SpaceSimCentral

From your description it just might be the game 'Hardwar' that you're looking for. Set on the moon Titan you fly a fighter from hangers in large buildings as you play in a sandbox type of game as either a trader, merc, bounty hunter, etc. Fits your criteria!

For those of us new to 4x games like this a good tutorial is necessary, unfortunitly the one with Star Ruler leaves a lot to be desired. The game is overwhelming. If someone could suggest a 10 turn walkthrough it'd be really apprecitated.