Simple Modular System for Avionics Bay Sleds

This is a quick post about a new design we’re playing with for avionics bay sleds that is lighter and more modular than my original design. I’m using the Fortissimo rocket (LEA-6) to experiment with some ideas. This is actually a Binder Excel kit with the dual deploy option that can fly in a short or long configuration. On the maiden flight, I flew with the full (long) configuration. I left the avionics bay empty, and just used motor ejection with a friction fit for deployment. It worked great.

For the second flight, I wanted to try electronic deployment for the first time, with a Featherweight Blue Raven altimeter. The av bay on this kit is a pretty common design, with two parallel lengths of threaded rod that connect stepped plywood bulkheads to a section of coupler tube.

Original Design

To build the original av bay sled, I just epoxied two lengths of aluminum tubing to a piece of 1/4″ birch plywood and drilled out the holes necessary to mount the altimeter. The aluminum tubes slide over the threaded rods, and are held in place with some standard hex nuts. Simple enough, but the design has some limitations.

• The bond between the mounting tubes and the plywood wasn’t very strong (the aluminum was too smooth) and one of the joints broke during the flight.
• Bonding the sled to the mounting tubes means that if you want to replace or change the size or shape of the sled, you need to start from scratch.
• It is heavier than it needs to be. This isn’t a big problem on a husky rocket like the Fortissimo, but it’s better to develop good habits now.

Improved Modular Design

I came up with a better approach that uses two 3D-printed mini sleds that can support either end of an av bay board. This board can be any material and length (within the constraints of the av bay) and attaches with small machine screws to the mini sleds at either end. This allows for custom boards to be designed for different avionics configurations and swapped out easily. Printing the parts in PLA (30% infill) keeps the weight down. And if we replace the stainless steel hex nuts with nylon hex nuts, we cut the weight further.

Below is a video that walks through the design in a bit more detail. And further down is a link to the STL file for the little connectors that attach the avionics board. Feel free to use and remix if it’s helpful.