Radial SRBs creating torques

[bewing]

I have a medium-heavy upper stage. In 1.0.5 I launched it with 5 kickbacks in a bottom stage, and 5 thumpers in a middle stage (created with symmetry), and I had big yaw and pitch problems during launch that I "solved" by slapping an enormous reaction wheel on the thing.

So, I am still having the problem in stock 1.1.2. The upper stage flies straight all by itself, merely controlled by the reaction wheel in a HECS. So the Pitch/Yaw problems are not coming from there.

I've tried rebuilding stage 2 and stage 3 from scratch, but nothing changes. In the atmosphere, the kickback stage can be controlled with aerodynamic control surfaces -- but you can see that there is a constant pitch and yaw control being applied. The thumper stage gets lit in the upper atmosphere, so the aerodynamic control surfaces are not effective to control that one. The reaction wheels have to apply more and more and more yaw/pitch until they max out -- and then the rocket starts to veer, of course.

If I simplify stage 2 to being just a single thumper, the problem goes away. If I add 2 radial thumpers to the central one, the problem is smaller than with 4. I've tried the experiment with hammers, and they seem to do something similar. If the thrusts are equal from the SRBs, then experiencing a torque would mean that the radial mountings are not exactly the same distance on one side as on the other.

Has anyone else seen this? Can anyone confirm it with an experiment or two?

 

 

[Hodari]

I always have this problem with larger SRB's as well.  It seems to be because their thrust is strong enough to cause the connection between them and the rest of the ship to flex slightly which means they are no longer firing in line with center of mass and thus producing some torque.  My usual solution is to add a couple struts to the top(and possibly the bottom as well) of each booster, either connecting to the ship itself for 2 way symmetry, or to the adjacent boosters if I'm using 4 way instead.  This is enough to hold everything in place.

[Plusck]

19 hours ago, Hodari said:

I always have this problem with larger SRB's as well.  It seems to be because their thrust is strong enough to cause the connection between them and the rest of the ship to flex slightly which means they are no longer firing in line with center of mass and thus producing some torque.  My usual solution is to add a couple struts to the top(and possibly the bottom as well) of each booster, either connecting to the ship itself for 2 way symmetry, or to the adjacent boosters if I'm using 4 way instead.  This is enough to hold everything in place.

Yeah, I really can't put this any better.

I'm sure that if you go back through the VAB and look at where decouplers and struts are located, something won't be aligned or will obviously flex off-centre. fix that and you should be sorted.