Stable station configurations wrt Gravity Gradient torque

[Zatnikitelman]

Several years ago, I completed my "Jump Station" in a near-circular low Earth Orbit. I constructed it similar to the old Freedom Station "power tower" design: http://i226.photobucket.com/albums/...Jump Station/orbiter2007-11-2119-01-11-64.jpg
I had thought that this configuration would let the station stabilize itself naturally with the scientific modules (near lower truss in that pic) being massive enough to have the whole station hang down. However, even under normal timewarp, the station didn't stabilize itself. How exactly does gravity-gradient torque work to keep objects stabilized, and how does that apply to a docked stack in Orbiter? Also, what is more important to Orbiter's engine in determining gravity-gradient torque in a docked system, mass or the PMI of a vessel, or something else?
Thanks,
Matt

 

[Loru]

I suggest "cheating Orbiter" - for the truss/tower create dummy PMI mesh to calculate it in the way it'll turn properly.
MeshCalc assumes even mass distribution in mesh so you have to cheat a bit.

I did it for mars impactor as it has to have most of it's mass in the nose, so I simply created reversed cone PMI calculations.

I'm not sure how orbiter calculates PMI shifts for attached objects.

 

[tblaxland]

I'm not sure how orbiter calculates PMI shifts for attached objects.

Simple: it is not calculated for attached objects. From the API Reference:

Attachments use a simplified physics engine: the root parent alone defines the object’s trajectory
(both for freespace and atmospheric flight). The children are assumed to have no influence on flight
behaviour.

For docked vessels, the PMIs for the superstructure are approximated. The method of approximation is described on page 8 of this paper by Martin Schweiger: Orbiter:A Free Spacecraft Simulation Tool.