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Liquid rocket

Liquid rocket

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A liquid-propellant rocket or a liquid rocket is a rocket engine
Rocket engine
A rocket engine, or simply "rocket", is a jet engineRocket Propulsion Elements; 7th edition- chapter 1 that uses only propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law...

 that uses propellant
Propellant
A propellant is a material that produces pressurized gas that:* can be directed through a nozzle, thereby producing thrust ;...

s in liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...

 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 pumps to pump the propellant from the tanks into the engines, which means that the propellants can be kept under low pressure. This permits the use of low mass propellant tanks, permitting a high mass ratio
Mass ratio
In aerospace engineering, mass ratio is a measure of the efficiency of a rocket. It describes how much more massive the vehicle is with propellant than without; that is, it is the ratio of the rocket's wet mass to its dry mass...

 for the rocket.

Liquid rockets have been built as monopropellant rocket
Monopropellant rocket
A monopropellant rocket is a rocket that uses a single chemical as its propellant.-Chemical-reaction monopropellant rockets:...

s using a single type of propellant, bipropellant rockets using two types of propellant, or more exotic tripropellant rocket
Tripropellant rocket
A tripropellant rocket is a rocket that uses three propellants, as opposed to the more common bipropellant rocket or monopropellant rocket designs, which use two or one fuels, respectively...

s using three types of propellant.
Bipropellant liquid rockets generally use one liquid fuel
Fuel
Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...

 and one liquid oxidizer, such as liquid hydrogen
Liquid hydrogen
Liquid hydrogen is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H2 form.To exist as a liquid, H2 must be pressurized above and cooled below hydrogen's Critical point. However, for hydrogen to be in a full liquid state without boiling off, it needs to be...

 or a hydrocarbon fuel such as RP-1
RP-1
RP-1 is a highly refined form of kerosene outwardly similar to jet fuel, used as a rocket fuel. Although having a lower specific impulse than liquid hydrogen , RP-1 is cheaper, can be stored at room temperature, is far less of an explosive hazard and is far denser...

, and liquid oxygen
Liquid oxygen
Liquid oxygen — abbreviated LOx, LOX or Lox in the aerospace, submarine and gas industries — is one of the physical forms of elemental oxygen.-Physical properties:...

. This example also shows that liquid-propellant rockets sometimes use cryogenic rocket engine
Cryogenic Rocket Engine
A cryogenic rocket engine is a rocket engine that uses a cryogenic fuel or oxidizer, that is, its fuel or oxidizer are gases liquefied and stored at very low temperatures...

s, where fuel or oxidizer are gases liquefied at very low temperatures.

Liquid propellants are also sometimes used in hybrid rocket
Hybrid rocket
A hybrid rocket is a rocket with a rocket motor which uses propellants in two different states of matter - one solid and the other either gas or liquid. The Hybrid rocket concept can be traced back at least 75 years....

s, in which they are combined with a solid or gaseous propellant.

History



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 Konstantin Tsiolkovsky. This seminal treatise on astronautics
Astronautics
Astronautics, and related astronautical engineering, is the theory and practice of navigation beyond the Earth's atmosphere. In other words, it is the science and technology of space flight....

 was published in 1903.

The only known claim to liquid propellant rocket engine experiments in the nineteenth century was made by a Peru
Peru
Peru , officially the Republic of Peru , is a country in western South America. It is bordered on the north by Ecuador and Colombia, on the east by Brazil, on the southeast by Bolivia, on the south by Chile, and on the west by the Pacific Ocean....

vian scientist named Pedro Paulet
Pedro Paulet
Pedro Paulet Mostajo was a Peruvian scientist who claimed he, in 1895, was the first person to build a liquid-fuel rocket engine and, in 1900, the first person to build a modern rocket propulsion system. Paulet is considered one of the "fathers of aeronautics"...

.
However, he did not immediately publish his work. In 1927 he wrote a letter to a newspaper in Lima
Lima
Lima is the capital and the largest city of Peru. It is located in the valleys of the Chillón, Rímac and Lurín rivers, in the central part of the country, on a desert coast overlooking the Pacific Ocean. Together with the seaport of Callao, it forms a contiguous urban area known as the 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
Max Valier
Max Valier was an Austrian rocketry pioneer. He helped found the German Verein für Raumschiffahrt that would bring together many of the minds that would later make spaceflight a reality in the 20th century.-Biography:Valier was born in Bozen , South Tyrol and in 1913 enrolled to study Physics at...

 and Willy Ley
Willy Ley
Willy Ley was a German-American science writer and space advocate who helped popularize rocketry and spaceflight in both Germany and the United States. The crater Ley on the far side of the Moon is named in his honor.-Life:...

