So I just discovered that you can acquire the rocket launcher way before you play the Dead City mission, but only if you've entered the DLC code for the sewer missions. There is a sewer access hatch on the cliff overlooking Kvasir's lair. Enter it and you'll fight your way through a relatively small sewer. The rocket launcher is sitting on the floor near the ladder that allows you to exit.

i don't know why i can't enter that sewer

Yeah, I found this too

Also the reason is for the Kvasir Sewers being small is because it is the same size and layout as the Ark Sewers and I think that is to reinforce the fact that Kvasir's lab is in an old Ark.