hey man thanks for the comment - I have been using sketchup on and off for a while now - probably under a year in total for little projects but amping up now
i can really see the potential in it as a concept art tool.
SU models look nice. Not to nitpick, but the artillery unit doesn't look like it supports any kind of elevation adjustments, which are essential for artillery. It certainly seems to be able to traverse left and right, the whole unit looks to rotate as well as the barrel itself mounted on that cylinder. If the cylinder were rotated 90 degrees, giving the barrel the ability to adjust elevation (up and down) it would be more believable as both an artillery or point defense system.
Nice work though, I like the thought process leaning towards reusable designs!