A railgun is a device that uses electromagnetic force to launch high velocity projectiles, by means of a sliding armature, accelerated along a pair of conductive rails. It is constructed as a weapon, the projectile does not contain explosives, instead relying on the projectile's high speed to inflict damage; the railgun uses a pair of parallel conductors -'rails' - along which a sliding armature is accelerated by the electromagnetic effects of a current that flows down one rail, into the armature and back along the other rail. It is based on principles similar to those of the homopolar motor; as of 2018, railguns have been researched as weapons utilising electromagnetic forces to impart a high kinetic energy to a projectile rather than using conventional propellants. While explosive-powered military guns cannot achieve a muzzle velocity of more than ~2 km/s, railguns can exceed 3 km/s. For a similar projectile, the range of railguns may exceed that of conventional guns; the destructive force of a projectile depends on its kinetic energy at the point of impact and due to the high velocity of a railgun-launched projectile, their destructive force may be much greater than conventionally launched projectiles of the same size.
The absence of explosive propellants or warheads to store and handle, as well as the low cost of projectiles compared to conventional weaponry, come as additional advantages. Not withstanding the above advantages, railguns are still much at the research stage, it remains to be seen whether or not railguns will be deployed as practical military weapons. Any trade-off analysis between electromagnetic propulsion systems and chemical propellants for weapons applications must factor in the novelty and complexity of the pulsed power supplies that are needed for electromagnetic launcher systems. In addition to military applications, NASA has proposed to use a railgun to launch "wedge-shaped aircraft with scramjets" to high-altitude at Mach 10, where they will fire a small payload into orbit using conventional rocket propulsion; the extreme g-forces involved with direct railgun ground-launch to space may restrict the usage to only the sturdiest of payloads. Alternatively long rail systems may be used to reduce the required launch acceleration.
The railgun in its simplest form differs from a traditional electric motor in that no use is made of additional field windings. This basic configuration is formed by a single loop of current and thus requires high currents to produce sufficient accelerations. A common variant of this configuration is the augmented railgun in which the driving current is channelled through additional pairs of parallel conductors, arranged to increase the magnetic field experienced by the moving armature; these arrangements reduce the current required for a given acceleration. In electric motor terminology, augmented railguns are series-wound configurations; some railguns use strong Neodymium magnets with the field perpendicular to the current flow to increase the force on the projectile. The armature may be an integral part of the projectile, but it may be configured to accelerate a separate, electrically isolated or non-conducting projectile. Solid, metallic sliding conductors are the preferred form of railgun armature but "plasma" or "hybrid" armatures can be used.
A plasma armature is formed by an arc of ionised gas, used to push a solid, non-conducting payload in a similar manner to the propellant gas pressure in a conventional gun. A hybrid armature uses a pair of "plasma" contacts to interface a metallic armature to the gun rails. Solid armatures may "transition" into hybrid armatures after a particular velocity threshold is exceeded. A railgun requires a pulsed DC power supply. For potential military applications, railguns are of interest because they can achieve much greater muzzle velocities than guns powered by conventional chemical propellants. Increased muzzle velocities with better aerodynamically streamlined projectiles can convey the benefits of increased firing ranges while, in terms of target effects, increased terminal velocities can allow the use of kinetic energy rounds incorporating hit-to-kill guidance, as replacements for explosive shells. Therefore, typical military railgun designs aim for muzzle velocities in the range of 2000–3500 m/s with muzzle energies of 5–50 Megajoules.
