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Solid-propellant

Solid-propellant

Solid rockets are rockets with a motor that uses solid propellants (fuel/oxidizer). The Chinese or the Arabs invented solid rockets and were using them in warfare by the 13th century. All rockets used some form of solid or powdered propellant up until the 20th century. Solid rockets are considered to be safe and reliable due to the long engineering history and simple design.

Basic Concepts

20th century A simple solid rocket motor consists of a casing, nozzle, grain (propellant charge), and igniter. The grain behaves like a solid mass, burning in a predictable fashion and producing exhaust gases. The nozzle dimensions are calculated to maintain a design chamber pressure, while producing thrust from the exhaust gases. Once ignited, a solid rocket motor cannot be shut off. Modern designs may also include; steerable nozzle for guidance, avionics, recovery hardware (parachutes), self destruct mechanisms, APU's, and thermal management materials.

Design

Design begins with the total impulse required, this determines the fuel/oxidizer mass. Grain geometry and chemistry are then chosen to satisfy the required motor characteristics. The following are chosen or solved simultaneously. The results are exact dimensions for grain, nozzle and case geometries;
- The grain burns at a predictable rate, given its surface area and chamber pressure.
- The chamber pressure is determined by the nozzle orifice diameter and grain burn rate.
- Allowable chamber pressure is a function of casing design.
- The length of burn time is determined by the grain 'web thickness'. The grain may be bonded to the casing, or not. Case bonded motors are much more difficult to design, since deformation of both the case and grain, under operating conditions, must be compatible. Common modes of failure in solid rocket motors are; fracture of the grain, failure of case bonding, and air pockets in the grain. All of these produce an instantaneous increase in burn surface area, and a corresponding increase in exhaust gas and pressure, and rupture of the casing. Another failure mode is casing seal design. Seals are required in casings that have to be opened to load the grain. Once a seal fails, hot gas will erode the escape path and result in failure. This was the cause of the Space Shuttle Challenger disaster.

Grain

Solid fuel grains are usually molded from a thermoset elastomer (which doubles as fuel), additional fuel, oxidizer, and catalyst. HTPB is commonly used for this purpose. Ammonium perchlorate is the most common oxidizer used today. The fuel is cast in different forms for different purposes. Slow, long burning rockets have a cylinder shaped grain, burning from one end to the other. Most grains, however, are cast with a hollow cross section, burning from the inside out (and outside in, if not case bonded), as well as from the ends. The thrust profile over time can be controlled by grain geometry. For example, a star shaped hole down the center of the grain will have greater initial thrust because of the additional surface area. As the star points are burned up, the surface area and thrust are reduced.

Casing

The casing may be constructed from a range of materials. Cardboard is used for model engines. Steel is used for the space shuttle boosters. Filament wound graphite epoxy casings are used for high performance motors.

Nozzle

A Convergent Divergent design accelerates the exhaust gas out of the nozzle to produce thrust. Sophisticated solid rocket motors use steerable nozzles for rocket control.

Performance

Solid fuel rocket motors have a typical specific impulse of 265 lbf·s/lb (2.6 kN·s/kg). This compares to 285 lbf·s/lb (2.8 kN·s/kg) for kerosene/Lox and ~389 lbf·s/lb (3.8 kN·s/kg) for liquid hydrogen/Lox¹. For this reason solids are generally used as initial stages in a rocket, with better performing liquid engines reserved for final stages. However, the venerable Star line motors manufactured by Thiokol have a long history as the final boost stage for satellites. This is due to their simplicity, compactness and high mass fraction. The ability of solid rockets to remain in storage for long periods, and then reliably launch at a moments notice, makes them the design of choice for military applications.

Amateur rocketry

Solid fuel rockets can be bought for use in model rocketry; they are normally small cylinders of fuel with an integral nozzle and a small charge that is set off when the fuel is exhausted. This charge can be used to ignite a second stage, trigger a camera, or deploy a parachute. Designing solid rocket motors is particularly interesting to amateur rocketry enthusiasts. The design is simple, materials are inexpensive and constructions techniques are safe. Early amateur motors were gunpowder. Later, zinc/sulfur formulations were popular. Typical amateur formulations in use today are; sugar (sucrose, dextrose, and sorbitol are all common)/potassium nitrate, HTPB (a rubber like epoxy)/magnesium/ammonium nitrate, and HTPB or PBAN/aluminum/ammonium perchlorate. Most formulations also include burn rate modifiers and other additives, and also possibly additives designed to create special effects, such as colored flames, thick smoke, or sparks. Amateur rocket builders are very active in hybrid motor research.

Advanced research

- Environmentally sensitive fuel formulations
- Ramjets with solid fuel
- Variable thrust designs based on variable nozzle geometry.
- hybrid rockets that use solid fuel and throttleable liquid or gaseous oxidizer

References

External links

- [http://www.braeunig.us/space/propuls.htm Robert A. Braeunig rocket propulsion page]
- [http://www.astronautix.com/articles/comlants.htm Astronautix Composite Solid Propellants ]
- [http://www.translatorscafe.com/cafe/MegaBBS/thread-view.asp?threadid=5069&messageid=63174]
- [http://www.esa.int/SPECIALS/Launchers_Access_to_Space/ASEDYQI4HNC_0.html Ariane 5 SRB]
- [http://www.tripoli.org/ Amateur High Power Rocketry Association]
- [http://www.nakka-rocketry.net/ Nakka-Rocketry (Design Calculations and Propellent Formulations)] Category:Spacecraft propulsion Category:Rocket fuels

Rocket

A rocket is a vehicle, missile or aircraft which obtains thrust by the reaction to the ejection of fast moving exhaust gas from within a rocket engine. Often the term rocket is also used to mean a rocket engine. In military terminology, a rocket generally uses solid propellant and is unguided. These rockets can be fired by ground-attack aircraft at fixed targets such as buildings, or can be launched by ground forces at other ground targets. During the Vietnam era, there were also air launched unguided rockets that carried a nuclear payload designed to attack aircraft formations in flight. A missile, by contrast, can use either solid or liquid propellant, and has a guidance system. This distinction generally applies only in the case of weapons, though, and not to civilian or orbital launch vehicles. In all rockets the exhaust is formed from propellant which is carried within the rocket prior to its release. Rocket thrust is due to accelerating the exhaust gases (see Newton's 3rd Law of Motion). There are many different types of rockets, and a comprehensive list can be found in spacecraft propulsion- they range in size from tiny models that can be purchased at a hobby store, to the enormous Saturn V used for the Apollo program. Rockets are used to accelerate, change orbits, de-orbit for landing, for the whole landing if there is no atmosphere (e.g. for landing on the Moon), and sometimes to soften a parachute landing immediately before touchdown (see Soyuz spacecraft). Most current rockets are chemically powered rockets (internal combustion engines). A chemical rocket engine can use solid propellant (see Space Shuttle's SRBs), liquid propellant (see Space shuttle main engine), or a hybrid mixture of both. A chemical reaction is initiated between the fuel and the oxidizer in the combustion chamber, and the resultant hot gases accelerate out of a nozzle (or nozzles) at the rearward facing end of the rocket. The acceleration of these gases through the engine exerts force ('thrust') on the combustion chamber and nozzle, propelling the vehicle (in accordance with Newton's Third Law). See rocket engine for details. Not all rockets use chemical reactions. Steam rockets, for example, release superheated water through a nozzle where it instantly flashes to high velocity steam, propelling the rocket. The efficiency of steam as a rocket propellant is relatively low, but it is simple and reasonably safe, and the propellant is cheap and widely available. Most steam rockets have been used for propelling land-based vehicles but a small steam rocket was tested in 2004 on board the UK-DMC satellite. There are proposals to use steam rockets for interplanetary transport using either nuclear or solar heating as the power source to vaporize water collected from around the solar system. Rockets where the heat is supplied from other than the propellant, such as steam rockets, are classed as external combustion engines. Other examples of external combustion rocket engines include most designs for nuclear powered rocket engines. Use of hydrogen as the propellant for external combustion engines gives very high velocities. Due to their high exhaust velocity (mach ~10+), rockets are particularly useful when very high speeds are required, such as orbital speed (mach 25). The speeds that a rocket vehicle can reach can be calculated by the rocket equation; which gives the speed difference ('delta-v') in terms of the exhaust speed and ratio of initial mass to final mass ('mass ratio'). Rockets must be used when there is no other substance (land, water, or air) or force (gravity, magnetism, light) that a vehicle may employ for propulsion, such as in space. In these circumstances, it is necessary to carry all the propellant to be used. Common mass ratios for vehicles are 20/1 for dense propellants such as liquid oxygen and kerosene, 25/1 for dense monopropellants such as hydrogen peroxide, and 10/1 for liquid oxygen and liquid hydrogen. However, mass ratio is highly dependent on many factors such as the type of engine the vehicle uses and structural safety margins. Often, the required velocity (delta-v) for a mission is unattainable by any single rocket because the propellant, structure, guidance and engines weigh so much as to prevent the mass ratio from being high enough. This problem is frequently solved by staging - the rocket sheds excess weight (usually tankage and engines) during launch to reduce its weight and effectively increase its mass ratio. Typically, the acceleration of a rocket increases with time (even if the thrust stays the same) as the weight of the rocket decreases as fuel is burned. Discontinuities in acceleration will occur when stages burn out, often starting at a lower acceleration with each new stage firing.

