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Hello again! Begginer's questions (Read 2066 times)
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Hello again! Begginer's questions
07/08/09 at 07:13:44
Yesterday i found this porgram and it's excellent! But there are some things that i'd like to know if they can be done.

  • Most important, i have problems with the standard version of GravSim (the one that's on the download page). I install it, run it, close, but then when i try to start it again it hangs and i have to kill it. The only solution i know is uninstalling the whole thing. I don't have this problem with the beta versions (i'm using Jan 08 now).
    I fixed this.
  • What is the maximum recommended simulation speed for simulations like the standard solar system (no moons) and such? I know that the full solar system simulation is VERY fragile; any speed higher than 1024 and the jupiter system goes very wrong...
  • Traces can be set to last only one orbit? After some time the screen fills up and its a bit difficult to track things.
  • Finally, is there some kind of replay feature? I mean, i leave the simulation doing the maths without drawing anything, all that data is recorded on a file, and then i can view it and look collisions, ejections, etc.

Excuse me for my bad english. If you don't understand something tell me and i'll try to explain.
Thanks for the help!
Edited for format fixing.
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Re: Hello again! Begginer's questions
Reply #1 - 07/17/09 at 03:54:24
The maximum speed, of course, depends on the simulation. The 'speed' number is the timestep, or number of seconds (or sometimes days, or years) that pass with each simulation step. I think Gsim still uses Euler method, so for reasonable accuracy, you should probably allow at least a few hundred steps for each complete orbit.  
As an example, onlyplanets.gsim's fastest-orbiting planet is Mercury, which takes about 88 days = 7603200 seconds per orbit. So, timesteps higher than around 7603200 / 500 ~= 15000 would be inaccurate; 16384 would be a good upper limit for that simulation (its default timestep is a lot higher, but onlyplanets.gsim is more of a demonstration sim anyway, so accuracy isn't so important there). You can often run simulations 'too fast' without things actually flying apart, but the orbits may precess a lot or get warped out of shape.
Though, for some reason, Gsim doesn't seem to have a tool for displaying a planet's period...am I missing something?  Huh
I don't know of a way to stop trails after one orbit...but you can turn them off and on to erase them.
There isn't that type of replay feature, as far as I know; it'd be too slow to constantly load all that data. There's an 'output file' tool in the File menu in recent beta versions that records orbital elements/vectors of objects over time, but I don't think you can re-simulate the data it records; it's mainly for making graphs of things. (And this tool has the period, but the Orbital Elements box doesn't...)
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Re: Hello again! Begginer's questions
Reply #2 - 07/17/09 at 16:33:51
You can compute orbital period with 2*pi*sqrt(a3/(GM), and the Orbital Elements box gives semi-major axis.  Also, you can open the View > Rotating Period Adjustment box, and choose an object from the dropdown list, and it will compute the period in seconds for you.
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