I want to modify a rocket for dual recovery. My plan is to have the motor release a streamer at apogee with its charge and then have an altimeter release a parachute at 500'. I have a 14"x4" payload bay for the main chute. My plan is to mount a bulkhead with two threaded rods and a hole for the charge holder about 4 inches in one end. Then I want to glue one bulkhead to the top of a coupler (which will house the altimeter) ontop of which will be the charge. My plan is to insert the coupler into the upper frame and secure it to the rods. The bottome bulkhead (the one which the streamer will attach to) will be lose for access to the bay.
My question is: How do I seal the lower bulkhead so no gasses will enter the altimeter bay? I was thinking building it so there is a 1/4" lip of coupler where I could apply putty. Would this work? is there a better way?
Bay design often depends upon a rocket's diameter. Rockets with a 4" and smaller diameter often use a coupler for the bay.
I recommend holding the bay in the rocket with screws. Two #6 screws are sufficient. Epoxy nuts (pem or T nuts) inside the bay. (Use a gel or paste epoxy like JB Weld.) Keep it as modular as possible so that damaged parts can be repaired or replaced without needing to replace the whole rocket.
Use aluminum threaded rod. It weighs a lot less than steel. You don't need
1/4" diameter threaded rod, either. #6 will work fine for a typical 4" diameter rocket. Mcmaster-Carr sells aluminum threaded rod and u-bolts.
My favored design uses three rocket airframe pieces.
Recovery/payload section
Sacrificial intersection coupler (a piece of airframe, not a tube coupler)
Fin section
The bay is a coupler that attaches to the recovery/payload section with screws. 1/2 of the bay is outside of the airframe. The sacrificial intersection coupler friction fits to the bottom of the bay and contains the drogue. The fin can ("anti-zipper" design) fits into the bottom of the sacrificial intersection coupler.
The scarificial interesection coupler takes the brunt of a landing rather than landing on the bay. If it gets damaged it is tossed and replaced. Mine have one to three layers of 6 ounce fiberglass cloth. I haven't replaced one yet.
The friction fit between the bay's bottom and the scarificial intersection coupler is tighter than the friction fit between the bottom of the sacrificial intersection coupler and the fin canister. The fit is adjusted with masking tape. You could also attach the intersection coupler to the bay with screws, but I haven't found that to be necessary.
Matt wrote:
A simple solution is to use two bulkheads on the tube coupler: a coupler bulkhead (fits inside the coupler) and an airframe bulkhead (same diameter as the coupler). The airframe bulkhead is fitted on top of the coupler bulkhead to create a cap.
Pictures, Matt, pictures! Can you draw a plan and get us a .jpg somehow? I just can't envision what you're describing without a picture, and you're about 850 words short of
Seal the lower bulkhead (drogue) instead of the upper (mains) Leave the upper loose for bay access. You could use a stack of a bulkhead and coupler bulkhead to make a cap. Add your threaded rods and whatever else you want to pass thru, them pour a thin layer of epoxy inside it to seal all the holes and affix the two bulkheads to the coupler.
I seal the lower bulkhead (Droge) for this reason: if any gas does freak the altimeter, it will be from the main popping and not the drogue, and the main's already popped at that point.
don't glue the bay to any tubes, screw them in like the others talked about. all good ideas they have.
This is all good Dean. The biggest issue I have found with the anti zipper designs I made is they are not good for Hybrid motors and if they were the fincan would be quite long. This can give you a quite long rocket. For instance my M powered hybrid has a fincan that's almost 6 feet long! As long as you keep this in mind, and possibly make the top of the section so the motor can stick out a bit then you are golden. I like the setup you propose, especially promoting the modular system and the sacrificial tube. That's how I like to build too.
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