I just drew up the fins for the Talos booster. Going to be 14 sheets thick of 1/32 ply, ouch! I may need to put holes in these layers to lighten them up. I thought I'd show how I drew them up, but then I thought some might also be interested in the whole design process. That might be helpful to anyone who hasn't done a scratch build like this before. So let's go way back...
I knew the rocket I wanted to build, the Black Brant XI. Pretty much a XII but no finless 4th stage, a good candidate for a 3 stage model. There is a nice scale drawing of the XII by Peter Alway in Rockets of the World, shared here in
post 38. First I drew it full scale in OpenRocket:
I only needed to modify the length of the sustainer which I estimated by proportion from the launch photo.
It's much easier to draw the fins on the full scale drawing and then scale the whole thing down. Complex fins can be drawn point by point after calculating the coordinates from the drawing:
The only bummer was the Taurus fins that are swept back 3 degrees and attached to the boat tail. Unfortunately there's no way to attach them to the transition in OpenRocket so the best approximation was to attach them to the body and slide them backward.
Once I had my prototype drawing, I needed to figure the scale. I knew I wanted to start with 4" for the Talos booster. Dividing the scale diameter 30.125 by 4 I get 7.53. Now I divide the other diameters by the same factor: 22.75/7.53=3.02 and 17.26/7.53=2.29. That's almost dead on for LOC 3" and 2.14" tubing ODs at 3.1 and 2.26. Next I figured if I rounded the factor to 7.5 to make it easy on myself what amount of error would I have in the body diameters? -0.4%, 2.2%, and -1.8%. The average of those looked pretty good to me so I used the OR scale function to scale my prototype drawing by a factor of 7.5.
From there in OR I replace body sections with actual component materials from the database but maintaining the scale lengths. That's my jumping off point to where I draw the complete build in OR. As I build it out I update my "as built" plan with true dimensions and masses of components for good flight simulations.