For comparison, 50MJ is equivalent to the kinetic energy of a school bus weighing 5 metric tons, travelling at 509 km/h. For single loop railguns, these mission requirements require launch currents of a few million amperes, so a typical railgun power supply might be designed to deliver a launch current of 5 MA for a few milliseconds; as the magnetic field strengths required for such launches will be 10 tesla, most contemporary railgun designs are "air-cored", i.e. they do not use ferromagnetic materials such as iron to enhance the magnetic flux. However, if the barrel is made of a magnetically permeable material, the magnetic field strength increases due to the increase in permeability; this automatically increases the force. Railgun velocities fall within the range of those achievable by two-stage light-gas guns. Another light gas gun, the Combustion Light Gas Gun in a 155 mm prototype form was
Eric Roberts Laithwaite was a British electrical engineer, known as the "Father of Maglev" for his development of the linear induction motor and maglev rail system. Eric Roberts Laithwaite was born in Atherton, Lancashire, on 14 June 1921, raised in the Fylde and educated at Kirkham Grammar School, he joined the Royal Air Force in 1941. Through his service in World War II, he rose to the rank of Flying Officer, becoming a test engineer for autopilot technology at the Royal Aircraft Establishment in Farnborough. On demobilization in 1946, he attended the University of Manchester to study electrical engineering, his work on the Manchester Mark I computer earned him his master's degree. His subsequent doctoral work started his interest in linear induction motors, he derived an equation for "goodness" which parametrically describes the efficiency of a motor in general terms, showed that it tended to imply that large motors are more efficient. He became professor of heavy electrical engineering at Imperial College London in 1964 where he continued his successful development of the linear motor.
He was involved in creating a self-stable magnetic levitation system called Magnetic river which appeared in the film The Spy Who Loved Me where it levitated and propelled a tray along a table to decapitate a seated dummy. He worked at applying linear motors on the Tracked Hovercraft until its cancellation. In the 1980s, he was involved in creating a device to extract energy from sea waves. Although the technology was successful in trials, it could not be made storm proof, so it never became a commercial success. Laithwaite was an able communicator. Notable among these were his Royal Institution Christmas Lectures to young people in 1966 and 1974; the latter of these made much of the surprising properties of the gyroscope. In 1974, Laithwaite was invited by the Royal Institution to give a talk on a subject of his own choosing, he decided to lecture about gyroscopes, a subject in which he had only become interested. His interest had been aroused by an amateur inventor named Alex Jones, who contacted Laithwaite about a reactionless propulsion drive he had invented.
After seeing a demonstration of Jones's small prototype, Laithwaite became convinced that "he had seen something impossible". In his lecture before the Royal Institution he claimed that gyroscopes weigh less when spinning and, to demonstrate this, he showed that he could lift a spinning gyroscope mounted on the end of a rod with one hand but could not do so when the gyroscope was not spinning; this was discussed in the BBC science series'Horizon - 2015-2016: 2. Project Greenglow - The Quest for Gravity Control'. In his 1974 lectures, Laithwaite suggested that Newton's laws of motion could not account for the behaviour of gyroscopes and that they could be used as a means of reactionless propulsion; the members of the Royal Institution rejected his lectures were not published. His lectures were subsequently published independently as Engineer Through The Looking-Glass. Despite this rejection and the fact that Laithwaite acknowledged that gyroscopes behave in accord with Newtonian mechanics, he continued to explore gyroscopic behaviour, maintaining the belief that some form of reactionless propulsion could be derived from them.
Laithwaite set up Gyron Ltd with William Dawson and, in 1993, applied for a patent entitled "Propulsion System". See US5860317, for the US, PCT application for patents respectively. A United States Patent, Number 5860317, was granted in 1999. Although Laithwaite is best known for his ideas concerning gyroscopes, he held an idea concerning moths, he proposed. He persisted in this belief after the pheromone which they use had been isolated and could be bought "over-the-counter" — contradicting his account. However, he had argued in 1960 that there must be two different mechanisms for detecting pheromones: The orthodox account of chemical-gradients, some method for long-distance detection when the wind was in an unfavourable direction — and the only credible solution had to be electromagnetic; this explanation did not account for where the necessary energy might come from — a matter taken up by P. S. Callahan, though he too suffered considerable controversy. Laithwaite retired from Imperial College in 1986, but was offered no other research post until 1990, when he became Visiting Professor at the University of Sussex.
