I’ve flown a number of rockets with 3D printed fins (
listed here). The low power ones are single piece fin cans, but the larger ones have bolt on fins. I use separate fins so that I can have a fin span larger than the print bed. I bolt them on for three reasons: it is easier (for me) to analyze, glue is messy, and I can replace the fins if they break (this was very important with my grid fin rockets).
For structural analysis, I use a copy of Solidworks to do FEA, but fins are simple enough that hand calcs are easy to do as well. The tricky part is the structural properties of the plastic. I print with PETG, which has a listed tensile strength of about 6 to 7 ksi depending on where you look. Sources usually don’t say if that is the base material or as printed, so I have been using 3 ksi as my maximum allowable stress to be on the safe side. Based on some load testing that I have done, the max stress is at least 1 ksi.
For flutter analysis I use the Apogee newsletter article:
https://www.apogeerockets.com/education/downloads/Newsletter291.pdf. The hard part again is the plastic properties. I have found a few different values for the shear modulus of PETG, but the lowest I have found so far is ~73 ksi. Based on the second flight of Checked Baggage, the lower limit is around 23 ksi.
As far as infill, layer height, etc, I generally use a 0.2mm layer height with a 0.4mm diameter nozzle on a Prusa I3 MK2s. I try to model my parts so they can be printed solid or nearly solid as this makes the analysis a lot easier to do. If somebody does want to print something non-solid, I found this article talking about strength as a function of infill:
https://my3dmatter.com/influence-infill-layer-height-pattern/. I know a lot of people worry about the strength between the layers. I have broken a few parts and they rarely break along the layer lines, but it is probably safer to treat the layers similar to a wood grain. As far as how much weaker the layer adhesion is, I don’t have an answer.
I rarely do analysis for the smaller rockets and use the traditional cargo cult “that looks thick enough” engineering. I started doing some simple analysis once I got to mid power, and I am doing a lot more now that I am in the level 1 and 2 range. I hope people find my ramblings helpful, and I look forward to what other people share.