, have given differing amounts of credence to Paulet's report. Paulet described laboratory tests of liquid rocket engines, but did not claim to have flown a liquid rocket.

The first flight of a liquid-propellant rocket took place on March 16, 1926 at Auburn, Massachusetts
Auburn, Massachusetts
Auburn is a town in Worcester County, Massachusetts, United States. The population was 16,188 at the 2010 census.- History :Auburn was first settled in 1789 and was officially incorporated in 1808 as the town of Ward, in honor of American Revolution General Artemas Ward...

, when American professor Robert H. Goddard
Robert H. Goddard
Robert Hutchings Goddard was an American professor, physicist and inventor who is credited with creating and building the world's first liquid-fueled rocket, which he successfully launched on March 16, 1926...

 launched a vehicle using liquid oxygen
Liquid oxygen
Liquid oxygen — abbreviated LOx, LOX or Lox in the aerospace, submarine and gas industries — is one of the physical forms of elemental oxygen.-Physical properties:...

 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 rockets were possible.

After Goddard's success, German engineers and scientists became enthralled with liquid fuel rockets and design better liquid fuel rockets testing them in the early 1930s in a field near Berlin.

Properties

  • Liquid systems enable higher specific impulse
    Specific impulse
    Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...

     (energy content).
  • Lower propellant container mass: In a typical liquid-fuel rocket, tanks hold liquids at low pressure, and pumps force these liquids into a separate high-pressure combustion chamber. In a solid-fuel rocket, by contrast, the propellant container also serves as the combustion chamber, and therefore must withstand high pressures, requiring a more massive structure.
  • Liquid propellant rockets can be throttle
    Throttle
    A throttle is the mechanism by which the flow of a fluid is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases , but usually decreased. The term throttle has come to refer, informally and incorrectly, to any mechanism by which...

    d in realtime; they can also be shut down, and, with a suitable ignition system or self-igniting propellant, restarted.
  • A liquid rocket engine (LRE) can be tested prior to use, whereas for a solid rocket motor a rigorous quality management
    Quality management
    The term Quality management has a specific meaning within many business sectors. This specific definition, which does not aim to assure 'good quality' by the more general definition , can be considered to have four main components: quality planning, quality control, quality assurance and quality...

     must be applied during manufacturing to ensure high reliability.
  • A LRE can be reused for several flights (as in the Space Shuttle
    Space Shuttle
    The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...

    ).



Use of liquid propellants can be associated with a number of issues:
  • Because the propellant is a very large proportion of the mass of the vehicle, the center of mass
    Center of mass
    In physics, the center of mass or barycenter of a system is the average location of all of its mass. In the case of a rigid body, the position of the center of mass is fixed in relation to the body...

     shifts significantly rearward as the propellant is used; one will typically lose control of the vehicle if its center mass gets too close to the center of drag.
  • When operated within an atmosphere, pressurization of the typically very thin-walled propellant tanks must guarantee positive gauge pressure at all times to avoid catastrophic collapse of the tank.
  • Liquid propellants are subject to slosh, which has frequently led to loss of control of the vehicle. This can be controlled with slosh baffles in the tanks as well as judicious control laws in the guidance system
    Guidance system
    A guidance system is a device or group of devices used to navigate a ship, aircraft, missile, rocket, satellite, or other craft. Typically, this refers to a system that navigates without direct or continuous human control...

    .
  • They can suffer from pogo oscillation
    Pogo oscillation
    Pogo oscillation is a potentially dangerous type of self-excited combustion oscillation in liquid fuel rocket engines. This oscillation results in variations of thrust from the engines, causing variations of acceleration on the rocket's structure, giving variations in fuel pressure and flow rate....

     where the rocket suffers from uncommanded cycles of acceleration.
  • Liquid propellants often need ullage motor
    Ullage motor
    Ullage motors are relatively small, independently-fueled rocket engines that may be fired to accelerate the rocket prior to main engine ignition, when the vehicle is in a zero-g situation....

    s in zero-gravity or during staging to avoid sucking gas into engines at start up. They are also subject to vortexing within the tank, particularly towards the end of the burn, which can also result in gas being sucked into the engine or pump.
  • Liquid propellants can leak, especially hydrogen
    Hydrogen
    Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

