Rocket engine

A rocket motor, also called rocket engine, actually propels a rocket-propelled guided missile or unguided rocket. It generates hot, fast-moving gases that are directed through an exhaust nozzle, and, as a reaction to the action of the moving gases, the motor develops thrust in the opposite direction to the exhaust gas flow.

There are two basic types of rocket motor: solid and liquid. Solid propellants allow much simpler construction, as they do not need pumps to move them into a combustion chamber. They are stable in storage for an appreciable period of time. Their greatest disadvantage is that, other than some experimental devices, they burn until there is no more propellant, and the burning rate cannot be adjusted in real time. The highest-energy solid propellants do not produce as much energy as the highest-energy liquid propellants, but this is not a major restriction; all modern military rockets used for propulsion, as opposed to directional adjustment, are solid-propellant.

Liquid engines usually mix two liquids, a fuel and an oxidizer. Every fuel-ozidizer system has advantages and disadvantages. The highest-energy mixes, such as hydrogen and oxygen, use cryogenic, intensely cold liquids, which are difficult to store and pump. Cryogenic systems also need an ignition system.

Storable liquid propellants, such as a hydrazine-based compound and red fuming nitric acid or a relative, can be held at normal temperatures, but tend to be very toxic and corrosive. The combinations are often hypergolic, igniting on contact. The hypergolic property removes the need for an ignition system, but can cause catastrophes if a leak brings them into unintended contact.