A generational spacecraft, haflway through its mission. Its name is the Arc-en-ciel. Back cover:
. . . the idle passengers found themselves face to face with these same problems - and this time there was no place to run and hide or to postpone their answers. For their miniature society was changing faster and faster. An the spaceship suddenly seemed destined to end as a star-bound coffin.
The "changes" considered sexual politics. The ship's society was changing from promiscuity to a more restricted view of personal liberties. I expect that was the crisis in the fifth generation as well.
Don't know why such a vessel would have to be aerodynamic.