    , possibly leading to the formation of an explosive mixture.
  • Turbopumps to pump liquid propellants are complex to design, and can suffer serious failure modes, such as overspeeding if they run dry or shedding fragments at high speed if metal particles from the manufacturing process enter the pump.
  • Cryogenic propellants, such as liquid oxygen, freeze atmospheric water vapour into very hard crystals. This can damage or block seals and valves and can cause leaks and other failures. Avoiding this problem often requires lengthy chilldown procedures which attempt to remove as much of the vapour from the system as possible. Ice can also form on the outside of the tank, and later fall and damage the vehicle. External foam insulation can cause issues as shown by the Space Shuttle Columbia disaster
    Space Shuttle Columbia disaster
    The Space Shuttle Columbia disaster occurred on February 1, 2003, when shortly before it was scheduled to conclude its 28th mission, STS-107, the Space Shuttle Columbia disintegrated over Texas and Louisiana during re-entry into the Earth's atmosphere, resulting in the death of all seven crew members...

    . Non-cryogenic propellants do not cause such problems.
  • Non-storable liquid rockets require considerable preparation immediately before launch. This makes them less practical than solid rocket
    Solid rocket
    A solid rocket or a solid-fuel rocket is a rocket engine that uses solid propellants . The earliest rockets were solid-fuel rockets powered by gunpowder; they were used by the Chinese in warfare as early as the 13th century and later by the Mongols, Arabs, and Indians.All rockets used some form of...

    s for most weapon systems.

Propellants



Thousands of combinations of fuels and oxidizers have been tried over the years. Some of the more common and practical ones are:
  • liquid oxygen (LOX
    Lox
    Lox is salmon fillet that has been cured. In its most popular form, it is thinly sliced—less than in thickness—and, typically, served on a bagel, often with cream cheese, onion, tomato, cucumber and capers...

    , O2) and liquid hydrogen
    Hydrogen
    Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

     (LH2, H2) – Space Shuttle
    Space Shuttle
    The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...

     main engines, Ariane 5
    Ariane 5
    Ariane 5 is, as a part of Ariane rocket family, an expendable launch system used to deliver payloads into geostationary transfer orbit or low Earth orbit . Ariane 5 rockets are manufactured under the authority of the European Space Agency and the Centre National d'Etudes Spatiales...

     main stage and the Ariane 5 ECA second stage, the first stage of the Delta IV
    Delta IV rocket
    Delta IV is an active expendable launch system in the Delta rocket family. Delta IV uses rockets designed by Boeing's Integrated Defense Systems division and built in the United Launch Alliance facility in Decatur, Alabama. Final assembly is completed at the launch site by ULA...

    , the upper stages of the Ares I
    Ares I
    Ares I was the crew launch vehicle that was being developed by NASA as part of the Constellation Program. The name "Ares" refers to the Greek deity Ares, who is identified with the Roman god Mars...

    , Saturn V, Saturn IB, and Saturn I as well as Centaur
    Centaur (rocket stage)
    Centaur is a rocket stage designed for use as the upper stage of space launch vehicles. Centaur boosts its satellite payload to geosynchronous orbit or, in the case of an interplanetary space probe, to or near to escape velocity...

     rocket stage, the first stage and second stage of the H-II
    H-II
    The H-II rocket was a Japanese satellite launch system, which flew seven times between 1994 and 1999, with five successes. It was developed by NASDA in order to give Japan a capability to launch larger satellites in the 1990s. It was the first two-stage liquid-fuelled rocket Japan made using only...

    , H-IIA
    H-IIA
    H-IIA is an active expendable launch system operated by Mitsubishi Heavy Industries for the Japan Aerospace Exploration Agency . The liquid-fueled H-IIA rockets have been used to launch satellites into geostationary orbit, to launch a lunar orbiting spacecraft, and to launch an interplanetary...

    , H-IIB
    H-IIB
    H-IIB is an expendable launch system used to launch H-II Transfer Vehicles towards the International Space Station. H-IIB rockets are liquid-fuelled with solid-fuel strap-on boosters and are launched from the Tanegashima Space Center in Japan...

  • liquid oxygen (LOX) and kerosene
    Kerosene
    Kerosene, sometimes spelled kerosine in scientific and industrial usage, also known as paraffin or paraffin oil in the United Kingdom, Hong Kong, Ireland and South Africa, is a combustible hydrocarbon liquid. The name is derived from Greek keros...

     or RP-1
    RP-1
    RP-1 is a highly refined form of kerosene outwardly similar to jet fuel, used as a rocket fuel. Although having a lower specific impulse than liquid hydrogen , RP-1 is cheaper, can be stored at room temperature, is far less of an explosive hazard and is far denser...

