I was wondering if you had considered that the nozzle is all one piece like in the photo with the gentleman in the suit. Maybe think of the nozzle extension less as "turkey feathers" like you would see on a jet engine, and more like "umbrella". Or, maybe even more accurately, (I'm going to loose my man card for even mentioning this) a woman's pleated skirt. The nozzle extension looks to be on the outside of the first stage tankage, represented by the dark grey texture applied to your model. The blueprint also shows what appears to be three (pyrotechnically released?) straps that hold the nozzle closed against the first stage until staging occurs. If Sea Dragon has a diameter of 75 feet, then my math comes out to be about 166 feet for the nozzle exit diameter when open. Anyway, just my 2 cents, could be wrong. Feel free to ignore if you already knew this.