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Propulsion
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== Vehicular propulsion == ===Air propulsion=== {{main|Powered aircraft}} [[File:Tupolev Tu-95 in flight.jpg|thumb|left|A [[turboprop]]-engined [[Tupolev Tu-95]] ]] An aircraft propulsion system generally consists of an [[aircraft engine]] and some means to generate thrust, such as a [[propeller (aircraft)|propeller]] or a [[propulsive nozzle]]. An aircraft propulsion system must achieve two things. First, the thrust from the propulsion system must balance the drag of the airplane when the airplane is cruising. And second, the thrust from the propulsion system must exceed the drag of the airplane for the airplane to accelerate. The greater the difference between the thrust and the drag, called the excess thrust, the faster the airplane will accelerate.<ref name=":0" /> Some [[aircraft]], like airliners and [[cargo planes]], spend most of their life in a cruise condition. For these airplanes, excess thrust is not as important as high engine efficiency and low fuel usage. Since thrust depends on both the amount of gas moved and the velocity, we can generate high thrust by accelerating a large mass of gas by a small amount, or by accelerating a small mass of gas by a large amount. Because of the aerodynamic efficiency of propellers and fans, it is more fuel efficient to accelerate a large mass by a small amount, which is why high-bypass turbofans and turboprops are commonly used on cargo planes and airliners.<ref name=":0" /> Some aircraft, like [[fighter planes]] or experimental high speed aircraft, require very high excess thrust to accelerate quickly and to overcome the high drag associated with high speeds. For these airplanes, engine efficiency is not as important as very high thrust. Modern [[combat aircraft]] usually have an afterburner added to a [[low bypass turbofan]]. Future [[hypersonic aircraft]] may use some type of [[ramjet]] or rocket propulsion.<ref name=":0" /> ===Ground=== [[File:Opel Ampera (front quarter).jpg|thumb|Wheels are commonly used in ground propulsion]] {{main|Ground propulsion}} Ground propulsion is any mechanism for propelling solid bodies along the ground, usually for the purposes of [[transportation]]. The propulsion system often consists of a combination of an [[engine|engine or motor]], a [[gearbox]] and [[wheel and axle]]s in standard applications. ===Maglev=== [[File:Transrapid 09.jpg|thumb|[[Transrapid]] 09 at the [[Emsland test facility]] in [[Germany]]]] {{main|Maglev}} Maglev (derived from '''mag'''netic '''lev'''itation) is a system of transportation that uses [[magnetic levitation]] to suspend, guide and propel vehicles with magnets rather than using mechanical methods, such as [[wheel and axle|wheels, axles and bearings]]. With maglev a vehicle is levitated a short distance away from a guide way using magnets to create both lift and thrust. Maglev vehicles are claimed to move more smoothly and quietly and to require less maintenance than wheeled [[mass transit]] systems. It is claimed that non-reliance on friction also means that acceleration and deceleration can far surpass that of existing forms of transport. The power needed for levitation is not a particularly large percentage of the overall energy consumption; most of the power used is needed to overcome air resistance ([[drag (physics)|drag]]), as with any other high-speed form of transport. ===Marine=== [[Image:Two Main Engines, V12.jpg|thumb|250px|A view of a ship's engine room]] {{main|Marine propulsion}} Marine propulsion is the mechanism or system used to generate [[thrust]] to move a [[ship]] or [[boat]] across water. While [[paddle]]s and [[sail]]s are still used on some smaller boats, most modern ships are propelled by mechanical systems consisting of a motor or engine turning a [[propeller]], or less frequently, in jet drives, an [[impeller]]. Marine engineering is the discipline concerned with the design of marine [[propulsion system]]s. [[Steam engine]]s were the first mechanical engines used in marine propulsion, but have mostly been replaced by [[two-stroke engine|two-stroke]] or [[four-stroke engine|four-stroke]] diesel engines, outboard motors, and [[gas turbine engine]]s on faster ships. [[nuclear marine propulsion|Nuclear reactors]] producing steam are used to propel [[warship]]s and [[icebreaker]]s, and there have been attempts to utilize them to power commercial vessels. [[Electric motor]]s have been used on [[submarine]]s and [[electric boat]]s and have been proposed for energy-efficient propulsion.<ref>{{cite web |url=http://www.ivt.ntnu.no/imt/electricship/ |title=Energy Efficient - All Electric Ship |access-date=2009-11-25 |url-status=dead |archive-url=https://web.archive.org/web/20090517060017/http://www.ivt.ntnu.no/imt/electricship/ |archive-date=2009-05-17 }}</ref> Recent development in liquified natural gas (LNG) fueled engines are gaining recognition for their low emissions and cost advantages. ===Space=== [[Image:Shuttle Main Engine Test Firing.jpg|thumb|left|A remote camera captures a close-up view of a [[Space Shuttle main engine]] during a test firing at the [[John C. Stennis Space Center]] in [[Hancock County, Mississippi]]]] {{main|Spacecraft propulsion}} Spacecraft propulsion is any method used to accelerate [[spacecraft]] and artificial [[satellite]]s. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. However, most spacecraft today are propelled by forcing a gas from the back/rear of the vehicle at very high speed through a [[rocket engine nozzle|supersonic de Laval nozzle]]. This sort of [[engine]] is called a [[rocket engine]]. All current spacecraft use chemical rockets ([[bipropellant rocket|bipropellant]] or [[solid rocket|solid-fuel]]) for launch, though some (such as the [[Pegasus rocket]] and [[SpaceShipOne]]) have used [[air-breathing engine]]s on their [[Multistage rocket|first stage]]. Most satellites have simple reliable chemical thrusters (often [[monopropellant rocket]]s) or [[resistojet rocket]]s for [[orbital station-keeping]] and some use [[momentum wheel]]s for [[Spacecraft attitude control|attitude control]]. Soviet bloc satellites have used [[Electrically powered spacecraft propulsion|electric propulsion]] for decades, and newer Western geo-orbiting spacecraft are starting to use them for north–south stationkeeping and orbit raising. Interplanetary vehicles mostly use chemical rockets as well, although a few have used [[ion thruster]]s and [[Hall-effect thruster]]s (two different types of electric propulsion) to great success. {{clear}} ===Cable=== {{main|Cable car (railway)}} A cable car is any of a variety of transportation systems relying on cables to pull vehicles along or lower them at a steady rate. The terminology also refers to the vehicles on these systems. The cable car vehicles are motor-less and engine-less and they are pulled by a cable that is rotated by a motor off-board.
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