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Rocket propellants undergo exothermic chemical reactions which produce hot gas which is used by a rocket for propulsive purposes. Image File history File links Question_book-3. ... For other uses, see Chemical reaction (disambiguation). ... This article is about vehicles powered by rocket engines. ... A remote camera captures a close-up view of a Space Shuttle Main Engine during a test firing at the John C. Stennis Space Center in Hancock County, Mississippi Spacecraft propulsion is any method used to change the velocity of spacecraft and artificial satellites. ...

Overview

Rockets create thrust by expeling mass backwards in a high speed jet (see Newton's Third Law ). Chemical rockets, the subject of this article, create thrust by reacting propellants into very hot gas, which then expands and accelerates within a nozzle out the back. The amount of the resulting forward force, known as thrust, that is produced is the mass flow rate of the propellants multiplied by their exhaust velocity (relative to the rocket), as specified by Newton's third law of motion. Thrust is therefore the equal and opposite reaction that moves the rocket, and not any interaction of the exhaust stream with air around the rocket (but see base bleed). Equivalently, one can think of a rocket being accelerated upwards by the pressure of the combusting gases in the combustion chamber and nozzle. This operational principle stands in contrast to the commonly held assumption that a rocket "pushes" against the air behind or below it. Rockets in fact perform better in space (where there is in theory nothing behind or beneath them to push against), because they do not need to overcome air resistance and atmospheric pressure on the outside of the nozzle. For other uses, see Mass (disambiguation). ... Newtons First and Second laws, in Latin, from the original 1687 edition of the Principia Mathematica. ... For other uses, see Gas (disambiguation). ... Mass flow rate is the movement of mass per time. ... Sir Isaac Newton FRS (4 January 1643 – 31 March 1727) [ OS: 25 December 1642 – 20 March 1727][1] was an English physicist, mathematician, astronomer, natural philosopher, and alchemist. ... Diagram of a base bleed unit. ... For a solid object moving through a fluid or gas, drag is the sum of all the aerodynamic or hydrodynamic forces in the direction of the external fluid flow. ...

The maximum velocity that a rocket can attain in the absence of any external forces is primarily a function of its mass ratio and its exhaust velocity. The relationship is described by the rocket equation: V_f = V_eln(M₀ / M_f). The mass ratio is just a way to express what proportion of the rocket is fuel when it starts accelerating. Typically, a single-stage rocket might have a mass fraction of 90% propellant, which is a mass ratio of 1/(1-0.9) = 10. The exhaust velocity is often reported as specific impulse. In aerospace engineering, mass ratio is a measure of the proportion of a rocket that is propellant. ... Tsiolkovskys rocket equation, named after Konstantin Tsiolkovsky who first derived it, considers the principle of a rocket: a device that can apply an acceleration to itself (a thrust) by expelling part of its mass with high speed in the opposite direction, due to the conservation of momentum. ... A single-stage to orbit (or SSTO) vehicle would be a vehicle that could reach orbital velocity without the use of multiple stages. ... In aerospace engineering, the mass fraction is an important measure of a rockets efficiency. ... Specific impulse (usually abbreviated Isp) is a way to describe the efficiency of rocket and jet engines. ...

The first stage will usually use high-density (low volume) propellants to reduce the area exposed to atmospheric drag and because of the lighter tankage and higher thrust/weight ratios. Thus, the Apollo-Saturn V first stage used kerosene-liquid oxygen rather than the liquid hydrogen-liquid oxygen used on the upper stages (hydrogen is highly energetic per kilogram, but not per cubic metre). Similarly, the Space Shuttle uses high-thrust, high-density SRBs for its lift-off with the liquid hydrogen-liquid oxygen SSMEs used partly for lift-off but primarily for orbital insertion. Atmospheric drag is a form of drag, which is the force that opposes an object moving through a liquid or gas. ... For the moon designated Saturn V, see Rhea. ... Kerosene or kerosine, also called paraffin oil or paraffin in British usage (not to be confused with the waxy solid also called paraffin wax or just paraffin) is a flammable hydrocarbon liquid. ... This article does not cite any references or sources. ... Liquid hydrogen is the liquid state of the element hydrogen. ... NASAs Space Shuttle, officially called Space Transportation System (STS), is the United States governments current manned launch vehicle. ... See also: SRB Srb is a litle town located in the southeastern part of Lika, Croatia. ... Space Shuttle Main Engine block The Space Shuttle orbiter has three main engines. ...

