My Level 3 rocket has three segments (a booster and two parachute bays), so it also needs two couplers to mate all the segments. For this, I used a pair of standard LOC 5.38″ cardboard couplers, which are 11 inches in length. I knew in advance that one of these would be purposed as an avionics bay (and I wanted both to look roughly the same), so I cut off a couple of 1 inch bands from the tube (a LOC 5.38″ cardboard airframe tube) from which I cut the parachute bays to act as switch bands. I glued these bands onto the coupler 5 inches from either end with wood glue.
I have a fairly small car, so I designed this rocket to be disassembled into multiple fairly small pieces. However, this means that as assembled, this rocket may require extra precautions to ensure everything stays together during all phases of flight. For this, the avionics bay coupler is held to the main parachute bay (forward) by twelve pop shank rivets in two offset patterns of six rivets, and to the drogue parachute bay (aft) by the same. (The planes of separation are at the forward end of the main parachute bay and the aft end of the drogue bay. The booster coupler is riveted with the same arrangement to the booster segment. The booster coupler and drogue bay are constrained by up to six #2 shear pins (nominally three are used). Since the airframe and coupler components are primarily cardboard, special consideration is given to preserve the life of these components. In particular, the couplers are thinner than the airframe. PLA backing rings were 3-D printed for the interior of the couplers for the riveted and shear pin joints and super glued in place. These should minimize the effect of dragging rivets and shear pins through the cardboard couplers. Additionally, the number and alignment of rivets was chosen to distribute the shearing load through the airframe.
I had originally planned to have two key switches to arm the altimeters, so I painstakingly drilled the two holes for those as well as two holes for LED indicators. Unfortunately, I discovered that the solder joints on the key switches I bought were a bit brittle and (more importantly) that the actual switch seating mechanism got busted after some number of turns (it appeared due to some seal or rubber component in the assembly unseating and jamming elsewhere in the mechanism. This inspired me to switch (pun intended) to a simpler, more elegant solution with PCB screw switches. For this, I 3-D printed a switch housing for the screw switches for easy accessibility as well as holes to mount indicator LED lights. Finally, I printed a switch housing cover, which is friction fit into the housing. The switch housing is the exact width of the switch band and a little less than a quarter the circumference of the switch band.
In order to get the indicator LEDs to function properly with any altimeter, a simple logic circuit involving a 5V regulator (I blew out too many transistors) and a resistor was created. Two of these simple circuits fit onto a small proto board. The proto board was fitted into a custom 3-D printed bracket, which was then epoxied just above the aft plastic backing ring on the avionics bay coupler. V+, V-, and ground wires were routed to the forward end of the coupler where they could be plugged into a terminal block on the each altimeter canister. V+ and V-, and ground wires were also routed to the corresponding indicator LEDs and screw switches.
The avionics bay coupler is capped by two 1/2″ plywood bulkheads. Each of these bulkheads has two holes for the altimeter canisters, which are housed in standard 38mm tubing. The canisters are retained with laser cut 1/4″ plywood brackets and four #6-32 stainless steel screws (and corresponding tee nuts) per bracket. Each bulkhead also features a 2″ steel U-bolt with 1/4-20 threads. The MAIN (forward) bulkhead also includes a terminal block with indicators for which wire to plug in where, whereas the DROGUE (aft) bulkhead contains no such feature. Ejection canisters are integrated into the altimeter canisters.
To complete the avionics bay coupler, I drilled four 1/4″ vent holes in the switch band in 90 degree intervals. I super glued in 5mm LED holders into these holes to preserve the integrity of the holes and artificially remove the burr from the cardboard.
The booster coupler features a 1/2″ plywood forward bulkhead with a 2″ steel U-bolt that ties into the drogue parachute system. Because the aft part of the coupler carries no loads, there is only a 1/4″ plywood aft centering ring less than 0.5 inches wide to add rigidity to the aft coupler.