
An ion thruster is one of several types of spacecraft propulsion that uses beams of ions for propulsion. The precise method for accelerating the ions may vary, but all designs take advantage of the high electric charge-to-mass ratio of ions to accelerate them to very high velocities. Ion thrusters are therefore able to achieve high specific impulse, reducing the amount of reaction mass required but increasing the amount of power required compared to chemical rockets. Ion thrusters can deliver performance approximately one orders of magnitude greater fuel efficiency than traditional liquid fuel rocket engines but they are generally constrained to very low thrusts by the available power. == Types of ion thruster == There are many types of ion thruster currently in development; some are currently in use, while others have not yet been installed in spacecraft. Some of the types of ion thruster are: * Electrostatic ion thrusters * Hall effect thrusters * Field Emission Electric Propulsion * Pulsed inductive thruster Other forms of high-efficiency electric thruster have also been proposed; see spacecraft propulsion. == General design == In the simplest design, an electrostatic ion thruster, ions are accelerated by passing them through highly-charged grids (similar in concept to a vacuum tube). Opposite charged ions are also fired into the ion beam and accelerated through the grid as they leave the thruster. This keeps the spacecraft and the thruster beams neutral electrically. The acceleration uses up very little reaction mass (i.e., the specific impulse, or Isp, is very high). == Energy usage == A major consideration is the amount of energy or power required to run the engine, partly to ionize the materials, but most especially to accelerate the ions to the extremely high speeds required to have any useful effect. Exhaust speeds of 30 km/s are not uncommon, which is far faster than the 3-4.5 km/s for chemical rockets, and makes for notably low propellent usage. With ion thrusters, most of the energy is lost in the high speed exhaust and this affects the thrust levels. It turns out that the overall thrust obtained from a given amount of energy is inversely proportional to exhaust speed (since energy consumption per kilogram of propellant is proportional to exhaust velocity squared, but the thrust per kilogram of propellant is only proportional to exhaust speed—see [1]). == Thrust == In practice, with currently practical energy sources of perhaps a few tens of kilowatts, and given a not untypical Isp of 3000 seconds (30 kN·s/kg), ion thrusters give only extremely modest forces (often tenths of a newton). With the weight of the energy sources and vehicle of hundreds of kilogram, the accelerations are typically ~gee (10 mm/s²). Since thrust goes down with higher specific impulse, as specific impulse increases, a mission takes longer to achieve and this can incur additional costs. Since many missions are attempting to minimise costs, some of which increase with the length of the mission, an optimum specific impulse can be calculated. == Lifespan == Given the low thrust, the life of the thruster becomes important. Ion drives have to be kept running a large part of the time to allow the milli-gee acceleration to build up into something meaningful. In the simplest design of engine, an electrostatic ion thruster, the ions often hit the grids on their way through the engine, which leads to the decay of the grids and their eventual failure. Smaller grids lower the chance of these accidental collisions, but decrease the amount of charge they can handle, and thus lower the thrust. == Missions == Of all the electric thrusters, ion engines have been the most seriously considered commercially and academically in the quest for interplanetary mission. Ion engines are seen as the best solution for these missions as interplanetary trajectories require very high ΔV (the overall change in velocity, taken as a single value) that can be built up over long periods of time (years). The Hall effect thruster is a type of ion thruster that has been used for decades for station keeping by the Soviet-Union and is now also applied in the West: the European Space Agency's satellite Smart 1 uses it. NASA has developed an ion engine called NSTAR for use in their interplanetary missions. This engine was tested in the highly successful space probe Deep Space 1. Hughes Aircraft has developed the XIPS (Xenon Ion Propulsion System) for performing stationkeeping on geosynchronous satellites. These are electrostatic ion thrusters and work by a different principle than Hall effect thrusters. In 2003 NASA ground-tested a new version of their ion engine called High Power Electric Propulsion, or HiPEP. The HiPEP engine differs from earlier ion engines because the xenon ions are produced using a combination of microwaves and spinning magnets. Previously the electrons required were provided by a cathode. Using microwaves significantly reduces the wear and tear on the engine by avoiding any contact between the speeding ions and the electron source. Most other electric spacecraft engine designs are based on the same principles, but attempt to avoid the grid degradation problem with a combination of other electric or magnetic fields. Other fuels have been considered for use with ion propulsion. Research has been invested in fullerenes for this purpose, specifically C60 (buckminsterfullerene), due in part to its large electron-impact cross section. This property gives the potential for ion engines with higher efficiency than current Xenon-based designs at Isp values of less than 3,000 lbf·s/lb (29 kN·s/kg). JP Aerospace has been working to build an orbital airship, which uses a combination of a balloon and ion thrusters to achieve orbit without any use of conventional rockets, for roughly one dollar per short ton per mile of altitude ($0.70/(tonne·km)). ==Ion thrusters in fiction== *Film creator and director George Lucas seems to have some confidence in ion propulsion: in the ''Star Wars'' movies, the technologically sophisticated Empire's TIE Fighters get their name from the TIEs used for propulsion — Twin Ion Engines... *Arthur C. Clarke's 1949 short story ''Breaking Strain'' features a cargo ship with an ion drive powered by "Atomic motors". *In Star Trek: The Original Series, The engineer of the USS Enterprise, Scotty, says: "Captain, they're using an ion drive on that ship! I bet they could teach us a thing or two". ==References== [1] Tsiolkovsky_rocket_equation#Energy ==See also== * Spacecraft propulsion * Nuclear electric rocket * Hall effect thruster * Field Emission Electric Propulsion * Pulsed inductive thruster * VASIMR ==External link== * [http://www.aip.org/tip/INPHFA/vol-6/iss-5/p16.pdf Plasma Propulsion in Space] Spacecraft propulsion
Question: Why is it important that the Ion thruster driven space engine stays electrically neutral? I tend to think that in an almost perfect vacuum (outer space) electrical charge doesn't matter. Because charge is relative to its surroundings - basically nothing in space. Or is there still too much dust around in "empty" space? Or is there electrical attraction to be expected from really far away objects (say, a planet)? :Charge isn't relative: like mass, it's a property of the object, whether or not there's anything nearby to be affected by it. Also, ion drives are being used in places that are, in terms of particles, far from empty, like near-Earth space. User:Vicki Rosenzweig 11:59, 29 Sep 2003 (UTC) The problem with needign to remain electricalyl neutral is that if the chassis becomes too electrically negative, then the device must perform extra work to remove the extra electrons adn ionize the workign fluid, plus the exhaust beign still ionized would be attracted to the highly negative chassis instead fo open space, producing no thrust. Its far more complex than that but the explanation should suffice. Also it needs to be noted in here that DS-1 will NOT be using an ION engine it will be using a HALL EFFECT engine, the difference is technical but ion drives are no longer used for future designs. My understanding of ion engines is that they can increase their specific impulse simply by increasing the grid voltage. However, they normally only operate with an isp of about 3000 because higher isp values would require too much power to produce a decent thrust. Is there any practical limit to how high the grid voltage and isp can get if power supply is not a problem? Could an ion engine vary its isp over a wide range of values by changing the ratio of power being used to ionize fuel to the power being used to accelerate the fuel?--User:Todd Kloos 23:01, 9 Jun 2004 (UTC) :Yes and no; ion drives have problems with ions striking electrodes and eroding the electrodes. If you just crank up the voltage, the ions will start to wreck the electrodes. Other problems may also occur; I'm guessing, but you may also have to avoid arcing and corona discharge in the ion stream, and there may be other concerns as well. But certainly power consumption is a major design issue: you have to trade off the mass of fuel required against the mass of the power generation equipment. For very large delta-v (comparable to the specific impulse) improving specific impulse is a very big win. But if delta-v is relatively small compared to exhaust speed, then the reaction mass is not very large, so boosting specific impulse may require you to increase launch weight to generate enough power. This becomes worse when you want to travel far from the sun; power generation gets more difficult, and if it requires an expendable resource, you might as well throw the expended resource out the back as extra reaction mass. --User:Aarchiba 06:53, 11 Jun 2004 (UTC) ::Thank you for replying. I know that the current generation of ion thrusters (HiPEP) has very significant advantages in thruster lifetime compared with previous ion engines. Presumably this would also allow higher voltages without wrecking the electrodes? Anyway, I know that higher specific impulse is not always better. However, for many missions using a constant isp value is not the most efficient way. I have seen profiles for optimized VASIMR missions to Mars where the spacecraft will start out with an isp of 3000, but the isp will slowly rise during the trip (sometimes reaching a peak as high as 50,000) before going back down to 3000 at the end of the trip. If ion engines can go through a similar variation of specific impulse, it should allow them to increase efficiency shorten mission durations considerably.--User:Todd Kloos 08:05, 11 Jun 2004 (UTC) ==Scotty Quote== Uh, unless my understanding of wikipedia format is totally off, the quote by Scotty is not appropriate
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Ion thruster
