I am looking for a (short?), possibly German, story. I read it around 1992, but it is certainly older.
It is a about a generation ship in which the human species has split into two; the ("lower") one being more savage-like, but still with intelligence, and controlling the parts of the ship with the air tanks. The human population is also still around, and there is a war between them.
There is no notion of memory loss (as in the movie Pandorum) and the aim of the mission is also still clear to the humans (unlike in Orphans of the Sky).
Humanity has since invented faster flight methods and the ship is indeed already expected at the destination...
Parts of the story are in the movie Pandorum, but then again lots is different.