What is wrong with My ship?

[Mr.Gundum]

So for the firs 1-2 minutes of flight the ship stays almost perfectly straight, after that it begins to pitch uncontrollably forward and starts doing flips.

Edited April 6, 2014 by Mr.Gundum

[Jagger]

1. Struts. Get the upper part strutted up to the bottom boosters.

2. Reaction wheels. On the top or bottom, or both.

3. Control surfaces. These always helps.

If nothing works, rework the design and make it not be so tall.

(sorry for any English mistakes an hope I helped!)

[TheTBBD]

Put wings and RCS thrusters

Might help.

[Vanamonde]

Why is there a lift vector pointing to the right?

[DeMatt]

The "Tail Fin", despite its name, does very poorly as a stabilizer in KSP's stock aerodynamics model. It applies lift like a wing when it shouldn't. If possible, use a control surface like the AV-R8 instead.

[cantab]

If there are four boosters attached, the one nearest to the camera is clearly off-centre. That won't help. Did you use symmetry and angle snap - the two icons above those for showing CoM/L/T - when placing the parts?

Complete loss of control without any input from yourself is a sign that something about the rocket is off-balance. Since your design doesn't look like it's got any need to be asymmetric, the best option is to correct the issue.

[Pecan]

As fuel from the lower stages the centre of mass will move upwards because you have so much remaining fuel up there. Most of it you probably need/want but that seems like a very large nosecone on top, wouldn't a smaller nosecone or, especially, a parachute help more?

As the centre of mass moves upwards, futher away from the engines, which are the only thing that can counter any rotation, you'll need more "command authority" from reaction wheels/SAS. Either add that or do a very gentle gravity turn until you're so far up that it doesn't flip.

[Claw]

I tried to replicate your ship and didn't have any problems with it. Although I also didn't have the lift vector pointing off randomly either. . .

Is it happening before or after you stage off the outer four engines? What mods are you using?

I don't see anything obvious in the picture except how high the boosters are mounted. Sometimes mounting them that high induces twist. As fuel burns off and TWR climbs, the twisting gets worse. However, that usually results in rolling and not pitching. Having winglets on twisting boosters can also make it twist more or wobble (especially if they are active and react with SAS).

Check to see if they are mounted straight, and I would recommend turning on angle snap (I see that it's off). If you don't know what angle snap is, it's the button that has the circle with a dot in the lower left corner. Click it and it turns into a hexagon with a dot. Also make sure the boosters are mounted centered on the decouplers. It's easy to miss the center and mount them on the edge of a decoupler.

[SRV Ron]

With the boosters mounted so high on the radical couplers, twisting is causing the flip over. It happened in this design until braces were properly installed.