     – Saturn V, Zenit rocket
    Zenit rocket
    Zenit is a family of space launch vehicles designed by the Yuzhnoye Design Bureau of Ukraine. Zenit was built in the 1980s for two purposes: as a liquid rocket booster for the Energia rocket and, equipped with a second stage, as a stand-alone rocket...

    , R-7 Semyorka
    R-7 Semyorka
    The R-7 was a Soviet missile developed during the Cold War, and the world's first intercontinental ballistic missile. The R-7 made 28 launches between 1957 and 1961, but was never deployed operationally. A derivative, the R-7A, was deployed from 1960 to 1968...

     family of Soviet
    Soviet Union
    The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....

     boosters which includes Soyuz, Delta
    Delta rocket
    Delta is a versatile family of expendable launch systems that has provided space launch capability in the United States since 1960. There have been more than 300 Delta rockets launched, with a 95 percent success rate. Two Delta launch systems – Delta II and Delta IV – are in active use...

    , Saturn I, and Saturn IB first stages, Titan I
    Titan (rocket family)
    Titan was a family of U.S. expendable rockets used between 1959 and 2005. A total of 368 rockets of this family were launched, including all the Project Gemini manned flights of the mid-1960s...

     and Atlas rockets
  • liquid oxygen (LOX) and alcohol (ethanol
    Ethanol
    Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...

    , C2H5OH) – early liquid fueled rockets, like German
    Germany
    Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

     (World War II
    World War II
    World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

    ) A-4, aka V-2, and Redstone
    Redstone (rocket)
    The PGM-11 Redstone was the first large American ballistic missile. A short-range surface-to-surface rocket, it was in active service with the U.S. Army in West Germany from June 1958 to June 1964 as part of NATO's Cold War defense of Western Europe...

  • liquid oxygen (LOX) and gasoline
    Gasoline
    Gasoline , or petrol , is a toxic, translucent, petroleum-derived liquid that is primarily used as a fuel in internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. Some gasolines also contain...

     – Robert Goddard's first liquid-fuel rocket
  • T-Stoff
    T-Stoff
    T-Stoff was the oxidizer part of a bipropellant rocket fuel combination used in Germany during World War II. It is a stabilized high test peroxide...

     (80% hydrogen peroxide, H2O2 as the oxidizer) and C-Stoff
    C-Stoff
    C-Stoff was a reductant used in bipropellant rocket fuels developed by Hellmuth Walter Kommanditgesellschaft in Germany during World War 2...

     (methanol, CH3OH, and hydrazine hydrate, N2H4n(H2O as the fuel) – Walter Werke HWK 109-509 engine used on Messerschmitt Me 163
    Messerschmitt Me 163
    The Messerschmitt Me 163 Komet, designed by Alexander Lippisch, was a German rocket-powered fighter aircraft. It is the only rocket-powered fighter aircraft ever to have been operational. Its design was revolutionary, and the Me 163 was capable of performance unrivaled at the time. Messerschmitt...

    B Komet a rocket fighterplane of (World War II
    World War II
    World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

    )
  • nitric acid
    Nitric acid
    Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...

     (HNO3) and kerosene – Soviet
    Soviet Union
    The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....

     Scud
    Scud
    Scud is a series of tactical ballistic missiles developed by the Soviet Union during the Cold War, and exported widely to other countries. The term comes from the NATO reporting name SS-1 Scud which was attached to the missile by Western intelligence agencies...

    -A, aka SS-1
  • inhibited red fuming nitric acid (IRFNA, HNO3 + N2O4) and unsymmetric dimethyl hydrazine (UDMH
    UDMH
    Unsymmetrical dimethylhydrazine is a toxic volatile hygroscopic clear liquid, with a sharp, fishy, ammoniacal smell typical for organic amines. It turns yellowish on exposure to air and absorbs oxygen and carbon dioxide. It mixes completely with water, ethanol, and kerosene. In concentration...

    , (CH3)2N2H2) – Soviet Scud
    Scud
    Scud is a series of tactical ballistic missiles developed by the Soviet Union during the Cold War, and exported widely to other countries. The term comes from the NATO reporting name SS-1 Scud which was attached to the missile by Western intelligence agencies...

