It's once per guardian per week. If you have three hunters, you probably only get one chance at a drop, so have one of each.

Bungie didnt say we could farm for it. But they made the chest the end so you can farm for the raid weapons and armor but the rocket is not in there. So I'm assuming that it's part of the weekly rewards, so 1 per character per week. Although in my opinon, there should be an extra option that has everything from the raid including the rocket launcher (besides the mods). Its still a extreamly low chance but it costs the max amount of spoils you can carry which is 250 (I think?). Doing this will grant an extra chance at the rocket for people who grinded the living hell out of raids.