There are three main types of propellants: solid, liquid, and hybrid.

Solid propellants

Main article: Solid-fuel rocket

The earliest rockets were created hundreds of years ago by the Chinese, and were used primarily for fireworks displays and as weapons. They were fueled with black powder, a type of gunpowder consisting of a mixture of charcoal, sulfur and potassium nitrate (saltpeter). Rocket propellant technology did not advance until the end of the 19th century, by which time smokeless powder had been developed, originally for use in firearms and artillery pieces. Smokeless powders and related compounds have seen use as double-base propellants. The Space Shuttle Columbia is initially launched with the help of solid-fuel boosters A solid rocket or a solid fuel rocket is a rocket with a motor that uses solid propellants (fuel/oxidizer). ... For other uses, see Fireworks (disambiguation). ... For other uses, see Weapon (disambiguation). ... Black powder was the original gunpowder and practically the only known propellant and explosive until the middle of the 19th century. ... Smokeless powder Gunpowder is a pyrotechnic composition, an explosive mixture of sulfur, charcoal and potassium nitrate that burns rapidly, producing volumes of hot gas which can be used as a propellant in firearms and fireworks. ... Charcoal is the blackish residue consisting of impure carbon obtained by removing water and other volatile constituents from animal and vegetation substances. ... This article is about the chemical element. ... R-phrases   S-phrases   Supplementary data page Structure and properties n, εr, etc. ... Salt peter( a. ... Smokeless powder Smokeless powder is the name given to a number of gunpowder-like propellants used in firearms which produce negligible smoke when fired, unlike the older black powder which it replaced. ...

Solid propellants (and almost all rocket propellants) consist of an oxidizer and a fuel. In the case of gunpowder, the fuel is charcoal, the oxidizer is potassium nitrate, and sulfur serves as a catalyst. (Note: sulfur is not a true catalyst in gunpowder as it is consumed to a great extent into a variety of reaction products such as K₂S. The sulfur acts mainly as a sensitizer lowering threshold of ignition.) During the 1950s and 60s researchers in the United States developed what is now the standard high-energy solid rocket fuel. The mixture is primarily ammonium perchlorate powder (an oxidizer), combined with fine aluminium powder (a fuel), held together in a base of PBAN or HTPB (rubber-like fuels). The mixture is formed as a liquid, and then cast into the correct shape and cured into a rubbery solid. Solid fueled rockets are much easier to store and handle than liquid fueled rockets, which makes them ideal for military applications. In the 1970s and 1980s the U.S. switched entirely to solid-fuelled ICBMs: the LGM-30 Minuteman and LG-118A Peacekeeper (MX). In the 1980s and 1990s, the USSR/Russia also deployed solid-fuelled ICBMs (RT-23, RT-2PM, and RT-2UTTH), but retains two liquid-fuelled ICBMs (R-36 and UR-100N). All solid-fuelled ICBMs on both sides have three initial solid stages and a precision maneuverable liquid-fuelled bus used to fine tune the trajectory of the reentry vehicle. An oxidizing agent is a substance that oxidizes another substance in electrochemistry or redox chemical reactions in general. ... Ammonium perchlorate is a chemical compound with the formula NH4ClO4. ... Aluminum redirects here. ... PBAN - Polybutadiene Acrylonitrile copolymer. ... Hydroxy-terminated polybutadiene (HTPB), a butadiene, is a stable and easily stored synthetic rubber, often used in tire manufacturing. ... The LGM-30 Minuteman is a United States nuclear missile, a land-based intercontinental ballistic missile (ICBM). ... Test launch of a Peacekeeper ICBM by the 576 Flight Test Squadron, Vandenberg AFB, CA (USAF) The LG-118A Peacekeeper was a land-based ICBM deployed by the United States starting in 1986. ... The R-36 (Russian: ) is a family of intercontinental ballistic missiles (ICBMs) and space launch vehicles designed by the Soviet Union during the Cold War. ... The UR-100N is an intercontinental ballistic missile in service with Russia. ...

