I'm not sure about the square root of energy, are you sure about that ? It kinda reminds me rather of atmospheric flight where drag (and thus energy expediture)

I'm not sure about the square root of energy, are you sure about that ? It kinda reminds me rather of atmospheric flight where drag (and thus energy expediture) rises by similar non-linear amount. As for shielding - there are ways around that. To shield from your own reactor you can use shadow shields & fit you ship quite comfortably into the protected cone it provides. If you are afraid of solar radiation, you can use that as well, just orient your ship accordingly when a solar flare is detected that could har you. For fixed stations you either are on some body & have more or less unlimited amounts of "dirt" for shielding or for an orbital installation you should be able to accumulate enough material over time & it should not really matter as such stations would likely rarely change their orbit, if ever. And lastly, you might be able to improve humans over time to better handle the space environment via new medical practices, new drugs or even various physical enhancements. Modern people live in many places where their predecessors would not survive (or at least not with any reasonable quality of life) and this seems like another such challenge & stepping stone for humans.