    -C, aka SS-1-c,-d,-e
  • nitric acid 73% with dinitrogen tetroxide
    Dinitrogen tetroxide
    Dinitrogen tetroxide is the chemical compound N2O4. It is a useful reagent in chemical synthesis. It forms an equilibrium mixture with nitrogen dioxide; some call this mixture dinitrogen tetroxide, while some call it nitrogen dioxide.Dinitrogen tetroxide is a powerful oxidizer, making it highly...

     27% (=AK27) and kerosene/gasoline mixture (=TM-185) – various Russian (USSR) cold-war ballistic missiles (R-12
    R-12
    The R-12 Dvina was a theatre ballistic missile developed and deployed by the Soviet Union during the Cold War. Its GRAU designation was 8K63, and it was given the NATO reporting name SS-4 Sandal...

    , Scud
    Scud
    Scud is a series of tactical ballistic missiles developed by the Soviet Union during the Cold War, and exported widely to other countries. The term comes from the NATO reporting name SS-1 Scud which was attached to the missile by Western intelligence agencies...

    -B,-D), Iran
    Iran
    Iran , officially the Islamic Republic of Iran , is a country in Southern and Western Asia. The name "Iran" has been in use natively since the Sassanian era and came into use internationally in 1935, before which the country was known to the Western world as Persia...

    : Shahab
    Shahab
    Shahab may refer to:* a male given name in Arabic and also Persian, meaning meteor or shooting star, or a bright star that illuminates the heavens* Shahab F.C., Iranian football club in Semnan...

    -5, North Korea
    North Korea
    The Democratic People’s Republic of Korea , , is a country in East Asia, occupying the northern half of the Korean Peninsula. Its capital and largest city is Pyongyang. The Korean Demilitarized Zone serves as the buffer zone between North Korea and South Korea...

    : Taepodong-2
    Taepodong-2
    The Taepodong-2 is a designation used to indicate a North Korean two or three-stage ballistic missile design that is the successor to the Taepodong-1.-Details:...

  • hydrogen peroxide and kerosene – UK
    United Kingdom
    The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

     (1970s) Black Arrow
    Black Arrow
    Black Arrow, officially capitalised BLACK ARROW, was a British satellite carrier rocket. Developed during the 1960s, it was used for four launches between 1969 and 1971...

    , USA
    United States
    The United States of America is a federal constitutional republic comprising fifty states and a federal district...

     Development (or study): BA-3200
  • hydrazine
    Hydrazine
    Hydrazine is an inorganic compound with the formula N2H4. It is a colourless flammable liquid with an ammonia-like odor. Hydrazine is highly toxic and dangerously unstable unless handled in solution. Approximately 260,000 tons are manufactured annually...

     (N2H4) and red fuming nitric acid – Nike Ajax Antiaircraft Rocket
  • Aerozine 50
    Aerozine 50
    Aerozine 50 is a 50/50 mix of hydrazine and unsymmetrical dimethylhydrazine . Originally developed in the late 1950s by Aerojet General Corporation as a storable, high-energy, hypergolic fuel for the Titan II ICBM rocket engines, Aerozine continues in wide use as a rocket fuel, typically with...

     and dinitrogen tetroxide – Titans 2–4
    Titan (rocket family)
    Titan was a family of U.S. expendable rockets used between 1959 and 2005. A total of 368 rockets of this family were launched, including all the Project Gemini manned flights of the mid-1960s...

    , Apollo lunar module, Apollo service module
    Service module
    A service module is a spacecraft compartment containing a variety of support systems used for spacecraft operations. Usually located in the uninhabited area of the spacecraft, the service module is jettisoned upon the completion of the mission, and usually burns up during atmospheric reentry...

    , interplanatary probes (Such as Voyager 1
    Voyager 1
    The Voyager 1 spacecraft is a 722-kilogram space probe launched by NASA in 1977, to study the outer Solar System and eventually interstellar space. Operating for as of today , the spacecraft receives routine commands and transmits data back to the Deep Space Network. At a distance of as of...

     and Voyager 2
    Voyager 2
    The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...

