Yesterday I ran across a picture in a book of a pipe which had been welded into a semi-spherical closed end. If you think of the top half of a tennis ball cut in half with a chop saw, then rotated 90° and again cut in half (so that the top half of the tennis ball was cut in quarters) and then the quarters were lifted straight up, if you just looked at that upper profile of the tennis ball and then if you layed out a proportional pattern on the end of a piece of pipe and cut away the material in between the "petals", then the tabs could be forged over into a semispherical shape and welded up to form the shape I saw yesterday.
I thought it looked cool. Naturally, being the curious type, I started thinking about how to lay it out on a pipe from first principles. Hoo, boy, that one is a tricky layout problem. Anyone know how to approach it?