He was persuaded by George Scelzo of PRT Maglev Systems in Chicago to submit a proposal to NASA for an electromagnetic launch assist track inspired by John C. Mankins of NASA, he died within weeks of the contract being awarded. The initial stage has been continued by William Dawson and the contract with PRT for this development is still active; the track uses both levitation coils and linear induction motors and it can be seen in the "Magnets" episode of Modern Marvels on the History Channel. Laithwaite was a keen entomologist and the co-author of The Dictionary of Butterflies and Moths, he married, in 1951. A Radiation Theory of the Assembling of Moths The Entomologis
A rotation is a circular movement of an object around a center of rotation. A three-dimensional object can always be rotated around an infinite number of imaginary lines called rotation axes. If the axis passes through the body's center of mass, the body is said to rotate upon itself, or spin. A rotation about an external point, e.g. the Earth about the Sun, is called a revolution or orbital revolution when it is produced by gravity. The axis is called a pole. Mathematically, a rotation is a rigid body movement which, unlike a translation, keeps a point fixed; this definition applies to rotations within both two and three dimensions All rigid body movements are rotations, translations, or combinations of the two. A rotation is a progressive radial orientation to a common point; that common point lies within the axis of that motion. The axis is 90 degrees perpendicular to the plane of the motion. If the axis of the rotation lies external of the body in question the body is said to orbit. There is no fundamental difference between a “rotation” and an “orbit” and or "spin".
The key distinction is where the axis of the rotation lies, either within or outside of a body in question. This distinction can be demonstrated for "non rigid" bodies. If a rotation around a point or axis is followed by a second rotation around the same point/axis, a third rotation results; the reverse of a rotation is a rotation. Thus, the rotations around a point/axis form a group. However, a rotation around a point or axis and a rotation around a different point/axis may result in something other than a rotation, e.g. a translation. Rotations around the x, y and z axes are called principal rotations. Rotation around any axis can be performed by taking a rotation around the x axis, followed by a rotation around the y axis, followed by a rotation around the z axis; that is to say, any spatial rotation can be decomposed into a combination of principal rotations. In flight dynamics, the principal rotations are known as yaw and roll; this terminology is used in computer graphics. In astronomy, rotation is a observed phenomenon.
Stars and similar bodies all spin around on their axes. The rotation rate of planets in the solar system was first measured by tracking visual features. Stellar rotation is measured by tracking active surface features; this rotation induces a centrifugal acceleration in the reference frame of the Earth which counteracts the effect of gravity the closer one is to the equator. One effect is that an object weighs less at the equator. Another is that the Earth is deformed into an oblate spheroid. Another consequence of the rotation of a planet is the phenomenon of precession. Like a gyroscope, the overall effect is a slight "wobble" in the movement of the axis of a planet; the tilt of the Earth's axis to its orbital plane is 23.44 degrees, but this angle changes slowly. While revolution is used as a synonym for rotation, in many fields astronomy and related fields, revolution referred to as orbital revolution for clarity, is used when one body moves around another while rotation is used to mean the movement around an axis.
Moons revolve around their planet, planets revolve about their star. The motion of the components of galaxies is complex, but it includes a rotation component. Most planets in our solar system, including Earth, spin in the same direction; the exceptions are Uranus. Uranus rotates nearly on its side relative to its orbit. Current speculation is that Uranus started off with a typical prograde orientation and was knocked on its side by a large impact early in its history. Venus may be thought of as rotating backwards; the dwarf planet Pluto is anomalous in other ways. The speed of rotation is given by period; the time-rate of change of angular frequency is angular acceleration, caused by torque. The ratio of the two is given by the moment of inertia; the angular velocity vector describes the direction of the axis of rotation. The torque is an axial vector; the physics of the rotation around a fixed axis is mathematically described with the axis–angle representation of rotations. According to the right-hand rule, the direction away from the observer is associated with clockwise rotation and the direction towards the observer with counterclockwise rotation, like a screw.
