shuttle out of control above 35km?

[uNiverselEgacy]

I built a rough replica of NASA's space shuttle. I put only one vector engine on the orbiter itself and another two under the ET (because they are too powerful and heavy if all three are on the orbiter). It launches quite well if not perfect. The SRBs separate around 12km and it will continue to fly just fine without wobbling or anything. But it starts to get out of control above 30km and really bad above 35km. With the help of MechJeb, it won't lose control completely but it's still not a smooth ride (you can see in the pics below that pitch, yaw, and roll controls all go crazy).

I thought it has to do with CoM shifting place as fuel drained at first so I tried pumping fuel around to keep CoM relatively still but it doesn't seem to make things any better if not even worse. I tried adjusting the thrust limit of the two engines on the ET to keep the thrust vector point at CoM but that doesn't seem to help either. I'm completely clueless now.

 

[Rocket In My Pocket]

Hard to say, shuttle replicas are always difficult to fly, probably one of the hardest things you can try to make in KSP.

I would assume it's a center of thrust issue combined with the engines gimbal, and KSP physics being notoriously wobbly.

You could try disabling the gimbal once you reach 30-35km up and see if it that helps, or download Claw's stock bug fixes which allows for tweakable gimbal rate/speed. 

[Val]

My guess would be, you're reaching altitudes where due to thinning atmosphere, aerodynamic control is failing and your thrust is too off-center for gimbal control to compensate.