I tried to download your SSU (Space shuttle ultra) 4.2 from the /resoures section, but there is no download link. It says "We ran into some problems: The addon is private".
Is there a way to download SSU 4.2?
Well, I installed a new Version of Blender on Linux. But I can import add-ons for Blender in Linux too. And actually I can build my own nozzles. But yes, it would be nice to compare. And It would even more nice, if I could use parts. Like the RCS thrusters from the SM. My Addon will be on top of a S-IVB and under a SM inside a Skylab-Shroud. I could use these parts in Blender to figure out the measurements.
For older versions fo blender there is a plugin by Vlad. For more recent versions of blender there is another plugin... can't remember who made it. There should be threads somewhere for both.
As for the legal side of things, usually the issue is distribution, but it depends on the source of the mesh.