Their simplicity also makes solid rockets a good choice whenever large amounts of thrust are needed and cost is an issue. The Space Shuttle and many other orbital launch vehicles use solid fuelled rockets in their first stages (solid rocket boosters) for this reason. NASAs Space Shuttle, officially called Space Transportation System (STS), is the United States governments current manned launch vehicle. ... A Saturn V launch vehicle sends Apollo 15 on its way to the moon. ... NASA Image of the final solid rocket booster (right) being mated to a Delta II rocket (blue). ...

However, solid rockets have a number of disadvantages relative to liquid fuel rockets. Solid rockets have a lower specific impulse than liquid fueled rockets. It is also difficult to build a large mass ratio solid rocket because almost the entire rocket is the combustion chamber, and must be built to withstand the high combustion pressures. If a solid rocket is used to go all the way to orbit, the payload fraction is very small. (For example, the Orbital Sciences Pegasus rocket is an air-launched three-stage solid rocket orbital booster. Launch mass is 23,130 kg, low earth orbit payload is 443 kg, for a payload fraction of 1.9%. Compare to a Delta IV Medium, 249,500 kg, payload 8600 kg, payload fraction 3.4% without air-launch assistance.) Specific impulse (usually abbreviated Isp) is a way to describe the efficiency of rocket and jet engines. ... Categories: Stub ... Pegasus rocket on the ground Pegasus rocket attached to bottom of carrier aircraft The Pegasus rocket is a winged space booster developed by Orbital Sciences Corporation (Orbital). ...

Another drawback to solid rockets is the difficulty in throttling or shutting them down before they run out of fuel. Essentially, the burning grain must be vented to lower the chamber pressure. Venting generally involves destroying the rocket, and is usually only done by a Range Safety Officer if the rocket goes awry. The third stages of the Minuteman and MX rockets have precision shutdown ports which, when opened, reduce the chamber pressure so abruptly that the interior flame is blown out. This allows a more precise trajectory which improves targeting accuracy. In the field of rocketry, a Range Safety Officer has responsibility for the safe transportation, storage and launch of rocket vehicles. ...

Finally, casting very large single-grain rocket motors has proved to be a very tricky business. Defects in the grain can cause explosions during the burn, and these explosions can increase the burning propellant surface enough to cause a runaway pressure increase, until the case fails.

Liquid propellants

Main article: Liquid rocket propellants

Liquid fueled rockets have better specific impulse than solid rockets and are capable of being throttled, shut down, and restarted. Only the combustion chamber of a liquid fueled rocket needs to withstand combustion pressures and temperatures. On vehicles employing turbopumps, the fuel tanks carry very much less pressure and thus can be built far more lightly, permitting a larger mass ratio. For these reasons, most orbital launch vehicles and all first- and second-generation ICBMs use liquid fuels for most of their velocity gain. The highest specific impulse chemical rockets use liquid propellants. ... A turbopump can refer to either of two types of pump. ...

The primary performance advantage of liquid propellants is the oxidizer. Several practical liquid oxidizers (liquid oxygen, nitrogen tetroxide, and hydrogen peroxide) are available which have much better specific impulse than ammonium perchlorate when paired with comparable fuels. This article does not cite any references or sources. ... Nitrogen tetroxide (or Dinitrogen tetroxide) (N2O4) is a hypergolic propellant often used in combination with a hydrazine-based rocket fuel. ... R-phrases , , , , S-phrases , , , ,, , , , Flash point Non-flammable Related Compounds Related compounds Water Ozone Hydrazine Except where noted otherwise, data are given for materials in their standard state (at 25 Â°C, 100 kPa) Infobox disclaimer and references Hydrogen peroxide (H2O2) is a very pale blue liquid which appears colourless in... Ammonium perchlorate is a chemical compound with the formula NH4ClO4. ...

Most liquid propellants are also cheaper than solid propellants. For orbital launchers, the cost savings do not, and historically have not mattered; the cost of fuel is a very small portion of the overall cost of the rocket, even in the case of solid fuel.

