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General >> Discussion >> Regina - What's going on in this sim?
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Message started by Mal on 11/10/06 at 22:21:02

Title: Regina - What's going on in this sim?
Post by Mal on 11/10/06 at 22:21:02

Regina sim (http://www.evildrganymede.net/temp/Regina.gsim)

This is a 5 planet sim from the Traveller RPG, with a white dwarf (Speck) around the primary star in a close orbit.

Lusor: primary (1.25 solar mass)
Speck: White Dwarf, close companion (1 solar mass, 0.05 AU)
Clement: planet (0.002 earth mass, 0.2 AU, 800km radius)
Ausun: planet (0.05 earth mass, 0.4 AU, 2400km radius)
Burgund: small gas giant (18 earth mass, 0.7 AU, 25,000 km radius)
Olybrius: planet (0.626 earth mass, 1 AU, 5600 km radius)
Assiniboia: large gas giant (1 jupiter mass, 1.6 AU, 70,000 km radius)

Eccentricity and inclination of all bodies is 0.000

If I remove Speck (the white dwarf) and run the sim, I get nice circular orbits of the planets, and things progress as normal.

But with the dwarf, the sim goes crazy - the planets just immediately head off in coily paths into space, even at a timestep as low as 8 secs.

Am I missing something here or doing something wrong? Do I need to do stuff with barycentres to make this work? Or is it really that unstable?!

Title: Re: Regina - What's going on in this sim?
Post by Tony on 11/10/06 at 23:02:18

In the "Create Objects" interface, check the "barycenter" option under Reference Object.

If you don't do this, then when Gravity Simulator converts your orbital elements to state vectors, it uses the mass and position of the Reference object only.  With barycenter checked, it adds the mass of the star + the white dwarf, computes their center of mass and uses that for the position.

Like I said in another thread, Gravity Simulator does not yet provide output for barycentric systems.  So your results will be orbital elements with respect to a single object, rather than with respect to a barycentric system.  (i.e. they'll be meaningless unless you're really good with Excel and the conversion between orbital elements and state vectors, as the math can be done externally to the simulation).

It's not that hard for me to incorporate such output as it would use existing blocks of code.  Just haven't gotten around to it yet.

Title: Re: Regina - What's going on in this sim?
Post by Mal on 11/11/06 at 00:15:59


Tony wrote:
In the "Create Objects" interface, check the "barycenter" option under Reference Object.

If you don't do this, then when Gravity Simulator converts your orbital elements to state vectors, it uses the mass and position of the Reference object only.  With barycenter checked, it adds the mass of the star + the white dwarf, computes their center of mass and uses that for the position.

Like I said in another thread, Gravity Simulator does not yet provide output for barycentric systems.  So your results will be orbital elements with respect to a single object, rather than with respect to a barycentric system.  (i.e. they'll be meaningless unless you're really good with Excel and the conversion between orbital elements and state vectors, as the math can be done externally to the simulation).

It's not that hard for me to incorporate such output as it would use existing blocks of code.  Just haven't gotten around to it yet.


Thought something weird was going on...

So I guess there's not much point in me trying this sort of thing until the output is sorted out for barycentres? I don't fancy doing all the conversions manually!

Title: Re: Regina - What's going on in this sim?
Post by Tony on 11/11/06 at 01:34:13

It's much easier to do the conversions in a programming language than in Excel.  I'll get some coding time after the school semester is over.

You can still draw some qualitative conclusions from the screen plots.

Title: Re: Regina - What's going on in this sim?
Post by Mal on 11/12/06 at 10:41:22

I don't quite follow how this barycentre thing is suppose to work. If I've already created the sim without it, can I add it later? Or does it have to be there at the start?

And when do I add it if I'm creating it from scratch? Can you tell me the order in which I'm supposed to create things if I want a barycentre there?

Title: Re: Regina - What's going on in this sim?
Post by Tony on 11/12/06 at 13:22:32

Imagine a barycenter as a ficticious object.  It's mass is the combined mass of 2 objects.  It's location is the center of mass of those 2 objects.  The center of mass is just the weighted average of the positions.

In reality a barycenter is not an object.  It is just an empty point in space.  But any object that orbits the pair of objects from a great enough distance will behave as if it is orbiting the ficticious object.

For example, let's create a 5 object system:
Star A: component 1 of the binary system.  1 Msol
Star B: component 2 of the binary system. 0.9 Msol in a tight 1AU binary orbit.
Planet Aa: Orbits star A
Planet Ba: Orbits star B
Planat ABa: Orbits the pair of stars from a distance.

File > New
Creates a 1 solar-mass star

Objects > Edit Objects
Change name to Star A and change color if you like

Object > Create Objects
Name: Star B
Color : your choice
Reference Object: Star A, leave "Barycenter" unselected
Mass: 0.9 Msol
SMA: 1 AU, a close binary system
Ecc: 0.3, moderately eccentric
Inc: 0, best to define the plane of the star orbits as the ecliptic
Size:1000000 km, this is not important
All others: not important

Press Create

This creates the binary pair.  Gravity Simulator uses their combined mass when converting the orbital elements you provided into the state vectors that Gravity Simulator uses to crunch the numbers.

