How to attach off-axis components togther?

[Alexandicity]

Hellos!

I have become a KSP addict... can't stop thinking about it!!

I have general purpose Munar lander. It consists of a large radius central section with four normal radius tank/engine/leg combos on the four sides. It flies and lands well. I sling rovers below, and ascent stages above, the central section. Very similar in style to this (but with four rather than six radial tanks):

(credit: Roboray)

Now I want to make it land me heavier, larger payloads (base sections, large ascent modules etc) that would constitute the whole of the central section. This actually works as well; I have radial decouplers that can detach the four side engine assemblies from the central section that is then free to do what it wants. When I do so, all four side modules break off and fall to the ground; the struts between them disappearing (presumably because there are no "real" parts between them).

That they fall to the side into a pile of rubble isn't a problem most of the time. But now I want those four side engines to stay together even when the central section is detached and removed. With the ring of four engines, I can fly away, leaving the payload where it landed. Or the lander can remain as an operational base once the crew has left. Presumably, to do this, I need to have "real" parts running between the four radial tanks.

So, the question is: how do you attach off-axis things together in the KAB? If I use a truss or girder, I can attached it to one side; but it's never the right length to intersect the neighbouring tanks.. Even if it was, how would I make sure that both sides of the girder are attached (as opposed to the one side I positioned with)?

Hope that makes sense; a little hard to explain!! It's been infuriating me for ages

[Amoun]

I am not sure if I don't understand or that is more complicated than it should be.

That just sounds like a sky-crane to me. Engines etc all ready to go, with a docking port releasing the payload. Done that a lot of times.

If it is DECOUPLERS you want to use...well that makes it a little more complicated. I would build the engines first, then try coupling the payload...but then that would end up being rather weird...hmm. I want to experiment and see if that would work. But I would still recommend going with method 1.

[Kasuha]

The whole construction you make in VAB/SPH are a tree - you start with a root component, then branch out. At this stage, you cannot make a circle, i.e. attach two branches together.

In your design, the lander capsule is probably the root and the four fuel tanks with legs are its branches. They are connected to the capsule and when you detach it, they fall off.

You can make circles ex-post if you use docking ports. As long as you put two docking ports on two construction branches so they exactly face each other on the ramp, they will stick together the moment physics simulation starts. Then you can decouple from them and they will stay together.

[Alexandicity]

Hello - thanks for the responses!

Amoun: Your second interpretation sounds about right. Four decouplers, one to each pod, support the middle section, which can be completely released (leaving a "hole" in the middle). This works well at the moment The question is... can I get the four pods to stay together when the middle section - currently the only "true" (non-strut) part holding them together - is decoupled.

Kasuha: I understand your explanation of the tree - makes perfect sense. I think I also get your idea of the docking port - basically, build two technically different, half-ring spacecraft, and have them immediately mate. Intriguing. However, I suspect a full circular logical ring isn't needed - if I can make a branch that wraps around the central section (joining the four pods with three links), I can get all four pods to be attached to the tree without linking branches. The structural stability I can achieve by putting struts between the first and last pods in the tree

So the tree would look like:

                                   Nosecone
                                       |
                        Pod 4          |          Pod 1
                         ||            |       /   ||
                         ||     Central Section    ||
                         ||            |           ||
                        Pod 3  ========|========  Pod 2
                                       |
                                 Launcher stages

(In this, a single line represents a decoupler, a double line a fixed link like a girder or truss).

When landed, Pod 1 decouples from the central section, releasing it from the entire ring of pods, which remain still attached.

Difficulties from this approach are it's messy: unbalanced and requiring some delicate placement of pods around the centre (rather than symmetry-attaching them). I will experiment tonight and see what I can achieve. I will use lots of struts to hold Pods 1 and 4 together (creating a mechanical, if not logical, ring) and to support all four pods to the central section (which otherwise would all be dangling off a single decoupler!)

Phew - hard idea to convey in text!

To put my diagram above in context, the existing lander I have now (that falls apart on decoupling the central section) is:

                                   Nosecone
                                       |
                        Pod 4          |          Pod 1
                               \       |       /   
                                Central Section    
                               /       |       \   
                        Pod 3          |          Pod 2
                                       |
                                 Launcher stages

And what I would ideally want, but is not possible in a tree, is:

                                   Nosecone
                                       |
                        Pod 4  ========|========  Pod 1
                         ||    \       |       /   ||
                         ||     Central Section    ||
                         ||    /       |       \   ||
                        Pod 3  ========|========  Pod 2
                                       |
                                 Launcher stages

[JumpsterG]

I'm scratching my head on this one, but sounds like a fun engineering challenge.

The way I see it, you can only do this using a single decoupler rather than 4. Due to the tree structure, 4 decouplers form their own separate trees, with no way for them to connect to parts on another tree.

The easiest (symmetrical) way is to make a skycrane (incorporate a stack decoupler on top, with some common stack piece that all the 4 pieces attach to). But this clashes with your design, because it would need to be on top, with nothing else above it.

It sounds messy, but to keep the concept of your design, you might manage this with a single radial decoupler on one corner, using structural pieces to space out the engines in a similar configuration and strutting to keep everything working the same. Keeping such a design symmetrical is the hardest part as the symmetry tool is no longer useful and you have to compensate for the sticky out bit of the decoupler in one corner.

(One final possibility is to use docking ports on the four corners and do some fancy construction outside the VAB, but I don't recommend this for sanity's sake)

[Alexandicity]

Hi Jumpster,

Yup, I think you're thinking the same as me.. there are some configurations to be played with! I will need to try them all...