Interstage decoupling

[seaces]

Hi All,

So I recently I started experimenting with KSP stock fairing used as interstage as I have a lander with dual engine configuration. So instead of putting two decouplers on each engine followed by bi-adapter I wanted to put these engines in an interstage which can be decoupled. Bellow is a screenshot so that you can see what I am talking about:

Now my problem here is I dont know how to decouple the interstage so that I am left with the clean lander above with engines ready to fire. I guess that I need to insert the decoupler but I dont know how. I tried inserting a 2.5 decoupler in one of the interstage nodes between the lander and the interstage plate but that did not work. Any suggestions?

[jimmymcgoochie]

The decoupler needs to go between the bottom node on the lander (probably the Rockomax-8 fuel tank) and the node you're attaching it to on the fairing. It might be easier making the fairing the root part, attaching the decoupler where you want it then turning off the additional nodes and reattaching the lander, then re-rooting back to the command pod.

If you have the Making History DLC you could use a 2.5m engine mount instead, set it to have two attach nodes for the engines and the bottom node has a built-in shroud (like engines do) which will work as an interstage without needing to use a fairing.

[Streetwind]

This is something for which there is no straightforward way to do it in the base game. As Jimmy said above, the engine plates included in the Making History DLC are specifically meant for this.

If you don't have them, you need to use surface attachment to circumvent the lack of dedicated parts.

Start by attaching the fuel tank of the lander to a fairing truss node, with a full size decoupler in between. No bi-couplers or anything here, don't ever use them in between stages, it never works. Just lander tank -> decoupler -> truss node.  This leaves you with a clean, full-size node stack top to bottom without any of the engines getting in the way.

Now, rightclick the fairing and turn off the interstage truss. Not the nodes, just the truss, so that your attached lander is magically free-floating but still attached. Yes, it looks stupid, don't worry, it'll be fine later. Then, you surface-attach engines to the bottom of the lander's tank inside the hollow decoupler, in such a way that they won't catch on the decoupler ring during stage separation.

But what if you want engines that cannot surface attach? Well, that's what the BZ-52 Radial Attachment Point is for. You surface attach a pair of those to the bottom of the tank, which gives you extra nodes outside the main node stack to attach the engines to.

Next, switch to the Move tool, select the decoupler, and move it down towards the fairing base until the engine bells are nice and close to it. Don't clip into it, that may not end well.

Finally, build the interstage fairing. Once it is closed, you'll no longer be looking at a magically floating lander - it is now (at least visually) supported by the interstage. Set the fairing to Not Staged, and it'll remain a fixed structure that never gets ejected until you leave it behind when the lander decouples.

 

Edited December 18, 2021 by Streetwind

[18Watt]

45 minutes ago, Streetwind said:

But what if you want engines that cannot surface attach? Well, that's what the BZ-52 Radial Attachment Point is for.

The cubic octagonal strut also works well for mounting things which otherwise aren’t radial-mount, even engines.

[jimmymcgoochie]

2 hours ago, 18Watt said:

The cubic octagonal strut also works well for mounting things which otherwise aren’t radial-mount, even engines.

This. Cubic struts are very light, surface attachable and have nodes to attach stuff too- I use them all the time to attach things which wouldn’t otherwise be attachable in a stack; the only real downsides are the increased part count and possibly some stability issues if you have heavy stuff attached to other heavy stuff via a tiny little cubic strut (so use normal or autostruts to fix that!). Engine plates also have some annoying issues with autostruts (they keep appearing and they keep mangling engines attached to the outer nodes) so I don’t tend to use them much, instead I use cubic struts.

[18Watt]

28 minutes ago, jimmymcgoochie said:

Engine plates also have some annoying issues with autostruts (they keep appearing and they keep mangling engines attached to the outer nodes)

I think I've been lucky in that I have not had that issue with engine plates, they work really well for me.  In particular, it solves the problem of how to stage a section with multiple engines, exactly what the original post seems to be having issues with.

The problem I see with engine plates is they mess up the dV display for stages.  Although I sometimes see incorrect dV values even when I'm not using engine plates.

I'm one of those players who goes to great lengths to keep part count down- to minimize the dreaded lag.  However, the cubic octagonal struts are often the best solution to a problem, I use them a lot.