The eternal flame - Greg Egan

The generation ship Peerless is suffering from a population explosion, and the only way to reduce the number of children is by drastically limiting the females' food intake. So population control consists of two barbaric choices: starvation, or suicide.

Trying to find a better way, a biologist starts experimenting with animals, and stumbles on a technique that radically alters the reproductive cycle. But while the advantages are obvious, there's a major drawback: while it spares women from their old role - reproduction without hope of survival - it will essentially wipe out an entire sex.

Amid the turmoil created by this new possibility, physicists on the ship are working to develop the technology they will need to complete the mission of the Peerless. One of the expedition's founders dreamed of discovering the Eternal Flame: a way to generate thrust without consuming any fuel at all.

The inhabitants on board the Peerless have some hard choices to make - and the wrong one could spell extinction for their entire race.