Attach a part at two different points

[CosmicCharlie]

I have those upper tanks attached to radial decouplers.  I want to attach the tanks underneath to the tank above and another radial decoupler.  It won't let me. What's the secret?

[Snark]

17 minutes ago, CosmicCharlie said:

I have those upper tanks attached to radial decouplers.  I want to attach the tanks underneath to the tank above and another radial decoupler.  It won't let me. What's the secret?

There isn't one.  You can't.

KSP craft are "tree" structures, meaning that every part can attach only to a single parent part.  It is physically impossible to have a closed loop of parts.

There are only two exceptions, strut connectors and fuel ducts.  Those can connect two parts.

What you need to do is, in the place where you were trying to attach a second decoupler (which is impossible), instead attach a strut connector joining the two sides.

When you decouple the decouplers, the struts will automatically break, too.

[Hotaru]

Struts.

Unfortunately there is no way to have multiple attach points in KSP: each part has to have exactly one "parent" part. The only exception to this is struts, which can connect any two parts. So in this case, you'd have to run some struts between the upper tanks of the boosters and the core. The struts will detach when the radial decouplers fire, so your boosters will still separate properly.

[FullMetalMachinist]

Unfortunately that sort of connection is impossible in KSP.  The craft structure must follow a "tree" pattern, where each part can have one, and only one, parent part. This means that closed loops (like you're trying to do here) simply don't work. The only exception to this is strut connectors and fuel lines. They can go from any part to any part. Everything else is bound by the "tree" logic.  

For your rocket I would suggest not using the bottom radial decouplers and use a few well placed struts instead.

[CosmicCharlie]

Thx all.

[FullMetalMachinist]

wow...mad rush to answer the guy, apparently. 

Another thing to love about this community!

[sgt_flyer]

there is one possibility through docking ports. (you place 1 decoupler radially on the core stage, + 1 docking port onto the decoupler. then, 1 docking port radially on the booster, then translate the docking port on the booster so it's aligned with the docking port on the core stage. upon physics loading, they will dock automatically. (it's still not considered as a loop though)

though, it's an heavy solution that will also generate drag. you'll be better off using struts for boosters  (check the strutting on real rockets, it works wonder in ksp too :)) they generally use two struts horizontally at the bottom of the booster connected to the core stage to absorb sideway forces (Soyuz, Shuttle SRBs, ariane V SRBs etc) , and you can add 1 strut at the top (placed more vertically (if possible to the upper stage), so the weight of the payload / upperstage is distributed vertically on the boosters. (again, that 3 point connection is quite standard in real life)

Edited January 16, 2016 by sgt_flyer

[Zosma Procyon]

I realize I'm rather late to the party, but I have been using what I think is a clever way to trick the game into allowing multiple attachment points in my mobile ground bases for some time. The trick is to hide docking ports inside the parts you want to attach. Put one port on each face/end/surface you wish to connect, and use the offset tool to slide one of them back into the part it is attached to. The two smaller sized docking ports work best for this. Now the tricky part is to position the docking ports so that they just barely touch each, so that as soon as the craft spawns they dock. To do this you have to zoom way into the part with the port offset inside it, so you're actually looking beneath the rendered skin of said part. It might be helpful to offset the entire craft off the vertical axis of the VAB, since the game will only zoom in so close to said axis. Then select the offset tool, and select the port you tucked inside the part/ This can be very tricking, usually takes me a few attempts. Once you have it selected, offset it to be in the proper position right up against the other port. The ports should NOT overlap or have a perceptible distance between them. On the outside of the craft, it should just look like the parts the ports are connecting are just attached to each other, as the ports are tucked inside one of them. If I were currently at my own computer, I'd provide you with picture demos. When you have all the ports set up, you should send it to the launch pad, zoom in on the part with the docking ports hidden inside it, and see if the ports are docking as desired. The newish "aim camera" option when you right click on a part in game play makes this easier.

Whenever i'm designing a craft, I always have the "part clipping" "cheat" turned on. I'm not sure if that is required to make this trick work. Also in some craft designs, like ring shaped space stations where some larger components overlap, the "unbreakable joints" cheat seems to be required if the final craft using this trick is sitting in a gravity well (i.e. on the launch pad). I don't know if that's because the ports are tucked inside of larger components, or because the method I use to construct ring stations involves overlapping larger components (like fuel tanks and hitchhikers). when sitting in the free fall of space, no cheats are needed to hold these stations together.

Lastly, if you haven't already you should go into the game settings and enable "Advanced Tweakables", so you have access to the "rigid connection" and "autostruct" features. You want the hidden docking ports to be rigidly connected to their mother parts, and this trick seems to work better when the ports are also autostructed. I used to autostruct them to different parts, one to the root part (usually a cockpit module or medium sized fuel tank) and the other docking port to the heaviest part; but I'm not sure that made a difference. And it might have contributed to the deadly "ground vehicle shimmy of death" or summoning the Kraken in the case of my space stations. More experimentation is needed.

I've tested the strategy extensively, including in a huge mobile ground base on Eve, and it seems to work beautifully. In fact in a few destructive tests when i've deliberately crashed my creations, often the two parts the ports are connecting will remain connected even after breaking off their own parent parts.

I hope this finds you well.

[Tyko]

Doesn't this mod enable multiple connections?