History

Origins of rocketry

staging The ancient Chinese invention of gunpowder by Taoist chemists, and their use of it in various forms of weapons: (fire arrows), bombs, and cannons, resulted in the development of the rocket. They were initially developed for religious proceedings that were related to the worship and celebration of the Chinese Gods in the ancient Chinese religion. They were the precursors to modern fireworks and, after extensive research, were adapted for use as artillery in warfare during the 10th century to 12th century. Some of the ancient Chinese rockets were stationed at the military fortification known as the Great Wall of China, and employed by the elite soldiers stationed there. Rocket technology first became known to Europeans following their use by the Mongols Genghis Khan and Ogodei Khan when they conquered Russia, Eastern Europe, and parts of Central Europe(i.e. Austria). The Mongolians had stolen the Chinese technology by conquest of the northern part of China and also by the subsequent employment of Chinese rocketry experts as mercenaries for the Mongol military. Additionally, the spread of rockets into Europe was also influenced by the Ottomans at the siege of Constantinople in 1453. Although it is very likely that the Ottomans themselves were influenced by the Mongol invasions of the previous few centuries. Nevertheless, for several more centuries rockets remained misunderstood curiosities to those in the West. For over two centuries, the work of Polish-Lithuanian Commonwealth nobleman Kazimierz Siemienowicz, "Artis Magnae Artilleriae pars prima" ("Great Art of Artillery, the First Part". also known as "The Complete Art of Artillery"), was used in Europe as a basic artillery manual. The book provided the standard designs for creating rockets, fireballs, and other pyrotechnic devices. It contained a large chapter on caliber, construction, production and properties of rockets (for both military and civil purposes), including multi-stage rockets, batteries of rockets, and rockets with delta wing stabilizers (instead of the common guiding rods). At the end of the 18th century, rockets were successfully used militarily in India against the British by Tipu Sultan of the Kingdom of Mysore during the first Mysore War. The British then took an active interest in the technology and developed it further during the 19th century. The major figure in the field at this time was William Congreve. From there, the use of military rockets spread throughout Europe. At the Battle of Baltimore in 1814, the rockets fired on Fort McHenry by the rocket vessel HMS Erebus were the source of the rockets' red glare described by Francis Scott Key in The Star-Spangled Banner. Early rockets were very inaccurate. Without the use of spinning or any gimballing of the thrust, they had a strong tendency to veer sharply off course. The early British Congreve rockets reduced this somewhat by attaching a long stick to the end of a rocket (similar to modern bottle rockets) to make it harder for the rocket to change course. The largest of the Congreve rockets was the 32 pound (14.5 kg) Carcass, which had a 15 foot (4.6 m) stick. Originally, sticks were mounted on the side, but this was later changed to mounting in the center of the rocket, reducing drag and enabling the rocket to be more accurately fired from a segment of pipe. gimbal The accuracy problem was mostly solved in 1844 when William Hale modified the rocket design so that thrust was slightly vectored to cause the rocket to spin along its axis of travel like a bullet. The Hale rocket removed the need for a rocket stick, travelled further due to reduced air resistance, and was far more accurate.

Modern rocketry

In 1903, high school mathematics teacher Konstantin Tsiolkovsky (1857-1935) published Исследование мировых пространств реактивными приборами (The Exploration of Cosmic Space by Means of Reaction Motors), the first serious scientific work on space travel. The Tsiolkovsky rocket equation—the principle that governs rocket propulsion—is named in his honor. His work was essentially unknown outside the Soviet Union, where it inspired further research, experimentation, and the formation of the Cosmonautics Society. His work was republished in the 1920s in response to Russian interest in the work of Robert Goddard. Among other ideas, Tsiolkovsky accurately proposed to use liquid oxygen and liquid hydrogen as a nearly optimal propellant pair and determined that building staged and clustered rockets to increase the overall mass efficiency would dramatically increase range. Early rockets were grossly inefficient because of the heat energy that was wasted in the exhaust gases. Modern rockets were born when, after receiving a grant in 1917 from the Smithsonian Institution, Robert Goddard attached a supersonic (de Laval) nozzle to a rocket engine's combustion chamber. These nozzles turn the hot gas from the combustion chamber into a cooler, hypersonic, highly directed jet of gas; more than doubling the thrust and enormously raising the efficiency. In 1923, Hermann Oberth (1894-1989) published Die Rakete zu den Planetenräumen ("The Rocket into Planetary Space"), a version of his doctoral thesis, after the University of Munich rejected it. This book is often credited as the first serious scientific work on the topic that received international attention. During 1920s, a number of rocket research organizations appeared in America, Austria, Britain, Czechoslovakia, France, Italy, Germany, and Russia. In the mid-1920s, German scientists had begun experimenting with rockets which used liquid propellants capable of reaching relatively high altitudes and distances. A team of amateur rocket engineers had formed the Verein für Raumschiffahrt (German Rocket Society, or VfR) in 1927, and in 1931 launched a liquid propellant rocket (using oxygen and gasoline). From 1931 to 1937, the most extensive scientific work on rocket engine design occurred in Leningrad, at the Gas Dynamics Laboratory. Well funded and staffed, over 100 experimental engines were built under the direction of Valentin Glushko. Work included regenerative cooling, hypergolic ignition, and fuel injector designs that included swirling and bi-propellant mixing injectors. Work was curtailed by Glushko's arrest during Stalinist purges in 1938. Similar but much less extensive work was also done by the Austrian professor Eugen Sänger. In 1932, the Reichswehr (which in 1935 became the Wehrmacht) began to take an interest in rocketry. Artillery restrictions imposed by the Treaty of Versailles limited Germany's access to long distance weaponry. Seeing the possibility of using rockets as long-range artillery fire, the Wehrmacht initially funded the VfR team, but seeing that their focus was strictly scientific, created its own research team, with Hermann Oberth as a senior member. At the behest of military leaders, Wernher von Braun, at the time a young aspiring rocket scientist, joined the military (followed by two former VfR members) and developed long-range weapons for use in World War II by Nazi Germany, notably the A-series of rockets, which led to the infamous V-2 rocket (initially called A4). In 1943, production of the V-2 rocket began. The V-2 represented the biggest step forward in rocketry ever. The V-2 had an operational range of 300 km (185 miles) and carried a 1000 kg (2204 lb) warhead, with an amatol explosive charge. The vehicle was only different in details from most modern rockets, with turbopumps, inertial guidance and many other features. Thousands were fired at various Allied nations, mainly England, as well as Belgium and France. While they could not be intercepted, their guidance system design and single conventional warhead meant that the V-2 was insufficiently accurate against military targets. 2,754 people in England were killed, and 6,523 were wounded before the launch campaign was terminated. While the V-2 did not significantly affect the course of the war, it provided a lethal demonstration of the potential for guided rockets as weapons. At the end of World War II, competing Russian, British, and U.S. military and scientific crews raced to capture technology and trained personnel from the German rocket program at Peenemünde. Russia and Britain had some success, but the United States benefited most. The US captured a large number of German rocket scientists (many of whom were members of the Nazi Party, including von Braun) and brought them to the United States as part of Operation Paperclip. There the same rockets that were designed to rain down on Britain were used instead by scientists as research vehicles for developing the new technology further. The V-2 evolved into the American Redstone rocket, used in the early space program. After the war, rockets were used to study high-altitude conditions, by radio telemetry of temperature and pressure of the atmosphere, detection of cosmic rays, and further research. This continued in the U.S. under von Braun and the others, who were destined to become part of the U.S. scientific complex. Independently, research continued in the Soviet Union under the leadership of Sergei Korolev. With the help of German technicians, the V-2 was duplicated and improved as the R-1, R-2 and R-5 missiles. German designs were abandoned in the late 1940s, and the foreign workers were sent home. A new series of engines built by Glushko and based on inventions of Aleksei Isaev formed the basis of the first ICBM, the R-7. The R-7 launched the first satellite, the first man into space and the first lunar and planetary probes, and is still in use today. These events attracted the attention of top politicians, along with more money for further research. Rockets became extremely military important in the form of ICBMs when it was realised that nuclear weapons carried on a rocket vehicle were essentially not defensible against once launched, and they became the delivery platform of choice for these weapons. Fuelled partly by the cold war, the 1960s became the decade of rapid development of rocket technology in the Soviet Union (Vostok, Soyuz, Proton) and in the United States (e.g. X-20 Dyna-Soar, Gemini), including research in other countries, such as Britain, Japan, Australia, etc., culminating at the end of the 60s with the manned landing on the moon via the Saturn V. Rockets remain a popular military weapon. The use of large battlefield rockets of the V-2 type has given way to guided missiles, but rockets are often used by helicopters and light aircraft for ground attack, being more powerful than machine guns, but without the recoil of a heavy cannon. In the 1950s there was a brief vogue for air-to-air rockets, including the formidable AIR-2 'Genie' nuclear rocket, but by the early 1960s these had largely been abandoned in favor of air-to-air missiles. However in the heart of many of the public, the most important use of rockets is manned spaceflight. Vehicles such as Soyuz for orbital tourism and Spaceship One for suborbital tourism show the way towards greater commercialisation of rocketry, away from government funding, and towards more widespread access to space.

