In Tom Godwin's story "The Cold Equations" -well, if you read it you know.
Good choice. I read that in a collection from the public library when I was about thirteen, upon Googling probably The Science Fiction Hall of Fame, Volume One, 1929–1964. Stunning.
Anyway, that's what you get for not having wiggle room on your delta-V.
I believe it's from "The Long, Twilight Struggle".