To say I would highly recommend KSP is an understatement. It is easily in my top games list of all time, and it is not even finished yet (version 0.23). This game is also really popular with a lot of people at NASA. It's also got tones of mods check out kerbalspaceport.com.
If you need some tutorials poke around here for our KSP thread, and check out scott manley on youtube.
Tutorials
http://www.youtube.com/playlist?list...-O2ak6mBYXWPAL