Juniper Lander has a spacious 3” by 9” coupler that makes it easy to mount multiple flight computers.
I mount a sled inside with an Altus Metrum Telemetrum and an Easy Mini. The sled is constructed to be both the backplane for mounting and wiring and also a structural member that is overbuilt to handle tensional forces in the case of a mishap.
The main body of the sled is a sheet of 1/16” G10 that extends the length of the bay plus forms a tab on either end. For added strength a piece of ¼” Kevlar with loops sewn in each end reinforces the sled for potential tensile loads. The loops are glued inside of the tabs on either end of the sled. They encircle holes D-shackles to connect to tethers. The tabs are thickened with layers of G10 to match the D-shackle opening. 3/16” stainless D-shackles with a 2200 lb load rating are used.
This sled is strong, lightweight, and does not block RF from the tracking electronics. Also note that in the event of a catastrophe such as a ballistic deployment of the main, the large tensile forces are directly transferred through the Kevlar reinforced spine. In a more traditional electronics bay the end caps become structural members that must transfer force from the threaded rods to the eye bolt.
At the “sky” end of the sled a G10 centering ring and end cap were glued around the tab. There is a short section of hollow carbon kite stick through which the deployment charge wires pass. This tube is plugged with plumbers putty during use to keep gasses out of the electronics bay. A hole in the tab allows charge wires to be secured with a cable tie.
At the “dirt” end of the altimeter bay a removable G10 end cap was created that is slotted to pass over the recovery tab. This end also has a short hollow tube for charges wires that faces away in this picture. This removable end cap is held on and in compression with a slotted sealing disk and two #6 screws threaded through a short aluminum closure bar. Plumber’s putty smashed between the slotted seal disk and the end cap creates a good seal around the tab.
Closing the electronics bay after completing wiring is quick and easy:
- Insert the sled through the coupler tube
- Put on the slotted end cap
- Make a snake of plumbers putty around the tab
- Put on the circular disk
- Push the aluminum bar through the square hole in the tab
- Install one #6 screw that was removed to allow the aluminum bar to come out
- Tighten the #6 screws, tensioning the electronics bay and smashing the plumber’s putty into a good seal
This electronics bay is configured with an Altus Metrum Telemetrum and an Easy Mini for redundant dual-deploy and tracking. The flight computers are mounted with #4 stainless screws. These thread into #4 PEM nuts pressed into the G10 backplane and glued in place with G5000 Rocketpoxy. Computers and batteries are mounted by the upper = sky end of the bay to get their mass as high as possible in the rocket. Don’t forget to configure your altimeter for antennae down if it matters.
The antennae of the Telemetrum is wrapped three times around a fiberglass tube with “pencil” diameter. This inductively center loads the antennae. While it is no longer a ¼ wave whip antennae, this works well and shortens the total length fit better into rockets without going through bulkheads.
Two sets of charge deployment wires must go from the dirt end to the sky end of the sled. These are separated as far as possible from the Telemetrum Antennae. They pass through holes for good cable management and strain relief.
Wiring is placed on the back side of the sled. This makes it easier to clearly see how the wires attach to the flight computes and the holes provide strain relief. While the wiring on the back side of the sled looks complex, it is actually fairly simple. Each computer has 4 wire pairs that connect: battery, screw switch, apogee charge, main charge. I provide a rail of 1/16” G10 with a row of 1/8” diameter holes for securing wires. Best practice is to keep the wires short and twist the pairs for RF immunity. This compact design provides stress relief so that wires do not tug on connectors/screw terminals as the rocket is assembled. All switches, batteries, wires, and deployment charge igniters are assembled, secured, and slipped into the rocket as one unit.
The two screw switches consist of thin brass sheet sandwiching 1/8” G10. On the inside of the switch there is a #4 brass nut soldered to one brass sheet. The fiberglass has a close fit hole for a #4 brass screw. The brass sheet on the screw head side of the switch has a larger hole so that the screw cannot wiggle and make contact until the head is screwed down. One construction note: I start by making the close fit hole in the G10. Then I solder the nut to the brass and drill/tap it. Then I epoxy it to the fiberglass centered around the screw hole and clean it out with a tap. The wires that solder onto the brass sheets pass through holes in the G10 to relieve strain.
Holes in the side of the rocket and coupler line up with these screws so that the rocket can be armed at the pad with a 1/8” screwdriver. There is no switch band and there are no wires to a coupler or airframe mounted switch. This eliminates long wires that fatigue, get tangled, and must be packed into a traditional bay design.
I mount a pair of 1s 380 mAH batteries in a fiberglass compartment that I refer to as the “battery coffin”. This is held in place by four #4 screws. I use polarized micro deans connectors to hook up the batteries.
Note that there are Roman numbers by some holes on the back of the board: I, II, and III are visible. IV is partially covered by screw switch wires. These numbered holes are used to insure the ejection charge wires end up on the proper flight computer terminals. I, and II are apogee charges, II being a larger backup charge. III and IV are main parachute charges.
When preparing the bay, I ohm out four igniters then number them I to IV on both ends. Apogee igniters I and II pass through the tube at the dirt end of the sled. They are routed through holes to stay in place as far from the antennae as possible. Before trimming the wires to length I verify and mark their number again between the sled and the flight computer terminal. So far I have avoided deploying the main at apogee by this technique. Plumber’s putty is added for sealing the wires in the tubes.
Some people like bulkheads for attaching igniters. I have tried that and move away from it. With this design it isn’t not difficult to route the igniter wires directly to the flight computers. Fewer connections means fewer chances of failure.
After assembling the entire bay but before adding any deployment powder, I test the electronics for proper beeps and telemetry information. Only then, with screw switches backed off to disarm electronics, do build the deployment charges and add powder. In this rocket I use surgical tubing and cable ties to hold the powder. I mark the tubes I to IV and make sure each tube has the correct amount of powder during assembly. Then I wrap each with 3 layers of electrical tape.
One comment on strength: on a test flight, this electronics bay survived a premature separation of the rocket at about Mach 1.4 with no damage and proper electrical function.
This bay also flew really well on my L3 certification. An M1350W took the rocket up to 23,783’ at Balls 2019. All four charges went off at proper height and in the proper order safely bringing the rocket down.