Why is this rocket wobbling?

[Dizzle]

I've been trying to launch this rocket for a while, and within a few seconds of ignition, it starts wobbling uncontrollably.

 

On the pad:

 

Without the fairing:

 

I've tried launching without the fairing, without the solar array, and without the ISRU. I've tried disabling torque on the reaction wheel. I've tried re-seating each part in case it's a clipping error. I've even tried moar struts, but nothing makes a difference. KJR doesn't seem to be working in the 1.2 Pre (either that or it's not helping), so no luck there. Any advice is appreciated.

[coredumpster]

I've found that wobble seems to happen when the engine gimbal fights the reaction wheels.  I've had best results from dialing back the engine gimbal.

I'd also add some tail fins to add a bit of drag at the back end to help with stability.

[Plusck]

A couple of reasons:

- large reaction wheel in a bad place. The large reaction wheel has weak joints, and should never be placed where it might suffer torque from another source. Here, there is a hugely long lever arm between the engines (causing torque through gimbal) and it, with the command pod on the other side. It's a recipe for disaster.

Difficult to say where to put that reaction wheel. At the bottom of the 2.5m stack it will seriously weaken the whole rocket - however, if you can strut over it (like you have between the 3.75 and 2.5m sections) it might work. Frankl, though, I'd ditch it and put a smaller reaction wheel on top of the command pod, and use fins and/or vernors for the massive parts of the rocket instead.

- large number of joints between the command pod and the engines: the command pod moves itself (and is moved by the large reaction wheel) while the rest of the rocket is still flexing; the command pod thinks it's facing the right way while the flex from engine gimbal is still working its way up the rocket; the flex hits the command pod sending it the other way, so it tells the torque wheels and engines to go the other way, and therefore the whole ship snaps back in a wobble in the opposite direction.

If I were you, I'd set a custom action group (personally, I always always use same key for this) to toggle gimbal on the engines. I do it for all engines systematically but at least do it on the Mammoth. Add a quartet of control fins to the bottom of the rocket and use them while you're low in the atmosphere. Toggle gimbal off for as long as you possibly can, which should be at least until the next stage.

I'd also set a second action group (again, I always use the same key for this) to toggle torque on the large torque wheel. I'd start by toggling both gimbal and torque off, using only aero surfaces for control, until it starts looking like the rocket is drifting off prograde.

[Dizzle]

1 hour ago, coredumpster said:

I've found that wobble seems to happen when the engine gimbal fights the reaction wheels.  I've had best results from dialing back the engine gimbal.

I'd also add some tail fins to add a bit of drag at the back end to help with stability.

Thanks, reducing engine gimbal to 15 worked.