Liquid-propellant rocket

A liquid-propellant rocket or liquid rocket is a rocket engine that uses liquid propellants. 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. A liquid-propellant rocket or liquid rocket is a rocket engine that uses liquid propellants. 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, 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. Some designs are throttleable for variable thrust operation and some may be restarted after a previous in-space shutdown. Liquid propellants are also used in hybrid rockets, with some of the advantages of a solid rocket. 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 school teacher Konstantin Tsiolkovsky. This seminal treatise on astronautics was published in May 1903, but was not distributed outside Russia until years later, and Russian scientists paid little attention to it. Pedro Paulet wrote a letter to a newspaper in Lima in 1927, 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, Willy Ley, and John D. Clark, 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. Goddard launched 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. The German-Romanian Hermann Oberth published a book in 1922 suggesting the use of liquid propellants. In Germany, engineers and scientists became enthralled with liquid-fuel rockets, building and 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 designed the V-2 rocket weapon for the Nazis. By the late 1930s, use of rocket propulsion for manned flight began to be seriously experimented with, as Germany's Heinkel He 176 made the first manned rocket-powered flight using a liquid-fueled rocket engine, designed by German aeronautics engineer Hellmuth Walter on June 20, 1939. The only production rocket-powered combat aircraft ever to see military service, the Me 163 Komet in 1944-45, also used a Walter-designed liquid-fueled rocket motor, the Walter HWK 109-509, which produced up to 1,700 kgf (16.7 kN) thrust at full power. 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.