For the past year, I have focused primarily on CAD using Fusion 360. Not to spark a holy war, it fits my kind of workflow and many other CAD options can do this. I get excited when I discover some design element that isn't straightforward and needs a bit of brain usage -- limited as it may be -- to solve. Please understand that I am self-taught, so there may be an easy button in Fusion to do many things that I demonstrate. Organic learning is my teacher, though it is, at times, a difficult schoolmaster.
Fin geometry has been my white whale, so I set out to overcome this. For example, the Nike Smoke has different variants, but most of us are familiar with the complex airfoil design (figure 1). It tapers from the center both horizontally and vertically, so a simple extrusion procedure will not work. After much trial and error (mostly error), I finalized a method for designing these complex fin geometries, which I will attempt to describe here. I will also follow with a video tutorial.
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Figure 1 - Nike Smoke Complex Airfoil
I recently started working on a 3-in Black Brant II upscale and while it isn't technically correct, I elected to use the BB-VC fin. The process works for most complex geometries and I will demonstrate using the BB-VC.
Design Process:
My first step is to find an engineering drawing of the fin. I found this image that was posted by @Bruiser (who is also my muse for this post). I also used the basic fin shape of the BB-II from an OpenRocket Sim design.

Figure 2 - BB-VC Engineering Rendering
I used this drawing for proportionate dimensions of the leading/trailing airfoil tapers going horizontally, and the center section with the tapers going vertically. Building a table in Excel to create proportions, I could then scale these segments to my fin (figure 3).

Figure 3 - Taper Scaling Calculations
Next, in Fusion, I sketched out the fin in two dimensions as I would with any other fin. It's not exact to scale, but it's approximate and reasonably resembles the fin shape. The 2D sketch acts as a centerline for the rest of the design. I switched the sketch to 3D and found my points at the leading and trailing edges and where the airfoil tapers begin both at the top and bottom of the fin and connected all the segments at the top and bottom. These values are split in half. In the end, I ended up with this sketch (figure 4).

Figure 4 - 3D Sketch
Next, I selected the top and bottom of one section of the airfoil (figure 5). I used the loft process to create the complex taper from top to bottom, center to edge. I repeated this 3 more times, then did the same for the top and bottom center sections.

Figure 5 - Lofting Segments
In total, I have 6 separate lofts:

Figure 6 - Lofted FWD and AFT Airfoils

Figure 7 - Lofted Center Section
Finally, I extruded the fin tab. Since it's in the middle, I use the symmetric method with 3mm on either side. I also elected not to taper the tab with the fin. Fin tabs are for structure, not show and all of this will be covered by the fillet.

Figure 8 - Extruded Fin Tab
Once all these steps are complete, you are rewarded with a wicked-looking fin that will be the envy of all your friends.

Figure - Completed Fin as a Single Body
Conclusion:
I know that there are a lot of parts here that I glossed over, such as how I got the figures for my measurements. I will discuss these details in the video. If you're like me, a guide like this is helpful but a video is 10x the value. I hope this helps you on your journey in designing rocket parts.
Fin geometry has been my white whale, so I set out to overcome this. For example, the Nike Smoke has different variants, but most of us are familiar with the complex airfoil design (figure 1). It tapers from the center both horizontally and vertically, so a simple extrusion procedure will not work. After much trial and error (mostly error), I finalized a method for designing these complex fin geometries, which I will attempt to describe here. I will also follow with a video tutorial.
.

Figure 1 - Nike Smoke Complex Airfoil
I recently started working on a 3-in Black Brant II upscale and while it isn't technically correct, I elected to use the BB-VC fin. The process works for most complex geometries and I will demonstrate using the BB-VC.
Design Process:
My first step is to find an engineering drawing of the fin. I found this image that was posted by @Bruiser (who is also my muse for this post). I also used the basic fin shape of the BB-II from an OpenRocket Sim design.

Figure 2 - BB-VC Engineering Rendering
I used this drawing for proportionate dimensions of the leading/trailing airfoil tapers going horizontally, and the center section with the tapers going vertically. Building a table in Excel to create proportions, I could then scale these segments to my fin (figure 3).

Figure 3 - Taper Scaling Calculations
Next, in Fusion, I sketched out the fin in two dimensions as I would with any other fin. It's not exact to scale, but it's approximate and reasonably resembles the fin shape. The 2D sketch acts as a centerline for the rest of the design. I switched the sketch to 3D and found my points at the leading and trailing edges and where the airfoil tapers begin both at the top and bottom of the fin and connected all the segments at the top and bottom. These values are split in half. In the end, I ended up with this sketch (figure 4).

Figure 4 - 3D Sketch
Next, I selected the top and bottom of one section of the airfoil (figure 5). I used the loft process to create the complex taper from top to bottom, center to edge. I repeated this 3 more times, then did the same for the top and bottom center sections.

Figure 5 - Lofting Segments
In total, I have 6 separate lofts:
- Bottom Forward airfoil
- Top Forward airfoil
- Bottom Aft airfoil
- Top Aft airfoil
- Bottom Center Section
- Top Center Section

Figure 6 - Lofted FWD and AFT Airfoils

Figure 7 - Lofted Center Section
Finally, I extruded the fin tab. Since it's in the middle, I use the symmetric method with 3mm on either side. I also elected not to taper the tab with the fin. Fin tabs are for structure, not show and all of this will be covered by the fillet.

Figure 8 - Extruded Fin Tab
Once all these steps are complete, you are rewarded with a wicked-looking fin that will be the envy of all your friends.

Figure - Completed Fin as a Single Body
Conclusion:
I know that there are a lot of parts here that I glossed over, such as how I got the figures for my measurements. I will discuss these details in the video. If you're like me, a guide like this is helpful but a video is 10x the value. I hope this helps you on your journey in designing rocket parts.