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PROPULSION
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Propulsion is a means of creating force leading tomovement.
A propulsion system has a source of mechanical
power (some type of engine or motor, muscles), andsome means of using this power to generate force,such as wheel and axles, propellers, a propulsivenozzle, wings, fins or legs.
The term propulsion is derived from two Latin words:pro meaning before or forwards and pellere meaningto drive.
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Air propulsion
An aircraft propulsion system generally consists of an aircraft
engine and some means to generate thrust, such as a
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. In fact, the greater
the difference between the thrust and the drag, called theexcess thrust, the faster the airplane will accelerate.
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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 byaccelerating 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.That is why we find high bypass fans and turboprops on cargo
planes and airliners.
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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 military aircraft typically employafterburners on a low bypass turbofan core. Future
hypersonic aircraft will employ some type of ramjet or rocket
propulsion.
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Spacecraft propulsion
Spacecraft propulsion is any method used to accelerate
spacecraft and artificial satellites. most spacecraft today are
propelled by forcing a gas from the back/rear of the vehicle at
very high speed through a supersonic de Laval nozzle. This
sort of engine is called as rocket engine.
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AIRCRAFT ENGINE
An aircraft engine is the component of the propulsion system
for an aircraft that generates mechanical power. Aircraft
engines are almost always either lightweight piston engines or
gas turbines.
Reciprocating (piston) engines
Jet engines
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Reciprocating (piston) engines
V-type engine
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In-line engine
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horizontally opposed air-cooled aero
engine
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Rotary engine
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Jet engine
A jet engine is a reaction engine discharging a fast
moving jet that generates thrust by jet propulsion in
accordance with Newton's laws of motion. This
broad definition of jet engines includes turbojets,turbofans, rockets, ramjets, and pulse jets. In
general, jet engines are combustion engines but non-
combusting forms also exist.
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The term jet engine loosely refers to an internal combustionair breathing jet engine (a duct engine). These typically consist
of an engine with a rotary (rotating) air compressor powered
by a turbine ("Brayton cycle"), with the leftover power
providing thrust via a propelling nozzle. Jet aircraft use these
types of engines for long-distance travel. Early jet aircraft used
turbojet engines which were relatively inefficient for subsonic
flight. Modern subsonic jet aircraft usually use high-bypass
turbofan engines. These engines offer high speed and greaterfuel efficiency than piston and propeller aero engines over
long distances.
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Jet engine types
Air breathing engine
Non Air breathing engine
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Commonly aircraft are propelled by air breathing jet
engines. Most air breathing jet engines that are in
use are turbofan jet engines which give good
efficiency at speeds just below the speed of sound.
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Turbine powered
Gas turbines are rotary engines that extract energy from a
flow of combustion gas. They have an upstream compressor
coupled to a downstream turbine with a combustion
chamber in-between. In aircraft engines, those three core
components are often called the "gas generator.
Turbojet
Turbofan
Turboprop and turboshaft
Propfan
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Ram powered (non-Turbine powered)
Ram powered jet engines are air breathing enginessimilar to gas turbine engines and they both followthe Brayton cycle. Gas turbine and ram poweredengines differ, however, in how they compress the
incoming airflow. Whereas gas turbine engines useaxial or centrifugal compressors to compressincoming air, ram engines rely only on aircompressed through the inlet or diffuser. Ram
powered engines are considered the most simpletype of air breathing jet engine because they cancontain no moving parts.
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Types
Pulsejet
Ramjet
Scramjet
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Turbojet engine
A turbojet engine is a gas turbine engine that works by
compressing air with an inlet and a compressor (axial,
centrifugal, or both), mixing fuel with the compressed air,
burning the mixture in the combustor, and then passing the
hot, high pressure air through a turbine and a nozzle. Thecompressor is powered by the turbine, which extracts
energy from the expanding gas passing through it. The
engine converts internal energy in the fuel to kinetic energy
in the exhaust, producing thrust. All the air ingested by theinlet is passed through the compressor, combustor, and
turbine, unlike the turbofan engine
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Turbofan
A turbofan engine is a gas turbine engine that is very similar
to a turbojet. Like a turbojet, it uses the gas generator core
(compressor, combustor, turbine) to convert internal energy in
fuel to kinetic energy in the exhaust. Turbofans differ from
turbojets in that they have an additional component, a fan.Like the compressor, the fan is powered by the turbine section
of the engine. Unlike the turbojet, some of the flow
accelerated by the fan bypasses the gas generator core of the
engine and is exhausted through a nozzle. The bypassed flowis at lower velocities, but a higher mass, making thrust
produced by the fan more efficient than thrust produced by
the core. Turbofans are generally more efficient than
turbojets at subsonic speeds, but they have a larger frontal
area which generates more drag.
