Building an Avionics Bay

There is no one right way to build a compartment for your electronics, any more than there is one perfect fin shape. It really depends on many different aspects of your overall rocket design. However, there are two important criteria that should be built into any avionics bay: it should protect the electronics inside through both the flight and recovery of the rocket, and it should allow reasonably easy access. Everything else is optional. The design that follows is one that I have used on many different rockets with a diameter of three inches or more, particularly on rockets using dual parachute deployment.

Design/Construction

Like most av bays, this design places the electronics inside the coupler tube, which is sealed on both ends by plywood discs (Click [HERE!] for an article on cutting rings with a router, [HERE!] for an article on cutting rings with a band saw, and [HERE!] for an article on making custom couplers) The forward bulkhead is cut to fit snugly inside the forward end of the coupler and is epoxied firmly in place. The aft bulkhead is made of two discs glued together. One disc has the same outside diameter as the coupler, and it is glued to a second disc that fits inside the coupler -- this makes a removable base that I later attach to my electronics mounts.

The base is held in place by the eyebolts used to attach the recovery harness. Use forged or welded eyebolts for added strength. The forward and aft bolts extend through the bulk plates and connect in the center of the avionics bay using a coupler nut. Threaded rod can be added if the bolts are not long enough to meet. I also add a wing nut to the aft eyebolt to tighten the whole assembly together after the two eyebolts are connected. This method will keep all the pieces of your recovery system together even if catastrophe strikes during a violent parachute deployment.

If your project is truly extreme, it would be optimal to reinforce the interior of the coupler with fiberglass -- but well save that process for another article. Most rockets will fare just fine with a phenolic coupler, but cardboard couplers can be ruggedized if they are soaked with thin cyanoacrylate (super glue). It completely transforms the cardboard from flimsy to tough.

Switches/Wiring

There also a number of different switch alternatives for arming and disarming your electronics. Whichever you choose, pick one that will withstand extreme jolts or g-forces. Phono plugs with integrated normally-closed switches are popular, but I know a number of flyers who disdain them because the switches are spring-loaded and may open with a hard knock. I use a two-position rotary switch made by Schurter (part number 0033.4501) available online from Allied Electronics (catalog number 287-1022). Click [HERE!] for link. It is low-profile, very resistant to gs, easy to mount, turns with a coin or screwdriver, and can arm two devices or safe two charges with a single switch.

Use braided wire as it is more resistant to breaking with continuous flexion. Think of wire like a pipe for electricity; thinner wire is more restrictive to the flow of electricity (higher ohm value). The thinner the wire, the harder your batteries have to work to do their job.

The fewer failure points in your wiring the better, so try to eliminate unnecessary connections. Attach your squibs or e-matches directly to your terminal output whenever possible. However, dont attach your squib wires directly to your electronics output terminals if there is any chance they will get yanked on deployment -- the solder joints to your circuit boards arent made for that. Use external terminal posts if the squib wires will bear any stress during the flight.

Location

Where the av bay is located in your rocket will depend on its design -- but this particular design allows you some flexibility and works for both conventional (Rocket A) and Anti-Zipper (Rocket B) configurations (Click [HERE!] for an article on anti-zipper designs). The av bay should be attached in place using small sheet metal screws installed into the bulkheads through the airframe tube. The switches can be installed after the coupler tube is in place. Ive had success centering the switch on the joint where the two airframe tubes come together -- this allows the coupler to be easily removed if needed. And DONT FORGET THE VENT HOLE if your av bay will hold a barometric altimeter.

Charge Holders

Take a 2-inch piece of 1/2-inch copper pipe and solder an end cap on one end. Drill a small hole in the end cap just large enough for a squib wire to pass through. Drill a 5/8-inch hole in your forward bulkplate and epoxy the holder in the hole (make certain the cap is inside the avionics bay). You can do the same for the aft plate, especially if you are using the zipper-proof design (Rocket B). Consider mounting terminal posts to the bottom of the aft bulkplate if you use the other design (Rocket A); attach squib wires to the terminals and place your charges down by the motor mount to push out your drogue chute. I make paper-wrapped charges to fit inside the charge holders; you can put your powder directly into the copper pipe, but make sure your squib wires do not short out against the metal.

Electronics Mounts

Nearly any sturdy material will do; I've built them out of 1/8-inch plywood, angled aluminum extrusions, and fiberglass. Hardware stores carry steel corner brackets that will solidly hold your electronics mount perpendicular to the av bay base. Take care when you install the mount that it doesnt interfere with your eyebolts, switches, or charge holders. Consult the instructions for your electronics to determine the correct mounting screws, standoffs, and orientation in the rocket.