Help with Little Dog DD 54

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mars

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Hi all, I am starting to build the Little Dog DD 54 for my L2 (it's going to be an energetic flight...) and I was hoping to get some feedback on my design, specifically how and where I am choosing to attach the bulkheads.

I figured I'd take the two, green stepped bulkheads and use them as end plates for the middle coupling section, which will be my avionics bay. I was thinking of attaching them via a threaded rod through the coupler.

I then figured I'd use the one black, non-stepped bulkhead at the bottom of the nose cone coupler. It fits loosely in the ID of the coupler and I'm not quite sure how to attach it best. Could someone provide some advice on that as well?

Lastly, I figured I'd epoxy the shock cord to the motor mount. Or, since I'd actually like to add some weight to the rocket, I might do another bulkhead and bolt it in radially, somewhere between the motor and the AV bay and attach the shock cord there. I plan on having eye bolts/nuts in all of the bulkheads for other shock cord attachment points.

I've attached a photo of the general setup as I see it. If anybody could provide some insight, that would be much appreciated! :smile:

Little Dog DD.jpg
 
One of my favorite rockets that I've flown on everything from Hs to AT J500Gs, the later being a very gratifying launch. Here is the end of the av-bay:
IMG_1133-X2.jpg


Small u-bolt, big enough charge canister from Doghouse Rocketry (now Binder), left the center hole open for the e-match that gets a direct connection to the PF Stratologger, and two threaded rods (#8 I think). I plug the center hole with Play Doh, which works better than I expected it to (still there, as you can see). The sled is 3D-printed from Additive. Lots of ways to set it up, this is just one. Hope this helps.
 
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What you describe for the avionics bay is the common approach. I typically use two forged eye-bolts with a coupler nut to join them together (my local HW store sells Stanley 1/4"-20 forged eyebolts with 2" and 4" shaft lengths, so I can do up to ~8" bays this way). Others use one run (or more) of all-thread nutted at both ends to hold the bay together as well as the sled inside, and U-bolts or separate eye-bolts attached to each bulkplate for the shock cord. I prefer my approach as there's one solid piece from bottom cord to top cord, not doing any transfer of loads via the bulkplates themselves (though for smaller rockets and FG bulkplates it should be much less of a deal, I split a plywood bulkplate in a 54mm rocket due to a late chute deployment when I used separate all-thread and eyebolts).

For the nose bulkplate, if you don't want any access to the inside of the nose you could just tack the bulkplate in place with a few drops of CA (krazy glue, etc) that cures quickly, then use an epoxy to get a better fillet around the entire thing (ideally on both sides, so I'd epoxy the bulkplate to the coupler first so you can access both sides, then after that's cured epoxy the coupler to the nosecone). But having access to the inside of the nose can be good for either putting in a GPS tracker and/or adding nose weight if necessary (I often find the tracker and battery is sufficient extra weight on its own), and I also use the APE-RC part captbk mentioned above. Or you could epoxy the bulkplate to the coupler and leave the coupler removable (i.e. attach with something removable like snap rivets, screws, etc).

One question: I'm not seeing a vent band for your avionics bay? If this was a kit (haven't heard of the Little Dog before) it probably should have come with a ~1" piece of airframe. This serves two purposes:

1) Keeps the avionics bay in place (otherwise it could slide up or down inside the airframe under acceleration, or even just standing upright on the pad, and would no longer couple the two airframe pieces together)
2) Gives the avionics bay a place to vent to the outside world, so that barometric altimeters can sample the air pressure

Now the other option to avoid a vent band (which is what I do on my rockets) is to epoxy (or rivet, screw, etc) the avionics bay coupler into the upper airframe section. Solves #1 since it can't move up/down, and for #2 you just drill your vent hole through the portion that's epoxied. Personally, I prefer the "parachute cannon" approach for the main, where the chute is pushed-out by the charge (I use charge wells on the avionics bay bulkplates), so the rocket separates below the avionics bay and at the nose, not immediately above/below the avionics bay as is standard for a lot of kits (where the avionics bay would have to pull the chute out of the tube by inertia alone). Biggest down-side is you have to drop the contents of the avionics bay down from the top, in smaller diameter rockets it can be a bit of a process if the sled is a tight fit (helps to round the leading edges so they don't hang-up on the coupler lip). The smaller drogue generally isn't a tight fit so pulling it out is much easier than the main.

On the lower shock cord attach, personally I'd say attach the cord to the MMT. The bulkhead wouldn't add much weight (so it doesn't help much), and will immediately limit either your parachute area and/or the length of the motor you can use, where without it you have more flexibility. If your Cg/Cp is marginal the bulkplate could keep the chute in place rather than letting it shift downward under acceleration, so that is a plus for having a wall there I guess. Oh, and if you plan to use motor eject (even as a backup) be sure to put some holes in that bulkplate. And if you think getting a bulkplate onto the nose coupler is tricky, wait until you try to get one several inches into the airframe. :)
 
Thanks everybody for the very helpful replies! Viperfixr, I really like your setup for the AV bay. My kit didn't come with a vent band, woferry, but I'm planning on purchasing one from Madcow. I think I'm going to go with the chute cannon approach for main that you mentioned and rivet the nose cone coupler into the nosecone, gluing in the bulkhead per your recommendations. I'll post back with some pictures when I get everything set up, thanks again to all!
 
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