Using Kevlar as recovery harness attachment on 38mm min diam project

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kjkcolorado

kjkcolorado

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Thinking through the details of the av-bay for a dual deploy, 38mm, minimum diameter rocket. I have been searching through 38mm minimum diameter build threads for ideas and have seen multiple examples of using looped or tied Kevlar through a hole (or holes) in the av-bay bulkhead instead of an eye bolt or u-bolt for recovery chain attachment. I’ve not done this before and want to try it. One thing I haven’t seen or read is how do you seal the hole(s) where the Kevlar passes through the bulkhead to prevent BP residue/gases from entering the av-bay. If you’ve done this or seen this, what are my options? Pictures appreciated if you have them. Thanks.
 
rubber expansion nuts aka well nuts also work well for sealing wire holes in avbay bulk plates, its what I use. I usually file a small groove along one side of the hole to allow the wire to lay flat in it which makes removal and insertion of the well nut easier.
 
If you have a spare brick of firestop hanging around, you could use that. I wouldn't go buying any, though. [grin]

Seriously, though, simple clay should do the trick. Or even duct tape. You just want to keep the ick out of the AV bay.
 
I've used the sticky poster stuff for years without issue. Put it on the pressure side for the best results. It conforms very well to the wires and you can squish it into the passageway. I generally back it up with a blob on the backside if the charge is on the larger size.


Tony
 
I’ve used ‘sticky stuff’ (poster tac) to seal wires for deployment charges. I was hoping for a more permanent solution to seal around the Kevlar anchors so I don’t have to re-do it every flight. Do you think a well nut (with a groove as Rich noted above) would do well with sealing around Kevlar (which would obviously have a larger diameter than an e-match)?
 
Remember, you heard this from a random person on the internet, so it must be true. 'Scuse me, a "random person on the internet who's never actually tried this".

In my current build, I'm experimenting with 3d printed bulkheads for the AvBay. I've been quite successful at printing in a slot at the periphery of the bulkhead, sized so my nylon webbing is squished pretty hard as it comes through the slot. The tight fit prevents any ick from getting to the electronics so far in my ground testing. If you were to put a slot on the upper and lower bulkheads, and perhaps a printed channel along the wall through the avbay, you should be good with a length of kevlar or nylon that terminates in a loop below and above the bay.
 
kjkcolorado said:
I’ve used ‘sticky stuff’ (poster tac) to seal wires for deployment charges. I was hoping for a more permanent solution to seal around the Kevlar anchors so I don’t have to re-do it every flight. Do you think a well nut (with a groove as Rich noted above) would do well with sealing around Kevlar (which would obviously have a larger diameter than an e-match)?
Click to expand...
While not permanent, the poster tac works very well in small spaces. Obviously a well nut would require more space and a larger diameter hole in the bulkhead. I have not used a well nut before but it does seem like a good solution for the anchors. One bit of caution though, the few times I've had kevlar fail it's almost always been at a 'hard joint', where the kevlar was compressed. One of the biggest weaknesses of Kevlar is its low compression strength. So just make sure when you compress it in the nut it's against the rubber portion and not trapped against a hard edge.

Please let us know what you do and how it works out.


Tony

according to this paper:

https://users.rowan.edu/~newell/Publications/22 Characterization of Structural Changes.pdf

"The compressive strength of Kevlar is 1/10 of its ultimate tensile strength"
 
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thx for that, very informative. Finally I understand what I always thought about kevlar being flawed and it's lateral/compressive weakness.
It has it place when used properly, but hardly the "perfect" shock cord so many think it is.
Now I have the proof.
 
manixFan said:
...One bit of caution though, the few times I've had kevlar fail it's almost always been at a 'hard joint', where the kevlar was compressed. One of the biggest weaknesses of Kevlar is its low compression strength. So just make sure when you compress it in the nut it's against the rubber portion and not trapped against a hard edge.

Please let us know what you do and how it works out.


Tony

according to this paper:

https://users.rowan.edu/~newell/Publications/22 Characterization of Structural Changes.pdf

"The compressive strength of Kevlar is 1/10 of its ultimate tensile strength"
Click to expand...

Sorry, I don't mean to be pedantic about it, but the words don't quite mean what you may think they mean.

Compression strength in the context of that article is based on rigid rod behavior along the fiber , not of a product like a braided rope being "compressed" by pinching it between two objects.

What you're describing as a 'hard joint' is a very real effect though. It's still a tension failure, but you're creating a stress concentration at the edge where the stress can be magnified 2x-5x due to a sudden change in stiffness. So a kevlar rope that normally would fail at 1000lb might now fail at 200lb in tension. This has nothing to do with compression strength. It's a tension failure.

