Fin cans for Estes Alpha III model rocket

Estes' bulk packs of model rockets are great, but having them all the same design limits opportunities to explore different fin profiles and makes them hard to tell apart. The Estes Orbis pack offers the opportunity to “grow your own plastic parts”, but these kits aren't available locally, so I tried using the Orbis models with the Alpha III kits.

The nose cone models can be resized to fit the 24 mm ID of the Alpha III body tube (134% worked for me), but this doesn't work for the fins as the Alpha III tube is too short to fit the engine. To get around this, I designed a ‘blank’ engine mount in TinkerCAD, and added fins by importing SVGs into PrusaSlicer. I've uploaded some three-fin designs, but if you want to design your own, instructions are below.

Stability

These designs alter the centre of pressure and centre of mass of the rocket as they weigh less than the stock Alpha III part (13.3 g), and have less fin area. Swing test your rocket for stability before flight! As a guideline, the pictured rocket with triangular fins, an Orbis ogive nosecone and a C6-5 engine needed about 20 g added to the nose.

Print settings

The models pictured were printed on a Bambu Labs P1S using the 0.28 mm Extra Draft profile with no modifications. Filament was old unbranded PLA I found in a cupboard, so there's room for optimisation in both the material choice and print settings.

Adding your own fins

• Create your fin profile in Inkscape or other vector drawing program. The only thing I kept the same was the height of 57 mm. Make sure you know the width (w) of the profile.
• Import the motor mount (nofins.stl) into PrusaSlicer. Note its x/y position (x, y)

• Import the fin profile SVG into the slicer. Set its thickness (t) to 1 mm using the object manipulation controls* and rotate 90 degrees in x so that it stands upright. Copy and paste to create the number of fins you want (f).

• Move one of the fins to (x - 12.5 - w/2, y + t/2**). This should give something like this:

• Select both the fin and motor mount, and rotate in z by 360/f degrees. Select the mount only, and record its new x/y position.

• Move an unplaced fin to (x - 12.5 - w/2, y + t/2). Repeat the rotation and placement process for each fin, then select everything and merge them into a single model. This is now ready to print!

* Using the SVG settings causes the object's ‘thickness’ to remain as 10 mm and cause some issues with placing the fins (as of PrusaSlicer 2.9.1).

** I don't know why the t/2 adjustment is needed, but I think it's related to the issue above. Possibly something to do with where the origin of the model ends up after rotation?