Propulsion - Internal Energy, Internal Reaction Mass

The classic rocket. The vehicle carries a supply of one or more substances that, singly or in combination, release energy (the 'fuel') and the energy is used to push a stream of mass (the 'reaction mass') out of the vehicle -- exhaust goes one way, vehicle accelerates the other way. It is a common case for the fuel and the reaction mass to actually be the same thing (as in a chemical rocket, where you burn two chemicals and the reaction products form the reaction mass) in which case they are more commonly called 'propellant'. But they may be separate things; in a nuclear-thermal rocket, the 'fuel' is the fissionable material in the reaction, and the 'reaction mass' is pushed through the reactor, made hot, and ejected through a nozzle.

Examples include: Hydrolox Engine Nuclear-Thermal Rocket Photon Rocket

Onboard power such as a nuclear reactor can also be used to supply electricity, which can be used to power various forms of electric rocket such as: [Ion Engine]