I made the decision to 3D print the transition sections using white ABS. My printer, a FlashForge Creator Pro, is a pretty nice unit with a fairly large build area–approximately 8.9 inches by 5.7 inches on the base, and 5.9 inches vertically–but this is also a really big rocket. Neither of the transitions could fit on the print bed, and even if they could, there still would not be enough room in the Z direction. My solution to this problem was to divide each transition into four segments. Additionally, the nose cone, while it could fit on the base, was too tall for my printer, so it too got sliced in half.
The immediate question that arises after splitting up a large print is how to join the pieces. Joining with an adhesive (say, epoxy), plastic welding, and melting the mating surfaces together with acetone were all viable options, but left some risk of the joint failing catastrophically, especially given that being such a large print, there was an increased probability for a wavy surface. (I’m still working on dialing in the bed level and print settings, but generally the mates are pretty good.) My solution was to employ one of these methods (leaning towards acetone melting), but also to design in alignment pins, which would relieve some mechanical strain from warping and other deformations. This would also allow for the optimal alignment of the parts for better surface continuity at the segment interface. Below are the pre-joined nose cone, forward transition, and aft transition.
After printing, I bored out the holes a bit with a 9/64 drill bit so the 1/8 pins would slide in. The pins (1.5 inch long steel) are secured into one of the segments with a drop of CA glue and then the opposite (horizontally) segment is mated to it. I have not joined these segments together yet; that will occur once I verify they fit snugly into their respective airframe tubes. For the transition units, the top and bottom halves will be mated such that the horizontal interface planes of each are perpendicular. This should theoretically increase the strength (or, rather, decrease the weakness) of each transition unit. I noticed when trying to join the top and bottom halves of the forward transition that the pins did not align and the diameters at the mating plane may have been slightly different. I figure I made an error when breaking up the forward transition, so this will have to be re-printed. Below are the aft transition, forward transition (the horizontal mates looked nice), and the nose cone when joined together.
I noticed that there is a little bit of sag in the prints, so I will have to do a bit of cosmetic work (sand and coat with epoxy) to the top-bottom mating interfaces. I anticipated this might happen, so that dictated the decision to print all components with the top-bottom interface at the bottom of the print. This meant that I would not compromise the circularity of the transition shoulders or the effective diameter, since the mate into the airframe tube is a far more important mate (and more difficult to repair) than the mate between each segment.