I have spent a week and a half or so working through the US's X-planes (x-15, x-35, etc) Wiki List_of X-planes Most of them can be reasonably modeled in Kerbal and after doing so I found each has it's own characteristics and quirks. I tried to match the real life missions on most of them. The X-4 is undercontrolled, the X-12 explodes when you stage, the X-17 suffers from spaghetti, the X-32 is underpowered, and the X-24B is unflyable but many of these problems aren't too far from reality. each has a description with a wiki blurb about it's importance etc. and any special controls. My personal favorites are the X-13, the X-15, X-27 and X-35 the X-35 is a modified F-35 by mareczex333 and the B-52 is slightly modded from allmhuran's design. Other than that, they are all entirely original. I skipped the few I was unable to build stock maybe some other time. Craft files