You mentioned Roll20, and I'm never too sure what someone means when they say "Hard SciFi" cause it always doesn't seem to fit.
Several authors, in various interviews, mentioned a SFWA definition from the 1960's... No more than three breaks from known physics. At the time, nuclear fusion power was one; FTL still is, super-high ISP (>10 k sec) and/or reactionless drives. Gravitics generally gives you artificial gravity, inertial compensation, and reactionless drives as one "break"... with lenient reviewers, at least. Most no longer count fusion, as we've got energy release in excess of energy to ignite; we don't have energy recaptured in excess yet, but current reactors are focused upon ignition and sustenance, not energy recapture.
Note that a reactionless drive (or a super-high Isp reaction drive) that generates thrust generates effective gravity via inertia.
One that generates a gravity well in front of the ship (and generates a second a bit beyond before the first decays), such as the Humanx Commonwealth setting's KK drive, generate artificial gravity by tidal force (That is, the ship is pulled based upon the center of mass; those inside experience gravity based upon compariing their centroid to the ship's - if closer to the distorition, down is forward (towards the distortion); if further, down is aft. A perfect midship deck generally feels microgravity...
A reactionless drive that grabs your bubble of N-Space and severs it from the local space potentially doesn't generate a gravity-like force. (An Alcubierre warp drive - which is somewhat low plausibility as FTL, but reasonable as a a high sub-C constant speed drive by this principle. (Which still potentially breaks causality a bit - if it indeed is exempted from dilation.)
