why does this flip?

[Perotis]

I'm trying to figure out why this rocket flips the way it does.  A few seconds after launch I lean it over about 5 degrees towards 90 and then just leave it.  not much later is just pulls over in the opposite direction and flips.  I can understand a flip, but why does it flip the the completely opposite direction?

 

https://www.youtube.com/watch?v=7-qTxatPxis&feature=youtu.be

[ToukieToucan]

I see you have no wings, which are kinda very important. 

 

Here's a nice post explaining why it flips (and how to fix it):

 

http://forum.kerbalspaceprogram.com/index.php?/topic/106190-why-does-my-fliping-rocket-always-flip-over-easy-picture-explanation/

Edited January 19, 2016 by ToukieToucan

[KerikBalm]

nose cones to reduce the drag in the front would help as well.

Were those lv-t30s or 45s? gimbal is good to be able to follow prograde closely

nose cones + fins at the back + follow prograde very closely = no flipping problems

[tutike2000]

Looks like a souposphere-era design

You need aerodynamic rockets now. Nosecones and fins at the bottom at least. Maybe move that outer stack downwards as well.

Edited January 21, 2016 by tutike2000

[MagicFireCaster]

yep you need all kind of aerodynamic parts to give it stability 

[Plusck]

Agreed on all the above (fins, nosecones etc are needed).

Also, it looks like you must have fuel lines from outer to inner stacks. And I'd guess that the fuel lines are feeding from the bottom fuel tank on the outer stack. That means that you have draggy and mostly-empty fuel tanks sitting high above the CoM, further compounding the problem. If you attach the fuel lines to the topmost tank in the outer stack, you'll get balanced fuel usage and less of a shift in the CoM.

[Perotis]

 

the fuel is actually going the other way.  two of the outer engines are feeding the other two which are feeding the inner ones.  first stage drops two empty outer tanks and the second drops the other two, leaving just the inner ones with full tanks.

 

I can still understand that my design problems make it flip. 
It just doesn't make sense why it would flip in the opposite direction.

 

[Snark]

To sum it up:  The ship flips because it is extremely aerodynamically unstable.  It does everything wrong, from an aero perspective.  It's like a poster child for a rocket that has been designed to flip. 

(Your rocket design would have been fine before 1.0.  However, 1.0 completely changed the rules of how aerodynamics work.  A different design is needed.)

Specific issues:

• Lightweight, draggy things are in front (you want heavy, streamlined things in front)
• Engines with no gimbal (you want engine gimbal, it can help a lot with steering control)
• Tall stacks of fuel tanks (you want to avoid vertical stacks, they drain from the top which is the exact opposite of what you want)
• Nothing to help with stability (you want fins)

So, some specific suggestions:

• Put nosecones on everything.  You really, really don't want a big flat surface facing forward.  This is important not just for stability, but also for efficiency:  if you don't fix this, you're wasting lots of fuel fighting drag.
• Those outer stacks need to be moved down.  Way down.  As in, the top of the outer stacks should be down near the bottom of the inner stage, not sticking out above the top of the command pod.
• If there's any way you can redesign to avoid vertical stacks of fuel tanks, it would help.  (If you can't do that-- well, if you fix all the other problems, maybe you can get away with this one.)
• Put fins on the rocket.  You want them way down low, as far behind the ship's CoM as possible.
• Replace the Reliants with Swivels so that you have gimbal available.

[Plusck]

44 minutes ago, Perotis said:

 

the fuel is actually going the other way.  two of the outer engines are feeding the other two which are feeding the inner ones.  first stage drops two empty outer tanks and the second drops the other two, leaving just the inner ones with full tanks.

 

I can still understand that my design problems make it flip. 
It just doesn't make sense why it would flip in the opposite direction.

Ah, well, the video certainly doesn't have the fuel and staging arrangement you're describing. All four outer tanks are set to drop together and fuel is draining equally from them. You are flipping at 180-200 m/s at 2000m, which is probably higher than terminal velocity for your rocket, when the outer tanks are nearly a third depleted meaning that the top tanks are probably empty.

Watching the video again, when you stop giving keyboard input SAS is trying to yaw back to a direction that is more vertical than prograde.

One possible problem is that your command pod is not very firmly attached to the rest of the rocket, so it is overcorrecting and the direction your stacks are facing is not in fact what the navball shows.

Edited January 19, 2016 by Plusck

[Frybert]

needs more fin

[Perotis]

This is what I did with your input.  it worked well.