A liquid-propellant rocket

A liquid-propellant rocket or a liquid rocket is a rocket engine that uses propellants in liquid form. Liquids are desirable because their reasonably high density allows the volume of the propellant tanks to be relatively low, and it is possible to use lightweight centrifugal turbopumps to pump the propellant from the tanks into the combustion chamber, which means that the propellants can be kept under low pressure. This permits the use of low-mass propellant tanks, resulting in a high 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 lower mass ratio, but are usually more reliable :186,187and are therefore used widely in satellites for orbit maintenance.

Liquid rockets have been built as monopropellant rockets using a single type of propellant, bipropellant rockets using two types of propellant, or more exotic tripropellant rockets using three types of propellant. Bipropellant liquid rockets generally use a liquidfuel and a liquid oxidizer, such as liquid hydrogen or a hydrocarbon fuel such as RP-1, and liquid oxygen. The engine may be acryogenic rocket engine, where the fuel and oxidizer, such as hydrogen and oxygen, are gases which have been liquefied at very low temperatures.

In this schematic A is the cross sectional area of the nozzle on its longitudinal axis, The subscript e refers to the exit of the nozzle, and the subscript 0 refers to atmospheric conditions.

Liquid propellant rockets can be throttled (thrust varied) in realtime, and have control of mixture ratio (ratio at which oxidizer and fuel are mixed); they can also be shut down, and, with a suitable ignition system or self-igniting propellant, restarted.

Liquid propellants are also sometimes used in hybrid rockets, in which a liquid oxidizer is combined with a solid fuel. :354-356

The idea of liquid rocket as understood in the modern context first appears in the book The Exploration of Cosmic Space by Means of Reaction Devices, by the Russian schoolteacher Konstantin Tsiolkovsky. This seminal treatise on astronautics was published in 1903, but was not distributed outside of Russia until years later, and Russian scientists paid little attention to it.

During the 19th century, the only known developer of liquid propellant rocket engine experiments was Peruvian scientist Pedro Paulet, who is considered one of the "fathers of aeronautics.". However, he did not publish his work. In 1927 he wrote a letter to a newspaper in Lima, claiming he had experimented with a liquid rocket engine while he was a student in Paris three decades earlier. Historians of early rocketry experiments, among them Max Valier and Willy Ley, have given differing amounts of credence to Paulet's report. Paulet described laboratory tests of, but did not claim to have launched a liquid rocket.

The first flight of a liquid-propellant rocket took place on March 16, 1926 at Auburn, Massachusetts, when American professor Dr. Robert H. Goddardlaunched a vehicle using liquid oxygen and gasoline as propellants. The rocket, which was dubbed "Nell", rose just 41 feet during a 2.5-second flight that ended in a cabbage field, but it was an important demonstration that liquid-fueled rockets were possible. Goddard proposed liquid propellants about fifteen years earlier and began to seriously experiment with them in 1921.

After Goddard's success, German engineers and scientists became enthralled with liquid fuel rockets and designed and built rockets, testing them in the early 1930s in a field near Berlin. This amateur rocket group, the VfR, included Wernher von Braun who became the head of the army research station that secretly built the V-2 rocket weapon for the Nazis. The German-Romanian Hermann Oberth published a book in 1922 suggesting the use of liquid propellants.

After World War II the American government and military finally seriously considered liquid-propellant rockets as weapons and began to fund work on them. The Soviet Union did likewise, and thus began the Space Race.