Rethinking my AV Bay designs

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Issus

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I've been trying to think of a better way to setup an AV bay than how I've been doing it with the two threaded rod rails running down the AV bay. I don't have a problem with it on 4" and larger rockets, but getting down to 54mm airframes with redundant dual deployment seems to be overly heavy for me.

I use centrifuge vials with grain of wheat bulbs in them as deployment charges. My track record is pretty bad, with a 10% failure rate, but its so cheap I don't care. I just use two altimeters and two sets of charges - easy and I've still maintained a 100% recovery rate.

I'm working on an idea for a rocket at the moment which i'm going to launch a prototype in bluetube using a 38mm motor before going carbon/glass and 54mm motor. As I'm going for performance, I'm trying to cut down weight where I can. To that end - this is what I've come up with.

Rather than using rails, I'm using a 1/4" eye bolt down one end, the sled plate will be epoxied to the AV bay end plate, and the nut for the eye bolt is embedded in the sled - the nut/eye-bolt assembly will also be epoxied in place. At the other end, I have two 1/4" bolts going to nuts embedded in the sled. The nuts in the sled will be epoxied in place however the bolts will be removable to facilitate access. The eye bolt on this end will be secured with a nut that is epoxied to the centring ring but will not touch the AV Bay sled.

In the past I've used "Eurostyle barrier terminal block"s before, however have had some difficulties with ensuring the tiny grain of wheat bulb wires are hit by the screw coming down inside the cylindrical assembly inside. To that end, I've gone with a two row 3 position barrier block - unfortunately I can't get a finer pitch than this off the shelf. Dual row is simply to make life easier when switching out vials - the altimeters always stay connected so there is no worry of a connection not being made properly on one wire in a group stuffed under one screw. The barrier blocks will be screwed in and sitting on a bed of hot glue "just to make sure" :)

I've mounted a voltage selection switch in the sled which will be JB welded in place. This keeps it out of the way and gives it plenty of clearance to the altimeters.

AVBayTop.jpg

Bottom:
Blue box at the left is a 180mAh 2S lipo - 25-40C nanotech from HobbyKing, more than enough to light my globes up.
The bottom has an Entacore AIM-USB. I'm not a huge fan of either of these altimeters but they do the job well enough.

The battery will get Velcro and zip ties.

AVBayBottom.jpg

Given the 150+ G's this is simulated to pull with a J motor, the 6mm plywood sled will get a couple of layers of glass on it just to be sure everything stays together.
 
I don't know that I would trust my sled to hold my rocket together, all I can see is an altimeter ripped in half.
 
I don't know that I would trust my sled to hold my rocket together, all I can see is an altimeter ripped in half.

I would agree. I am not a fan of the sled have force or weight on it like that.
 
I am in agreement as well on not liking that much force on the sled..For 3" and smaller airframes I use 1 threaded rod down the center, or off center if it is tight, to bear the load..
 
150G? What motor would push it to that extreme?
 
Can you explain the wheat bulbs?
I use these: https://www.ebay.com/itm/ws/eBayISAPI.dll?ViewItem&item=390172484789&ssPageName=ADME:L:OC:AU:3160 coupled with a centrifuge vial. I use a pair of cheap crappy wire crimpers to just squash the bulb enough that it shatters on the tip, but leaves the filament intact. I rub the bottom of the vial over a piece of sandpaper to flatten off the bottom (to make it easier to drill) then make a 2mm hole in the bottom with a drill press. Finally, I poke the wires through the holes in the bottom of the vials, then put a dollop of glue around the leads and pull it through so the still hot glue seals the bottom of the vial and holds the bulb in place. Then simply add powder, fill the cap of the vial with barf or quest/estes wadding and close the vial (if you're using smaller rockets, add more wadding to the vial before closing). You can add a bit of tape over the top to ensure it will stay shut.

I don't know that I would trust my sled to hold my rocket together, all I can see is an altimeter ripped in half.
Hmm, that's a good point - I was thinking of it under boost/deceleration not after recovery. I'll try one with several layers of tip to... end? glass, with the glass over the nuts as well and do some static tests on it.

150G? What motor would push it to that extreme?
On this little 38mm rocket, a I1299N will happily give it a kick in the pants that big, even with my old AV bay design which has two 1/4" rods through it (ie: heavy). When going 54mm motors... take your pick.
 
Very ambitious plan. I will continue to follow. 150G is a little more kick that I usually fly.
 
It's more than I usually fly too, but I like to push the boundaries :) We have a "mach madness" contest each year here - My Fiancee won the H/I motor group this year, so I really need to step up my game.
 
