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Ion engine efficency.

  1. Jun 5, 2009 #1
    I was arguing with my NASA friend the other day about what would make a more efficent propellent for an ion engine. Is it a lighter ion or a heavier ion. THis was what I derived.

    Assume a charge q of mass m is accelerated through a potential V. The mass of the ship is M. Working through the problem with the tools an intro physics student could comprehend, I arrived at an equation that gives you the change in velocity for the ship after consuming ONE available ion.

    v = Sqrt[2qVm/(M^2)]



    I'm pretty confident in this. The units work out and all variables are in the correct place intuitively.

    i'm confused on one point. I already have velocity as a function of mass of one particle. Do I need to divide by mass of one particle to get to velocity change per unit mass?

    I think i'm confusing particle mass with bulk mass somehow.
     
  2. jcsd
  3. Jun 7, 2009 #2

    Redbelly98

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    You're equation is correct. Not sure why the velocity change per unit mass would be useful here. If we assume a certain number of ions are generated per unit time, then multiply your equation by the rate at which ions are generated to get the acceleration.
     
  4. Jun 7, 2009 #3
    Well, you want to carry as little propellent to keep the weight down. A higher change in velocity per unit mass would allow you to carry less propellent to achieve the same change in velocity.
     
  5. Jun 7, 2009 #4

    Redbelly98

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    Thinking about this a little more, and I am thinking the quantity of interest is the velocity change due to burning a fixed mass of fuel, since the "energy budget" would include a certain amount of fuel by mass.

    Multiplying your v expression times the number of particles in a total fuel mass mf would give Δv for that mass of fuel. This number of particles is simply mf/m, so we have

    Δv = Sqrt[2qVmf2 / (m2M2)] = Sqrt[2qV] mf/(mM) ​

    I guess this is pretty much what you were getting at ... essentially divide by the mass m to get the velocity change per unit mass of fuel.

    EDIT:
    I posted this before reading your post #3. (Yes, I had this edit window open and in progress for nearly an hour.) I agree with what you said.

    So it appears that a smaller particle mass results in a greater thrust.
     
  6. Jun 7, 2009 #5
    Well, if that's the case, why are they using xenon? It's heavy. Is xenon particularly easy to ionize multiple times? I mean, all that you need this particle to do is get ionized and fly out the back. Other than ease of storage, all other properties are irrelevant.
     
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