AvBay Design Block

[soopirV]

Hi- This isn't necessarily High Power, since I'm talking 38mm diameter, but I figured more experience with AvBay design is in this zone. I've got 6 or 7 DD rockets, and am trying now to equip a MiniDX3 (1.6"/38mm with 29mm motor), and it's kicking my butt. I can fit my Eggtimer Quark, a 2S 350mah Lipo, and a small mag switch of my own design, but I can't figure out how to hold the whole thing together! My normal go-to is to run a threaded rod or two axially, secured with wing nuts on one side, or a threaded eye nut if I can find it...
Trouble is, all of this is too bulky for the current state of things. My sled is G10, so I can probably use it to transmit the axial load, if I can find a way to anchor the bulk plates to the sled. Small angle iron has come to mind, but strength is a concern. I thought of using kevlar cord to bear the load, with the caps secured only as barriers to gas entry, but that's sketchy...what else am I missing? What elegant solution has been previously implemented in this scenario that I CANNOT FIND NOR THINK OF!

Thanks, as always!

DJ

 

[3stoogesrocketry]

Hi- This isn't necessarily High Power, since I'm talking 38mm diameter, but I figured more experience with AvBay design is in this zone. I've got 6 or 7 DD rockets, and am trying now to equip a MiniDX3 (1.6"/38mm with 29mm motor), and it's kicking my butt. I can fit my Eggtimer Quark, a 2S 350mah Lipo, and a small mag switch of my own design, but I can't figure out how to hold the whole thing together! My normal go-to is to run a threaded rod or two axially, secured with wing nuts on one side, or a threaded eye nut if I can find it...
Trouble is, all of this is too bulky for the current state of things. My sled is G10, so I can probably use it to transmit the axial load, if I can find a way to anchor the bulk plates to the sled. Small angle iron has come to mind, but strength is a concern. I thought of using kevlar cord to bear the load, with the caps secured only as barriers to gas entry, but that's sketchy...what else am I missing? What elegant solution has been previously implemented in this scenario that I CANNOT FIND NOR THINK OF!

Thanks, as always!

DJ

How long does your ebay need to be? Years ago I built a 6 inch long ebay with no threaded rod because of rf interference . I simply used two , three inch sections of coupler with a bulkhead glued into one side of each coupler . For recovery attatchment , I used a short section of 1/8 kevlar cord tied in a loop , and glued to the backside of the bulkhead. I then glued one of the couplers one inch into a 2 inch section of body tube . The av sled slid into the BT and coupler half way . Then the other 3 inch section of coupler was slid over the av sled , and 1 inch into the BT . 4 number 8 or 10 screws secured the section of coupler into the BT . If you put little blocks on the inside of the bulkheads you can keep the av sled from rotating inside the bay.

Eric

 

[soopirV]

How long does your ebay need to be? Years ago I built a 6 inch long ebay with no threaded rod because of rf interference . I simply used two , three inch sections of coupler with a bulkhead glued into one side of each coupler . For recovery attatchment , I used a short section of 1/8 kevlar cord tied in a loop , and glued to the backside of the bulkhead. I then glued one of the couplers one inch into a 2 inch section of body tube . The av sled slid into the BT and coupler half way . Then the other 3 inch section of coupler was slid over the av sled , and 1 inch into the BT . 4 number 8 or 10 screws secured the section of coupler into the BT . If you put little blocks on the inside of the bulkheads you can keep the av sled from rotating inside the bay.

Eric

That's a neat idea, would like to see a drawing/photo so I'm sure I understand completely? My avbay is 4" long, kinda stuck with that one.

 

[OverTheTop]

This is the NC "sled" from my Nike Apache.


It has a 6mm eyebolt screwed into the bottom IIRC. Top attachment is M4 allthread.

Using G10 gives you this option because of strength, especially in tension.

Feel free to steal the idea. I don't think it was mine anyway.

Full details here: https://forum.ausrocketry.com/viewtopic.php?f=6&t=5019

 

[soopirV]

This is the NC "sled" from my Nike Apache.
View attachment 327860

It has a 6mm eyebolt screwed into the bottom IIRC. Top attachment is M4 allthread.

Using G10 gives you this option because of strength, especially in tension.

Feel free to steal the idea. I don't think it was mine anyway.

Full details here: https://forum.ausrocketry.com/viewtopic.php?f=6&t=5019

I toyed with an idea like this, but how did you ensure the metal bits bonded to the fiberglass? My experience in the past have been mixed, maybe I didn't rough up each surface, and clean it sufficiently (I'd like to think I did). I can bond a coupling nut to the sled, and engage an eyebolt on one side, but the other side hasn't the room for that, so still looking for other suggestions (I'd LOVE to post photos...will try to do so tonight).

 

[OverTheTop]

I toyed with an idea like this, but how did you ensure the metal bits bonded to the fiberglass?

No bonding. Bolted through. More details on link in earlier post.

I have been known to use U-bolts in a similar situation. They work quite well too.

 

[soopirV]

No bonding. Bolted through. More details on link in earlier post.

I have been known to use U-bolts in a similar situation. They work quite well too.

oops, my bad, sorry, I didn't visit your link, but HOLY CRAP- that 3D printed work is amazing! I'm glad I did find the pic, because that would certainly take some heartburn out of it for me...I can cross-drill the lower couplernut to both inhibit rotation of the eyebolt and spindle, AND secure it to the sled. Then I just need to find a small eyenut to secure the other bulkhead...

I had toyed with the idea of buying a bunch of #10 eye nuts from Amazon, but balked at the prospect of paying so much...I think it might've been the absolutely right decision- I could ditch the coupler entirely, and just run all thread through, nice, trim #10 allthread. I may have to go back to square one and stop being so cheap!

 

[mpitfield]

Assuming that it is not a nosecone sled.

I have both sleds in the following links. The addicted has more mounting holes but both work well. To attach your recovery harness, one option to consider is to drill two holes in the AV bay lids, just large enough to insert some kevlar cord and knot it up to secure it on the inside of the lid. I have used this method on two rockets, one a 11lb loaded 54mm MD that has 10 flights up to 18K, Mach 1.8, and some hard deployments. The other is a 3" rocket with a 54mm MMT, 2 flights up to 14K and Mach 1.4, 16.5lbs loaded.

https://www.additiveaerospace.com/collections/altimeter-sleds/products/38mm-smart-sled

https://www.missileworks.com/store/#!/RRC2+-Modular-Sled-Systems/c/9045099/offset=0&sort=normal

 

[soopirV]

alright, so I think I'm going to go back to plan A:
1) Buy m3 eye nuts from amazon (~13$- originally would take 5-6 weeks to arrive, but apparently my hesitation means that some portion of that is nearby, since I can get them now in 2 days).
2) Use M3 rod up through the center (the stainless versions I've seen have sufficient strength to satisfy the mass of this rocket)

Thanks gang!

 

[OverTheTop]

If you want skinny rods with lots of tensile strength consider custom bike spokes. Good shops will have the threading machine for the 2-56 thread so you can get whatever length you want. If you want to go the whole hog you can go titanium spokes which end up with a 280kg breaking strain each. You can use nuts or nipples on the ends.

 

[soopirV]

If you want skinny rods with lots of tensile strength consider custom bike spokes. Good shops will have the threading machine for the 2-56 thread so you can get whatever length you want. If you want to go the whole hog you can go titanium spokes which end up with a 280kg breaking strain each. You can use nuts or nipples on the ends.

View attachment 327875

I used to work in a bike shop and still have a pile of nipples in a drawer (that sounds horrifying), and this idea is genius!!


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