The word rocket has commonly been used in two ways: one, as a flying vehicle (a missile, spacecraft, or space launch vehicle) driven by a rocket propulsion system; and two, as the propulsion system itself. A rocket, in its conventional form, is an internal-combustion engine that needs no outside air to operate. It carries both fuel and oxidizer, which are burned together in a combustion chamber and produce hot gases that are discharged through a nozzle. Inside the combustion chamber the burning gases exert pressure in all directions. If the chamber were sealed, all these pressures would be balanced and the rocket would not move. The gases are allowed to escape at high speed through the nozzle, however, causing an imbalance in the chamber. Because the pressure exerted on the rocket in the forward direction is much greater than in the backward direction, the rocket shoots forward. It obeys Newton's second law of motion: an unbalanced force acting on an object produces an acceleration in the direction of the force, directly proportional to the force, and inversely proportional to the mass of the object.
The same principles apply to other types of rockets that have been developed, such as electric rockets. All are able to operate both within the Earth's atmosphere and in the near-vacuum of space. This means that all the various kinds of rockets can be used to maneuver artificial satellites, probes, or manned craft in outer space (see space exploration), or to power missiles for military purposes. Some rockets are also sufficiently powerful to launch craft directly into space.For many years after World War II the costly development of rockets was carried out mainly by national governments. Companies that built rockets with such funding later began to use unsold rockets for private satellite launches. Beginning in the late 1980s, companies were developing and employing rocket fleets of their own (see space exploration, commercial).