This is my first time posting anything here, so hello everybody! Anyway, I recently came up with an idea for a screenplay, and I don't want to divulge too much about it, but it is a sci-fi series that involves a large military space station. I've never really written much before, so this is mostly just for fun, but I'm trying to make it as realistic as possible. My problem is that I don't have a very in-depth knowledge of military ranking, so I have a few questions about the titles of some of my characters. First, I have the actual leader of the station. He commands the 2200 assorted military personnel aboard. I figured he would either be "Commander", or "Captain", but I'm not sure which. Second is a character who acts as a secondary pilot of the station. However, the character is new and inexperienced so I don't know what I would call him. And, finally, a character who leads the actual infantry aboard the vessel. She is only in charge of the crew who go out on combat missions, seeing as there are also many non-combatants aboard the ship. If I had to guess, I'd call her a Sergeant, but I have no clue if that's right or not. Thanks in advance to anyone who can help.