Regulation

Under international law, the nationality of the owner of a launch vehicle determines which country is responsible for any damages resulting from that vehicle. Due to this, some countries require that rocket manufacturers and launchers adhere to specific regulations to indemnify and protect the safety of people and property that may be affected by a flight. In the US any rocket launch that is not classified as amateur, and also is not "for and by the government," must be approved by the Federal Aviation Administration's Office of Commercial Space Transportation (FAA/AST), located in Washington, DC.

Accidents

Because of the enormous chemical energy in all useful rocket fuels (greater weight for weight than in explosives), accidents can and have happened. The number of people injured or killed is usually small because of the great care typically taken, but this record is not perfect. See List of space disasters

Future

- Nuclear thermal rockets have also been developed, but never deployed, they are particularly promising for interplanetary use because of their high efficiency.
- [http://www.neofuel.com Neofuel] - Nuclear/solar steam rockets for interplanetary use, using abundant extraterrestrial ice.
- Nuclear pulse propulsion rocket concepts give very high thrust and exhaust velocities. Another class of rocket-like thrusters in increasingly common use are ion drives, which use electrical rather than chemical energy to accelerate their reaction mass.

Patents of interest

- - Rocket apparatus - R. H. Goddard
- - Rocket apparatus - R. H. Goddard

External links

; Governing agencies
- [http://ast.faa.gov/ FAA Office of Commercial Space Transportation]
- [http://www.nasa.gov National Aeronautics and Space Administration (NASA)]
- [http://www.nar.org National Association of Rocketry]
- [http://www.tripoli.org Tripoli Rocketry Association]
- [http://www.canadianrocketry.org Canadian Association of Rocketry]
- [http://www.hobby.org Hobby Industry Association]
- [http://www.rchta.org Radio Control Hobby Trade Association]
- [http://www.ja-r.net Japan Association of Rocketry (site in Japanese)]
- [http://www.isro.org Indian Space Research Organisation] ; Information sites
- [http://www.astronautix.com/lvs/ Encyclopedia Astronautica - Rocket and Missile Alphabetical Index]
- [http://space.skyrocket.de Gunter's Space Page - Complete Rocket and Missile Lists] Category:Rocket-powered aircraft Category:Rocketry ja:ロケット ms:Roket

Fuel

:For information on the band, see Fuel (band). :For the workstation, see SGI Fuel. Fuel is material with one type of energy which can be transformed into another usable energy. A common example is potential energy being converted into kinetic energy, (as heat and mechanical work). In many cases this is just something that will burn.

Fuels

Solid fuels

burn There are many different types of fuel. Solid fuels include coal, wood and peat. All these types of fuel are combustible, they create fire and heat. Coal was burnt by steam trains to heat water into steam to move parts and provide power. Peat and wood are mainly used for domestic and industrial heating, though peat has been used for power generation, and wood-burning steam locomotives were common in times past. Steam power is becoming more and more desirable as oil and gas supplies begin to run out, this is because of the wide number of possible things that can burn to heat water.

Liquid and gas fuels

Non-solid fuels include petroleum and gas (both fuel types have myriad varieties including petrol (gasoline) and natural gas). The former is widely used in the internal combustion engine while both are used in power generation.

Nuclear fuels

In a nuclear reaction a radioactive fuel will undergo fission. This provides a useful source of energy without combustion. Also, in stars (and our sun), hydrogen (a gas) is the fuel for the nuclear fusion.

Other fuel

nuclear fusion Hydrogen also features as an upcoming fuel for automobiles with Oxygen in the Fuel Cell. This involves a reaction where the hydrogen and oxygen react to produce water (H₂O) and electrical energy, which then can supply an electrical motor in order to run a car (or a variety of other uses). In this reaction the chemical energy of the chemicals is converted into electrical energy due to redox. Carbohydrates, fats, and proteins, derived from food, are the fuels for biological systems. For instance, glucose (a simple carbohydrate) combines with oxygen to produce water, carbon-dioxide, and a release of energy. In the bodies of most animals, the released energy is used by the muscles.

Fuel values

Main article: Fuel value. The fuel value is the quantity of potential energy in a food or other substance.

Oxidizer

An oxidizing agent is a substance that oxidizes another substance in electrochemistry or redox chemical reactions in general. In doing so, the oxidizing agent, sometimes called an oxidizer, becomes reduced. Because the plum oxidizing agent receives electrons it is also known as an electron acceptor.

Common oxidizing agents

- Hypochlorite and other hypohalite compounds
- Bleach (probably the most common household oxidizer)
- Iodine and other halogens
- Chlorite, chlorate, perchlorate, and other analogous halogen compounds
- Permanganate compounds
- Cerium compounds
- Hexavalent chromium compounds such as chromic and dichromic acids and chromium trioxide and chromate/dichromate compounds
- Peroxide compounds
- Tollen's Reagent
- Swern oxidation
- Kornblum oxidation
- Ozone
- Osmium tetroxide (OsO4)
- Pyridinium chloride (PCC)

Common oxidizing agents and their products

There are many other oxidizing agents too numerous to list here.

Arabs

The Arabs ((Arabic: عرب ʻarab) are a large ethnic group widespread in the Middle East and North Africa, originating in the Arabian Peninsula of southwest Asia.

Who is an Arab?

The definition of who an Arab is has several aspects:
- Ethnic identity: someone who considers himself to be an Arab (regardless of racial or ethnic origin) and is recognized as such by others.
- Linguistic: someone whose first language is Arabic (including any of its varieties); this definition covers more than 200 million people.
- Genealogical: someone who can trace his or her ancestry back to the original inhabitants of the Arabian Peninsula.
- Political: someone who is a resident or citizen of a country where Arabic is an official or national language, or is a member of the Arab League or is part of the wider Arab world; this definition would cover more than 300 million people, but it is rather simplistic and rigid in that it excludes the entire Diaspora but includes indigenous or migrant minorities The relative importance of these factors is estimated differently by different groups. Most people who consider themselves Arabs do so on the basis of the overlap of the political and linguistic definitions. However, some members of groups which fulfill both criteria reject the identity on the basis of the genealogical definition; Lebanese Maronites, for example, may reject the Arab label in favor of a narrower Phoenecian-Lebanese national identity. Groups which use a non-Arabic liturgical language - such as Copts in Egypt - are especially likely to be considered non-Arab. Not many people consider themselves Arab on the basis of the political definition without the linguistic one—thus, Kurds or Berbers do not usually identify themselves as Arab—but some do (for instance, some Berbers do consider themselves Arabs, and Kurds were in some historical circumstances seen as Arabs or Turks or Persians). In addition, a majority of the population of Qatar and the United Arab Emirates is made up of non-citizen non-Arab immigrants and so the political definition does not apply there either. A hadith of questionable authenticity[http://www.islamtoday.com/show_detail_section.cfm?q_id=266&main_cat_id=11], related by Ibn Asakir in Târîkh Dimashq and attributed by its narrator Salmân b. `Abd Allah to Islam's prophet Muhammad, expresses a common sentiment in declaring that: :"Being an Arab is not because of your father or mother, but being an Arab is on account of your tongue. Whoever learns Arabic is an Arab." According to Habib Hassan Touma (1996, p.xviii), "An 'Arab', in the modern sense of the word, is one who is a national of an Arab state, has command of the Arabic language, and possesses a fundamental knowledge of Arabian tradition, that is, of the manners, customs, and political and social systems of the culture." On its formation in 1946, the Arab League defined an "Arab" as follows: :"An Arab is a person whose language is Arabic, who lives in an Arabic speaking country, who is in sympathy with the aspirations of the Arabic speaking peoples." As a number of the Prophet companions were of non-Arab descent, Salman the Persian, Suhaib the Roman and Bilal from Abisinia. The genealogical definition was widely used in medieval times (Ibn Khaldun, for instance, does not use the word Arab to refer to "Arabized" peoples, but only to those of originally Arabian descent), but is usually no longer considered to be particularly significant.

