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Relativistic propulsion

  1. May 29, 2015 #1
    Okay, I tried searching for this in the forums and didn't really find what I was looking for. So, I'll start a new thread and if anybody has seen this before, feel free to drop a link to an old thread or whatever.

    Many of our older space probes use/used thermoelectric radioisotope batteries to power their electronics as they move/moved through space. Solar sails are a good option when it comes to free propulsion in space near to the sun, but further out they stop working and lose their effectiveness if further acceleration is desired. Besides that, there are plenty of material science challenges with development of sails (though I think a lot of them have been solved and some space probes recently have made use of solar sails).

    As an alternative, why not use a thermoelectric radioisotope battery to power an electron accelerator to allow for continued propulsion beyond the known solar system, to perhaps make it to a neighboring system? On that note, I wanted to do a simple analysis, but I'm having trouble figuring out how the momentum equations change to accommodate relativity, assuming the electrons are accelerated to relativistic speeds.

    Other than requiring a ridiculous length to accommodate an accelerator inductor system, I think this might work. Any problems anybody wants to point out before I waste too much time on this (if there are major problems I'm probably overlooking). Would there be enough electrons in deep space to keep the craft close to a neutral charge (as this would quickly eat into the power required per electron I'm thinking).
     
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  3. May 29, 2015 #2

    Simon Bridge

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    to what? afaik solar sails are not currently used.
    ... because there are better designs of ion-drive in use.
    Google "ion drive".
     
  4. May 29, 2015 #3
    You're right, they have not been used for propulsion, I misunderstood some reading I did a few years back. Japan has run some tests though using IKAROS, so maybe one day it will become a system of propulsion. I'm familiar with the basics of ion propulsion, I was just hoping to skip the inconvenience of carrying ionizing material and ionizing the propulsive elements before accelerating them, which an "electron" drive would be able to do theoretically.

    As for the conservation of momentum equations framed in relativistic terms, anybody mind linking to an example or posting some here?

    Of course, I guess stripping a material of electrons would require the same energy as ionizing the propulsion material?
     
    Last edited: May 29, 2015
  5. May 29, 2015 #4

    Orodruin

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    They look the same as for classical physics with the addition that you must use the relativistic expression for momentum.
     
  6. May 29, 2015 #5
    Yup, so that's what I'm asking for. Link or equation?
     
  7. May 29, 2015 #6

    Orodruin

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    This question is of the nature that you will probably be best helped by typing "relativistic momentum" into Google or another search engine. This should give you several relevant links.
     
  8. May 29, 2015 #7

    pervect

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    I'd suggest looking at ion thrusters, which do much the same thing. However, heavy ions are preferred over electrons in the only designs I've heard of. I assume this is to get better acceleration.

    You might also look at Vasimir, and some of the threads on PF about it (including the difficulty of finding a power supply for it.)

    I don't think thermoelectric power supplies have a very good weight/power ratio, unfortunately.

    As far as relativistic momentum goes, it's gamma * m * v. You might also find the relativisitic rocket equations http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html [Broken] interesting.
     
    Last edited by a moderator: May 7, 2017
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