Hi niko, also a note on this part:

> Just for reference's shake,this union is sucessful in Rhino...

That's because Rhino processes booleans between open surfaces differently than MoI does.

Rhino uses the positive normal direction of open surfaces to decide which side to keep. That can sometimes be pretty confusing because it means that something like Boolean Union actually behaves more like Boolean Difference instead if the normals happen to be reversed from what you were thinking.

For example with your original model here, load that into Rhino and then select the bigger piece and run the Flip command on it (it is able to be flipped since it is not a closed solid). Now try the boolean union command in Rhino again after that, and you'll see a much different result.

It's kind of a matter more of luck that the normals for that larger piece were aligned in a more expected direction as if it was a closed solid.

In situations like this MoI instead tries to slice up pieces with each other and try to extract whatever different volume chunks it can, rather than having this sensitivity to the surface normal direction. That helps to avoid this one kind of confusing situation of having Union behave as if it was Difference as in Rhino...

- Michael