I like the centrifuge vials seated in the bulkplates. Good thinking. I use the vials all the time for deployment charges, I should try this. However, I am always worried about a portion of the ejection charge INSIDE the av-bay!
 
Interesting idea. What if the charge is not in the tube and the tube is used a a sealable canal through which the wires run. Just an idea.
 
I like the centrifuge vials seated in the bulkplates. Good thinking. I use the vials all the time for deployment charges, I should try this. However, I am always worried about a portion of the ejection charge INSIDE the av-bay!

I really don't see how this works reliably. At least a third of the time when I use centrifuge tubes they split down the side, which in this design, would expose the inside of the av-bay to the ejection charge.
 
Agreed. This was my concern. I have 100% success in blowing off the centrifuge vial cap, but it does seem very easy to shatter/crack the whole thing, sending shrappnel and ejection charge through the e-bay. Yeah, not worth the risk.
 
I pack my charges to the brim for one of my rockets and have done a heap of static tests with them - I've never had any failure of the plastic body - the cap is just too easy to release pressure through. The hot glue in the bottom of the vial seals the bottom completely. It also means you don't need terminal blocks outside the AV bay, meaning no BP on them. My avionics and terminal blocks all look brand new.

So i've been re-thinking how to do this for strength. I was cutting up some 1/4" threaded rod on the lathe earlier and I had an idea - rather than using bolts going into nuts glued into the wood, what about simply having a short length of threaded rod in the wood, secured by 1-2 nuts all of which are JB welded in place, then glass over the wood and bolts. This would leave sections of rod sticking out of the top plate for nuts to go onto and would give a lot more contact point for the sled/bolt. I could even just use a bolt facing the other way - all embedded in the sled.

...

Cut to one hour later of talking to one of my mechanical engineer friends, and we've worked out where all the loads will be and have come up with this:
Bolt facing the other way to the original design, filleted behind the bolt head to improve stress lines.

EndRework.jpg

The wood will be glassed after coming of the mill, I don't have any Uni glass on hand, so it's just going to be standard plain weave, 2 layers on either side giving a total of ~0.75mm of glass. The glass will then be cut out for the various drills and slots. I'll be adding stands of CF along the outer edge of the wood under and between glass layers to handle some of the stress there. There will also be some strips of CF along the wood next to the switch which is also a weak point.

Now we have a glassed piece of wood, the switch will be installed and JBwelded to the wood, on either side - and then glass or carbon over the top of it to either side to take some of the extra load across there.

The terminal strips will be installed and then the bolts will be installed and heavily covered in JB weld, with JB weld slathered on either side of them too (longitudinally) up onto the terminal strip. Obviously, trying to cut down on weight here by taking this approach so it will not be too excessive as it gets to the point where its not doing anything more structurally.

A single nut with a bolt going into it on 6mm plywood of this same configuration is easily handling 20kg of loading without any signs of compression or deformation of the wood.

Most of the above is just overdoing things because we can, not that it's really needed. Simple JB weld onto the wood should be enough to secure everything under the loads it will see.
 
On this little 38mm rocket, a I1299N will happily give it a kick in the pants that big, even with my old AV bay design which has two 1/4" rods through it (ie: heavy). When going 54mm motors... take your pick.

The I1299 isn't surprising. On 54mm though, you really won't exceed 100G very often, simply because the motor weight is too high. There are a few exceptions (K2045 for example, though to hit 150G on a 2045, you'd still need a rocket less than a pound), but for the most part, even if your rocket weighed nothing at all, a 54mm motor won't exceed 100G.

As for the idea of the sled as a structural member? I actually kind of like the idea. I would dramatically overbuild it though, for the same reason as chadrog mentioned above. I also think that the centrifuge vials might need to be replaced by something a little sturdier - it seems to me that there's too much risk of cooking the electronics with your current design.
 
True, not all vials are equal. Orange vial on the left seems more brittle. Clear one on the right seems more pliable, and I have good success blowing off the cap.

DSC00579.JPG

I have a 38mm alt bay where the sled is a structural member, I guess. It is glued to the couplers, and the couplers have t-nuts for screws through both the booster and payload airframes. No problems on H motors and deployment events.

