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Thread: Ion Thruster Usage in an Orbital Context

  1. #1
    Join Date
    Nov 2003
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    206

    Ion Thruster Usage in an Orbital Context

    Forget for the moment that every instance of space travel is essentially orbital in nature, it occured to me that there may be some unique uses for Ion Thruster-drive craft in the Earth or Inner Planet orbital environment. These would take advantage of the fact that such a drive system can potentially build up a higher velocity than conventional chemical rockets.

    The basic idea would be putting craft in regular orbits about the earth, or in an earth-mars orbital path, that did not require significant thrust or where 'travel time' was unimportant. The eventual result being a far higher 'end velocity' than otherwise attainable - and with the resource of that velocity itself, either for short eventual travel times or by trading this velocity for kinetic energy for very specific applications where it would be useful and moer difficult to acquire otherwise.

    I have a few potential uses in mind for both resources, but I would like to explore the difficulties involved before fantasizing too much about the applications. From my understanding, there are three main issues with a scenario which requireds an ion thruster to run for the duration necessary to reach the high velocity being referenced here. These are Power, Propellant and what my tendencies for alliteration compel me to label 'Pith'.

    Drives of this kind, be they Electrostatic Ion or Electromagnetic Thrusters, require relatively large amounts of power. This currently means a considerable required mass, which of course is undesirable. However this problem could be solved through the use of low mass power generation systems and is not insurmountable even with currently technology.

    The propellant issue is also one which I believe can be addressed with existing levels of development, or through near term advances. Argon, Bismuth, Caesium, Hydrogen, Lithium and the most widely used Xenon, may give way to even better materials with lower power needs and a higher specific impulse...or methods of 'refueling' may be developed, or some combination of the two. Again, not problem I see as lacking solutions.

    The 3rd problem, which I called Pith, refers to vital elements of the engine which require high levels of strength, to offset the unavoidable damage caused through the normal functioning of the engine. In the case of an electrostatic gridded ion thruster, grid erosion is the main problem here, with a lifetime of between 20,000 and 30,000 hours or so being the likely upper limit, though the actual limit is uncertain. A hall thruster has a similar erosion problem with the discharge chamber, with lifetimes of a few thousand hours being the norm.

    The lifetime of the engine, and thus the duration of active thrust, is the real problem here. I have no doubt that advances in materials science will allow us to increase that lifetime, but to what degree I do not know.

    The question then is this - how long would an ion thruster have to run in order for the craft in question to reach (for the purposes referenced above) usefully high levels of velocity?

    GOCE (Gravity field and steady-state Ocean Circulation Explorer) for example will run for about 17280 hours, and is currently in orbit. However, it is only producing 10 mN of thrust. Deep Space 1, using NSTAR, produced 92 mN, for 678 days or 16272 hours, shutting down in December 2001...and reaching a final speed of 4.5 km/s. I reference these two specifically as bookends, but they are just two examples to give a general idea of our current capabilities.

    There are new technologies of course, such as HDLT (Helicon Double Layer Thruster), which do not have some of the 'PPP' issues mentioned above, but it is early in development...

    So, using existing technology, just how fast could we realistically expect to go?

  2. #2
    Join Date
    Sep 2008
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    5,892
    Quote Originally Posted by Anthrage View Post
    ...
    I see Dawn is expected to achieve a delta-v of 10km/s. That's with one gravity assist, and three of those NSTAR engines. Mission time is round 7 yrs. There's a diagram of its planned trajectory there.

    It took 14 months to reach Mars, delta-v of 1.8 km/s, on one engine.

  3. #3
    Join Date
    Oct 2006
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    3,767
    Smart 1 also used Ion engines to go from GTO to lunar orbit.

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