Liquid-propellant rocket

A liquid-propellant rocket or liquid rocket utilizes a rocket engine that uses liquid propellants. Gaseous propellants may also be used but are not common because of their low density and difficulty with common pumping methods. Liquids are desirable because they have a reasonably high density and high specific impulse (Isp). This allows the volume of the propellant tanks to be relatively low. The rocket propellants are usually pumped into the combustion chamber with a lightweight centrifugal turbopump, although some aerospace companies have found ways to use electric pumps with batteries, allowing the propellants to be kept under low pressure. This permits the use of low-mass propellant tanks that do not need to resist the high pressures needed to store significant amounts of gasses, resulting in a low mass ratio for the rocket. An inert gas stored in a tank at a high pressure is sometimes used instead of pumps in simpler small engines to force the propellants into the combustion chamber. These engines may have a higher mass ratio, but are usually more reliable, and are therefore used widely in satellites for orbit maintenance. 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. Most designs of liquid engines are throttleable for variable thrust operation and some may be restarted after a previous in-space shutdown. Liquid oxidizer propellants are also used in hybrid rockets, with some of the advantages of a solid rocket. The idea of a liquid rocket as understood in the modern context first appeared in 1903 in the book Exploration of the Universe with Rocket-Propelled Vehicles, by the Russian school teacher Konstantin Tsiolkovsky.The magnitude of his contribution to astronautics is astounding, including the Tsiolkovsky rocket equation, multi staged rockets and using liquid oxygen and liquid hydrogen in liquid propellant rockets.