Wow! This was a fun Heinlein-esque book. Young kids and ex-astronaut build a spaceship to go to Mars. Better than that makes it sound; Essentially a what-if that involves the equivalent of an Orion-drive without nukes (i.e. what could you do if you had unlimited thrust?). A little bit hot-and-heavy re the young kids (2 ~19-20 year old couples) though not *really* dirty (sex never "happens" during the narrative)... There is even a plot device that makes them clean up their language. The other good device (I think) is that the genius who comes up with their power source is sort of an idiot savant, so nobody goes into much technobabble.
It presents a good example of a hard SF story that takes one basic "what if," and then plays fair as to what the consequences would be, and how you might use that idea.