Flight Question Centre of Gravity

[Warped]

When I have to spacecraft docked together, and I try to put one spacecraft into an auto pilot, both ships start wobbling and swinging.

I know that this has to do with the Centre of Gravity and is normal.

Is there anyway that I can negate these effects if, for example, I wanted to take a mothership docked to a small craft to another planet, or do I just have to deal with it?

 

[Lucy]

You might try this: http://www.orbiter-forum.com/showthread.php?t=16008

The search function is your friend. There have been a lot of threads on this subject.

Nevertheless, to the forums, since I haven't seen you before.

 

[Warped]

You might try this: http://www.orbiter-forum.com/showthread.php?t=16008

The search function is your friend. There have been a lot of threads on this subject.

Nevertheless, to the forums, since I haven't seen you before.

Thanks! iv'e lurked around here for a while.

I tried searching, but I didn't know the correct definition of what I was asking.

 

[Lucy]

To be fair, actually, I only found that one by searching "invisible ship docking" or something similar - which I knew because I'd already seen the thread.

 

[martins]

If it is just about main thrust, you could try gimballing the engines (if the spacecraft supports it) to compensate for the shift in cog. For attitude control with the RCS thrusters it's more tricky. In some cases it may be possible to compensate by fine-tuning the relative thrust of the RCS thruster pair that performs a particular action, but often you may have to engage additional thrusters to eliminate unwanted parasite rotations.

 

[Warped]

If it is just about main thrust, you could try gimballing the engines (if the spacecraft supports it) to compensate for the shift in cog. For attitude control with the RCS thrusters it's more tricky. In some cases it may be possible to compensate by fine-tuning the relative thrust of the RCS thruster pair that performs a particular action, but often you may have to engage additional thrusters to eliminate unwanted parasite rotations.

One thing i'm having trouble with is getting to Mars with to ships docked together. In this case, I'm using a Deepstar with a DGIV docked to it's nose. Unfortunately, the Deepstar doesn't support any gimbaling that I know off.

Also, i'm trying to get there using IMFD, the extra mass causes the Deepstar to over adjust it's attitude when i'm auto-burning it for ejection.

 

[ex-orbinaut]

The problem with gimballing the thrusters is that the thruster needs to point at the CG in order to negate the swing. Once that is accomplished, the vessel no longer "thrusts" along its own longitudinal (Z) axis, but along an axis defined between the thruster and the CG.

This is not okay when you are doing long range flights, as TransX, for example, appears to use the "true" axes of the vessel (its natural Z) as the reference for its computations, apparently assuming that the vessel is thrusting straight along it. It leads to large errors in navigation.

That said, it is creditable, realistic behavior that points at the excellent physics employed in the core Orbiter, and it is a feature the solution to which has evidently been largely neglected by developers, when it is actually a very worthy challenge to rise to. I see this complaint often, and it is not the fault of the add on users. The solutions are there, as simple or as complex as you like, in the API, and how you implement them.