# General QuestionRigid body model and angular momentum?

#### Nikogori

##### Donator
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I'm translating orbiter scenario descriptions and now I'm stuck at The rotation model.scn.
I only understand high school level physics and don't know some of the concepts below.

Orbiter's rigid body model in action. Check how the angular momentum oscillates between the principal axes of the glider.

1. What are the principal axes of the glider? Is it something like this?

2. What does "the angular momentum oscillates between the axes" mean?
Is it something like a movement of a spinning top?

#### kuddel

##### Donator
Donator
1. What are the principal axes of the glider? Is it something like this?
Yes, those are the principal axes. Arrows in that image show the positive direction of the axes (+X, +Y, +Z)

2. What does "the angular momentum oscillates between the axes" mean?
If you understand "angular momentum", then the "oscillation between the axes" should not be the problem I hope

The movement of a spinning top (precession) is another phenomenon - although possibly related (dont't qoute me on that )

#### Linguofreak

##### Well-known member
Basically, if you have an object that has different moments of inertia around all three axes, then rotations around the axis with the minimum moment of inertia and the maximum moment of inertia are stable: if those axes are close to the direction of the angular momentum vector, they will tend to remain near the angular momentum vector as the object rotates. But the axis with the intermediate moment of inertia is not stable, it will tend to move away from the angular momentum vector.

"The angular momentum oscillates between the axes" is a somewhat awkward way of phrasing it: as long as the body is not acted upon by any external forces, conservation of angular momentum applies, so the angular momentum vector stays pointed in the same direction as seen by the outside universe, it doesn't really "oscillate". But as seen in a coordinate system where the principal axes of the object are fixed, the angular momentum vector will be seen to wander.

#### BrianJ

You might find my "Rotating Wingnut" add-on amusing and interesting from an angular-momentum point of view.

As a historical note, Orbiter's handling of angular momentum was not quite correct until Orbiter2010-P1.
This was only noticed when I tried making an attitude-control algorithm for the Giotto spacecraft (a spin-stabilised vessel), as discussed here.

Cheers,
BrianJ

#### Nikogori

##### Donator
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Yes, those are the principal axes. Arrows in that image show the positive direction of the axes (+X, +Y, +Z)

Basically, if you have an object that has different moments of inertia around all three axes, then rotations around the axis with the minimum moment of inertia and the maximum moment of inertia are stable: if those axes are close to the direction of the angular momentum vector, they will tend to remain near the angular momentum vector as the object rotates. But the axis with the intermediate moment of inertia is not stable, it will tend to move away from the angular momentum vector.

You might find my "Rotating Wingnut" add-on amusing and interesting from an angular-momentum point of view.

Thank you for the replies. I have googled based on your comments and found this video (actually it was posted in BrianJ's thread):

And I learned this phenomenon can be seen in Orbiter.

#### Linguofreak

##### Well-known member
And I learned this phenomenon can be seen in Orbiter.

Of course! That's basically exactly the physics that The rotation model.scn is meant to demonstrate.

#### Nikogori

##### Donator
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I mean, The rotation model.scn only shows rotation around intermediate axis. I needed to see what will happen around 1st and 3rd principal axis.

#### kuddel

##### Donator
Donator
...so I guess the translation is now delayed by "first hand playing to grasp the phenomenon", right?

#### Nikogori

##### Donator
Donator
...so I guess the translation is now delayed by "first hand playing to grasp the phenomenon", right?

Finished translating up to Demo scenarios.
Machine translation is useless for this kind of job. So it takes time...

https://github.com/orbinautjp/scn-localization-JP

I really wish Orbiter was compatible with UTF-8. I need to force VSCode to use cursed Shift-JIS and I don't like it.

#### kuddel

##### Donator
Donator
A UTF-8 Orbiter might need UTF-8 Addons, too - although most of the ASCII Strings will work for them as well.
But remember someone has to constantly maintennce a translated version ... volumteers?
Most tutorials will still be written in English and when it says "click Button doodeldidoo", will a localized user get it?
Please get me right, I am not against that, but I think Orbiter is so special that understandig english should not be a hurdle (My view - totally biased )

I once (several years ago) tried to make the universal camera addon to display kyrillic characters to mimic the KURS System and I whished it would have worked better.
So and UTF-8 capable Orbiter Core might actually be a good option.
If string-related API functions (graphics client) will be possible, that might be feasable and beneficial....now that I think about it
An issue (feature request) on Orbiter github might get the right people started.

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#### Nikogori

##### Donator
Donator
Please get me right, I am not against that, but I think Orbiter is so special that understandig english should not be a hurdle (My view - totally biased )

When you play Orbiter, you don't have to read lengthy English text - you just have to remember what buttons to push. Orbiter is not a RPG. You don't have to read hefty conversation logs.
I think scenario description is one of a few exceptions. People (including addon developers) do expect users to read them because it provides some background information. (historical events etc.)

An issue (feature request) on Orbiter github might get the right people started.

Maybe I'll submit a request when OpenOrbiter has been released. I heard people are now working hard creating 64bit Orbiter?

#### Arvil

##### Well-known member
Also, a UTF-8 Orbiter will be able to display planet feature names that use UTF-8 characters, which there are a number of them.

#### kuddel

##### Donator
Donator
Also, a UTF-8 Orbiter will be able to display planet feature names that use UTF-8 characters, which there are a number of them.
Those are already UTF-8 (in the "Textures\<PLANET>\Archive\Label.tree" that is) and are converted to wide-character in D3D9Client.
I don't know whether the inline client and/or D3D7Client handles those the same way.

#### Linguofreak

##### Well-known member
A UTF-8 Orbiter might need UTF-8 Addons, too - although most of the ASCII Strings will work for them as well.
But remember someone has to constantly maintennce a translated version ... volumteers?
Most tutorials will still be written in English and when it says "click Button doodeldidoo", will a localized user get it?
Please get me right, I am not against that, but I think Orbiter is so special that understandig english should not be a hurdle (My view - totally biased )

Even if Orbiter isn't localized at all, UTF-8ification is desireable from a modernization perspective: basically all software these days uses Unicode, and Windows is one of the few platforms whose API uses UTF-16 instead of UTF-8. Even on Windows, most applications write UTF-8 to disk and for anything being sent out over the network, and only use UTF-16 internally. Windows-1252 is dead, Jim; and for the English text that dominates Orbiter, you don't actually need to do any conversion of the text itself to convert to UTF-8, just the code that handles it, since a high-bit-zero byte in both UTF-8 and 1252 is going to be an ASCII character, and English text generally sticks to ASCII.

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