Why does this rocket flip over?

[Destroyer713]

I'm literally bashing my head against my desk trying to figure out why this rocket refuses to fly straight, even after I put fins on it. The instant it drifts a single degree away from the prograde marker as it approaches Mach 1, it'll just flip wildly!

Just to note: There are fuel pumps leading from those outer tanks to the center one.

And before you ask, no, I don't have fairings. I'm too early in Career to have unlocked those yet.

Edited May 10, 2015 by Destroyer713

[basic.syntax]

See if putting a nose cone above the probe core will help. I think the final stage, attached with a very small item (48-7S?) / node to the stack decoupler and FL-800 tank below it, could be too small / too weak to remain solid, against for the forces being applied to it. As soon as you drift off prograde, drag is probably causing the top part to bend at that small node connection. If you have access to struts, it needs those. If you don't, see what other engine or part you can use to keep all center stack nodes the same size / strength. Might help.

[sewerbird]

Played around with it a bit - the two main factors in my attempts to orbit were

1 - It's a lot of rocket for such a light payload. I tuned the thrust on the boosters down to 50.

2 - The center of mass shifts quite close to the center of lift as the boosters approach empty (easier to flip!). I shifted the boosters downwards a bit with respect to the central rocket, so that the CoM goes 'up' the rocket instead of down as the side tanks empty.

With a very 'meh' attempt at flying, I did get one wonky wobble but the rocket corrected itself quickly. Made 200x200 orbit with likely enough fuel to orbit Mun or Minmus.

[allmhuran]

With a TWR over 3 with full tanks and 5 just before staging you're probably going too fast. You could double the length of the central column (add another FLT800) and still have a plenty-good-enough TWR. This will also move your payload further away from the stabilizers, keeping the CoM closer to the front. You could also swap in SRB's for booster columns to save some cash. You'd probably need to thrust limit them somewhat, and you'd still be launching with only ~25% throttle or less, then ramp it up to 100 when the boosters stage.

Edit:

Looks like sewerbird ninja'd most of this while I was typing my post.

Edited May 10, 2015 by allmhuran

[basic.syntax]

I tip my hat to sewerbird - nice work.