What is going on here

[magnemoe]

Well this is an new one.

Has run into that rockets oscillates during launch, that is nose tip up and down during mechjeb launches but this is an problem in  vacuum. 
 
Made an simple tanker to transport nuclear fuel from Minmus to LKO for future interplanetary crafts.
Stock outside of the planetary bases nuclear storage container who is an fuel tank with 3 ton dry mass. 

3.75 meter heat shield the 1.25 meter probe core, the nuclear tank, reaction wheels on the side nodes, fuel tank, an drag plate made of 4 wing parts as drag plate, a bit clipping in corners, its type D or E wing connectors, metal plates as landing legs, 4 twitch engines they clip trough the wing sections. 

then an second nuclear storage module, and the second stage, 
Rocket during launch

Using 12 fins at rear it was stable during atmospheric flight. 

The problem was then either telling ship to rotate to node it would not dampens it rotation but swing forward and back, an mechjeb burn would start to swing wildly at once. 
If I put it on node marker and did an normal burn even set to point towards node. 

See two problems here clipping however not anything I have not done before or the reaction wheels on the sides of the tank, 

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I have tested this with removed the drag plate wings and replaced the two reaction wheels with one below the fuel tank did not help. 

Edited March 26, 2018 by magnemoe

[Cpt Kerbalkrunch]

Any chance you've activated some trim? This could help explain it. During atmospheric flight, the fins and the gimbal on the Mainsail might've been enough to overcome it; which would be why it didn't effect you there. A quick way to check is to hit Alt-X and see if that does the trick.

[magnemoe]

1 minute ago, Cpt Kerbalkrunch said:

Any chance you've activated some trim? This could help explain it. During atmospheric flight, the fins and the gimbal on the Mainsail might've been enough to overcome it; which would be why it didn't effect you there. A quick way to check is to hit Alt-X and see if that does the trick.

Good tips but its not that, its like the reaction wheels has very little dampening, looks like an restart helped some for SAS but not for mechjeb. 
Not an major issue manual control works well enough. 

[Cpt Kerbalkrunch]

5 minutes ago, magnemoe said:

Good tips but its not that, its like the reaction wheels has very little dampening, looks like an restart helped some for SAS but not for mechjeb. 
Not an major issue manual control works well enough. 

Hmm. Been hearing about a lot of oddball issues since 1.4 came out. Not sure if it's the base game or mod interaction for most of them, but I think the next update will be a significant one. Might take awhile, though. Probably a lot of things to look into.

[magnemoe]

Still in 1.31 here and first time I ran into something like this even if mechjeb has an wobble tendency.