ElChristou wrote:I think I love you, you know?
heh
Well, I love your models so I'm happy to assist.
ElChristou wrote:Now of course I want to know what do PrecessionRate and EquatorAscendingNode... ...because I was really far away to use those commands...
Can you tell me? (or someone else for sure because you have done more than enough
)
I was hoping someone with more knowledge than me would respond... oh well, I'm not very good at explaining things but will give it a go.
Disclaimer:
I'm still learning the ssc stuff so don't take any of this as 'fact', it's just my understanding of it so far.
When using the LongLat commands, the model will adopt the same rotational axis (poles) as the parent body, so LB7s rotational axis (or its North pole) should point to Polaris, the same as Earths.
It seems you have rotated LB7 90 degrees before exporting, I think it should have been 90 degrees in the opposite direction. The result of your rotation initially renders LB7 inverted (i.e. the base is pointing to Polaris). The only difference this makes is once LB7 is pointing where you want it, the rotation is retrograde. It's no big deal, but instead of rotating in your soft you can use the Orientation command. If LB7 in your soft has its nose pointing up then "Orientation [ 90 1 0 0 ]" should correct it so the nose points to Polaris in Celestia. Well that's how it worked on a test model I made in 3dsMax.
PrecessionRate:
Every day Earth rotates 360 degrees about its axis and 'almost' 1 degree around Sol. This combined rotation is automatically transferred to LB7 because it's in a (equivalent) geo-stationary orbit.
Now this is where you had problems-
As soon as you add RotationPeriod to your model, Celestia discards the rotation transferred from Earths motion. This leaves the models rotational axis (pole) constantly looking at the same point in space. So the model appears to tumble slowly backwards as Earth rotates. The ProcessionRate value of 360.98...etc approximates Earths daily rotation and keeps the model in a static position relative to Earth. So if you want rotation on your model and are using the LongLat command, adding this procession rate value is the first thing to do before pointing the model where you want.
Orientation:
This is not the command you want to use to point LB7 in the desired direction. It seems to be best used only to align the rotational axis. If you rotate the model before export then you don't need this command. (unless you rotate it the wrong way before export
).
Obliquity and EquatorAscendingNode:
By default the poles of LB7 should be the same as Earth. To change this, you tilt the axis (in degrees) using Obliquity, but this command doesn't define which way to tilt it, you define the direction of tilt (in degrees) using the EquatorAscendingNode command.
Note: the ssc I wrote for LB7 uses 260 degrees for Obliquity because the model was initially upside down. In normal practice you should not need a value higher than 180, I should have added "Orientation [ 180 1 0 0 ]" to flip the model the right way up and then the obliquity value would be 80, (260 - 180 = 80).
Summary:
Set the LongLat to desired value.
Set RotationPeriod (if required).
Set ProcessionRate 360.9856414794 (only required if RotationPeriod is used and you want the model to be static relative to Earth).
Set Obliquity value from 1 to 180 to tilt axis from North through to South.
Set EquatorAscendingNode value 0 to 360 for direction of tilt (negative numbers can also be used).
Just FYI: If you would like to try exporting LB7 'without' rotating it first, try this ssc. The result should be the same as the current release but with 'opposite' rotation. Of course I'm not asking you to update the current release, but it may help with other projects.
WARNING:
The following ssc is NOT written for the current release of Liberty Bell 7.
It is only an example for ElChristou to test. If you use it with the current release it will NOT have the correct orientation.
Code: Select all
"Liberty Bell 7" "Sol/Earth"
{
Class "spacecraft"
Mesh "lb7.cmod"
MeshCenter [ 0 0 0.003 ] # for correcting any eccentric rotation.
Orientation [ 90 1 0 0 ] # corrects the rotational axis (poles).
Radius 0.003
# approx position at 2'30" from take off before extension of periscope:
LongLat [ -79.8 28.3 70 ]
RotationPeriod 0.09
PrecessionRate 360.9856414794 # approximates Earths daily rotation.
Obliquity 80 # axis tilt in degrees.
EquatorAscendingNode -25 # direction of tilt in degrees.
Albedo 0.10
InfoURL "http://www.celestiamotherlode.net/creators/elchristou/HTML/LB7/"
}
Hope this helps. Let me know if there is anything that's not clear.
Thank you again for your great work!
cheers,
sundog
... mmhuu... YES THAT IS!!!! 
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