examples of jet engines that make use of a nonchemical source of propulsion. In a nuclear rocket, for example, a source of nuclear energy, such as a fission or fusion reactor, is used to heat a working fluid, such as liquid hydrogen. The hot gases formed in this process are then released from the rear of the rocket, providing its forward thrust.
Various kinds of electrical rockets have been designed. In one type, a fluid within the engine is first ionized. The ions thus formed are then attracted and/or repelled by strong electrostatic fields created within the engine. The escape of the ionized fluid provides the rocket with its forward thrust.