My Rocket appears to be rotating for no reason.

[Chris the Mage]

My Rocket appears to be slowly and not so slowly rotating mid flight, and I have no idea what is causing it. The rotation begins immediately but speeds up severely once the thumpers have been detached and the rocket is being propelled by the reliants, if I can get any help as to what is causing this it would be a boon. I've attached photos below and will try answer questions as soon as I can.  

Spoiler

 

[Superfluous J]

There are a combination of factors I can think of:

• Holy cow that's a lot of thrust. The faster you go the less stable you are in atmosphere. Also, lots of thrust means parts flex and once they've flexed in one direction, the high thrust tends to make them continue pointing that way.
• Trilateral symmetry. There are rounding errors in the math that show up when you don't do 2-, 4-, or 8-fold symmetry.
• Lack of control. Solid boosters and Reliants don't have gimballing, and your fins don't provide any control either. That little reaction wheel can only do so much.
• SAS tries to hold rotation but doesn't really care about it. It won't correct you back to where you were, just continue to try to keep you where you are now. As soon as the rotational forces caused by the above things overcomes its tiny strength, the ship will just continue to rotate.

My suggestions start with:

• Go down to 2-way symmetry. Or even 1 reliant and 2 side boosters, not 3.
• Your TWR in the 1st stage is too high, I prefer 1.2 to 1.3.
• Your TWR in the 2nd stage is ACME-level powerful. It should also be in the 1.2 range.
• Your 3rd stage (A terrier?) can be about 1.5 to 2x the size. I think you're showing Vacuum dV numbers. 1500-2000m/s is a good target for a booster stage. Especially an upper stage, as that's more efficient. It's easier for a reliant to lift the fuel for an efficient engine than for itself.
• What does all this have to do with rotation? Smaller rockets with less symmetry and lower TWR are easier to control.

[Aegolius13]

The engineer's report says your rocket has 255 parts. That seems like some kind of glitch.  You might want to try removing parts and putting them back on until you see that number normalize.  I've had quite a bit of trouble with the symmetry on those bi/tri-couplers, where it stacks multiple instances of the same part in each of the nodes. So I'd suspect that first and foremost.

Otherwise, I agree with everything @5thHorseman said, though I tend to like my initial TWR more in the 1.4 to 1.6 range.  But since SRBs increase more than LF engines in TWR as they burn out, starting a bit lower with an SRB stage might be prudent.

Another thing to consider is using Swivels on the second stage.  As mentioned, you have more than enough TWR, so the hit from the Swivels isn't much.  And they would likely provide better overall specific impulse - their sea level ISP is lower, but by the time the SRBs burn out, they may have closed the gap, and will continue to do so as you climb. Plus, the engine gimbal should help with your roll issue (two or more gimbal engines can control roll).

If you downsize to 2-way symmetry and need to add back some rocket, it looks like you could add some more fuel to that second stage.  

 

[VoidSquid]

1 hour ago, Aegolius13 said:

The engineer's report says your rocket has 255 parts

Actually it says the rocket has 50 parts, and the maximal allowed number would be 255, VAB expansion level 2.

I often had similar odd behaviour too when not using a single center engine but a symmetric three or more fold arrangement you have with that tricoupler, I'd check that.

Everything else I could add has already been said by @5thHorseman and @Aegolius13

Edited August 29, 2019 by VoidSquid

[Chris the Mage]

Thank you all very much. I've switch to a twin symmetry with a single core and it's done it's job well, got the satellite up there, then de-orbited. 

[Aegolius13]

14 hours ago, VoidSquid said:

Actually it says the rocket has 50 parts, and the maximal allowed number would be 255, VAB expansion level 2.

Whoops.   So it does.   Was thinking that was some kind of stage/ total thing. 

[VoidSquid]

Happens

And such things happen to me all the time, oopsy, embarrassing but then, laughing about myself later