The main difficulties with liquid propellants are also with the oxidizers. These are generally at least moderately difficult to store and handle, either due to extreme toxicity (nitric acids), moderately cryogenic (liquid oxygen), or both (liquid fluorine is a perennial favorite of wild-eyed enthusiasts). Several exotic oxidizers have been proposed: liquid ozone (O₃), ClF₃, and ClF₅, all of which are unstable, energetic, and toxic. The chemical compound nitric acid (HNO3), also known as aqua fortis and spirit of nitre, is an aqueous solution of hydrogen nitrate (anhydrous nitric acid). ...

Liquid fuelled rockets also require potentially troublesome valves and seals and thermally stressed combustion chambers, which increase the cost of the rocket. Many employ specially designed turbopumps which raise the cost enormously due to difficult fluid flow patterns that exist within the casings.

Though all the early rocket theorists proposed liquid hydrogen and liquid oxygen as propellants, the first liquid-fuelled rocket, launched by Robert Goddard on March 16, 1926, used gasoline and liquid oxygen. Liquid hydrogen was first used by the engines designed by Pratt and Whitney for the Lockheed CL-400 Suntan reconnaissance aircraft in the mid-1950s. In the mid-1960s, the Centaur and Saturn upper stages were both using liquid hydrogen and liquid oxygen. Robert Goddard Robert Hutchings Goddard (October 5, 1882 – August 10, 1945) was one of the pioneers of modern rocketry. ... Pratt & Whitney is an American owned aircraft engine manufacturer whose products are widely used in both civil and military aircraft. ... Suntan was the code-name of a prototype reconnaissance aircraft program, with the goal of creating a much faster and higher-altitude successor to the U-2, enabled by the use of liquid hydrogen as fuel. ...

The highest specific impulse chemistry ever test-fired in a rocket engine was lithium and fluorine, with hydrogen added to improve the exhaust thermodynamics (making this a tripropellant)^[1]. The combination delivered 542 seconds (5.32 kN·s/kg, 5320 m/s) specific impulse in a vacuum. The impracticality of this chemistry highlights why exotic propellants are not actually used: to make all three components liquids, the hydrogen must be kept below -252 °C (just 21 K) and the lithium must be kept above 180 °C (453 K). Lithium and fluorine are both extremely corrosive, lithium ignites on contact with air, fluorine ignites on contact with most fuels, and hydrogen, while not hypergolic, is an explosive hazard. Fluorine and the hydrogen fluoride (HF) in the exhaust are very toxic, which damages the environment, makes work around the launch pad difficult, and makes getting a launch license that much more difficult. The rocket exhaust is also ionized, which would interfere with radio communication with the rocket. This article is about the chemical element named Lithium. ... Distinguished from fluorene and fluorone. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... A Tripropellant rocket is a form of spacecraft propulsion that uses two fuels and one oxidizer. ... Hypergolic rocket fuels spontaneously ignite when their two components come into contact with each other. ...

The common liquid propellant combinations in use today:

• LOX and kerosene (RP-1). Used for the lower stages of most Russian and Chinese boosters, the first stages of the Saturn V and Atlas V, and all stages of the developmental Falcon 1 and Falcon 9. Very similar to Robert Goddard's first rocket. This combination is widely regarded as the most practical for civilian orbital launchers.

• LOX and liquid hydrogen, used in the Space Shuttle, the Centaur upper stage, the newer Delta IV rocket, and most stages of the European Ariane rockets.

• Nitrogen tetroxide (N₂O₄) and hydrazine (N₂H₄), MMH, or UDMH. Used in military, orbital and deep space rockets, because both liquids are storable for long periods at reasonable temperatures and pressures. This combination is hypergolic, making for attractively simple ignition sequences. The major inconvenience is that these propellants are highly toxic, hence they require careful handling. Hydrazine also decomposes energetically to nitrogen and hydrogen, making it a fairly good monopropellant.

