In F1, and space shuttle engines (I think), the regenerative cooling tubing goes through the inner wall of the nozzle. The russian engines (RD-180 and likes) seem to have smooth inner and outer surfaces for the nozzle. However, I think the outer surface of the nozzle has the cooling tubing, which is then covered up by another sheet of metal. Is that correct? What are the advantages that either layouts may have over the other?