The Falcon BMS combat flight simulator is free in itself but in order to respect the original license agreement, you will first have to get your hands on a copy of the original Falcon 4 from Microprose. You will need it to unlock the installation of BMS. You will not need to install the Original Falcon 4 but you will be asked to point to it's installer when you first start the BMS install process.
There are still some copies of Falcon 4 original available on the internet. Do a quick search and you will find a copy.