Objects > Create Objects
Reference Object: Star A, leave "barycenter" unchecked
Name: Planet Aa
Color : your choice
Mass : 1 Mearth, or your choice
SMA  : 0.1 AU, or your choice, but keep it close to the star.
Ecc   : 0, or your choice, but make sure that its apastron doesn't wander too far from the star.
Inc    : 0, or your choice, but keep it low so Kozai doesn't destabalize it
Size   : 12000 km, or your choice
All others: unimportant

Press Create
This creates a planet in orbit around star A.  Gravity Simulator uses the combined masses of Star A and Planet Aa when converting the orbital elements you provided into the state vectors that Gravity Simulator uses to crunch the numbers.


Objects > Create Objects
Reference Object: Star B, leave "barycenter" unchecked
Name: Planet Ba
Color : your choice
Mass : 1 Mearth, or your choice
SMA  : 0.1 AU, or your choice, but keep it close to the star.
Ecc   : 0, or your choice, but make sure that its apastron doesn't wander too far from the star.
Inc    : 0, or your choice, but keep it low so Kozai doesn't destabalize it
Size   : 12000 km, or your choice
All others: unimportant

Press Create
This creates a planet in orbit around star B.  Gravity Simulator uses the combined masses of Star B and Planet Ba when converting the orbital elements you provided into the state vectors that Gravity Simulator uses to crunch the numbers.


Objects > Create Objects
Reference Object: Star A, this time select "barycenter".
Name: Planet ABa
Color : your choice
Mass : 1 Mearth, or your choice
SMA  : 10 AU, or your choice, but keep it far.
Ecc   : 0, or your choice, but make sure that its apastron doesn't wander too far from the star.
Inc    : 0, or your choice.
Size   : 12000 km, or your choice
All others: unimportant

Press Create
This creates a planet in orbit around the barycenter of star A and star B.  Gravity Simulator creates a ficticious object whose mass is the combined mass of Star A and Star B, and whose position is the weighted average of the position of Star A and Star B.  It then uses the combined masses of the ficticous object and Planet ABa when converting the orbital elements you provided into the state vectors that Gravity Simulator uses to crunch the numbers.  The ficticious object is not used in the number crunching, only in creating the object, for the purpose of computing the state vectors of the planet.

You should now have the 5-object system described above.

When viewing the system, select Star A as the focus object.  Then use the F/A button on the Graphics Options interface to toggle back and forth between Floating Mode and absolute Mode.  Pressing F places you in the floating mode, and the screen is centered on the system barycenter.  Pressing A returns you to absolute mode and the screen is centered on Star A.

http://orbitsimulator.com/gravity/images/binarysystem.GIF

In the case of your system, all the planets orbit the pair, so you would select barycenter when creating all objects except the white dwarf.

Title: Re: Regina - What's going on in this sim?
Post by Mal on 11/12/06 at 13:37:51

ah, gotcha now. I'll give that a go.

Title: Re: Regina - What's going on in this sim?
Post by Mal on 11/12/06 at 22:59:27

Huh. I did what you said, and the planets all immeditaely shot off at tangents to their orbits with a 0.25 second timestep...!

Here's the new gsim file, can anyone identify what's wrong?
regina2.gsim (http://www.evildrganymede.net/temp/regina2.gsim)

Title: Re: Regina - What's going on in this sim?
Post by Tony on 11/13/06 at 03:12:21

The one you sent did exactly what you said, with everything but the secondary star flying away at fast speeds.

I tried creating it from scratch, and mine works fine.  I used 0.6 Msun for Speck, as in the simulation you posted.  All other values were taken from the first post in this thread.

Create New Simulation:
http://orbitsimulator.com/gravity/images/mr1.GIF


Edit the star into Lusor, changing name, color, mass
http://orbitsimulator.com/gravity/images/mr2.GIF


Create Speck.  DON'T choose barycenter option
http://orbitsimulator.com/gravity/images/mr3.GIF


Create Clement. DO choose barycenter option
http://orbitsimulator.com/gravity/images/mr4.GIF

Create Ausun.  DO choose barycenter option
Forgot to hig Ptrscr for Ausun.

Create Burgund.  DO choose barycenter option
http://orbitsimulator.com/gravity/images/mr5.GIF


Create Olybrius.  DO choose barycenter option
http://orbitsimulator.com/gravity/images/mr6.GIF


Create Assiniboia.  DO choose barycenter option
http://orbitsimulator.com/gravity/images/mr7.GIF


Run the sim.  Everything is in nice circular orbits.
http://orbitsimulator.com/gravity/images/mr8.GIF

http://www.orbitsimulator.com/gravity/simulations/Lusor.gsim




Title: Re: Regina - What's going on in this sim?
Post by Mal on 11/13/06 at 09:13:58

*scratches head*

Thanks for posting all that, but that's almost exactly what I did. The only differences are that Lusor was 1.51 solar radii, I actually entered "0" into the Eccentricity and Inclination boxes for all the objects, and I put a leading "0" before all the values that were less than 1 (i.e. 0.626 instead of .626). Otherwise it was the same.

I'll try to make it again following your instructions exactly and see if it still screws up.

Though as a suggestion, one thing you might consider is to actually make the barycentre an object in the program - that way we can explicitly tell it to use it as the reference object instead of the star.

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