Here is the plot; it was written at least 30 years ago, but I don't know the author/title. Any ideas?
In brief: a long space voyage from earth to some star; the psychologist is lively and normal, everyone else, including the main character (MC), is much less so; MC notices an older member always hanging out at one spot in the craft. Later, the older member dies (natural causes). MC investigates the spot, sees a crack in the ship(!) and sees that they are not on a voyage at all but on an airstrip. The project is a simulation of a long voyage, to study its effects; the psychologist has his own escape hatch to the outside world, and is there to observe them. I forget the ending, but that is the essence of the plot.