Spacecraft wobbles at a stack decoupler

[guitarxe]

I have a craft that wobbles a lot, and the wobbling is happening at a stack decoupler. You can see a video of it happening below.

Never mind the payload wobbling at the top, I am talking about the wobbling happening at the TR-18A Stack Decoupler in the center of the craft. You can See the whole craft compress and then bounce like a slinky when I burn.

Although in the video I am using the fairings mod and the wobble happens between the stack decoupler and the fairings base, if I replace the fairings base with a rockomax adapter with no fairings, the exact same wobbling happens at the same point.

Why does this happen and what can I do to prevent it?

Edited September 23, 2014 by guitarxe
change to answered

[Yasmy]

This is just a choice by the developers to make decoupler joints be less rigid than normal joints.

Use strut connectors, or deal the the flex.

[Godot]

I agree with Yasmy ...

Struts between the side mounted parts and the main body should help.

Maybe also struts between the fairing base and either the side mounted parts or the main body

[MarkyJ279]

Combination of what Yasmy said and the fact you heavy a fairly heavy payload at the front, a very powerful engine at the back and that single decoupler in the middle transfering the force between the two.

The centre cannot hold.

In the future, add extra struts to help reduce the stress on that decoupler. To salvage what you've got, set your thrust limiter to something low. I noticed even on low thrust you were reaching over 2G. Set a low thrust limiter and keep your acceleration low (0.5G maybe?) and your burns will be longer but your rocket won't stretch or wobble as much.

[tjsnh]

You seem to have zero struts.

This would be less than the 324299 struts a craft like that needs.

Place struts inside the payload area before attaching the fairing sides.

[Ruedii]

That is quite common. It's due to delays between the response of the physics and the actions taken by the (A)SAS. You can reduce it by moving your SAS units to the center and turning off gimbling. You can toggle your SAS units so that only those at the center are on.

[Carter]

I typically put a pair of modular girder segments just underneath stack decouplers and strut them to the stage just above them, it works pretty well. With ferram's Kerbal Joint Reinforcement, I only ever have to do this if I'm attaching a larger diameter stage underneath a smaller diameter stage, like shown in your video.

[guitarxe]

I see. I actually have a lot of trouble with connecting struts. They never seem to want to connect from one place to another, and the game doesn't give you any indication whether a connection is viable or not, so you end up clicking and the strut doesn't connect, forcing you to pixel-find it on your craft (because it is often half-buried inside whatever you placed it on) and repeat the whole thing dozens of times.

The idea of using some girders to act as the strut connection points is pretty good though, I'll try that. Though I shudder to think what ugly designs that would lead to. It'd be like sending a cantilever bridge to space rather than a rocket

[Carter]

I see. I actually have a lot of trouble with connecting struts. They never seem to want to connect from one place to another, and the game doesn't give you any indication whether a connection is viable or not, so you end up clicking and the strut doesn't connect, forcing you to pixel-find it on your craft (because it is often half-buried inside whatever you placed it on) and repeat the whole thing dozens of times.

The idea of using some girders to act as the strut connection points is pretty good though, I'll try that. Though I shudder to think what ugly designs that would lead to. It'd be like sending a cantilever bridge to space rather than a rocket

That's why I try to keep strut-work minimal, and identify areas that actually need it. For aesthetics, you can try using small hardpoints instead of girders. I typically only need to strut like that on ascent stages though so girders don't bother me because they'll be in the ocean and I'll be in space.

[xtoro]

Try using Kerbal Joint Reinforcement. Works great and helps with wobbles by strengthening decouplers and interstages, which in real life are quite a bit stronger than the ones in KSP. Doesn't help much with docking ports but that's fine. It also stabilizes your ship when you come out of warp or load the ship by making it temporarily indestructible and turning off the physics. It then makes your ship "destructible" again and slowly ramps up the physics back to normal. This prevents large ships from exploding, or parts breaking off, or weird bends and spins when you load it or come out of timewarp.

[guitarxe]

Yes, I might try that mod if I don't like the strut/girder solution. Thanks guys