The laws of physics are believed to be invariant under any fixed rotation. In modern physical cosmology, the cosmological principle is the notion that the distribution of matter in the universe is homogeneous and isotropic when viewed on a large enough scale, since the forces are expected to act uniformly throughout the universe and have no preferred direction, should, produce no observable irregularities in the large scale structuring over the course of evolution of the matter field, laid down by the Big Bang. In particular, for a system which behaves the same regardless of how it is oriented in space, its Lagrangian is rotationally invariant. According to Noether's theorem, if the action (the integral over ti
CRRC Qingdao Sifang
CRRC Qingdao Sifang Co. Ltd. known as CSR Qingdao Sifang Locomotive & Rolling Stock Co. Ltd. is a Chinese rolling stock manufacturer based in Qingdao, Shandong province. China Railway CRH1 as a joint venture between Bombardier Transportation CRH2 as a joint venture between Kawasaki Heavy Industries CRH380A Fuxing Hao Class CR400AF MTR Corporation Vibrant Express id: Kereta Cepat Indonesia–China Beijing Subway Line 1, Line 4, Daxing line, Line 8, Line 14, Line 16, Batong line, Changping line New Airport line Guangzhou Metro Line 4, Line 5 and Line 6 in a joint venture with Kawasaki Heavy Industries Zhengzhou Metro Line 1 MTR Corporation MTR Urban Lines Vision Train Chengdu Metro Line 1 Line 2 Shenyang Metro Line 2 Tianjin Metro Line 3, Line 6 Singapore Mass Rapid Transit North South MRT line, East West MRT line, Thomson-East Coast MRT line Kawasaki Heavy Industries & CSR Qingdao Sifang C151A Kawasaki Heavy Industries & CSR Qingdao Sifang C151B Kawasaki Heavy Industries & CSR Qingdao Sifang CT251 Kawasaki Heavy Industries & CRRC Sifang C151C Qingdao Metro Chongqing Rail Transit Line 4, Line 5, Line 10 Chicago Transit Authority 7000 series China Railway CRH6 Trenes Argentinos CSR EMU Qingdao Tram zh:青岛有轨电车 as a joint venture with Skoda Transportation China Railway zh:中国铁路25T型客车 as a joint venture between Bombardier Transportation Coaches for Turkmenistan SEPTA Bi-level cars for SEPTA Regional Rail lines Sri Lanka Railways Sri Lanka Railways S9 Sri Lanka Railways S10 Sri Lanka Railways S12 Iraqi Republic Railways manufacture of permanent magnet straddled-type monorail train manufacture of bi-level cars Bombardier Sifang Transportation Ltd was established in 1998 as a joint venture between Bombardier Transportation and Sifang Locomotive and rolling stock company limited as a company for the production of high speed trains and high quality coaches.
By 2009 it had delivered over 1000 units, including the CRH1E high speed sleeper trains, had secured an order for 80 CRH380D high speed trains in an order estimated to be worth €2.7 billion in total. Kawasaki Heavy Industries co-operated with CRRC Sifang Co Ltd. in year 2009 to produce the C151A trains, the fourth generation MRT train for SMRT Trains, in Singapore. A total of 22 trainsets were built with 6 carriages each. By 2010 half of the trainsets are completed, testing was done in 2011 by Kawasaki Heavy Industries, before full delivery in December 2011; these trains now serves the North South East West Line in Singapore. Another 78 cars of C151A trains which in production to be delivered by 2014. In 2012, KHI and CSR Sifang will collaborate to manufacture the new 168 cars of C151B trains and will deliver from 2015 till 2017. An Additional 174 cars of C151B trains were ordered in 2014 and will be delivered from 2017 till 2019 and a total of 57 trains but they have been reduced to 45 set as they announced and the first trainset has delivered in 2015, Another 12 set of C151C trains are expected to be added by 2019 which ordered in 2015.
In 2013, Kawasaki Heavy Industries planned to sue CSR Sifang for patent infringement after their partnership was dissolved. KHI said it regretted entering into the partnership. KHI subsequently dropped the action. In 2014, LTA had ordered the new 364 cars of T251 Trains with manufacture by KHI and CSR Sifang for future Thomson-East Coast Line and will have automated and driverless trains, the first trains in Singapore to have 5 doors on each side and each carriage, These 91 new trains will deliver from 2018 onwards. On 5 July 2016, a Hong Kong Based non-profit news organization FactWire had broken the news of SMRT C151A suffering from multiple defects relating to Chinese-made materials and posted the entire investigative works in YouTube. and most of its claims are subsequently acknowledged by the rail operator SMRT and the transport authorities in Singapore, Land Transport Authority. The entire issue has since generated a huge amount of controversies in Hong Kong and Singapore with some rumors spreading in the Internet as well.
