Well the interpolation is what I want to achieve as it what modern FPS implement although my maths are a bit rusty after so many years Anyway, I will look into it once I understand how to spawn and update a vessel as I don't know much of the KSP API. Right now vessels that are closer than 20km are updated at 30FPS, the ones that are farther away at 5FPS and the ones that are really far (other planets) at 1FPS
Regarding UDP, with the modern network hardware is quite reliable and even if you work with old hardware, lidgren is what's called a "reliable UDP" library. Some packets cant be sent in reliable mode (like chat messages etc) and some others in unreliable mode (like vessel position updates, sync time requests,...) all of this is taken into account in my fork. Using both TCP and UDP at the same time is a bad choice as some packets will affect each other (like this paper describes: https://www.isoc.org/INET97/proceedings/F3/F3_1.HTM)