Religions

Before the coming of Islam, most Arabs followed a religion featuring the worship of a number of deities, including Hubal, Wadd, Al-Lat, Manat, and Uzza, while some tribes had converted to Christianity or Judaism, and a few individuals, the hanifs, had apparently rejected polytheism in favor of a vague monotheism. The most prominent Arab Christian kingdoms were the Ghassanid and Lakhmid kingdoms. With the expansion of Islam, the majority of Arabs rapidly became Muslim, and the pre-Islamic polytheistic traditions disappeared. At present, most Arabs are Muslims. Sunni Islam dominates in most areas, overwhelmingly so in North Africa; Shia Islam is prevalent in Bahrain, southern Iraq and adjacent parts of Saudi Arabia, northern Yemen, and southern Lebanon, as well as parts of Syria. The tiny Druze community, belonging to a secretive offshoot of Islam, is usually considered Arab, but sometimes considered an ethnicity in its own right. Reliable estimates of the number of Arab Christians, which in any case depends on the definition of "Arab" used, vary. According to [http://arabworld.nitle.org/texts.php?module_id=6&reading_id=63&sequence=4 Fargues 1998], "Today Christians only make up 9.2 per cent of the population of the Near East". In Lebanon they now number only about 40 per cent of the population, in Syria they make up about 10 to 15 per cent, in the Palestinian territories the figure is 3.8 per cent, and in Israel Arab Christians constitute 2.1 per cent. In Egypt, they constitute 5.9 per cent of the population, and in Iraq they presumably comprise 2.9 per cent of the populace. Most North and South American Arabs (about two-thirds) are Arab Christians, particularly from Syria, Palestine, and Lebanon. Arabic-speaking Jews - mainly Mizrahi Jews and Yemenite Jews - are today usually not categorised as Arab. Prior to the emergence of the term Mizrahi, the term "Arab Jews" (Yehudim ‘Áravim, יהודים ערבים) was used to describe Jews of Arab world. The term is rarely used today. The few remaining Jews in the Arab countries reside mostly in Morocco. Between the late 1940s and early 1960s, following the creation of the state of Israel, most Arabic-speaking Jews left their countries of birth. Most are now concentrated in Israel, but many also live in France (see Jewish exodus from Arab lands).

History

The first written attestation of the ethnonym "Arab" occurs in an Assyrian inscription of 853 BC, where Shalmaneser III lists a King Gindibu of mâtu arbâi (Arab land) as among the people he defeated at the Battle of Karkar. Some of the names given in these texts are Aramaic, while others are the first attestations of Proto-Arabic dialects. The Hebrew Bible likewise refers occasionally to peoples called `Arvi (or variants thereof), translated as "Arab" or "Arabian". The scope of the Hebrew term at this early stage is unclear, but it seems to have referred to various desert-dwelling tribes in the Syrian Desert and Arabia. Its earliest attested use referring to the southern "Qahtanite" Arabs is much later. Proto-Arabic, or Ancient North Arabian, texts give a clearer picture of the Arabs' emergence into history. The earliest such texts are written not in the modern Arabic alphabet, nor in its Nabataean ancestor, but in variants of the Epigraphic South Arabian musnad, beginning in the 8th century BC with the Hasaean inscriptions of eastern Saudi Arabia, and continuing from the 6th century BC on with the Lihyanite texts (in southeastern Saudi Arabia) and the Thamudic texts (found throughout Arabia and the Sinai, and not in reality connected with Thamud). Later come the Safaitic inscriptions (beginning in the 1st century BC) and the many Arabic personal names attested in Nabataean inscriptions (which are, however, written in Aramaic.) From about the 2nd century BC, a few inscriptions from Qaryat al-Faw (near Sulayyil) reveal a dialect which is no longer considered "Proto-Arabic", but Pre-Classical Arabic. By the fourth century AD, the Arab kingdoms of the Lakhmids in southern Iraq and Ghassanids in southern Syria had emerged just south of the Fertile Crescent and ended up allying respectively with the Sassanid and Byzantine Empires. Thus they were constantly at war with each other on behalf of their imperial patrons. However, their courts were responsible for some notable examples of pre-Islamic Arabic poetry, and for some of the few surviving pre-Islamic Arabic inscriptions in the Arabic alphabet. The Lakhmid kingdom was dissolved by the Sassanids in 602, while the Ghassanids would hold out until engulfed by the expansion of Islam. During the 8th and 9th centuries, the Arabs (specifically the Umayyads, and later Abbasids) forged an empire whose borders touched southern France in the west, China in the east, Asia Minor in the north, and the Sudan in the south. This was one of the largest land empires in history. Throughout much of this area, the Arabs spread the religion of Islam and the Arabic language (the language of the Qur'an) through conversion and assimilation. Many groups came to be known as "Arabs" not through descent but through Arabization. Thus, over time, the term Arab came to carry a broader meaning than the original ethnic term. Many Arabs in Sudan, Morocco, Algeria and elsewhere became Arab through Arabization. Arab nationalism declares that Arabs are united in a shared history, culture and language. Arab nationalists believe that Arab identity encompasses more than outward physical characteristics, race or religion. A related ideology, Pan-Arabism, calls for all Arab lands to be united as one state. Anti-Arabism is hate or prejudice against Arabs. It is usually also associated with anti-Muslim hatred.

Traditional genealogy

Medieval Arab genealogists divided the Arabs into three groups:
- the "ancient Arabs", tribes that had been destroyed or vanished, such as Ad and Thamud; they are often alluded to in the Qur'an as examples of God's power to destroy wicked peoples.
- the "Pure Arabs" of South Arabia, descending from Qahtan. The Qahtanites (Qahtanis) are said to have migrated the land of Yemen following the destruction of the Ma'rib Dam (sadd Ma'rib). The Qahtanite Arabs were responsible for the ancient civilizations of Yemen, notably including that of the Sabaeans (known in the Bible as Sheba.)
- The "Arabized Arabs" (musta`ribah) of North Arabia, descending from Adnan, supposed to be a descendant of Ishmael (Ismail), the eldest son of Abraham and Hagar. The Arabic language as it is spoken today in its classical Quranic form was the result of a mix between the original Arabic tongue of Qahtan and the northern Arabic which shares a great deal with northern Semitic languages from the Levant. The Arabs take a great pride in their language and it's survival as usable and comprehendable language for over thousand years. In Jewish and Christian traditions, the identification of the Ishmaelites, described in the Bible as a people of the Arabian wilderness, with Arabs began at least by the time of Josephus, and became standard centuries prior to Islam (in which the term "Hagarenes", a pun on the Arabic muhajir and the name of Hagar, was commonly used.) Efforts to reconcile the Biblical and Arab genealogies later led to the identification of Joktan with Qahtan, probably due to his Biblical identification as the ancestor of Hazarmaveth (Hadramawt) and Sheba.