See the main articles for more details. Bombardier Transportation - Facilities in China - Three Manufacturing Joint Ventures, Bombardier Transportation, 2009 CRRC Qingdao Sifang http://mothership.sg/2016/07/we-summarise-wth-is-going-on-with-mrt-train-cars-being-shipped-back-to-china/
Maglev is a system of train transportation that uses two sets of magnets, one set to repel and push the train up off the track another set to move the'floating train' ahead at great speed taking advantage of the lack of friction. Along certain "medium range" routes Maglev can compete favorably with high-speed rail and airplanes. With Maglev technology, there are no moving parts; the train is the only moving part. The train travels along a guideway of magnets which control the train's speed. Maglev trains are therefore quieter and smoother than conventional trains, have the potential for much higher speeds. Maglev vehicles have set several speed records and Maglev trains can accelerate and decelerate much faster than conventional trains; the power needed for levitation is not a large percentage of the overall energy consumption of a high speed maglev system. Overcoming drag, which makes all land transport more energy intensive at higher speeds, takes up the most energy. Vactrain technology has been proposed as a means to overcome this limitation.
Maglev systems have been much more expensive to construct than conventional train systems, although the simpler construction of maglev vehicles makes them cheaper to manufacture and maintain. Despite over a century of research and development, maglev transport systems are in operation in just three countries; the incremental benefits of maglev technology have been hard to justify against cost and risk where there is an existing or proposed conventional high speed train line with spare passenger carrying capacity, as in high-speed rail in Europe, the High Speed 2 in the UK and Shinkansen in Japan. In the late 1940s, the British electrical engineer Eric Laithwaite, a professor at Imperial College London, developed the first full-size working model of the linear induction motor, he became professor of heavy electrical engineering at Imperial College in 1964, where he continued his successful development of the linear motor. Since linear motors do not require physical contact between the vehicle and guideway, they became a common fixture on advanced transportation systems in the 1960s and 70s.
Laithwaite joined one such project, the Tracked Hovercraft, although the project was cancelled in 1973. The linear motor was suited to use with maglev systems as well. In the early 1970s, Laithwaite discovered a new arrangement of magnets, the magnetic river, that allowed a single linear motor to produce both lift and forward thrust, allowing a maglev system to be built with a single set of magnets. Working at the British Rail Research Division in Derby, along with teams at several civil engineering firms, the "transverse-flux" system was developed into a working system; the first commercial maglev people mover was called "MAGLEV" and opened in 1984 near Birmingham, England. It operated on an elevated 600 m section of monorail track between Birmingham Airport and Birmingham International railway station, running at speeds up to 42 km/h; the system was closed in 1995 due to reliability problems. High-speed transportation patents were granted to various inventors throughout the world. Early United States patents for a linear motor propelled train were awarded to German inventor Alfred Zehden.
The inventor was awarded U. S. Patent 782,312 and U. S. Patent RE12,700. In 1907, another early electromagnetic transportation system was developed by F. S. Smith. A series of German patents for magnetic levitation trains propelled by linear motors were awarded to Hermann Kemper between 1937 and 1941. An early maglev train was described in U. S. Patent 3,158,765, "Magnetic system of transportation", by G. R. Polgreen; the first use of "maglev" in a United States patent was in "Magnetic levitation guidance system" by Canadian Patents and Development Limited. In 1959, while delayed in traffic on the Throgs Neck Bridge, James Powell, a researcher at Brookhaven National Laboratory, thought of using magnetically levitated transportation. Powell and BNL colleague Gordon Danby worked out a MagLev concept using static magnets mounted on a moving vehicle to induce electrodynamic lifting and stabilizing forces in specially shaped loops, such as figure of 8 coils on a guideway; these were patented in 1968-1969.
Transrapid 05 was the first maglev train with longstator propulsion licensed for passenger transportation. In 1979, a 908 m track was opened in Hamburg for the first International Transportation Exhibition. Interest was sufficient that operations were extended three months after the exhibition finished, having carried more than 50,000 passengers, it was reassembled in Kassel in 1980. In 1979, in the USSR, in the town of Ramenskoye was built an experimental test site for running experiments with cars on magnetic suspension; the test site consisted of a 600-meter ramp, extended to 980 meters. From the late 1970s to the 1980s five prototypes of cars were built that received designations from TP-01 to TP-05; the early cars were supposed to reach the speed up to 100 km/h. The construction of a maglev track using the technology from Ramenskoye started in Armenian SSR in 1987 and was planned to be completed in 1991; the track was supposed to connect the cities of Sevan via the city of Abovyan. The original design speed was 250 km/h, lowered to 180 km/h.
