Write down a short note on "Jet Propelled"
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Answer:
Jet propulsion is the propulsion of an object in one direction, produced by ejecting a jet of fluid in the opposite direction. By Newton's third law, the moving body is propelled in the opposite direction to the jet. Reaction engines operating on the principle of jet propulsion include the jet engine used for aircraft propulsion, the pump-jet used for marine propulsion, and the rocket engine and plasma thruster used for spacecraft propulsion. Biological systems include the propulsion mechanisms of certain marine animals such as cephalopods, sea hares, arthropods, and fish.
Contents
1 Physics
1.1 Specific impulse
1.2 Thrust
2 Types of reaction engine
2.1 Jet engine
2.2 Rocket engine
2.3 Plasma engine
2.4 Pump-jet
3 Jet-propelled animals
4 See also
5 References
Physics
Jet propulsion is produced by some reaction engines or animals when thrust is generated by a fast moving jet of fluid in accordance with Newton's laws of motion. It is most effective when the Reynolds number is high—that is, the object being propelled is relatively large and passing through a low-viscosity medium.[1]
In animals, the most efficient jets are pulsed, rather than continuous,[2] at least when the Reynolds number is greater than 6.[3]
Specific impulse
Main article: Specific impulse
Specific impulse (usually abbreviated Isp) is a measure of how effectively a rocket uses propellant or jet engine uses fuel. By definition, it is the total impulse (or change in momentum) delivered per unit of propellant consumed[4] and is dimensionally equivalent to the generated thrust divided by the propellant mass flow rate or weight flow rate.[5] If mass (kilogram, pound-mass, or slug) is used as the unit of propellant, then specific impulse has units of velocity. If weight (newton or pound-force) is used instead, then specific impulse has units of time (seconds). Multiplying flow rate by the standard gravity (g0) converts specific impulse from the mass basis to the weight basis.[5]
A propulsion system with a higher specific impulse uses the mass of the propellant more effectively in creating forward thrust and, in the case of a rocket, less propellant needed for a given delta-v, per the Tsiolkovsky rocket equation.[4][6] In rockets, this means the engine is more effective at gaining altitude, distance, and velocity. This effectiveness is less important in jet engines that employ wings and use outside air for combustion and carry payloads that are much heavier than the propellant.
Specific impulse includes the contribution to impulse provided by external air that has been used for combustion and is exhausted with the spent propellant. Jet engines use outside air, and therefore have a much higher specific impulse than rocket engines. The specific impulse in terms of propellant mass spent has units of distance per time, which is an artificial velocity called the "effective exhaust velocity". This is higher than the actual exhaust velocity because the mass of the combustion air is not being accounted for. Actual and effective exhaust velocity are the same in rocket engines not utilizing air.
Specific impulse is inversely proportional to specific fuel consumption (SFC) by the relationship Isp = 1/(go·SFC) for SFC in kg/(N·s) and Isp = 3600/SFC for SFC in lb/(lbf·hr).
Thrust
From the definition of specific impulse thrust in SI units is:
{\displaystyle F={\dot {m}}V_{e}}{\displaystyle F={\dot {m}}V_{e}}
where Ve is the effective exhaust velocity and {\displaystyle {\dot {m}}}{\dot {m}} is the propellant flow rate.
Types of reaction engine
Main article: Reaction engine
Reaction engines produce thrust by expelling solid or fluid reaction mass; jet propulsion applies only to engines which use fluid reaction mass.
Jet engine
Main article: Jet engine
A jet engine is a reaction engine which uses ambient air as the working fluid, and converts it to a hot, high-pressure gas which is expanded through one or more nozzles. Two types of jet engine, the turbojet and turbofan, employ axial-flow or centrifugal compressors to raise the pressure before combustion, and turbines to drive the compression. Ramjets operate only at high flight speeds because they omit the compressors and turbines, depending instead on the dynamic pressure generated by the high speed (known as ram compression). Pulse jets also omit the compressors and turbines, but can generate static thrust and have limited maximum speed.
Rocket engine
Main article: Rocket engine
The rocket is capable of operating in the vacuum of space, because it is dependent on the vehicle carrying its own oxidizer instead of using the oxygen in the air, or in the case of a nuclear rocket, heats an inert propellant (such as liquid hydrogen) by forcing it through a nuclear reactor.
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