The trouble with the whole large ship thing is that mass increases exponentially with speed. So as the "large" ship approaches the speed of light, it's mass gets too much and the energy needed to propel said ridiculously heavy ship also approaches much more than can be contained or carried by the ship. It's physically impossible in such a way that the ship will not get to that nearest planet in 70 years.
Anyhoo, on the subject of time travel, Godel, the greatest logician to have ever lived, showed that it is possible, if Einstein was right. It's linked, somewhat, to the idea of a rotating universe. Poul Andersen's Tau Zero kind of deals with that idea, using a sort of rotational travel to achieve time travel.
My 2 pence.