However, the Spitak earthquake in 1988 and the Nagorno-Karabakh war caused the project to freeze. In the end the overpass was only constructed. In the early 1990s, the maglev theme was continued by th
London is the capital and largest city of both England and the United Kingdom. Standing on the River Thames in the south-east of England, at the head of its 50-mile estuary leading to the North Sea, London has been a major settlement for two millennia. Londinium was founded by the Romans; the City of London, London's ancient core − an area of just 1.12 square miles and colloquially known as the Square Mile − retains boundaries that follow its medieval limits. The City of Westminster is an Inner London borough holding city status. Greater London is governed by the Mayor of the London Assembly. London is considered to be one of the world's most important global cities and has been termed the world's most powerful, most desirable, most influential, most visited, most expensive, sustainable, most investment friendly, most popular for work, the most vegetarian friendly city in the world. London exerts a considerable impact upon the arts, education, fashion, healthcare, professional services and development, tourism and transportation.
London ranks 26 out of 300 major cities for economic performance. It is one of the largest financial centres and has either the fifth or sixth largest metropolitan area GDP, it is the most-visited city as measured by international arrivals and has the busiest city airport system as measured by passenger traffic. It is the leading investment destination, hosting more international retailers and ultra high-net-worth individuals than any other city. London's universities form the largest concentration of higher education institutes in Europe. In 2012, London became the first city to have hosted three modern Summer Olympic Games. London has a diverse range of people and cultures, more than 300 languages are spoken in the region, its estimated mid-2016 municipal population was 8,787,892, the most populous of any city in the European Union and accounting for 13.4% of the UK population. London's urban area is the second most populous in the EU, after Paris, with 9,787,426 inhabitants at the 2011 census.
The population within the London commuter belt is the most populous in the EU with 14,040,163 inhabitants in 2016. London was the world's most populous city from c. 1831 to 1925. London contains four World Heritage Sites: the Tower of London. Other landmarks include Buckingham Palace, the London Eye, Piccadilly Circus, St Paul's Cathedral, Tower Bridge, Trafalgar Square and The Shard. London has numerous museums, galleries and sporting events; these include the British Museum, National Gallery, Natural History Museum, Tate Modern, British Library and West End theatres. The London Underground is the oldest underground railway network in the world. "London" is an ancient name, attested in the first century AD in the Latinised form Londinium. Over the years, the name has attracted many mythicising explanations; the earliest attested appears in Geoffrey of Monmouth's Historia Regum Britanniae, written around 1136. This had it that the name originated from a supposed King Lud, who had taken over the city and named it Kaerlud.
Modern scientific analyses of the name must account for the origins of the different forms found in early sources Latin, Old English, Welsh, with reference to the known developments over time of sounds in those different languages. It is agreed; this was adapted into Latin as Londinium and borrowed into Old English, the ancestor-language of English. The toponymy of the Common Brythonic form is much debated. A prominent explanation was Richard Coates's 1998 argument that the name derived from pre-Celtic Old European *lowonida, meaning "river too wide to ford". Coates suggested that this was a name given to the part of the River Thames which flows through London. However, most work has accepted a Celtic origin for the name, recent studies have favoured an explanation along the lines of a Celtic derivative of a proto-Indo-European root *lendh-, combined with the Celtic suffix *-injo- or *-onjo-. Peter Schrijver has suggested, on these grounds, that the name meant'place that floods'; until 1889, the name "London" applied to the City of London, but since it has referred to the County of London and Greater London.
