You don't know what you're talking about. Bone loss in microgravity has been stopped by demonstrated countermeasures. Even if that were not the case, all it means is that we would have to use artificial gravity (i.e., rotating spacecraft) for long-duration trips.
Why not just send robots, and forget about your magic rotating space ship? Robots don’t need to return. you might as well insist upon warp speed...the problem all science fiction writers encounter is the time for travel—the years in flight, with the bones and other vital organs deteriorating. Exercise helps—but only a little. Suspended animation, worm holes— even the new entrepreneurs like elin musk are stuck reinventing the wheel.
Yeah bone loss is relatively easy to deal with. The biggest problem as I see it is the fact that we’re limited to what we can lift into orbit which isn’t very much.
As far as propulsion is concerned we’re again limited by what we can hoist to orbit. Something as simple as steam exhaust can carry you to a significant percentage of the speed of light but requires billions of gallons of water.
Project Deadelus would have used nuclear pulse engines but ION drives or VASMIR drives could be scaled up 1000 fold or so.
http://news.discovery.com/space/private-spaceflight/tau-zero-project-daedalus-icarus-110119.htm