An ion thruster is one of several types of spacecraft propulsion that uses beams of ions for propulsion. The precise method for accelerating the ions may vary, but all designs take advantage of the high electric charge-to-mass ratio of ions to accelerate them to very high velocities. Ion thrusters are therefore able to achieve high specific impulse, reducing the amount of reaction mass required but increasing the amount of power required compared to chemical rockets. Ion thrusters can deliver performance approximately one orders of magnitude greater fuel efficiency than traditional liquid fuel rocket engines but they are generally constrained to very low thrusts by the available power. == Types of ion thruster == There are many types of ion thruster currently in development; some are currently in use, while others have not yet been installed in spacecraft. Some of the types of ion thruster are: * Electrostatic ion thrusters * Hall effect thrusters * Field Emission Electric Propulsion * Pulsed inductive thruster Other forms of high-efficiency electric thruster have also been proposed; see spacecraft propulsion. == General design == In the simplest design, an electrostatic ion thruster, ions are accelerated by passing them through highly-charged grids (similar in concept to a vacuum tube). Opposite charged ions are also fired into the ion beam and accelerated through the grid as they leave the thruster. This keeps the spacecraft and the thruster beams neutral electrically. The acceleration uses up very little reaction mass (i.e., the specific impulse, or Isp, is very high). == Energy usage == A major consideration is the amount of energy or power required to run the engine, partly to ionize the materials, but most especially to accelerate the ions to the extremely high speeds required to have any useful effect. Exhaust speeds of 30 km/s are not uncommon, which is far faster than the 3-4.5 km/s for chemical rockets, and makes for notably low propellent usage. With ion thrusters, most of the energy is lost in the high speed exhaust and this affects the thrust levels. It turns out that the overall thrust obtained from a given amount of energy is inversely proportional to exhaust speed (since energy consumption per kilogram of propellant is proportional to exhaust velocity squared, but the thrust per kilogram of propellant is only proportional to exhaust speed—see [1]). == Thrust == In practice, with currently practical energy sources of perhaps a few tens of kilowatts, and given a not untypical Isp of 3000 seconds (30 kN·s/kg), ion thrusters give only extremely modest forces (often tenths of a newton). With the weight of the energy sources and vehicle of hundreds of kilogram, the accelerations are typically ~gee (10 mm/s²). Since thrust goes down with higher specific impulse, as specific impulse increases, a mission takes longer to achieve and this can incur additional costs. Since many missions are attempting to minimise costs, some of which increase with the length of the mission, an optimum specific impulse can be calculated. == Lifespan == Given the low thrust, the life of the thruster becomes important. Ion drives have to be kept running a large part of the time to allow the milli-gee acceleration to build up into something meaningful. In the simplest design of engine, an electrostatic ion thruster, the ions often hit the grids on their way through the engine, which leads to the decay of the grids and their eventual failure. Smaller grids lower the chance of these accidental collisions, but decrease the amount of charge they can handle, and thus lower the thrust. == Missions == Of all the electric thrusters, ion engines have been the most seriously considered commercially and academically in the quest for interplanetary mission. Ion engines are seen as the best solution for these missions as interplanetary trajectories require very high ΔV (the overall change in velocity, taken as a single value) that can be built up over long periods of time (years). The Hall effect thruster is a type of ion thruster that has been used for decades for station keeping by the Soviet-Union and is now also applied in the West: the European Space Agency's satellite Smart 1 uses it. NASA has developed an ion engine called NSTAR for use in their interplanetary missions. This engine was tested in the highly successful space probe Deep Space 1. Hughes Aircraft has developed the XIPS (Xenon Ion Propulsion System) for performing stationkeeping on geosynchronous satellites. These are electrostatic ion thrusters and work by a different principle than Hall effect thrusters. In 2003 NASA ground-tested a new version of their ion engine called High Power Electric Propulsion, or HiPEP. The HiPEP engine differs from earlier ion engines because the xenon ions are produced using a combination of microwaves and spinning magnets. Previously the electrons required were provided by a cathode. Using microwaves significantly reduces the wear and tear on the engine by avoiding any contact between the speeding ions and the electron source. Most other electric spacecraft engine designs are based on the same principles, but attempt to avoid the grid degradation problem with a combination of other electric or magnetic fields. Other fuels have been considered for use with ion propulsion. Research has been invested in fullerenes for this purpose, specifically C60 (buckminsterfullerene), due in part to its large electron-impact cross section. This property gives the potential for ion engines with higher efficiency than current Xenon-based designs at Isp values of less than 3,000 lbf·s/lb (29 kN·s/kg). JP Aerospace has been working to build an orbital airship, which uses a combination of a balloon and ion thrusters to achieve orbit without any use of conventional rockets, for roughly one dollar per short ton per mile of altitude ($0.70/(tonne·km)). ==Ion thrusters in fiction== *Film creator and director George Lucas seems to have some confidence in ion propulsion: in the ''Star Wars'' movies, the technologically sophisticated Empire's TIE Fighters get their name from the TIEs used for propulsion — Twin Ion Engines... *Arthur C. Clarke's 1949 short story ''Breaking Strain'' features a cargo ship with an ion drive powered by "Atomic motors". *In Star Trek: The Original Series, The engineer of the USS Enterprise, Scotty, says: "Captain, they're using an ion drive on that ship! I bet they could teach us a thing or two". ==References== [1] Tsiolkovsky_rocket_equation#Energy ==See also== * Spacecraft propulsion * Nuclear electric rocket * Hall effect thruster * Field Emission Electric Propulsion * Pulsed inductive thruster * VASIMR ==External link== * [http://www.aip.org/tip/INPHFA/vol-6/iss-5/p16.pdf Plasma Propulsion in Space] Spacecraft propulsion
Ion thruster
Question: Why is it important that the Ion thruster driven space engine stays electrically neutral? I tend to think that in an almost perfect vacuum (outer space) electrical charge doesn't matter. Because charge is relative to its surroundings - basically nothing in space. Or is there still too much dust around in "empty" space? Or is there electrical attraction to be expected from really far away objects (say, a planet)? :Charge isn't relative: like mass, it's a property of the object, whether or not there's anything nearby to be affected by it. Also, ion drives are being used in places that are, in terms of particles, far from empty, like near-Earth space. User:Vicki Rosenzweig 11:59, 29 Sep 2003 (UTC) The problem with needign to remain electricalyl neutral is that if the chassis becomes too electrically negative, then the device must perform extra work to remove the extra electrons adn ionize the workign fluid, plus the exhaust beign still ionized would be attracted to the highly negative chassis instead fo open space, producing no thrust. Its far more complex than that but the explanation should suffice. Also it needs to be noted in here that DS-1 will NOT be using an ION engine it will be using a HALL EFFECT engine, the difference is technical but ion drives are no longer used for future designs. My understanding of ion engines is that they can increase their specific impulse simply by increasing the grid voltage. However, they normally only operate with an isp of about 3000 because higher isp values would require too much power to produce a decent thrust. Is there any practical limit to how high the grid voltage and isp can get if power supply is not a problem? Could an ion engine vary its isp over a wide range of values by changing the ratio of power being used to ionize fuel to the power being used to accelerate the fuel?--User:Todd Kloos 23:01, 9 Jun 2004 (UTC) :Yes and no; ion drives have problems with ions striking electrodes and eroding the electrodes. If you just crank up the voltage, the ions will start to wreck the electrodes. Other problems may also occur; I'm guessing, but you may also have to avoid arcing and corona discharge in the ion stream, and there may be other concerns as well. But certainly power consumption is a major design issue: you have to trade off the mass of fuel required against the mass of the power generation equipment. For very large delta-v (comparable to the specific impulse) improving specific impulse is a very big win. But if delta-v is relatively small compared to exhaust speed, then the reaction mass is not very large, so boosting specific impulse may require you to increase launch weight to generate enough power. This becomes worse when you want to travel far from the sun; power generation gets more difficult, and if it requires an expendable resource, you might as well throw the expended resource out the back as extra reaction mass. --User:Aarchiba 06:53, 11 Jun 2004 (UTC) ::Thank you for replying. I know that the current generation of ion thrusters (HiPEP) has very significant advantages in thruster lifetime compared with previous ion engines. Presumably this would also allow higher voltages without wrecking the electrodes? Anyway, I know that higher specific impulse is not always better. However, for many missions using a constant isp value is not the most efficient way. I have seen profiles for optimized VASIMR missions to Mars where the spacecraft will start out with an isp of 3000, but the isp will slowly rise during the trip (sometimes reaching a peak as high as 50,000) before going back down to 3000 at the end of the trip. If ion engines can go through a similar variation of specific impulse, it should allow them to increase efficiency shorten mission durations considerably.--User:Todd Kloos 08:05, 11 Jun 2004 (UTC) ==Scotty Quote== Uh, unless my understanding of wikipedia format is totally off, the quote by Scotty is not appropriate
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