Mk3 Passenger Module only wants to attach at its bulkheads - why?

[Jamin]

I saw a month or few ago a beautiful space station, roughly of the Space Odyssey wheel kind. The outer ring was made of Mk3 Passenger Modules joined at bulkheads with perhaps a 15-degree bend at each attachment, plus overlap as needed to close the narrow cleft caused by the bend at the attachment. Very pretty! Kudos, Kerbal architect.

And so I wanted to build my own, a design with my own quirks and choices. One thing I wanted was to have the bottom of the Passenger Module face "down" - that is away from the hub. So, building from the hub outward, I find myself wanting to (e.g.) place a BZ-52 Radial Attachment on the end of a spoke, ragged end outward, and then attach the center of the roof of a Mk3 Passenger Module to that ragged end. But what I want, the passenger module very much does NOT want. It only wants to attach at either of its two bulkheads. (And before you say "just kludge it with parts internal to the passenger module etc", I should mention that instead of the BZ-52 I might also want a pair of mated Clamp-o-tron Docking Ports so that the station could be assembled in orbit.)

So... I edit the CREW.cfg file that I found under GameData/Squad/Parts/FuelTank/mk3Fuselage (oh, hello, do you think the kerbals on the station realize they are living in a fuel tank???), changing attachRules:

18c18

< attachRules = 1,0,1,1,0

---

> attachRules = 1,1,1,1,0

(actually, I made a JAMIN.cfg alongside CREW.cfg, renamed a few things inside it and apparently did not break too many things).

This change _helps_ but is not perfect. Now the Jamin Passenger Module will attach on its roof side, but the attachment doesn't occur AT the roof, but more like at the center of the module. The clamp-o-trons and an entire structural fuselage segment attach plunged into the interior of the passenger module. (I wonder what the passengers make of this???)

So, there is apparently at least one more thing for me to fix to have the attachment occur AT the roof rather than through the roof, and I would like to know what that is exactly, but I also have a larger philosophical question:

Why were these Mk3 Passenger Modules set up to be so finicky about attachments in the first place? For that matter, this same question applies to basically the complete line of Mk3 parts - fuel tanks, cargo holds - because they seem to all behave this way. Is there some other benefit obtained by restricting attachment?

Edited July 20, 2015 by Jamin

[Sharpy]

You must use the subassembly trick.

Start a new craft. Place the Passenger Module as a root part. Attach the BZ-52 Radial Attachment wherever you want it. Attach whatever dummy part you want for that radial attachment's node. Set that dummy part as the new root, then save the remainder of the craft (the attachment point and the cabin) as a subassembly. Now as you load the subassembly into your space station construction, it will be attachable right by that node.

[Jamin]

Your trick solved my immediate problem. (My philosophical "why were these parts designed this way in the first place" question stands, but hmm seems less crucial now.)

[Sharpy]

It's the game mechanics of VAB/SPH. You have parts that are attachable only by nodes to nodes, and surface-attachable - that can be attached anywhere, to anything (and have some nodes, possibly) (*). You can attach a surface-attachable to any point on node-attachable, but not vice-versa.

Still, that's only the editor limitation, not inherent to the physics engine, only to the construction process. Since you can change the root part of a vessel, after attaching surface-attachable to node-attachable and changing the root to be at, or past the surface-attachable, you're technically creating the "forbidden" situation of node-attachable attached by a non-node surface to the main vessel. But since the process would require starting the construction at a point really ill-suited to be root (say, build an airplane starting with the nose cone of its under-wing missile) there comes the subassembly trick, that allows you to break the process at a reasonable point (say, build the missile up to the structural pylon by which it will hang from the wing), then restart it from the natural root (plane cockpit) and bring in the subassembly with the node or surface-attach capability where you want it.

Reasons? I guess the entry-level difficulty curve, so that the simple rockets all snap themselves right through the nodes. I don't really know.

(*) the situation isn't entirely -as- simple, as there are other weirdo parts, like ones that can be surface-attached *by* one surface, but have things attached to them *to* another surface, like the aforementioned structural pylon - a useful thing, because it can separate from whatever it's attached to, cleanly, without leaving any stubs. Thing is, "it's attached to", not "thing attached to it" so if you want the missile to fly away without carrying the pylon on the side, you must attach the pylon to the missile, not vice versa.)