To me, "hard science fiction" evokes the old school writers like Arthur C. Clarke who would explore ideas with a slide rule or a calculator when planning a story. Even if he had to use a little hand waving and some unobtainium to make Ringworld work.
Maybe the neatest part of that with Ringworld is when fans proved that the theoretical structure itself is orbitally unstable... which he then came up with explanations for and used as a major plot point in a follow-up book.
