I’m currently completely an utterly obsessed with Kerbal Space Program, a rocket-building sandbox game currently in development (demo (old) version available at link). It is really, really great fun and supported by a very active modding community (even new features can be modded in!). I find myself semi-roleplaying a space program by starting off with comsats and rovers before moving into manned landers and flight.
I’m documenting my progress over on Broken Forum in a Let’s Play. Come have a read, and go give KSP a try!