"London" is sometimes written informally as "LDN". In 1993, the remains of a Bronze Age bridge were found on the south foreshore, upstream of Vauxhall Bridge; this bridge either reached a now lost island in it. Two of those timbers were radiocarbon dated to between 1750 BC and 1285 BC. In 2010 the foundations of a large timber structure, dated to between 4800 BC and 4500 BC, were found on the Thames's south foreshore, downstream of Vauxhall Bridge; the function of the mesolithic structure is not known. Both structures are on the south bank. Although there is evidence of scattered Brythonic settlements in the area, the first major settlement was founded by the Romans about four years after the invasion
Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. Space propulsion or in-space propulsion deals with propulsion systems used in the vacuum of space and should not be confused with launch vehicles. Several methods, both pragmatic and hypothetical, have been developed each having its own drawbacks and advantages. Most satellites have simple reliable chemical thrusters or resistojet rockets for orbital station-keeping and some use momentum wheels for attitude control. Soviet bloc satellites have used electric propulsion for decades, newer Western geo-orbiting spacecraft are starting to use them for north-south station-keeping and orbit raising. Interplanetary vehicles use chemical rockets as well, although a few have used Ion thrusters and Hall effect thrusters to great success. Artificial satellites must be launched into orbit after which they must be placed in their nominal orbit. Once in the desired orbit, they need some form of attitude control so that they are pointed with respect to the Earth, the Sun, some astronomical object of interest.
They are subject to drag from the thin atmosphere, so that to stay in orbit for a long period of time some form of propulsion is necessary to make small corrections. Many satellites need to be moved from one orbit to another from time to time, this requires propulsion. A satellite's useful life is over once it has exhausted its ability to adjust its orbit. For interplanetary travel, a spacecraft must use its engines to leave Earth's orbit. Once it has done so, it must somehow make its way to its destination. Current interplanetary spacecraft do this with a series of short-term trajectory adjustments. In between these adjustments, the spacecraft moves along its trajectory with a constant velocity; the most fuel-efficient means to move from one circular orbit to another is with a Hohmann transfer orbit: the spacecraft begins in a circular orbit around the Sun. A short period of thrust in the direction of motion accelerates or decelerates the spacecraft into an elliptical orbit around the Sun, tangential to its previous orbit and to the orbit of its destination.
The spacecraft falls along this elliptical orbit until it reaches its destination, where another short period of thrust accelerates or decelerates it to match the orbit of its destination. Special methods such as aerobraking or aerocapture are sometimes used for this final orbital adjustment; some spacecraft propulsion methods such as solar sails provide low but inexhaustible thrust. The concept has been tested by the Japanese IKAROS solar sail spacecraft. No spacecraft capable of short duration interstellar travel has yet been built, but many hypothetical designs have been discussed; because interstellar distances are great, a tremendous velocity is needed to get a spacecraft to its destination in a reasonable amount of time. Acquiring such a velocity on launch and getting rid of it on arrival remains a formidable challenge for spacecraft designers; when in space, the purpose of a propulsion system is to change the v, of a spacecraft. Because this is more difficult for more massive spacecrafts, designers discuss spacecraft performance in amount of change in momentum per unit of propellant consumed called specific impulse.
Higher the specific impulse, better the efficiency. Ion propulsion engines have high specific impulse and low thrust whereas chemical rockets like monopropellant or bipropellant rocket engines have a low specific impulse but high thrust; when launching a spacecraft from Earth, a propulsion method must overcome a higher gravitational pull to provide a positive net acceleration. In orbit, any additional impulse very tiny, will result in a change in the orbit path; the rate of change of velocity is called acceleration, the rate of change of momentum is called force. To reach a given velocity, one can apply a small acceleration over a long period of time, or one can apply a large acceleration over a short time. One can achieve a given impulse with a large force over a short time or a small force over a long time; this means that for manoeuvring in space, a propulsion method that produces tiny accelerations but runs for a long time can produce the same impulse as a propulsion method that produces large accelerations for a short time.
When launching from a planet, tiny accelerations cannot overcome the planet's gravitational pull and so cannot be used. Earth's surface is situated deep in a gravity well; the escape velocity required to get out of it is 11.2 kilometers/second. As human beings evolved in a gravitational field of 1g, an ideal propulsion system would be one that provides a continuous acceleration of 1g; the occupants of a rocket or spaceship having such a propulsion system would be free from all the ill effects of free fall, such as nausea, muscular weakness, reduced sense of taste, or leaching of calcium from their bones. The law of conservation of momentum means that in order for a propulsion method to change the momentum of a space craft it must change the momentum of something else as well. A few designs take advantage of things like magnetic fields or light pressure in order to chan