    )
  • unsymmetric dimethylhydrazine (UDMH) and dinitrogen tetroxide – Proton rocket
    Proton rocket
    Proton is an expendable launch system used for both commercial and Russian government space launches. The first Proton rocket was launched in 1965 and the launch system is still in use as of 2011, which makes it one of the most successful heavy boosters in the history of spaceflight...

     and various Soviet rockets
  • monomethylhydrazine
    Monomethylhydrazine
    Monomethylhydrazine is a volatile hydrazine chemical with the chemical formula CH3 NH2. It is used as a rocket propellant in bipropellant rocket engines because it is hypergolic with various oxidizers such as nitrogen tetroxide and nitric acid...

     (MMH, (CH3)HN2H2) and dinitrogen tetroxide – Space Shuttle orbiter
    Space Shuttle Orbiter
    The Space Shuttle orbiter was the orbital spacecraft of the Space Shuttle program operated by NASA, the space agency of the United States. The orbiter was a reusable winged "space-plane", a mixture of rockets, spacecraft, and aircraft...

    's Orbital maneuvering system (OMS) engines and Reaction control system
    Reaction control system
    A reaction control system is a subsystem of a spacecraft whose purpose is attitude control and steering by the use of thrusters. An RCS system is capable of providing small amounts of thrust in any desired direction or combination of directions. An RCS is also capable of providing torque to allow...

     (RCS) thrusters.


One of the most efficient mixtures, oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 and hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

, suffers from the extremely low temperatures required for storing hydrogen and oxygen as liquids (around 20 K or −253 °C)) and low fuel density (70 kg/m³), necessitating large and heavy tanks. The use of lightweight foam to insulate the cryogenic tanks caused problems for the Space Shuttle Columbia
Space Shuttle Columbia
Space Shuttle Columbia was the first spaceworthy Space Shuttle in NASA's orbital fleet. First launched on the STS-1 mission, the first of the Space Shuttle program, it completed 27 missions before being destroyed during re-entry on February 1, 2003 near the end of its 28th, STS-107. All seven crew...

's STS-107
STS-107
-Mission parameters:*Mass:**Orbiter Liftoff: **Orbiter Landing: **Payload: *Perigee: *Apogee: *Inclination: 39.0°*Period: 90.1 min- Insignia :...

 mission, as a piece broke loose, damaged its wing and caused it to break up and be destroyed on atmospheric reentry
Atmospheric reentry
Atmospheric entry is the movement of human-made or natural objects as they enter the atmosphere of a celestial body from outer space—in the case of Earth from an altitude above the Kármán Line,...

.

For storable ICBMs and interplanetary spacecraft, storing cryogenic propellants over extended periods is awkward and expensive. Because of this, mixtures of hydrazine
Hydrazine
Hydrazine is an inorganic compound with the formula N2H4. It is a colourless flammable liquid with an ammonia-like odor. Hydrazine is highly toxic and dangerously unstable unless handled in solution. Approximately 260,000 tons are manufactured annually...

 and its derivatives in combination with nitrogen oxides are generally used for such rockets. Hydrazine has its own disadvantages, being a very caustic and volatile chemical as well as being toxic and carcinogenic. Consequently, hybrid rocket
Hybrid rocket
A hybrid rocket is a rocket with a rocket motor which uses propellants in two different states of matter - one solid and the other either gas or liquid. The Hybrid rocket concept can be traced back at least 75 years....

s have recently been the vehicle of choice for low-budget private and academic developments in aerospace technology. Also the RP-1
RP-1
RP-1 is a highly refined form of kerosene outwardly similar to jet fuel, used as a rocket fuel. Although having a lower specific impulse than liquid hydrogen , RP-1 is cheaper, can be stored at room temperature, is far less of an explosive hazard and is far denser...

/LOX
Lox
Lox is salmon fillet that has been cured. In its most popular form, it is thinly sliced—less than in thickness—and, typically, served on a bagel, often with cream cheese, onion, tomato, cucumber and capers...

 combination has become a popular choice for reliable and cost-sensitive commercial spaceflight applications.

Injectors


The injector implementation in liquid rockets determines the percentage of the theoretical performance of the nozzle that can be achieved. A poor injector performance causes unburnt propellant to leave the engine, giving extremely poor efficiency.

Additionally, injectors are also usually key in reducing thermal loads on the nozzle; by increasing the proportion of fuel around the edge of the chamber, this gives much lower temperatures on the walls of the nozzle.

Types of injectors


Injectors can be as simple as a number of small diameter holes arranged in carefully constructed patterns through which the fuel and oxidiser travel. The speed of the flow is determined by the square root of the pressure drop across the injectors, the shape of the hole and other details such as the density of the propellant.

