A very interesting Project to model the Space Shuttle in Flight Gear, specially after the latest update that made progresses in higher atmosphere modelling... All for free, the base sim, available cross-platform ( Windows, Linux, Mac ), but Space-Shuttle and the ISS !
The Manual for the latest released version is in itself a really worth reading PDF, lot's of interesting information on Space Shuttle operation in it.
http://www.science-and-fiction.org/FG/shuttle.html
Teasing video: https://www.youtube.com/watch?v=LOpKt2gXQoE