http://kerbalspaceprogram.com/
an example: http://www.youtube.com/watch?v=9RvVjysJKB4
I also recommend Orbiter 2010, but it is a simulator and is not easy.
http://orbit.medphys.ucl.ac.uk/
I love it though.
http://kerbalspaceprogram.com/
an example: http://www.youtube.com/watch?v=9RvVjysJKB4