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There are two general types of turbofan engines, lowbypass and high bypass. Low bypass turbofans have abypass ratio of around 2:1 or less, meaning that for eachkilogram of air that passes through the core of the engine,two kilograms or less of air bypass the core. Low bypass
turbofans often used a mixed exhaust nozzle meaning thatthe bypassed flow and the core flow exit from the samenozzle. High bypass turbofans have larger bypass ratios,sometimes on the order of 5:1 or 6:1. These turbofans canproduce much more thrust than low bypass turbofans or
turbojets because of the large mass of air that the fan canaccelerate, and are often more fuel efficient than lowbypass turbofans or turbojets.
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Turboprop engines Turboprop engines are jet engine derivatives, still gas
turbines, that extract work from the hot-exhaust jet to turna rotating shaft, which is then used to produce thrust bysome other means. While not strictly jet engines in thatthey rely on an auxiliary mechanism to produce thrust,turboprops are very similar to other turbine-based jet
engines, and are often described as such. In turboprop engines, a portion of the engines' thrust is
produced by spinning a propeller, rather than relying solelyon high-speed jet exhaust. As their jet thrust is augmentedby a propeller, turboprops are occasionally referred to as a
type of hybrid jet engine. While many turboprops generatethe majority of their thrust with the propeller. Turbopropsgenerally have better performance than turbojets orturbofans at low speeds where propeller efficiency is high,but become increasingly noisy and inefficient at high
speeds.
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Turboshaft
Turbo shaft engines are very similar to turboprops, differingin that nearly all energy in the exhaust is extracted to spinthe rotating shaft, which is used to power machinery ratherthan a propeller, they therefore generate little to no jetthrust and are often used to power helicopters. Turboshaft
engines are used primarily for helicopters and auxiliarypower units. A turboshaft engine is very similar to aturboprop, with a key difference: In a turboprop thepropeller is supported by the engine and the engine isbolted to the airframe. In a turboshaft, the engine does not
provide any direct physical support to the helicopter'srotors. The rotor is connected to a transmission, whichitself is bolted to the airframe, and the turboshaft enginesimply feeds the transmission via a rotating shaft.
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Propfan
A propfan engine (also called "unducted fan", "open rotor",or "ultra-high bypass") is a jet engine that uses its gasgenerator to power an exposed fan, similar to turbopropengines. Like turboprop engines, propfans generate most oftheir thrust from the propeller and not the exhaust jet. The
primary difference between turboprop and propfan designis that the propeller blades on a propfan are highly swept toallow them to operate at speeds around Mach 0.8, which iscompetitive with modern commercial turbofans. Theseengines have the fuel efficiency advantages of turboprops
with the performance capability of commercial turbofans.While significant research and testing (including flighttesting) has been conducted on propfans, no propfanengines have entered production.
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Pulse jets
Pulse jets are mechanically simple devices thatin arepeating cycledraw air through a no-return valve atthe front of the engine into a combustion chamber andignited it. The combustion forces the exhaust gases outthe back of the engine. It produces power as a series ofpulses rather than as a steady output, hence the name.The only application of this type of engine was theGerman unmanned V1 flying bomb of World War II.Though the same engines were also used
experimentally for ersatz fighter aircraft, the extremelyloud noise generated by the engines causedmechanical damage to the airframe that was sufficientto make the idea unworkable.
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Ramjet
Ramjets are the most basic type of ram powered jetengines. They consist of three sections; an inlet tocompress incoming air, a combustor to inject and combustfuel, and a nozzle to expel the hot gases and producethrust. Ramjets require a relatively high speed to efficiently
compress the oncoming air, so ramjets cannot operate at astandstill and they are most efficient at supersonic speeds.A key trait of ramjet engines is that combustion is done atsubsonic speeds. The supersonic incoming air isdramatically slowed through the inlet, where it is then
combusted at the much slower, subsonic, speeds. Thefaster the incoming air is, however, the less efficient itbecomes to slow it to subsonic speeds. Therefore ramjetengines are limited to approximately Mach 5.
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S j
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Scramjet Scramjets are mechanically very similar to ramjets. Like a
ramjet, they consist of an inlet, a combustor, and a nozzle.
The primary difference between ramjets and scramjets isthat scramjets do not slow the oncoming airflow tosubsonic speeds for combustion, they use supersoniccombustion instead. The name "scramjet" comes from"supersonic combusting ramjet." Since scramjets use
supersonic combustion they can operate at speeds aboveMach 6 where traditional ramjets are too inefficient.Another difference between ramjets and scramjets comesfrom how each type of engine compresses the oncomingair flow: while the inlet provides most of the compressionfor ramjets, the high speeds at which scramjets operateallow them to take advantage of the compressiongenerated by shock waves, primarily oblique shocks. Veryfew scramjet engines have ever been built and flown. InMay 2010 the Boeing X-51 set the endurance record for thelongest scramjet burn at over 200 seconds
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