Using 2 stage rockets - problem with decoupler

[Daraxir]

Hello, I'm building 2 stage rocket similar to Falcon 9 with reusable first stage, but as I use decoupler showed on image, as I launch a rocket, second stage is wobbly and it immediately snap from first stage. What is wrong with it. I've built multiple 2 stage rockets before but it happened to me for a first time today. Under decoupler is Remote Guidance Unit to control that first stage at the reentry and Advanced Reaction Wheel Module. 

 

Thanks for some advice.

 

 

[JTeam_]

I think you just need to use a bigger decoupler, man. If that can't be done, rework the design. Try using the adapter parts in the Structure Category to make engines and stuff fit together better.

[Curveball Anders]

There's a simple answer ...

(Everyone together ....)

Moar Struts!

Actually, designs like that need struts.

[Daraxir]

This was the biggest decoupler available I will use structs then. 

[Tyko]

Aerodynamics matter in KSP...all of those changes to diameter are going to cause problems. Each change causes drag. 

Suggest smoothing it out.

1. Use a cone shaped transition from your lower stage to your upper. 
2. Try putting your reaction wheel and your RGU in a material bay so they match the rest of your rocket
3. Use a taller thinner 2nd stage fuel tank that's the same diameter as your engine. 

TL:DR - keep the rocket sides smooth going from smaller on top to larger on the bottom and use angled transition pieces to move from one diameter to another.

[Daraxir]

I dont know where to hide guidance unit/battery/reaction wheel module/advanced inline stabilizer. They are too big for service bay so I want to hide them but i dont know how. There are some structural tubes but its empty tube and I cant put anything inside. 

I need to shield this part with something similar to protective shell used on a top of the second stage(cargo):

Edited January 8, 2019 by Daraxir

[Aegolius13]

4 hours ago, Tyko said:

going from smaller on top to larger on the bottom

Unless you want to be like Delta III and Ares I.  Which, uh, you probably don't. 

[bewing]

2 hours ago, Daraxir said:

I need to shield this part with something similar to protective shell used on a top of the second stage(cargo):

That is what fairings are for. Add a fairing, right click to open the window for the fairing, then click the "interstage nodes" button.