Atomic Rockets- an encyclopedia for science fiction writers

I recently learned of Atomic Rockets and it seems a great resource for planning space flight in stories in ways that make sense.

This site was mainly intended for science fiction authors who wanted a little scientific accuracy so they can write SF "the way God and Heinlein intended" (Arlan Andrews's Law). The technical term is Hard Science Fiction.
But anybody who is interested can play with the toys contained within, designing their own Planet Rangers Rocketships. It is assumed that the reader has enough knowledge to know the difference between a star and a planet, high school mathematics, and enough skill to use a pocket calculator. Computer spreadsheet and computer programming skills are a plus, spreadsheets in particular will make your life much easier.