Hi @timob2567
Rhino doesn’t support non-manifold geometry (eg. spheres within spheres) out of the box. In order to create such, you need to:
Select the two ellipsoids and run NonManifoldMerge
Select the resulting geometry and run CreateRegions
You are now left with 3 individual geometries sitting on top of each other: An outer shell, an inner shell and the combination of the two, so delete the inner and outer shells (best done in either ghosted or wireframe mode) and you are left with a hollow body.
Note that those non-manifold objects are not supported by all commands and file formats.
Another way to go is to mesh the two ellipsoids separately, flip the direction of the normals for the inner surface and Join them into a disjoint mesh.
All that said: From your file I can’t really decide which part you are looking to make hollow and why?
