Rocket still pogo'ing/oscillating even after struts are attached

[WarpPrime]

I recently built an ICBM out of stock parts (excluding the warheads). When I launched the missile, the upper stage immediately began oscillating, even though I added struts between the booster and the upper stage. At 2000 m, I was at Mach 2, and the missile began oscillate more and more. Finally, the upper stage (and the warheads) broke off of the booster stage and tumbled toward the ground. I ended up destroying most of the KSC as a result. How do I stop this from happening?

[Snark]

56 minutes ago, WarpPrime said:

I recently built an ICBM out of stock parts (excluding the warheads). When I launched the missile, the upper stage immediately began oscillating, even though I added struts between the booster and the upper stage. At 2000 m, I was at Mach 2, and the missile began oscillate more and more. Finally, the upper stage (and the warheads) broke off of the booster stage and tumbled toward the ground. I ended up destroying most of the KSC as a result. How do I stop this from happening?

Depends a lot on the nitty-gritty details of just how you've designed your craft.  It would really help if you could post a photo of it.

Just from your description, it sounds as though there's some sort of runaway resonance that's amplifying the oscillations rather than damping them.  There are various things that could cause such a phenomenon, including:

• insufficient rigidity of where the control point (e.g. your probe core) is mounted
• too much control authority on your aerodynamic control surfaces (assuming you have some)
• too much engine gimbal range
• issues with overall aerodynamics (such as where your CoM is, affecting aerodynamic stability)

I realize that's so vague as to be pretty unhelpful, but it's hard to say more specifically without seeing what we're dealing with.  Could you post a photo?

[Serenity]

1 hour ago, WarpPrime said:

even though I added struts between the booster and the upper stage. 

Are you using autostruts or the simple struts?

Go to Settings->Enable Advanced Tweakables then right click to any part to enable them individually.

They are better than the simple ones, and they are hidden, you can google for more info/guides, i  suggest using grandparent ones.

[Empiro]

This often happens with long rockets that have SAS enabled. Because of the aerodynamic stresses, your rocket is bending slightly. SAS tries to counter this, but your control point is at the top while your thrust vectoring and winglets are at the back of your rocket. This very easily causes an over-correction, which results in your rocket oscillating back and forth.

• Adding struts will decrease the amount of flex your rocket has
• Disabling SAS can prevent the oscillation from happening, but if your rocket is bending too much, then it may tumble out of control or break apart
• Reducing speed while low in the atmosphere will reduce the overall aerodynamic stress on your craft

[Guest]

Also check for any part clipping which can cause weird behaviour as well.

[Cavscout74]

4 hours ago, WarpPrime said:

At 2000 m, I was at Mach 2, and the missile began oscillate more and more

That's a bit excessive speed in the lower atmosphere, and is probably increasing the effects of any other problems. 

[WarpPrime]

3 hours ago, Empiro said:

This often happens with long rockets that have SAS enabled. Because of the aerodynamic stresses, your rocket is bending slightly. SAS tries to counter this, but your control point is at the top while your thrust vectoring and winglets are at the back of your rocket. This very easily causes an over-correction, which results in your rocket oscillating back and forth.

• Adding struts will decrease the amount of flex your rocket has
• Disabling SAS can prevent the oscillation from happening, but if your rocket is bending too much, then it may tumble out of control or break apart
• Reducing speed while low in the atmosphere will reduce the overall aerodynamic stress on your craft

I have about 16 struts in the interstage.

[Empiro]

1 hour ago, WarpPrime said:

I have about 16 struts in the interstage.

That should be more than enough. You should see what happens if you turn off SAS. Like others have pointed out, mach 2 at 2000m is quite fast. It may simply be very hard to keep a long rocket stable at those speeds.