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| A '''Pulsed Plasma Thruster (PPT)''', also known as a plasma jet engine, is a form of [[Electrically powered spacecraft propulsion|electric spacecraft propulsion]].<ref name = NASAPPT>{{cite web|url=http://www.nasa.gov/centers/glenn/about/fs23grc.html|title=NASA Glenn Research Center PPT|work=National Aeronautics & Space Administration (NASA)|accessdate=5 July 2013}}</ref> PPTs are generally considered the simplest form of electric spacecraft propulsion and were the first form of electric propulsion to be flown in space, having flown on two Soviet probes ([[Zond 2]] and [[Zond 3]]) starting in 1964.<ref name = SURREY_UNI>{{cite web|url=http://epubs.surrey.ac.uk/26851//|title=University of Surrey PPT Thesis Abstract|work=Doctoral Thesis - University of Surrey|accessdate=5 July 2013}}</ref> PPTs are generally flown on [[spacecraft]] with a surplus of electricity from abundantly available solar energy.
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| == Operation ==
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| [[File:SchematiclayoutofaPulsedPlasmaThruster.png|thumb|Schematic layout of a Pulsed Plasma Thruster]]
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| Most PPTs use a solid material (normally [[Polytetrafluoroethylene|PTFE]], more commonly known as Teflon) for [[propellant]], although a minority use liquid or gaseous propellants. The first stage in PPT operation involves an [[electric arc|arc of electricity]] passing through the fuel, causing [[ablation]] and [[Sublimation (phase transition)|sublimation]] of the fuel. The heat generated by this arc causes the resultant gas to turn into [[plasma (physics)|plasma]], thereby creating a charged gas cloud. Due to the force of the ablation, the plasma is propelled at low speed between two charged plates (an [[anode]] and [[cathode]]). Since the plasma is charged, the fuel effectively completes the circuit between the two plates, allowing a current to flow through the plasma. This flow of electrons generates a strong electromagnetic field which then exerts a [[Lorentz force]] on the plasma, accelerating the plasma out of the PPT exhaust at high velocity.<ref name = NASAPPT/> The pulsing occurs due to the time needed to recharge the plates following each burst of fuel, and the time between each arc. The frequency of pulsing is normally very high and so it generates an almost continuous and smooth thrust. While the thrust is very low, a PPT can operate continuously for extended periods of time, yielding a large final acceleration.
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| The energy used in each pulse is stored in a capacitor.<ref name = THE_ENGINEER>{{cite web|url=http://www.theengineer.co.uk/aerospace/news/plasma-thrusters-could-double-the-lifetime-of-mini-satellites/1011817.article|title=Plasma thrusters could double the lifetime of mini satellites|work=The Engineer (Magazine)|accessdate=5 July 2013}}</ref> By varying the time between each capacitor discharge, the thrust and power draw of the PPT can be varied allowing versatile use of the system.<ref name = SURREY_UNI/>
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| == Comparison to chemical propulsion ==
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| The equation for the change in velocity of a spacecraft is given by the [[Tsiolkovsky rocket equation | rocket equation]] as follows:
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| :<math>\Delta v = v_\text{e} \ln \frac {m_0} {m_1}</math>
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| where:
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| :<math>\Delta v\ </math> is delta-v - the maximum change of speed of the vehicle (with no external forces acting),
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| :<math>v_\text{e}</math> is the [[effective exhaust velocity]] (<math>v_\text{e} = I_\text{sp} \cdot g_0</math> where <math>I_\text{sp}</math> is the [[specific impulse]] expressed as a time period and <math>g_0</math> is [[Standard gravity|Standard Gravity]]),
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| :<math>\ln</math> refers to the [[natural logarithm]] function,
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| :<math>m_0</math> is the initial total mass, including propellant,
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| :<math>m_1</math> is the final total mass.
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| PPTs have much higher exhaust velocities than chemical propulsion engines, but have a much smaller fuel flow rate. From the Tsiolkovsky equation stated above, this results in a proportionally higher final velocity of the propelled craft. The exhaust velocity of a PPT is of the order of tens of km/s while conventional chemical propulsion generates [[thermal velocity|thermal velocities]] in the range of 2-4.5 km/s. Due to this lower thermal velocity, chemical propulsion units become exponentially less effective at higher vehicle velocities, necessitating the use of electric spacecraft propulsion such as PPTs. It is therefore advantageous to use an electric propulsion system such as a PPT to generate high interplanetary speeds in the range 20–70 km/s.
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| [[National Aeronautics and Space Administration|NASA's]] research PPT (flown in 2000) achieved an exhaust velocity of 13,700 m/s, generated a [[thrust]] of 860 µN, and consumed 70W of electrical power.<ref name = NASAPPT/> | |
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| == Advantages and disadvantages ==
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| PPTs are very robust due to their inherently simple design (relative to other electric spacecraft propulsion techniques), and draw very little electrical power relative to other comparable thrusters. As an electric propulsion system, PPTs benefit from reduced fuel consumption compared to traditional chemical rockets, reducing launch mass and therefore launch costs, as well as high specific impulse improving performance.<ref name = NASAPPT/>
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| However, due to energy losses caused by late time ablation and rapid [[conductive heat transfer]] from the propellant to the rest of the spacecraft, propellant efficiency is very low compared to other forms of electric propulsion, at around just 10%.
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| == Uses ==
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| PPTs are well-suited to uses on relatively small spacecraft with a mass of less than 100 kg (particularly [[Cubesat|CubeSats]]) for roles such as [[attitude control]], [[Orbital station-keeping|station keeping]], de-orbiting manoeuvres and deep space exploration. Using PPTs could double the life-span of these small satellite missions without significantly increasing complexity or cost due to the inherent simplicity and relatively low cost nature of PPTs.<ref name = THE_ENGINEER/> A PPT was flown by NASA in November, 2000, as a flight experiment on the [[Earth Observing-1]] spacecraft. The thrusters successfully demonstrated the ability to perform roll control on the spacecraft and also demonstrated that the [[electromagnetic interference]] from the pulsed plasma did not affect other spacecraft systems.<ref name = NASAPPT/> Pulsed Plasma Thrusters are also an avenue of research used by universities for starting experiments with electric propulsion due to the relative simplicity and lower costs involved with PPTs as opposed to other forms of electric propulsion such as [[Hall effect]] [[ion thrusters]].<ref name = SURREY_UNI/>
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| ==References==
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| {{Reflist}}
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| {{spacecraft propulsion}}
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| ==External links==
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| * [http://www.aerojet.com/capabilities/spacecraft.php Aerojet (Redmond, WA USA) - Pulsed Plasma Thruster Vendor]
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| * [http://alfven.princeton.edu/papers/tem_jpc2002abstext.htm Design of a High-energy, Two-stage Pulsed Plasma Thrust]
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| * [http://web.archive.org/web/20110716152726/http://eo1.gsfc.nasa.gov/miscPages/TechForumPres/25-PPT.pdf EO1 Pulsed Plasma Thruster]
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| * [http://w3.pppl.gov/ppst/docs/chen.pdf Gas-Fed Pulsed Plasma Thrusters: From Sparks to Laser Initiation]
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| [[Category:Spacecraft propulsion]]
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| [[Category:Soviet inventions]]
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