Etymology

The term "Arab" or "Arabian" (and cognates in other languages) has been used to translate several different but similar sounding words in ancient and classical texts which do not necessarily have the same meaning or origin. The etymology of the term is of course closely linked to that of the place name "Arabia". Although the term mâtu arbâi describing Gindibu in Assyrians texts is conventionally translated of Arab land, nothing is known with certainty about the exact location or extent of the land being referred to, nor what literal meaning the name had. In fact several different ethnonyms are found in Assyrian texts that are conventionally translated "Arab": Arabi, Arubu, Aribi and Urbi. The presence of Proto-Arabic names amongst those qualified by the terms arguably justifies the translation "Arab" although it is not certain if they all in fact represent the same group. In Hebrew the words `arav and `aravah literally mean "desert" or "steppe". In the Hebrew Bible the latter feminine form is used exclusively for the Arabah, a region associated with the Nabateans, who spoke Arabic. The former masculine form is used in Isaiah 21:13 and Ezekiel 27:21 for the region of the settlement of Kedar in the Syrian Desert. 2 Chronicles 9:14 contrasts “kings of `arav " with “governers of the country” when listing those who brought tribute to King Solomon. The word is typically translated Arabia and is the name for Arabia in Modern Hebrew. The New Revised Standard Version of the Bible uses instead the literal translation “desert plain” for the verse in Isaiah. The adjectival noun `aravi formed from `arav is used in Isaiah 13:20 and Jeremiah 3:2 for a desert dweller. It is typically translated Arabian or Arab and is the modern Hebrew word for Arab. The New Revised Standard Version uses the translation "nomad" for the verse in Jeremiah. In the Bible, the word `arav is closely associated with the word `erev meaning a "mix of people" which has identical spelling in unvowelled text. Jeremiah 25:24 parallels "kings of `arav " with "kings of the `erev that dwell in the wilderness". The account in 1 Kings 10:15 matching 2 Chronicles 9:14 is traditionally vowellized to read "kings of the `erev ". The people in question are understood to be the early Nabateans who do indeed appear to have been a mix of different tribes. The medieval writer Ibn an-Nadim, in Kitab al-Fihrist, derived the word from a Syriac pun by Abraham on the same root: in his account, Abraham addresses Ishmael and tells him u`rub, from Syriac `rob, "mingle". The early Nabateans are also referred to as `arvim in Nehemiah 4:7 and the singular `arvi is applied to Geshem a leader who opposed Nehemiah. This term is identical to `aravi in unvowelled text but traditionally vowelized differently. It is usually translated "Arabian" or "Arab" and was used in early 20th century Hebrew to mean Arab. However it is unclear if the term related more to `arav or to `erev. On the one hand its vowelization resembles that of the term `arvati (Arbathite) which is understood as an adjective formed from `aravah; thus it is plausibly a variant of `aravi. On the other hand it is used in 2 Chronicles 21:16 for a seemingly different people located in Africa plausibly the same Africans referred to as an `erev (mix of people) in Ezekiel 30:5. The words `aravim (plural of `aravi) and `arvim appear the same in unvowelled texts as the word `orvim meaning ravens. The occurrences of the word in 1 Kings 17:4-6 are traditionally vowellized to read `orvim. In the Talmud (Chullin 5a) a debate is recorded as to whether the passage refers to birds or to a people so named, noting a Midianite chieftain named Oreb (`orev: raven) and the place of his death, the Rock of Oreb. Jerome understood the term as the name of a people of a town which he described as being in the confines of the Arabians. (Genesis Rabba mentions a town named Orbo near Beth Shean.) One meaning of the root `-r-b in Hebrew is "exchange/trade" (la'arov: "to exchange", ma`arav: "merchandise") whence `orvim can also be understood to mean "exchangers" or "merchants", a usage attested in the construct form in Ezekiel 27:27 which speaks of `orvei ma`aravekh: "exchangers of thy merchandise". The Ferrar Fenton Bible translates the term as "Arabians" in 1 Kings 17:4-6. In Hebrew, the word `arav has the same triconsonantal root as the root meaning "west" (ma`arav) "setting sun" or "evening" (ma`ariv, `erev). The direct Arabic cognate of this is gharb ("west", etc.) rather than `arab; however, in Ugaritic, a language which normally preserves proto-Semitic gh, this root is found with `ayin, adding confusion. The Assyrian forms may plausibly be borrowings from Aramaic or Canaanite of either root, referring to land lying to the west in the latter case; the latter possibility is perhaps strengthened by the later Greek use of the term Saracen, with the parallel meaning in Arabic of "Easterners" (sharqiyyûn.) One meaning of the word Arab in Arabic is clear; clear as in comprehensible rather than as in pure. Bedouin elders still use this term with the same meaning; those whose speech they comprehend (ie Arabic-speakers) they call Arab, and those whose speech is of unknown meaning to them, they call Ajam (ajam or ajami). This is similar to how the ancient Greeks used the term Barbarian to desribe non-Greeks - Barbarian essentially meant that when they spoke their speech sounded like "Bar Bar Bar", ie. incomprehensible. In the Persian Gulf region, the term Ajam is often used to refer to the Persians. Another explanation derives the word from an old Semitic stem `.R.B., with a metathetical alternative `.B.R., both meaning travelling around the land, that is, nomadic. From that root, the terms Arab(Arabi) and Hebrew(Ebri), meaning nomads, are derived.
References

- Habib Hassan Touma (1996). The Music of the Arabs, trans. Laurie Schwartz. Portland, Oregon: Amadeus Press. ISBN 0931340888.
- Edward Lipinski, Semitic Languages: Outlines of a Comparative Grammar, 2nd ed., Orientalia Lovanensia Analecta: Leuven 2001
- [http://www.newadvent.org/cathen/01663a.htm The Catholic Encyclopedia, Robert Appleton Company, 1907, Online Edition, K. Night 2003: article Arabia]
- http://www.cia.gov/cia/publications/factbook/geos/le.html#People
See also

- Ababda
- Arabia
- Arab League
- Arab World
- Arabic alphabet
- Arabic language
- Bedouin
- Nabataeans
- Pan-Arabism
- Semitic
External links

- [http://www.aaiusa.org/arab_world.htm Maps of the Arab World]
- [http://www.albawaba.com News from Arabic countries]
- [http://www.ameinfo.com Business news from Arab countries]
- [http://www.bayt.com Jobs and Careers in the Arab World]
- [http://nabataea.net/arabia.html Arabia in ancient history] - with a discussion of the ancient usage of the word Arab
- [http://arabworld.nitle.org An Online Resource on Arab Culture and Civilization]
- [http://www.geocities.com/martinkramerorg/ArabNationalism.htm Arab Nationalism: Mistaken Identity] by Martin Kramer
- [http://www.al-islam.org/al-tawhid/arabnationalism.htm A Criticism of the Idea of Arab Nationalism] als:Araber ko:아랍인 ja:アラブ人

Propellant

A propellant is a material that is used to move an object by applying a motive force. This may or may not involve a chemical reaction. It may be a gas, liquid, plasma, or, before the chemical reaction, a solid. Common propellants are gasoline, jet fuel and rocket fuel.

Aerosol sprays

In aerosol spray cans, the propellant is simply a pressurized vapour in equilibrium with its liquid. As some gas escapes to expel the payload, more liquid evaporates, maintaining an even pressure. (See aerosol spray propellant for more information.)

Solid fuelled rockets and projectiles

In ballistics and pyrotechnics, a propellant is a material which burns very rapidly but controllably, to produce thrust by gas pressure and thus accelerate a projectile or rocket. In this sense, common or well known propellants include, for firearms, artillery and solid fuel rockets:
- Gunpowder
- Nitrocellulose
- Cordite and other smokeless powders
- Composite propellants made from a solid oxidizer such as ammonium perchlorate or ammonium nitrate, a rubber such as HTPB or PBAN, and usually a powdered metal fuel such as aluminum.
- Some amateur propelants use potassium nitrate, combined with sugar, epoxy, or other fuels / binder compounds.
- Potassium perchlorate has been used as an oxidizer, paired with asphalt, epoxy, and other binders.

Liquid fuelled rockets

Common propellants for liquid fuelled rockets include:
- RFNA and kerosene or RP-1
- RFNA and UDMH
- Dinitrogen tetroxide and UDMH, MMH and/or Hydrazine
- Liquid oxygen and kerosene or RP-1
- Liquid oxygen and liquid hydrogen
- Hydrogen Peroxide and alcohol or RP-1

Auxiliary power unit

An Auxiliary Power Unit (APU) is a relatively small self-contained generator used in aircraft to start the main engines, usually with compressed air, and to provide electrical power and air conditioning while the aircraft is on the ground. In many aircraft, the APU can also provide electrical power in the air. A gasoline piston engine APU was first used on the Pemberton-Billing P.B.31 Night Hawk Scout aircraft in 1916. The Boeing 727 in 1963 was the first jetliner to feature a gas turbine APU, allowing it to operate at smaller, regional airports, independent from ground facilities. Although APUs have been installed in many locations on various military and commercial aircraft, they are usually mounted at the rear of modern jet airliners. The APU exhaust can be seen on most modern airliners as a small pipe exiting at the aircraft tail. In most cases the APU is powered by a small gas turbine engine that provides compressed air from within or drives an air compressor (load compressor). Recent designs have started to explore the use of the Wankel engine in this role. The Wankel offers power-to-weight ratios better than normal piston engines and better fuel economy than a turbine. APUs fitted to ETOPS airplanes are more critical than others, as they supply backup electrical and compressed air in place of the dead engine during emergencies. While most APUs may or may not be startable while the aircraft is in flight, ETOPS compliant APUs must be flight-startable at all altitudes. If such APUs malfunction, the airplane cannot be released for ETOPS flight and is forced to take a longer route. APUs are even more critical for space shuttle flight operations, since the main engines are designed only for liftoff. The APUs provide hydraulic pressure for certain vehicle subsystems, including landing gear and brakes, rocket engine gimballing, and moving the shuttle's control surfaces. Category:Electrical generators Category:Aircraft components

O-ring

An O-ring is a loop of elastomer with a round (o-shaped) cross-section used as a mechanical seal. They are designed to be seated in a groove and compressed during assembly between two or more parts, creating a seal at the interface. The joint may be static, or (in a few circumstances) have relative motion between parts and o-ring (rotating pump shafts and hydraulic cylinders, for example). Joints with motion usually require lubrication of the o-ring to reduce wear. This is often accomplished with the fluid being sealed. O-rings are one of the most popular seals used in machine design because they are inexpensive and easy to make, reliable, and have simple mounting requirements. They can seal tens of mega pascals (thousands of psi) pressure. In some cases, O-rings are used with back-up rings.