DSC00577.JPG
 
Mine look very similar to the one on the right which Buckeye listed. In trying to replicate the shattering/breakage reported... i've hit it with a hammer several times as hard as i can, bent in in half, squashed it until the walls touch, hit it with a hammer again, hit it with a hammer on the base (causing it to shoot across the shop, spending 10 mins looking for it,) Jumped on it, found it fits in the hole of a compressor fitting, so applied 120PSI to it. Finally, I hit it with a hammer several more times and then spent about 20 minutes bending it and squashing it every which way I could while on the phone. The plastic appears to be a polypropylene or polyethylene of some sort. If it is PET, with a wall thickness this large I'd expect a bursting pressure of 800-1200PSI on it. If the cap can stay on through that, I will be well impressed.

Good point cjl, i have two 54mm motors which simulate to ~120G but most are staying around 60-70 (the big ones anyway). 14000ft and M2.2 sounds quite good, I'm sure i can optimise the rocket to perform much better though - it presently has more cosmetic fins that functional fins :)

Back to the AV bay - I've realised my plan of securing one end cap to the sled is a bad idea. With the barrier blocks inside the AVBay it would make it almost impossible to wire up the "open" end unless stupidly long wires are used. Therefore, I'll just mirror the end features with the two bolts to the other side and add a cutout for the nut on the eyebolt. Hopefully this wont challenge the strength at that end too much.
 
Good idea. The times that I have cracked for blown up the vial it was directly related to the amount of powder in the vial - go figure.
 
More or less powder? I have a 4" rocket that I pack the vials to the brim in and then tape shut and wrap tape around in order to secure with no problems.
 
IMG_0894.jpg

I've milled a test sled, the rotary switch in reality doesnt match up with their drawings in the datasheet. Therefore, i need to modify the model to get optimal strength based on the real thing.

Note; the bolts are just some 6mm ones i had on hand - I happened to have some 6mm washers which are the same width as a 1/4" bolt head/nut so I used those to get get things to fit. I normally don't have 1/4" stuff around - I need to head to the hardware store some time this week.
 
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Very nice sled. I have never mounted a switch like that but I like the way you have done it.
 
Cutting the large notch out for the switch makes me think you've taken a lot of strength out of the sled.
 
The thinnest material width is up at the bolt heads, and that is where a tensile failure will occur. If the plywood wasn't in a body tube and could flex, it would certainly fail behind the switch. The switch will also be JB welded in place, with a layer of JB weld over the top of it. This should transfer any load from one side of the plywood to the other. As far as I'm aware, JB weld is an order of magnitude stronger than plywood.

Simulation (such that you can do on cheap plywood lol) show the the bolt head area as the failure point every time, which is why i'm going to put 2 layers of glass on top of the whole sled which should keeep everything together, even if the wood does break.
 
Very nice sled. I have never mounted a switch like that but I like the way you have done it.

Thanks! I like the voltage switches - even if they are really huge compared to other options. They have a very nice hard click to them with big detents so it's a very positive lock when you're fumbling around through a static hole.
 
If the shock of deployment can open unwelded eyebolts (hasn't happened to me, but it has happened), those little tabs of ply against the nuts don't stand a chance, even with a ton of glass. What about running a single length of allthread through with an eyenut on each end.

Or, you can go minimalist like Adrian, and run a length of kevlar harness through the avbay, with loops on each end outside. The shock is then taken by the kevlar and not the bay or sled. Lighter, too.

Just my $0.02.
-Ken
 
On a larger, heavier rocket i'd be inclined to agree - the rocket this will be going in is only about 1.2kg including propellant. I need to post a build thread up for the rocket at some point. I'm hoping to mill the fins friday night and get them at least milled before I make the build thread as I'm trying something new with those too, and want to see how it goes before I embarrass myself :)

Hopefully I can get to a hardware store tonight or tomorrow night and then I can demonstrate how i'm planning to mount the bolts. The size of the ply tabs shouldn't matter too much once I'm done.

I cant run any threaded rod down the length of the bay due to the hole patterns and switch.


Its funny you mention kevlar, I've been talking about that with my engineer friend as a backup incase the sled does break in flight.

I'm more than happy to destructively test this sled and see what breaks and throw out this design if it doesn't work out - for me, designing and building the rocket is just as much, if not more fun than flying it :)

Hopefully these replies are not coming off as "sorry but your opinion is invalid" - I really do appreciate the feedback and different issues which go with this design.
 
I like the Kevlar design, but I think you AV Bay will take it in this light of a rocket.
 
I'm far more worried about using 1/8th" kevlar for the recovery than the AV bay at this point. The place I bought it from says it's from Wildman but I can't get specs on it - so it could be anywhere from 600lb to 2000lb depending on the quality of manufacture... Thats a discussion for a build thread on the rocket though. I normally don't build smaller rockets, and I rarely would buy a part I can't get a datasheet for...
 
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