In a rocket, propellants are burned in a combustion chamber and the combustion products are exhausted through a nozzle. The individual exhaust molecules can be thought of as little sailors jumping from the rear of the rocket at very high velocity. Although each molecule may not weigh much, its individual action imparts a small reaction to the rocket and accelerates it forward, just like the small boat in the example. When one ton of combustion products exit the rear of a rocket at supersonic speeds—every second—they can generate enough force, or thrust, to push the rocket into space.

Typical launch vehicle propulsion systems generate thrust through the combustion of a fuel and an oxidizer. By definition, a rocket propulsion system does not rely on the oxygen in the atmosphere. Liquid-fueled engines use liquid propellants—such as kerosene and liquid oxygen—which must be rapidly pumped into the combustion chamber at a suitable mixture ratio.