History

The o-ring was invented in 1936 by a then 72 year old Danish-born man, Niels Christensen. During the second world war, the US government "bought" critical war-related patents after finding out the big businesses were in violation of Christensen's patent right. Christensen got a lump sum payment of US$75,000 for it.

Theory and design

Successful o-ring joint design requires a rigid mechanical mounting that applies a predictable deformation to the o-ring. This introduces a calculated mechanical stress at the o-ring contacting surfaces. As long as the pressure of the fluid being contained does not exceed the contact stress of the o-ring, leaking cannot occur. The seal is designed to have a point contact between the o-ring and sealing faces. This allows a high local stress, able to contain high pressure, without exceeding the yield stress of the o-ring body. The flexible nature of o-ring materials accommodates imperfections in the mounting parts. O-rings are available in a large number of standard sizes and materials. Manufacturers or reference books supply application and machining data for the mounting. O-rings are one of the most common and important elements of machine design.

Material

fluid fluid O-ring selection is based on chemical compatibility, application temperature, sealing pressure, lubrication requirements, quality, quanitity and cost. SYNTHETIC RUBBERS:
- Acrylonitrile butadiene copolymers (NBR)
- Butadiene rubber (BR)
- Butyl rubber (IIR)
- Chlorosulfonated polyethylene (CSM)
- Epichiorohydrin (ECH, ECO)
- Ethylene propylene diene monomer (EPDM)
- Ethylene propylene monomer (EPM)
- Fluoroelastomers (FKM)
- Perfluoroelastomer (FFKM)
- Polyacrylate (ACM)
- Polybutadiene (PB)
- Polychloroprene (CR)
- Polyisoprene (IR)
- Polysulfide rubber (PSR)
- Semi-Conductive Fluorocarbon with nano carbon tubes.
- Silicone rubber (SiR)
- Styrene butadiene rubber (SBR) THERMOPLASTICS:
- Thermoplastic Elastomer (TPE) styrenics
- Thermoplastic polyolefin (TPO) LDPE, HDPE, LLDPE, ULDPE
- Thermoplastic Polyurethane (TPU) polyether, polyester
- Thermoplastic etheresterelastomers (TEEEs) copolyesters
- Thermoplastic polyamide (PEBA) Polyamides
- Melt Processable Rubber (MPR)
- Thermoplastic Vulcanizate (TPV)

Other seals

Similar devices with a non-round cross-section are called seals or gaskets. See also washer (mechanical).

Challenger disaster

The failure of an O-ring seal was determined to be the cause of the Space Shuttle Challenger disaster on January 28, 1986. A contributing factor was cold weather prior to the launch. This was famously demonstrated on television by Caltech physics professor Richard Feynman, when he placed a small O-ring into his ice water, and subsequently showed its loss of pliability before an investigative committee. The material of the failed o-ring was Viton (Registered Trade name with DuPont) and the manufacturer of that particular o-ring was Morton-Thiokol in Utah, USA. Viton is not a good material for cold temperature applications. When an o-ring is frozen there is a T_g (glass transition temperature) where it will not bounce back. Even when an o-ring does not reach T_g, the o-ring, once compressed, will take longer than normal to return to its original shape. The o-rings (and all other seals) work by creating positive pressure against a surface thereby preventing leaks. On the night before the launch, exceedingly low air temperatures were recorded. On account of this, Nasa technicians performed an inspection. The ambient temperature was within launch parameters, and the launch sequence was allowed to proceed. However, the temperature of the rubber O-rings remained significantly lower than that of the surrounding air. glass transition temperature During his investigation of the launch footage, Dr. Feynman observed a small outgassing event from the SRB at the joint between two segments in the moments immediately preceding the explosion. This was blamed on a failed O-ring seal. The high temperature gas reacted explosively with the external tank, and the entire booster stack was destroyed as a result. Among the wreckage recovered later were the emergency life support packs used by the crew. These had been triggered and used for approximately the amount of time required for a free-falling object to travel the distance between the explosion point and the ocean surface. The interpretation of this data was that the crew had remained alive, and perhaps conscious, until the crew compartment impacted the ocean surface. The rubber industry has gone through its share of transformation after the accident. All o-rings come with batch number and serial date, just like the medicine industry, to control and track precise distribution. O-Rings can, if need be, recalled off the shelf. Furthermore, O-rings and other seals are routinely batch-tested for quality control by the manufacturer, and often times undergo Q/A several more times by the distributor and ultimate OEM.

External links

- Marco Rubber, the O-Ring Specialists http://www.marcorubber.com
- O-Rings, Inc. http://www.oringsusa.com
- Lutz Sales Company, Inc. An O-ring distributor website: http://www.lutzsales.com
- Engineering Fundamentals website: http://www.efunda.com/designstandards/oring/oring_intro.cfm
- FAS Space Policy Project Challenger Accident website: http://www.fas.org/spp/51L.html ja:Oリング Category:Seals (mechanical)

Hydroxy-terminated polybutadiene

Hydroxy-terminated polybutadiene (HTPB), a polymer of butadiene, is a stable and easily stored synthetic rubber, often used in tire manufacturing. HTPB is sold as one part of a two part thermoset elastomer, with a diisocyanate as the other component, which is mixed prior to molding. The diisocyante serves to cross-link the molecules of polybutadiene between the ends where the -OH (hydroxyl) sites are found. HTPB is used to bind the fuel and oxidizer into a solid mass for solid rocket motors. It is also used as a hybrid rocket fuel. Together with N₂O (nitrous oxide, or "laughing gas") as the oxidizer, it is used to power the SpaceShipOne hybrid rocket motor. It also powers all 3/4 stages of the Japanese M-V-5 satellite launchers. JAXA describe the fuel as "HTPB/AP/Al=12/68/20" which means, proportioned by mass, HTPB 12% (binder and fuel), Ammonium Perchlorate 68% (oxidiser), and Aluminium powder, 20% (fuel). Category:Organic polymers Category:Rocket fuels

Ammonium perchlorate

Properties
General
Name	Ammonium perchlorate
Chemical formula	NH₄ ClO₄
Appearance	White solid
Physical
Formula weight	117.5 amu
Melting point	Decomposes at 45785 K (240 °C)
Density	2.0 ×10³ kg/m³
Crystal structure	?
Solubility	?
Thermochemistry
Δ_fH⁰_solid	-295.77 kJ/mol
S⁰_solid	184.18 J/mol·K
Safety
NFPA 704	100px
DOT Classes	100px 100px
Ingestion	Gastrointestinal irritation can occur.
Inhalation	May cause respiratory tract irritation or pulmonary edema.
Skin	May cause irritation.
Eyes	Irritation, chance of more serious problems.
More info	[http://ull.chemistry.uakron.edu/erd/chemicals/7/6025.html Hazardous Chemical Database]
SI units were used where possible. Unless otherwise stated, standard conditions were used. Disclaimer and references

Ammonium perchlorate is a chemical compound with the formula N H₄Cl O₄. It is the salt of ammonia and perchloric acid. Like other perchlorates, it is a powerful oxidizer. This salt is used as an explosive in mining, due to the low temperature elevation that follows its decomposition. It is produced by reaction between ammonia and perchloric acid, or by double decomposition between an ammonium salt and sodium perchlorate. It crystallises in colorless rhombohedra with a relative density of 1.95. It is the least soluble of all ammonia salts with 20 g in 100g water at 0°C. It decomposes before fusion. It is an important oxidizer used in solid rocket propellants, such as the Space Shuttle Solid Rocket Boosters, as well as many other solid rockets including some fireworks, amateur and hobby high power rockets, and larger rockets used for space launch and military purposes. The PEPCON disaster happened at an ammonium perchlorate manufacturing plant. The resulting explosions measured 3.5 on the Richter scale. Category:Ammonium compounds Category:Perchlorates Category:Pyrotechnic chemicals Category:Rocket fuels ja:過塩素酸アンモニウム

