Re: Orbital simulator alogorithms

From: John Ladasky (ladasky_at_my-deja.com)
Date: 09/24/04

  • Next message: Rodney Kelp: "Re: A first blueprint for interstellar travel" 
    To: sci-space-tech@moderators.isc.org
Date:  24 Sep 2004 13:21:01 -0700

    Hop David <hopspageHATESSPAaMmM@tabletoptelephone.com> wrote in message news:<41531C4A.5010801@tabletoptelephone.com>...
    > I'm trying to model orbits with Lunar perturbations in Microsoft Excel
    >
    > My first step is just trying to get an ordinary ellipse without lunar
    > peturbations. I use
    >
    > a = GMe/r0^2
    > V1 = V0 + a*t
    > r1 = r0 + V0t + .5*a*t^2
    >
    > Then the next row of cells uses the last row's r1 as r0, etc.
    >
    > So a path over (say) a ten second time increment gives me the endpoints
    > of a small parabola fragment. The r1 at the end of a parabola fragment
    > is slightly larger than the r1 end of a tiny ellipse fragment.
    >
    > Given this error I'd expected a slowing growing spiral rather than an
    > ellipse and this is exactly what I get.
    >
    > There is no point in putting the moon's influence into each row of cells
    > when my model of a simple two body is horribly inaccurate.
    >
    > I am hoping someone here will suggest better equations to put in the cells.

    Greetings, David... or is it Hop?

    I've been where you've been, and I've done what you've done. The
    problem that you are having is that you are trying to model continuous
    motion with finite time steps. This problem is intrinsic to all
    discrete modeling.

    We discussed this a few years ago on sci.astro. There are many ways
    to improve your formulas, which vary in accuracy and
    understandability. Generally speaking, the easier something is to
    implement, the less accurate it is. But you can obtain considerable
    improvements with relatively small changes. Have a look here:

    http://groups.google.com/groups?&selm=c09b237b.0210231811.35fdbf95%40posting.google.com

    Hope that helps. 

    --
Rainforest laid low.
"Wake up and smell the ozone,"
Says man with chiansaw.
John J. Ladasky Jr., Ph.D.
  • Next message: Rodney Kelp: "Re: A first blueprint for interstellar travel"

    Relevant Pages

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