This behavior can be seen in metals too. Any sudden change in stiffness along the part can cause a stress concentration. That's why you machine a gentle radius on metallic parts where thickness changes, or you risk starting a crack in the sharp corner.
 
AeroAggie, thanks for that great explanation. I guess I knew about the issue but found the wrong explanation for it.

I have a lot of minimum diameter rockets where space and weight are the driving factors in material selection. In that case, an aramid fiber (I actually use Technora which is what the military typically uses) is the best choice I have found. In my larger rockets tubular nylon seems to be a better choice.

And for what it's worth, I very rarely use knots with aramid cord. I use the 'finger trap' method, which has never failed me. From all my research, the loop is nearly as strong as the material and does not introduce the weak point that so many knots do. I use a very simple thin copper wire jig that allows me to very quickly create a loop.

However, the loop must always be under tension in use. I don't use Quicklinks but simply loop/unloop the cord as needed. Where the cord is in direct contact with an edge, I use heat shrink to help protect the cord.


Tony

loop.jpg
 
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No problem! I just wanted to head off something that could have turned into an old wives' tale which leads to misunderstanding and bad design.

I think knots are fine as long as you use an appropriate one for the task. I use climbing knots on mine like a Figure 8 for end loops or Alpine Butterfly for middle loops, and a Double or Triple Fisherman to join ends. They are designed for load bearing ropes and can usually be untied without cutting. The finger trap is also great when set up properly, but for me is more tediuous than just tying a good knot. I just haven't played with that method enough to become comfortable with it yet. I don't use quicklinks either. I use a girth hitch to make all my harness connections. They're almost as fast as a quicklink but don't add weight or take up space. I'd probably use a quicklink on something 4" or bigger if I had the volume. Maybe.
 
Tony, I always sew my kevlar loops. But yours looks great. Can you post your jig or how it is done? I am thinking of doing like yours and sewing them to boot. thanks, Dave.
 
Fingertraps are the way to go! the trick is to add a few stitches with kevlar thread along trap- takes very little to absolutely guarantee against failure. Every kevlar harness I make is trapped and sewn. I also tend to make large loops eliminating quick-links.
 
I also like to use large loops. Makes getting them through eye bolts that are already installed simple. Also more room to thread chutes in them. I purchased me a Phaff 130 sewing machine, German made from back in the 50's. All metal and have sewn 5/8" tubular kevlar with it. What tool are you using to make the trap?
 
warnerr said:
Fingertraps are the way to go! the trick is to add a few stitches with kevlar thread along trap- takes very little to absolutely guarantee against failure. Every kevlar harness I make is trapped and sewn. I also tend to make large loops eliminating quick-links.
Click to expand...

A few questions out of curiousity for those combining finger trap and sewing - it seems to me that the stitching would prevent the finger trap from actually working, since that relies on the individual tows in the braids to slip past each other under tension in order to create the grip. If you sew it, you're preventing that from happening unless you sew it with a bit of tension on it in the first place. Can you show pictures and elaborate on your process? Have you tested any to failure?
 
try this test yourself: pinch the far end of the trap- it is now impossible to remove. It takes only slight resistance to allow the trap to tighten. Without that slight resistance it is possible to slip out if tensioned un-tensioned tensioned .... 20# kevlar adds a lot of security for me.
 
manixFan said:
AeroAggie, thanks for that great explanation. I guess I knew about the issue but found the wrong explanation for it.

I have a lot of minimum diameter rockets where space and weight are the driving factors in material selection. In that case, an aramid fiber (I actually use Technora which is what the military typically uses) is the best choice I have found. In my larger rockets tubular nylon seems to be a better choice.

And for what it's worth, I very rarely use knots with aramid cord. I use the 'finger trap' method, which has never failed me. From all my research, the loop is nearly as strong as the material and does not introduce the weak point that so many knots do. I use a very simple thin copper wire jig that allows me to very quickly create a loop.

However, the loop must always be under tension in use. I don't use Quicklinks but simply loop/unloop the cord as needed. Where the cord is in direct contact with an edge, I use heat shrink to help protect the cord.


Tony

View attachment 406752
Click to expand...
Mind posting a video in the "Techniques" sub-forum on how you use your copper wire jig?
 
You can also use heat shrink to secure the loop, which still allows the finger trap to work as intended. Not as secure as sewing but still allows fast removal. I’ll try and post a vid in the next day or so of the copper wire trick.


Tony
 
how are you laying out the avbay/computers?

no MD here but i have a 38mm sustainer that i have been putting off via claustrophobia
 

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