Graphite-Epoxy Motor

The Graphite-Epoxy motor is a type of rocket engine used as a booster in the Delta II rocket, among others. Category:Rocketry

De Laval nozzle

A de Laval nozzle (or convergent-divergent nozzle, CD nozzle or con-di nozzle) is a tube that is pinched in the middle, making an hourglass-shape. It is used as a means of accelerating the flow of a gas passing through it. It is widely used in some types of steam turbine and is an essential part of the modern rocket engine. The nozzle was developed by Swedish inventor Gustaf de Laval in the 19th century. Its operation relies on the different properties of gases flowing at subsonic and supersonic speeds. The speed of a subsonic flow of gas will increase if the pipe carrying it narrows because the mass flow rate is constant (grams or pounds per second). The gas flow through a de Laval nozzle is isentropic (gas entropy is nearly constant) and adiabatic (heat loss or gain is nearly zero). At subsonic flow the gas is compressible; sound, a small pressure wave, will propagate through it. Near the nozzle "throat", where the cross sectional area is a minimum, the gas velocity locally becomes transonic (Mach number = 1.0). As the nozzle cross sectional area increases the gas continues to expand and the gas flow increases to supersonic velocities where a sound wave will not propagate through the gas (Mach number > 1.0). This expansion process accelerates the exhaust from the nozzle, improving its thrust. A de Laval nozzle using hot air at a pressure of 1,000 psi (6.9 MPa or 68 atm), temperature of 1470 K, would have a pressure of 540 psi (3.7 MPa or 37 atm), temperature of 1269 K at the throat, and 15 psi (0.1 MPa or 1 atm), temperature of 502 K at the nozzle exit. The expansion ratio, nozzle cross sectional area at exit divided by area at throat, would be 6.8. The specific impulse would be 151 s (1480 N·s/kg). This principle was used in a rocket engine by Robert Goddard. Walter Thiel's implementation of it made the V2 rocket possible. Category:Rocket engines Category:Jet engines

Specific impulse

The specific impulse (commonly abbreviated I_sp) of a propulsion system is the impulse (change in momentum) per unit of propellant. Depending on whether the amount of propellant is expressed in mass or in weight (by convention weight on the Earth) the dimension of specific impulse is that of speed or time, respectively, differing by a factor of g, the gravitational acceleration at the surface of the Earth.

General considerations

Essentially, the higher the specific impulse, the less propellant is needed to gain a given amount of momentum. In this regard a propulsion method is more fuel-efficient if the specific impulse is higher. This should not in any way be confused with energy-efficiency, which can even decrease as specific impulse increases, since many propulsion systems that give high specific impulse require high energy to do so. In addition it is important that thrust and specific impulse not be confused with one another. The specific impulse is a measure of the thrust per unit of propellant that is expelled, while thrust is a measure of the momentary or peak force supplied by a particular engine. In fact, propulsion systems with very high specific impulses (such as ion thrusters: 3,000 seconds) are power limited to producing low thrusts, due to the relatively high weight of power generators. When calculating specific impulse, only propellant that is carried with the vehicle before use is counted. For a chemical rocket the propellant mass therefore would include both fuel and oxidizer; for air-breathing engines only the mass of the fuel is counted, not the mass of air passing through the engine.

Examples

Specific impulse of various propulsion technologies

An example of a specific impulse measured in time is 459 seconds, or, equivalently, an effective exhaust velocity of 4500 m/s, for the Space Shuttle Main Engines when operating in vacuum. An air-breathing engine typically has a much larger specific impulse than a rocket: a jet engine may have a specific impulse of 2000-3000 seconds or more at sea level. In some ways, comparing specific impulse seems unfair in the case of jet engines and rockets. However in rocket or jet powered aircraft, specific impulse is approximately proportional to range, and rockets do indeed perform much worse than jets at sea level. The highest specific impulse for a chemical propellant ever test-fired in a rocket engine was lithium, fluorine, and hydrogen (a tripropellant): 542 seconds (5320 m/s). However, the combination is impractical, see rocket fuel. Nuclear thermal rocket engines differ from conventional rocket engines in that thrust is created strictly through thermodynamic phenomena, with no chemical reaction. The nuclear rocket typically operates by passing hydrogen gas over a superheated nuclear core. [http://www.lascruces.com/~mrpbar/rocket.html Testing in the 1960s] yielded specific impulses of about 850 seconds (8340 m/s), about twice that of the Space Shuttle engines. A variety of other non-rocket propulsion methods, such as ion thrusters, give much higher specific impulse but with much lower thrust; for example the Hall effect thruster on the Smart 1 satellite has a specific impulse of 1640 s (16100 m/s) but a maximum thrust of only 68 millinewtons. The hypothetical Variable specific impulse magnetoplasma rocket(VASIMR) propulsion should yield a minimum of 10,000-300,000 m/s but will probably require a great deal of heavy machinery to confine even relatively diffuse plasmas, so they will be unusable for very-high-thrust applications such as launch from planetary surfaces.

Specific impulse in seconds

For all vehicles specific impulse (impulse per unit weight-on-Earth of propellant) in seconds can be defined by the following equation: :

\mathrm=I_ \cdot \frac   \cdot g_ \,

where: Thrust is the thrust obtained from the engine. I_sp is the specific impulse measured in seconds.

\frac

is the mass flow rate, which is minus the time-rate of change of the vehicle's mass, since fuel is being expelled. g₀ is the acceleration at the Earth's surface. This I_sp in seconds value is somewhat physically meaningful - it is the number of seconds a unit weight of fuel would last if the engine would apply a unit force (if an engine could be scaled proportionately). As such it is a value that can be used to compare engines; much like 'miles per gallon' is for cars. The advantage that this formulation has is that it may be used for rockets, where all the reaction mass is carried onboard, as well as aeroplanes, where most of the reaction mass is taken from the atmosphere. In addition, it gives a result that is independent of units used (provided the unit of time used is the second).

Rocketry - specific impulse in seconds

In rocketry, where the only reaction mass is the propellent, an equivalent way of calculating the specific impulse in seconds is also frequently used. In this sense, specific impulse is defined as the change in momentum per unit weight-on-Earth of the propellent: :

I_=\frac

where I_sp is the specific impulse measured in seconds

v_

is the average exhaust speed along the axis of the engine g₀ is the acceleration at the Earth's surface It may seem odd that the acceleration or weight at the Earth's surface is in the definition, while the rocket may be far from the Earth. However, accelerations are often measured in terms of g₀; for example, astronauts should not be subjected to an acceleration more than a few times this value. Additionally, in Imperial units the relationship between force and mass is defined to involve the acceleration due to gravity. Thus pounds (force) and pounds (mass), both used in rocketry, when divided, must be additionally multiplied by g₀ to get the acceleration in more usual units. The official Imperial unit of mass the slug, which is not popular for obvious reasons, was introduced to make Imperial units more like the SI units and avoid this multiplication. This, the common use of pounds for both force and mass, is in fact the chief reason g₀ enters so often into rocketry definitions, and is likely the reason two definitions of specific impulse are in common use. When expressed in units of seconds, the specific impulse can be interpreted in the following ways:
- the impulse divided by the sea-level weight of a unit mass of propellant
- the time one kilogram of propellant lasts if a force equal to the weight of one kilogram is produced, for example for a hypothetical hovering over the Earth (imagine the fuel to be supplied from outside, so that the mass on which the thrust is applied does not reduce by spending fuel)
- the time one pound mass of propellant lasts if a force of one pound is produced, for example for a hypothetical hovering vehicle over the Earth (imagine the fuel to be supplied from outside, so that the mass on which the thrust is applied does not reduce by spending fuel)
- alternatively, for engines that can not produce a large thrust: approximately the time one kilogram of propellant lasts if an acceleration of 0.01 g of a mass of one 100 kilogram is produced
- 100 times the time an acceleration g can be produced (i.e. a thrust equal to the weight on Earth of the current mass) with a propellant mass of 1 % of the current total mass (100 times the time it takes in this case to reduce the total mass by 1 %)
- the time an acceleration g can be produced with a propellant mass of 63.2 % of the initial total mass (the time it takes in this case to reduce the total mass by a factor e, to 36.8 %)
- twice the net power to produce an acceleration of 1 m/s² to a mass which at Earth has a weight of 1 N (i.e. a mass of 102 grams) e.g. for hydrogen/oxygen, with a specific impulse of 460 seconds (4500 m/s):
- one kilogram of propellant lasts 460 seconds if an acceleration g of a mass of one kilogram is produced
- one kilogram of propellant lasts 460 seconds if an acceleration of 0.01 g of a mass of 100 kilogram is produced
- it takes 4.6 seconds to reduce the total mass by 1 % if an acceleration g is produced
- an acceleration g during 460 seconds can be produced with a propellant mass of 63.2 % of the initial total mass (it is the time it takes in this case to reduce the total mass by a factor e, to 36.8 %)
- the net power to produce an acceleration of 1 m/s² to a mass of 102 grams is 230 W.
- Need help on this non native speaker The reason that the specific impulse of a turbo fan is so high is that the atmosphere gives the oxidant, so the plane does not carry it. A very simplificated example can make this point clear: Let look at and hydrogen based motor: The ideal termo chemistry of reaction is like: 2H₂+O₂=2H₂O +467kJ/mol If the O₂ came from a tank in a rocket the specific gives (again over-simplificated) mv2/2=467kJ. Where the mass is 18g (2
- H+O, 2g+16) Solving for v , we get: 5093m/s about 5000 under ideal conditions (ejection temperature 0K) In case that we don’t have to carry the oxygen the mass is now 2g, but energy still is 467kJ, so we know get: 15280m/s. We can improve that by pushing great amounts of non-combustion air. This is possible because the Energy is proportional to the square power of the ejection speed but the “force” is proportional to the speed. The presence of nitrogen makes things even better. If we see the diagrams of big, efficient turbo fans we will see that this is important part of the optimization guides. (http://anirudh.net/seminar/ge90.pdf by example)

