To get the rocket launcher, you must first defeat nemesis in the garbage disposal area to get the card key, then take the facility key to the machine that is next to the place where you put in the water sample. Use the facility key in the ultraviolet ray machine and it will put codes on it. You can the return to the room with the steam coming out of the pipes and go on the elevator. Use the facility key at the door at the end and you will get the Rocket Launcher