Kerosene or kerosine, also called paraffin oil or paraffin in British usage (not to be confused with the waxy solid also called paraffin wax or just paraffin) is a flammable hydrocarbon liquid. ... RP-1 (alternately, Rocket Propellant-1 or Refined Petroleum-1) is a highly refined form of kerosene outwardly similar to jet fuel, used in the United States as a rocket fuel. ... For the moon designated Saturn V, see Rhea. ... Atlas V is a launch vehicle formerly built by Lockheed Martin and now built by the Lockheed Martin-Boeing joint venture United Launch Alliance in Decatur, Alabama. ... The Falcon 1 is a partially reusable launch system, designed and manufactured by SpaceX, a space-transportation startup company founded by entrepreneur and PayPal founder Elon Musk to provide commercial launch-to-space services. ... The Falcon 9 is an EELV class launch vehicle planned by SpaceX and scheduled to launch in 2008. ... NASAs Space Shuttle, officially called Space Transportation System (STS), is the United States governments current manned launch vehicle. ... It has been suggested that some sections of this article be split into a new article entitled Delta IV launches. ... The Ariane 4 Ariane is a series of a European civilian expendable launch vehicles for space launch use. ... Nitrogen tetroxide (or Dinitrogen tetroxide) (N2O4) is a hypergolic propellant often used in combination with a hydrazine-based rocket fuel. ... Hydrazine is the chemical compound with formula N2H4. ... Monomethylhydrazine (MMH) is a volatile hydrazine with the chemical formula CH3N2H3. ... Unsymmetrical dimethylhydrazine (UDMH) (1,1-Dimethylhydrazine) is a hypergolic rocket fuel ingredient, often used in combination with the oxidiser nitrogen tetroxide. ... Hypergolic rocket fuels spontaneously ignite when their two components come into contact with each other. ... A (usually liquid) rocket propellant that can be used by itself, without the need for a second component. ...

Air Propellants

An air propellant, usually involves some sort of compressed air shooting out through a hole as a jet stream. Air propellants are also, usually used for high speed projectiles. For more information regarding high speed projectiles, see gun, cannon, and projectile. In high speed projectiles, air usually plays an important role as a "pushing force". Nozzles can help control the jet stream.

Hybrid propellants

A hybrid rocket usually has a solid fuel and a liquid or gas oxidizer. The fluid oxidizer can make it possible to throttle and restart the motor just like a liquid fuelled rocket. Hybrid rockets are also cleaner than solid rockets because practical high-performance solid-phase oxidizers all contain chlorine, versus the more benign liquid oxygen or nitrous oxide used in hybrids. Because just one propellant is a fluid, hybrids are simpler than liquid rockets. A hybrid rocket propulsion system comprises propellants of two different states of matter, the most common configuration being a rocket engine composed of a solid propellant lining a combustion chamber into which a liquid or gaseous propellant is injected so as to undergo a strong exothermic reaction to produce hot...

Hybrid motors suffer two major drawbacks. The first, shared with solid rocket motors, is that the casing around the fuel grain must be built to withstand full combustion pressure and often extreme temperatures as well. However, modern composite structures handle this problem well, and when used with nitrous oxide or hydrogen peroxide relatively small percentage of fuel is needed anyway, so the combustion chamber is not especially large. For other uses, see Nitrous oxide (disambiguation). ... R-phrases , , , , S-phrases , , , ,, , , , Flash point Non-flammable Related Compounds Related compounds Water Ozone Hydrazine Except where noted otherwise, data are given for materials in their standard state (at 25 Â°C, 100 kPa) Infobox disclaimer and references Hydrogen peroxide (H2O2) is a very pale blue liquid which appears colourless in...

The primary remaining difficulty with hybrids is with mixing the propellants during the combustion process. In solid propellants, the oxidizer and fuel are mixed in a factory in carefully controlled conditions. Liquid propellants are generally mixed by the injector at the top of the combustion chamber, which directs many small swift-moving streams of fuel and oxidizer into one another. Liquid fuelled rocket injector design has been studied at great length and still resists reliable performance prediction. In a hybrid motor, the mixing happens at the melting or evaporating surface of the fuel. The mixing is not a well-controlled process and generally quite a lot of propellant is left unburned^{[citation needed]}, which limits the efficiency and thus the exhaust velocity of the motor. Additionally, as the burn continues, the hole down the center of the grain (the 'port') widens and the mixture ratio tends to become more oxidiser rich.