The first injectors used on the V-2 created parallel jets of fuel and oxidizer which then combusted in the chamber. This gave quite poor efficiency.

Injectors today classically consist of a number of small holes which aim jets of fuel and oxidiser so that they collide at a point in space a short distance away from the injector plate. This helps to break the flow up into small droplets that burn more easily.

The main type of injectors are
  • Shower Head type
  • Self Impinging doublet type
  • Cross impinging triplet type
  • Centrifugal or Swirling type

Other injector types include the pintle injector, which potentially permits good mixture control over a wide range of flow rates. The pintle injector was used on the Apollo Lunar Module
Apollo Lunar Module
The Apollo Lunar Module was the lander portion of the Apollo spacecraft built for the US Apollo program by Grumman to carry a crew of two from lunar orbit to the surface and back...

 engines and the Merlin
Merlin (rocket engine)
Merlin is a rocket engine developed by SpaceX for use on its Falcon 1 and Falcon 9 rockets. Merlin uses RP-1 and liquid oxygen as propellants in a gas-generator power cycle...

 and Kestrel
Kestrel (rocket engine)
The Kestrel engine is an LOX/RP-1 pressure-fed rocket engine. The Kestrel engine was developed by SpaceX for upper stage use on the Falcon 1 rocket....

 engines designed by SpaceX
SpaceX
Space Exploration Technologies Corporation, or more popularly and informally known as SpaceX, is an American space transport company that operates out of Hawthorne, California...

.

The Space Shuttle Main Engine
Space Shuttle main engine
The RS-25, otherwise known as the Space Shuttle Main Engine , is a reusable liquid-fuel rocket engine built by Pratt & Whitney Rocketdyne for the Space Shuttle, running on liquid hydrogen and oxygen. Each Space Shuttle was propelled by three SSMEs mated to one powerhead...

 uses a system of fluted posts, which use heated hydrogen from the preburner to vaporize the liquid oxygen flowing through the center of the posts and this improves the rate and stability of the combustion process; previous engines such as the F-1 used for the Apollo program had significant issues with oscillations that led to destruction of the engines, but this was not a problem in the SSME due to this design detail.

Valentin Glushko
Valentin Glushko
Valentin Petrovich Glushko or Valentyn Petrovych Hlushko was a Soviet engineer, and the principal Soviet designer of rocket engines during the Soviet/American Space Race.-Biography:...

 invented the centrifugal injector in the early 1930s, and it has been almost universally used in Russian engines. Rotational motion is applied to the liquid (and sometimes the two propellants are mixed), then it is expelled through a small hole, where it forms a cone-shaped sheet that rapidly atomizes. Goddard's first liquid fuel engine used a single impinging injector. German scientists in WWII experimented with impinging injectors on flat plates, used successfully in the Wasserfall missile.

Combustion stability


To avoid instabilities such as chugging which is a relatively low speed oscillation the engine must be designed with enough pressure drop across the injectors to render the flow largely independent of the chamber pressure. This is normally achieved by using at least 20% of the chamber pressure across the injectors.

Nevertheless, particularly in larger engines, a high speed combustion oscillation is easily triggered, and these are not well understood. These high speed oscillations tend to disrupt the gas side boundary layer of the engine, and this can cause the cooling system to rapidly fail, destroying the engine. These kinds of oscillations are much more common on large engines, and plagued the development of the Saturn V
Saturn V
The Saturn V was an American human-rated expendable rocket used by NASA's Apollo and Skylab programs from 1967 until 1973. A multistage liquid-fueled launch vehicle, NASA launched 13 Saturn Vs from the Kennedy Space Center, Florida with no loss of crew or payload...

, but were finally overcome.

Some combustion chambers, such as the SSME
Space Shuttle main engine
The RS-25, otherwise known as the Space Shuttle Main Engine , is a reusable liquid-fuel rocket engine built by Pratt & Whitney Rocketdyne for the Space Shuttle, running on liquid hydrogen and oxygen. Each Space Shuttle was propelled by three SSMEs mated to one powerhead...

 uses Helmholtz resonators as damping mechanisms to stop particular resonant frequencies from growing.

To prevent these issues the SSME injector design instead went to a lot of effort to vapourise the propellant prior to injection into the combustion chamber. Although many other features were used to ensure that instabilities could not occur, later research showed that these other features were unnecessary, and the gas phase combustion worked reliably.

Testing for stability often involves the use of small explosives. These are detonated within the chamber during operation, and causes an impulsive excitation. By examining the pressure trace of the chamber to determine how quickly the effects of the disturbance die away, it is possible to estimate the stability and redesign features of the chamber if required.

