Rocket engineering or rocket science is the specialized branch of engineering concerned with spacecraft.
It is very complex and requires in depth knowledge of other related topics such as mechanics, mathematics, material science, avionics, etc.
Here is a selection of some recommended Rocket Engineering sites.
DiscoverEngineering.org - Aerospace Engineering section of DiscoveringEngineering: Contains an introduction to rocket engineering, accomplishments of various aerospace engineers, and links to fun stuff.
Please feel free to share your favorite Rocket Engineering websites (with a short description).