There has been much less development of hybrid motors than solid and liquid motors. For military use, ease of handling and maintenance have driven the use of solid rockets. For orbital work, liquid fuels are more efficient than hybrids and most development has concentrated there. There has recently been an increase in hybrid motor development for nonmilitary suborbital work:

• The Reaction Research Society (RRS), although known primarily for their work with liquid rocket propulsion, has a long history of research and development with hybrid rocket propulsion.

• Several universities have recently experimented with hybrid rockets. Brigham Young University, the University of Utah and Utah State University launched a student-designed rocket called Unity IV in 1995 which burned the solid fuel hydroxy-terminated polybutadiene (HTPB) with an oxidizer of gaseous oxygen, and in 2003 launched a larger version which burned HTPB with nitrous oxide.

• Portland State University also launched several hybrid rockets in the early 2000's.

• The Rochester Institute of Technology is currently creating a HTPB hybrid rocket to launch small payloads into space and to several near Earth objects. Its first launch is scheduled for Summer 2007. http://meteor.rit.edu

• Scaled Composites SpaceShipOne, the first private manned spacecraft, is powered by a hybrid rocket burning HTPB with nitrous oxide. The hybrid rocket engine was manufactured by SpaceDev. SpaceDev partially based its motors on experimental data collected from the testing of AMROC's (American Rocket Company) motors at NASA's Stennis Space Center's E1 test stand. Motors ranging from as small as 1000 lbf (4.4 kN) to as large as 250,000 lbf (1.1 MN) thrust were successfully tested. SpaceDev purchased AMROCs assets after the company was shut down for lack of funding.

, Brigham Young University Brigham Young University (BYU), located in Provo, Utah, is the flagship university of The Church of Jesus Christ of Latter-day Saints (LDS or Mormon Church) and is THE university in Utah. ... The University of Utah (also The U or the U of U or the UU), located in Salt Lake City, is the flagship public research university in the state of Utah, and one of 10 institutions that make up the Utah System of Higher Education. ... Utah State University (USU) is a land-grant university whose main campus is located in Logan, Utah. ... Scaled Composites SpaceShipOne SpaceShipOnes patch The Scaled Composites Model 316 SpaceShipOne is an experimental air-launched suborbital spaceplane that uses a hybrid rocket motor. ...

Mixture ratio

The theoretical exhaust velocity of a given propellant chemistry is a function of the energy released per unit of propellant mass (specific energy). Unburned fuel or oxidizer drags down the specific energy. Surprisingly, most rockets run fuel-rich.

The usual explanation for fuel-rich mixtures is that fuel-rich mixtures have lower molecular weight exhaust, which by reducing M supposedly increases the ratio which is approximately equal to the theoretical exhaust velocity. This explanation, though found in some textbooks, is wrong. Fuel-rich mixtures actually have lower theoretical exhaust velocities, because decreases as fast or faster than M.

The nozzle of the rocket converts the thermal energy of the propellants into directed kinetic energy. This conversion happens in a short time, on the order of one millisecond. During the conversion, energy must transfer very quickly from the rotational and vibrational states of the exhaust molecules into translation. Molecules with fewer atoms (like CO and H₂) store less energy in vibration and rotation than molecules with more atoms (like CO₂ and H₂O). These smaller molecules transfer more of their rotational and vibrational energy to translation energy than larger molecules, and the resulting improvement in nozzle efficiency is large enough that real rocket engines improve their actual exhaust velocity by running rich mixtures with somewhat lower theoretical exhaust velocities.

The effect of exhaust molecular weight on nozzle efficiency is most important for nozzles operating near sea level. High expansion rockets operating in a vacuum see a much smaller effect, and so are run less rich. The Saturn-II stage (a LOX/LH₂ rocket) varied its mixture ratio during flight to optimize performance.