Engine cycles


For liquid propellant rockets four different ways of powering the injection of the propellant into the chamber are in common use.

Generally speaking, pumping losses are small compared to the heat energy lost in the nozzle. For atmospheric use, high pressure engine cycles are desirable as it improves the efficiency of the nozzle. For vacuum use, pumps aren't usually required.
  • pressure fed cycle
    Pressure-fed cycle (rocket)
    The pressure-fed cycle is a class of rocket engine designs. A separate gas supply, usually helium, pressurizes the propellant tanks to force fuel and oxidizer to the combustion chamber. To maintain adequate flow, the tank pressures must exceed the combustion chamber pressure.Pressure fed engines...

     – the propellants are forced in from pressurised (relatively heavy) tanks. The heavy tanks mean that a relatively low pressure is optimal. The pressurant used is frequently helium due to its lack of reactivity.
  • expander cycle – cryogenic fuel is used to cool the walls of the combustion chamber and nozzle. Absorbed heat vaporizes and expands the fuel which is then used to drive the turbopumps before it enters the combustion chamber. No heat or propellant is lost, so efficiency is very high. Pump power and combustion pressure are constrained by available heat transfer.
  • gas generator cycle
    Gas-generator cycle (rocket)
    The gas generator cycle is a power cycle of a bipropellant rocket engine. Some of the propellant is burned in a gas-generator and the resulting hot gas is used to power the engine's pumps. The gas is then exhausted...

     – a small percentage of the propellants are burnt in a preburner to power a turbopump and then exhausted through a separate nozzle, or low down on the main one. This usually gives a small reduction in performance.
  • staged combustion cycle
    Staged combustion cycle (rocket)
    The staged combustion cycle, also called topping cycle or pre-burner cycle, is a thermodynamic cycle of bipropellant rocket engines. Some of the propellant is burned in a pre-burner and the resulting hot gas is used to power the engine's turbines and pumps...

     – the high pressure outlet from the turbopump is fed back to power a burner which then powers the turbopump in a self starting cycle. The still high pressure exhaust from the turbine is then fed directly into the main chamber, thus essentially all the energy goes through the nozzle, giving no pumping losses at all, and permitting very high pressures.

Cooling



Injectors are commonly laid out so that a fuel-rich layer is created at the combustion chamber wall. This reduces the temperature there, and downstream to the throat and even into the nozzle and permits the combustion chamber to be run at higher pressure, which permits a higher expansion ratio nozzle to be used which gives a higher ISP and better system performance.
A liquid rocket engine often employs regenerative cooling
Regenerative cooling
Regenerative cooling in rockets is where some or all of the propellant is passed through tubes, channels or otherwise in a jacket around the combustion chamber or nozzle to cool the engine because the fuel in particular and sometimes the oxidizer are good coolants...

, which uses the fuel or the oxidiser to cool the chamber and nozzle.

Ignition


Ignition can be performed in many ways, but perhaps more so with liquid propellants than other rockets a consistent and significant ignitions source is required; a delay of ignition (in some cases as small as) a few tens of milliseconds can cause overpressure of the chamber due to excess propellant. A hard start
Hard start
A hard start is a rocketry term referring to an overpressure condition during start of a rocket engine at ignition. In the worst cases this takes the form of an explosion.-Rocket ignition:...

 can even cause an engine to explode.

Generally, ignition systems try to apply flames across the injector surface, with a mass flow of approximately 1% of the full mass flow of the chamber.

Safety interlocks are sometimes used to ensure the presence of an ignition source before the main valves open; however reliability of the interlocks can in some cases be lower than the ignition system. Thus it depends on whether the system must fail safe, or whether overall mission success is more important. Interlocks are rarely used for upper, unmanned stages where failure of the interlock would cause loss of mission, but are present on the SSME, to shut the engines down prior to liftoff of the Space Shuttle. In addition, detection of successful ignition of the igniter is surprisingly difficult, some systems use thin wires that are cut by the flames, pressure sensors have also seen some use.

Methods of ignition include pyrotechnic, electrical (spark or hot wire), and chemical. Hypergolic propellants have the advantage of self igniting, reliably and with less chance of hard starts. In the 1940s, the Russians began to start engines with hypergolic fuel, then switch over to the primary propellants after ignition. This was also used on the American F-1 rocket engine on the Apollo program.

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