Second Heinlein collection in this book (the first being The Man Who Sold the Moon). Now we have two related 1940s novellae fixed-up into a single novel in the 1960s. Oh, science fiction publishing, you are so fun.
Orphans of the Sky is one of the ur–generation ship tales. Heinlein immediately seizes on the possibility that something could go so disastrously wrong during the voyage such that the entire crew forgets it is on…