Vessel Rotating in Orbital Flight

[firefly77]

Hi everyone!
I usually don't post in the forum much, but I've run into a problem that's driving me a bit crazy. Now I'm not new to KSP I've been a player since I bought .13 way back when. However I can't for the life of me figure out this Duna interplanetary transfer vehicle I'm trying to design keeps rotating in orbit when I throttle up the engine. As you can see below it's docked to a "mock-up" lander similar to the one I plan on using. Through testing I've found that if I undock, the vehicle flies just fine, but while it's docked it rotates. The SAS isn't keeping it stable, and I've checked to make sure I hadn't accidentally set a trim in my game. I even tried it with multiple engines (was testing to see what would be the best to use for the flight to Duna) and they have the same result. I have noticed that if I shut the SAS of it takes more time for the ship to start rotating oddly enough. 

I'm really at a loss as to what's wrong with the design, and any help would be greatly appreciated. 

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[TheDestroyer111]

Looks like a much less nooby thread than most ones here. If everything is perfectly symmetrical (maybe the engine is slightly rotated? It doesn't look like so, but it could be), then idk. However, I spotted a similar issue on many of my larger planes. My B-747 keeps rolling as if it had something heavy on the right side, even though I counted its parts and it was right with the part count, and didn't find a single assymetrical part. Did you try rebuilding the vehicle from the ground up (on a different craft) but with exactly the same design?

//EDIT: Did you try redocking, or checking if the docking ports aren't rotated? It looks like the crew vehicle is a little misaligned relative to the rocket.

Edited August 16, 2016 by TheDestroyer111

[OHara]

Also try right-click and 'control-from-here' on the probe core that is oriented the same way as the active rocket engine,
or shut down that engine and use the one oriented with your capsule instead.

[firefly77]

I think I may have solved it, though I'm still not sure of the cause. I did a quick rebuild of the "mock-up" and attached struts to the docking port on the other vessel. I'm not sure which fixed it but it's flying fine now. Now I'm gonna actually put the finishing touches on the "real" lander and hopefully it'll work the same.  

[Spricigo]

5 hours ago, firefly77 said:

I think I may have solved it, though I'm still not sure of the cause. I did a quick rebuild of the "mock-up" and attached struts to the docking port on the other vessel. I'm not sure which fixed it but it's flying fine now. Now I'm gonna actually put the finishing touches on the "real" lander and hopefully it'll work the same.  

I bet the struts fixed it.  Your vessels was flexing and the docking port in particular was not stiff enough to damp the oscillation.   To add injury to offense SAS try to overcorrect your trajectory adding even more oscillation. 

 

[firefly77]

I suppose it probably was the struts that fixed it. Honestly never came across that problem before. 
Anyways I flew the design to Duna, landed, and returned to Kerbin without issue. All is well now!