Still a bit concerned about proper exposure though. I wonder if I should set it on manual or Tv mode and let the meter figure it out.
I think what I would do is lock my exposure in for the scene without the rocket lit up. If you find that the scene is quite contrasty, you might want to favor the highlights, but I am going to assume that the rocket flames will be blown out highlights regardless. Hopefully you can be on the sunny side of the rocket itself so that you can keep it well exposed.

