NEEDS OODLES OF PICTURES
The fin can is generally the most robust part of a high-power rocket. This article is geared towards the fin can for a typical L1 rocket – although the techniques used are largely scalable to larger rockets. (See the Clustering and Removable Fin Can sections for more.)
Materials:
- One body tube
- One motor mount tube
- All necessary fins
- Epoxy
- High-temp epoxy (optional, but recommended for motors K and higher.)
- Centering rings
- Sandpaper
- Razor blade
- Dremel/Mill (material dependent)
- Coupler
- Bulkhead for baffle (optional)
Slotting the body tube
The first step in this process is to slot the body tube. This allows the fins to go through the outer wall of the rocket and provide structural support for the motor mount tube, and give them a secure bonding point that will not be directly subject to the outside world.
This process will depend greatly based on the materials you are using. Cardboard can be slotted with careful use of a razor, but fiberglass/carbon fiber will require a dremel tool, or preferably, a mill to slot the tube. It is critical that the fin slots are aligned with the centerline of the rocket! To make sure of this, methods like:
- Using the mill table’s alignment (if using mill). Make sure to use the team's carbide end mills, as composites are hard to cut and wear out high speed steel end mills quickly.
- Propping the tube in a door frame corner and using the frame to draw a line
- Using a T-square
Are simple, but important when marking slots for the body tube.
Building the motor mount
The motor mount must be built and secured before adding the fins. If using screw-washer retention – don’t forget the tee nuts! Epoxy the centering rings to the motor mount tube using high-temperature epoxy according to your design. If using Aeropack, make sure you leave space on one end of the tube. Using regular epoxy, attach the motor mount assembly to the inside of your slotted body tube on the side with slots.
Fin installation
This is the most annoying step. Longer-duration epoxy (not 5 minute) is recommended.
Apply HT epoxy to the part of the fin that will be touching the motor mount, slide the fin through the slot, and spread the epoxy on the area that the fin will be touching. Repeat a few times for each fin. If you do not have a fin alignment jig – it is recommended to do the fins 1 or 2 at a time.
If possible, use a fin alignment jig to make sure your fin is perpendicular to the rest of your fin can. If you have 4 fins, an easy way to ensure alignment is to use U-shaped pieces of plywood, and clamp them around 2 opposite fins using small clamps. (would recommend)
Once all the fins are attached, aligned, and dried, use epoxy to create fillets for each fin on the outside of the body tube. The fillet radius should be about the same as the end of a craft stick for a typical L1, but can vary in size.
That pretty much does it. You’ll probably need to add a coupler to the other side to interface to the rest of the rocket, and affix the motor retention if you haven’t already.
Special Cases:
Clustering
If you’re planning on flying a cluster of motors, the motor mount assembly can be much more difficult. Recommendations:
Design such that all the motor mount tubes can fit inside one tube that can effectively be just one motor mount tube, which the fins attach to.
Design centering rings that can be cut on a laser cutter that are the shape you need to constrain the motor mount tubes.
Removable Fin Can:
The fin can also be designed to slide out of its body tube for easy repair. This was profoundly useful for projects Odyssey and Therion. Below are some pictures of the implementation, please see the Removable Fin Can article for more.