Normal at position t1 of a space curve 3D

[laurent_delrieu]

Hello
I can be wrong but I think this is better
https://developer.rhino3d.com/api/rhinocommon/rhino.geometry.curve/perpendicularframeat

Frenet frame abruptly shifting along 3D curve

Perp Frames component already have uniform/continue orientations: [loop] Maybe use its X(U) vectors as reference? Hey, this is interesting! Rollercoaster!