I've built an X-34, a design I haven't seen anyone replicate in KSP before. I did build one way back in 0.25, but it couldn't actually fly if I remember correctly.
Long story short, X-34 was supposed to be a testbed for SSTO technologies or something like that. Mine flies with the help of clipped reaction wheels, something I haven't done in a long time, because it's just too unresponsive to controls without. Was intended for it to launch from a Lockheed Tristar, which I don't have at the moment.