Liquid-propellant rocket

A liquid-propellant rocket or liquid rocket uses a rocket engine burning liquid propellants. (Alternate approaches use gaseous or solid propellants.) Liquids are desirable propellants because they have reasonably high density and their combustion products have high specific impulse (I_sp). This allows the volume of the propellant tanks to be relatively low.

Liquid rockets can be monopropellant rockets using a single type of propellant, or bipropellant rockets using two types of propellant. Tripropellant rockets using three types of propellant are rare. Liquid oxidizer propellants are also used in hybrid rockets, with some of the advantages of a solid rocket. Bipropellant liquid rockets use a liquid fuel such as liquid hydrogen or RP-1, and a liquid oxidizer such as liquid oxygen. The engine may be a cryogenic rocket engine, where the fuel and oxidizer, such as hydrogen and oxygen, are gases which have been liquefied at very low temperatures.

Most designs of liquid rocket engines are throttleable for variable thrust operation. Some allow control of the propellant mixture ratio (ratio at which oxidizer and fuel are mixed). Some can be shut down and, with a suitable ignition system or self-igniting propellant, restarted.

Hybrid rockets apply a liquid or gaseous oxidizer to a solid fuel.

The use of liquid propellants has a number of advantages:

Use of liquid propellants can also be associated with a number of issues:

Liquid rocket engines have tankage and pipes to store and transfer propellant, an injector system and one or more combustion chambers with associated nozzles.

Typical liquid propellants have densities roughly similar to water, approximately 0.7 to 1.4 g/cm³ (0.025 to 0.051 lb/cu in). An exception is liquid hydrogen which has a much lower density, while requiring only relatively modest pressure to prevent vaporization. The density and low pressure of liquid propellants permit lightweight tankage: approximately 1% of the contents for dense propellants and around 10% for liquid hydrogen. The increased tank mass is due to liquid hydrogen's low density and the mass of the required insulation.