Freespace is open source. or at least fs2 is.
I am not familiar with the.. um.. I guess you could call it the landscape of the freespace community but this was a top search result.
and here is a github project that looks correct.
https://github.com/scp-fs2open/fs2open.github.com
And now, for free, an opinion. I enjoyed descent, but did not like freespace, I am sure there is fun subtlety to the game, but I bounced off it hard, combat in open space ended up being just these really boring circle battles. Much funner combat when you have tight claustrophobic corridors to deal with.