The Rolling Stones (1952) - a shorter version was serialized in Boys Life (4 parts in 1952 as "Tramp Space Ship.") The Stone family - 3 generations worth - buy a used spaceship and it's "look out, Solar System!" Heinlein has a real juggling act with 7 family members all with their own ideas of what to do, but he pulls it off. Grandmother Hazel Stone is the same character as Hazel Stone in The Moon is a Harsh Mistress written a dozen years later, but I don't feel that this is a true novel and prequel because the societies and social backgrounds don't really match up - think of it as an inside joke for observant readers. And the flatcats are marvelous - I think it's obvious where the idea for "tribbles" came from, 15 years later.