Help with Modded Parts Stability?

[krakken232]

So I thought I would fire up blender and try to make some parts for personal use-- mainly trying to create some parts that mimic my common assemblies (i.e. three stacked fuel tanks, triple stacked triple fuel tanks) so I can get some bigger ships airborne with less stress on my processor.

Well I made some awesome looking parts and put them in my game. But here\'s where I run into trouble. Take my first part: it\'s one part that is the same exact size and shape as three liquid fuel tanks stacked (I know because I imported the DAE\'s of the fuel tanks into blender to base my model off of). It looks great, and works great in the VAB (all the connection points line up, etc). I modified the CFG file to change the mass, fuel capacity, and dry mass to exactly 3 times a regular fuel tank. All in all, it should be pretty much exactly the same as having three fuel tanks. I can snap them on and launch just fine.

Now, when I try to create even a slightly complicated ship, it just falls apart. For instance, a lander on top of my part, with four more of my parts attached symmetrically will barely stand up on the pad, whereas if I stack three regular fuel tanks on top of each other, and duplicate that four times, it\'s perfectly stable. Likewise, if I try one stage bigger, it wobbles around briefly and falls apart and explodes, whereas if I build the same ship out of multiple regular fuel tanks it stands tall and proud (and laggy).

So how come a single part with a single collision mesh with the same overall stats as three regular fuel tanks is so much more wobbly and unstable than three regular fuel tanks put together? Are there other stats I need to modify? Is it a problem with the collision mesh vs the attach node values? Help?

[Technical Ben]

The game will use the actual shape of the node_collider to detect the collisions and shape on the launch pad. This means, if your node_attach position is far away from the top or bottom of the node_collider, you have a gap. The rocket will tend to pivot and wobble along that open joint.

So, if you bring your attachment node up close to the fuel tank, it should sit a bit more flush and solid. You can also change the tanks shape (and the node_collider to match) to help give it a better contact too. For example a cone would have little support on the top, but lots on the bottom.

Oh, unless it\'s just the extra weight! That will pull the rocket apart. It\'s not made of titanium you know.

[krakken232]

Ok-- yeah, so the part itself is long and skinny (just like three stacked fuel tanks would be). So that makes sense, the node attach Y dimension is zero, the middle of the part, so if I were to set the node attach Y to be up near the top of the part, and use struts to secure the bottoms (again, just like you would three stacked fuel tanks) theoretically it should be a lot more stable.

I will have to try it out when I get home!

[Technical Ben]

Yep, that would work. Don\'t make it past the top, else it will wobble again.

Or you could keep it secured in the middle, but you would need 2 struts to help secure the top and bottom, instead of just 1 extra if you move the securing point.

Once the 'adjustable sizes' of parts are supported or implemented things might be a bit more stable. Or is some sort of multiple node connection could be made. But for now struts will help!

[krakken232]

Alright -- so I moved the attach node up and things were way more stable. However, I then realized that the engine wobbles on the bottom of the tank. No matter how many decimal places out I took the \'y\' of the bottom attach node, I could not hit the sweet spot between not connecting and the engine being wobbly.

I did what I should have done in the first place -- gone back to the drawing board with some rudimentary objects for experimentation. I\'m using Blender, again, and I am wondering if there are any setup options on the interface or the DAE exporter I need to tweak? Here\'s the deal -- if I make a cylinder that is 4 meters tall exactly in Blender, and set my node positions to 20 and -20 (cause of the .1 scale), perfect match. If I make a cylinder that is 4.5 meters tall, and set my node positions to 22.5 and -22.5, the engine is wobbly. Even if I move the node down to like 22.4 or 22.3, it still wobbles.

So I can achieve a perfectly solid connection if I keep everything in nice, even, meter increments apparently, but if I mix in decimals I can\'t? I have no way to test at the moment but I am also curious if I move the node like two or three tenths inside the collision mesh and set allow_collisions to 1 - will it result in a solid non-wobbly connection?

[Technical Ben]

If you set the scene to 'metric' and leave the scale at 1.0 you can write down the positions by hand and put them in the part.cfg.

I just did if for my shuttle engine, and it worked perfectly snug. I did round the numbers up a little, from 5.30934 to 5.3094, and it seems to work fine so far.