LOX/hydrocarbon rockets are run only somewhat rich (O/F mass ratio of 3 rather than stoichiometric of 3.4 to 4), because the energy release per unit mass drops off quickly as the mixture ratio deviates from stoichiometric. LOX/LH₂ rockets are run very rich (O/F mass ratio of 4 rather than stoichiometric 8) because hydrogen is so light that the energy release per unit mass of propellant drops very slowly with extra hydrogen. In fact, LOX/LH₂ rockets are generally limited in how rich they run by the performance penalty of the mass of the extra hydrogen tankage, rather than the mass of the hydrogen itself. Stoichiometry (sometimes called reaction stoichiometry to distinguish it from composition stoichiometry) is the calculation of quantitative (measurable) relationships of the reactants and products in chemical reactions (chemical equations). ...

Another reason for running rich is that off-stoichiometric mixtures burn cooler than stoichiometric mixtures, which makes engine cooling easier. And as most engines are made of metal or carbon, hot oxidizer-rich exhaust is extremely corrosive, where fuel-rich exhaust is less so. American engines have all been fuel-rich. Some Soviet engines have been oxidizer-rich.

Additionally, there is a difference between mixture ratios for optimum specific impulse (I_sp) and optimum thrust. During launch, shortly after takeoff, high thrust is at a premium. This can be achieved at some temporary reduction of Isp by increasing the oxidiser ratio initially, and then transitioning to more fuel-rich mixtures. Since engine size is typically scaled for takeoff thrust this permits reduction of the weight of rocket engine, pipes and pumps and the extra propellant use can be more than compensated by increases of acceleration towards the end of the burn by having a reduced dry mass. Specific impulse (usually abbreviated Isp) is a way to describe the efficiency of rocket and jet engines. ...

Propellant density

Although liquid hydrogen gives a high Isp, its low density is a significant disadvantage: hydrogen occupies about 7x more volume per kilogram than dense fuels such as kerosene. This not only penalises the tankage, but also the pipes and fuel pumps leading from the tank, which need to be 7x bigger and heavier. (The oxidiser side of the engine and tankage is of course unaffected.) This makes the vehicle's dry mass much higher, so the use of liquid hydrogen is not such a big win as might be expected. Indeed, some dense hydrocarbon/LOX propellant combinations have higher performance when the dry mass penalties are included.

Due to lower Isp, dense propellant launch vehicles have a higher the vehicle may well end up cheaper. Liquid hydrogen is quite an expensive fuel to produce and store, and causes many practical difficulties with design and manufacture of the vehicle.

Because of the higher overall weight, a dense-fuelled launch vehicle necessarily requires higher takeoff thrust, but it carries this thrust capability all the way to orbit. This, in combination with the better thrust/weight ratios, means that dense-fuelled vehicles reach orbit earlier, thereby minimizing losses due to gravity drag. Thus, the effective delta-v requirement for these vehicles are reduced. In astrodynamics, gravity drag is inefficiency encountered by a spacecraft thrusting while moving against a gravitational field. ...

However, liquid hydrogen does give clear advantages when the overall mass needs to be minimised; for example the Saturn V vehicle used it on the upper stages; this reduced weight meant that the dense-fuelled first stage could be made proportionately smaller, saving quite a bit of money.

References

1. ^ ARBIT, H. A., CLAPP, S. D., DICKERSON, R. A., NAGAI, C. K., Combustion characteristics of the fluorine-lithium/hydrogen tripropellant combination. AMERICAN INST OF AERONAUTICS AND ASTRONAUTICS, PROPULSION JOINT SPECIALIST CONFERENCE, 4TH, CLEVELAND, OHIO, Jun 10-14, 1968.

See also

• Category: Rocket fuels
• Comparison: Aviation fuel
• Nuclear propulsion
• Ion thruster

An aviation fuel truck. ... Nuclear propulsion can include a wide variety of methods, the commonality of which is the use of some form of nuclear reaction as their primary power source. ... An ion engine test This article focuses on electrostatic ion thrusters - for a more general description, refer to electric propulsion. ...

External links

• NASA page on propellants
• History of solid rocket fuels
• Detailed list of rocket fuels, practical and theoretical
• Rocket Man Short essay by S. Abbas Raza about development of solid rocket fuel at 3 Quarks Daily