Rocketry - specific impulse as a speed (effective exhaust velocity)

In rocketry the specific impulse as the impulse per unit mass of propellant used is simply the effective exhaust velocity: :

I_=v_ \,

where I_sp is the specific impulse, as defined above, and measured in metres per second (in the U.S. feet/second). v_e is the effective exhaust velocity measured in metres per second. It is related to the thrust, or forward force on the rocket by the equation: :

\mathrm=I_ \cdot \frac   \,

where

\frac

is the mass flow rate, which is minus the time-rate of change of the vehicle's mass, since fuel is being expelled. A rocket must carry all its fuel with it, so the mass of the unburned fuel must be accelerated along with the rocket itself. Minimizing the mass of fuel required to achieve a given push is crucial to building effective rockets. Using Newton's laws of motion it is not difficult to verify that for a fixed mass of fuel, the total change in velocity (in fact, momentum) it can accomplish can only be increased by increasing the exhaust velocity. A spacecraft without propulsion follows an orbit determined by the gravitational field. Deviations from the corresponding velocity pattern (these are called delta-v) are achieved by sending exhaust mass in the direction opposite to that of the desired velocity change. Due to the law of conservation of momentum, to change the speed of the spacecraft by an amount equal to 1% of the exhaust speed, approximately requires an exhaust mass equal to 1% of the mass of the spacecraft, including the fuel that has not yet been spent. As a useful rule of thumb the delta-v that can be produced with a propellant mass of 63.2 % of the initial total mass is equal to the exhaust velocity (see Rocket equation.) The speed is also approximately twice the power per unit thrust For a delta-v that is much smaller than the specific impulse, the fuel required is approximately proportional to the delta-v. For a delta-v that is larger than the specific impulse, this requirement of carrying the fuel and spending much of the fuel on accelerating the fuel, gives rise to an exponential increase in fuel requirement (and larger tanks which also add to the mass). See spacecraft propulsion calculations and Tsiolkovsky rocket equation for details. e.g for hydrogen/oxygen, with a specific impulse of 4500 m/s (460 seconds):
- the effective exhaust speed is 4,500 m/s
- the impulse produced per unit mass of propellant used is 4,500 N·s per kg
- the thrust is 4,500 N if the propellant mass flow rate is 1 kg/s
- the delta-v that can be produced with a propellant mass of 1 % of the current total mass (the delta-v that reduces the mass by 1%) is 45 m/s
- the delta-v that can be produced with a propellant mass of 63.2 % of the initial total mass (the delta-v that reduces the total mass by a factor e, to 36.8 %) is 4,500 m/s
- the power-thrust ratio is 2,250 W/N

Liquid oxygen

Liquid oxygen (also LOx, LOX or Lox in the aerospace industry) is the liquid form of oxygen. It has a pale blue colour and is strongly paramagnetic. Liquid oxygen has a density of 1140 kg/m³ and is moderately cryogenic (freezing point: −219 °C, boiling point: −183 °C). Oxygen is found naturally in the air. For industrial applications it is obtained from air by fractional distillation. Liquid oxygen is a powerful oxidising agent: organic materials will burn rapidly and energetically in liquid oxygen, hence LOx is a common liquid oxidizer propellant for spacecraft rocket applications usually in combination with liquid hydrogen or kerosene. It was used in the very first rocket applications like the V2 missile and Redstone, R-7 or Atlas boosters. LOX is useful in this role because it creates a high specific impulse. LOx was also used in some early ICBMs although more modern ICBMs do not use LOX because its cryogenic properties and need for regular replenishment to replace boiloff make it harder to maintain and launch quickly. LOX also had extensive use in making oxyliquit explosives. Liquid nitrogen has a significantly lower boiling point (77 K) than oxygen (90 K), and vessels containing liquid nitrogen can condense oxygen from air: when most of the nitrogen has evaporated from such a vessel there is a risk that liquid oxygen remaining can react violently with organic material. Conversely, liquid air can be oxygen-enriched by letting it stand in open air; atmospheric oxygen dissolves in it, while nitrogen evaporates preferentially.

External link

- LOx enhanced combustion: [http://www.showmenews.com/2005/Jul/20050703Comm006.asp Lighting a barbeque with liquid oxygen] Do not try this yourself ms:Oksigen cair ja:液体酸素 Category:Rocket fuels

Liquid hydrogen

Liquid hydrogen is a common liquid fuel for rocket applications. In the aerospace industry, its name is often abbreviated to LH2. Hydrogen is found naturally in the molecular H₂ form, thus the H2 part of the name. Hydrogen at normal temperature and pressure is a gas and to exist as a liquid must be pressurised and cooled to a very low temperature, 20.268 K (−423.188 °F). Once liquified it can be maintained as a liquid in pressurised containers. Liquid hydrogen has a very low density of 70.8 kg/m³ (at 20 K), so storage tanks for it have to be quite large. Category:Rocket fuels ja:液体水素

References

In general, a reference is something that refers or points to something else, or acts as a connection or a link between two things. The objects it links may be concrete, such as books or locations, or abstract, such as data, thoughts, or memories. The object which is named by a reference, or to which the reference points, is the referent. The term reference is used with different specialized meanings in a variety of fields, as follows:

Semantics

In semantics, reference is generally construed as the relation between nouns or pronouns and objects that are named by them. Hence the word "John" refers to John; the word "it" refers to some previously specified object. The objects referred to are called the "referents" of the word. Sometimes the word-object relation is called "denotation". Reference is not in general the same as meaning, as words can often be meaningful without having a referent. Fictional and mythological names such as "Bo-Peep" and "Hercules" show that this is possible. As Frege discovered, reference cannot be treated as identical with meaning: "Hesperos" (an ancient Greek name for the evening star) and "Phosphorus" (an ancient Greek name for the morning star) both refer to Venus, but the astronomical fact that '"Hesperos" is "Phosphorus"' can still be informative, even if the 'meanings' of both "Hesperos" and "Phosphorus" are already known. This problem led Frege to distinguish between the sense of a word and its reference.

Art

In Art, a reference is an item from which a work is based. This may include an existing artwork, a reproduced (i.e. photo) or directly observed (i.e. person) object, or the artist's memory.

Computer science

In computer science, references are datatypes which refer to an object elsewhere in memory, and are used to construct a wide variety of data structures such as linked lists. Most programming languages support some form of reference. See reference (computer science). The C++ programming language has a specific type of reference also referred to as a reference; see reference (C Plus Plus).

Geometry

A reference point is a location used to describe another one, by giving the relative position. Similarly we have the concept of frame of reference (both in physics and figuratively), etc.

Libraries

In a library, the word reference may refer to a dictionary, encyclopedia, or other reference work that contains many brief articles that cover a broad scope of knowledge in one book, or a set of books. However, the word reference is also used to mean a book that cannot be taken from the room, or from the building. Many of the books in the reference department of a library are reference works, but some are books that are simply too large or valuable to loan out. Conversely, selected reference works may be shelved with other circulating books, and may be loaned out.

Scholarship

A reference may also be a text<