Kevlar Cord instead of eye-bolts?

[Off Grid Gecko]

was buying an eye-bolt for my new nose cone today and got to thinking.

The safe working limit on steel is pretty pathethic for the weight.

Anyone considered permanently or semi-permanently attached loops of cordage instead of steel to anchor shock cords to?

The idea is simple enough. Couple loops of 1600lb test kevlar attached to MM or NC. Maybe some reinforcement of the knot with epoxy... doesn't seem that far fetched.


I'm not building this, just thinking out loud.. Would be much lighter AND MUCH stronger than off the shelf hardware, and fairly cheap.

 

[cls]

Yes! I do it that way too. Rarely use eye bolts anymore.

And skip the quick links, too. Just adds weight and another couple possible failures.

For shock cords, long kevlar with double figure 8 loop at each end. The right size loop can slip around a nose cone and double back through itself... Hmmm, sorry no pix right now.

 

[heada]

U-bolt is much better than an eye-bolt and will outlast kevlar. Stronger than both too.

 

[scotte]

was buying an eye-bolt for my new nose cone today and got to thinking.

The safe working limit on steel is pretty pathethic for the weight.

Anyone considered permanently or semi-permanently attached loops of cordage instead of steel to anchor shock cords to?

The idea is simple enough. Couple loops of 1600lb test kevlar attached to MM or NC. Maybe some reinforcement of the knot with epoxy... doesn't seem that far fetched.


I'm not building this, just thinking out loud.. Would be much lighter AND MUCH stronger than off the shelf hardware, and fairly cheap.

Just did this on a Super Big Bertha that I bought the plywood fins / centering rings upgrade for. Ditched the Estes "elastic shock cord and glued tri-fold paper" solution in favor of kevlar attached directly to the MM. I'm confident this will be a better solution. My intent is to fly this rocket with up to a G80 motor, and possibly beyond, so I needed to bulk up the recovery system. It's still in the workshop but should be ready to fly soon!

 

[David Schwantz]

I use kevlar loops on a lot of smaller rockets where both weight and space is at a premium.

 

[Titan II]

I do it frequently using kevlar. I also do it on avionics bay bulkheads.

 

[heada]

I use kevlar loops on a lot of smaller rockets where both weight and space is at a premium.

In LPR and MPR, sure. This is in the HPR section so space and weight shouldn't be as much of a concern

 

[Titan II]

In LPR and MPR, sure. This is in the HPR section so space and weight shouldn't be as much of a concern

I previously mentioned that I frequently use kevlar loops. I only fly HP. I like low thrust long burn motors. Weight reduction is typically a major design factor.

I also have several HED models. Kevlar loops in the NC is my norm in these cases.

 

[Off Grid Gecko]

Good to know I'm not the only crazy person out there to think of this. hehe.

There was one comment about the longevity and strength of U-Bolts, and I'm curious to know where the data was aquired that U-bolts would "last longer" and "be stronger." Looking at 3/8" U-bolts the highest I saw was around 1000lbs rating. I'm sitting here with a spool of 1500lb test kevlar from the kitestring lady, and the cord appears to be about 3/16" in size. Three wraps in a loop through standard u-bolt mounting holes would prolly snap the plywood before breaking (assuming holes are cut cleanly to avoid cutting the rope).
I can see wear and tear taking it's toll on kevlar over time from friction, cutting of the outer sheath, and would need to be inspected periodically. On a larger diameter rocket though (4" plus) there's enough room to get a hand in there with some kind of wire threading tool to replace the cordage. Assuming my math isn't off, 3 wraps of this stuff should hold 9000 pounds (though I wouldn't suggest dangling a truck from it by the tow hitch).

Question now I guess is why this isn't a more common solution. This all started from thinking, build bigger (for say an L3 build), means build heavier in most cases, which means more strength needed, which makes it heavier, etc, etc. Seems kevlar coordage would be a rather simple and elegant way to get around that with attachment points, plus it would be easier to distribute the pressure on the bulkhead. IE 3 loops through 6 holes spaced evenly around the MMT rather than a rigid mount on one side. And fancy knot-work could give you further flexibility to redirect some of the forces involved as well.

 

[captbk]

I went with these instead of the big eyes to save space and weight.

 

[Hobie1dog]

 

[Adrian A]

I went with these instead of the big eyes to save space and weight.

Where did you get that?

 

[Off Grid Gecko]

HAHA sorry like I said just a brain experiment from me. Maybe some of the others who have utilized it will give us color graphics.

 

[Neutronium95]

Where did you get that?

It looks like it might be some sort of rod end bolt, possibly drilled out to a larger diameter. I've considered them before, but they are kind of pricey. I would certainly like to know what exact part this is though.

McMaster link

 

[Titan II]

This forum also has a pretty good search function.....

https://www.rocketryforum.com/threa...khead-for-recovery-harness-attachment.164956/

 

[DarthMuffin]

Yes, I do that when space or weight is a concern. Especially if the "other side" of the yoke is reachable so it can be serviced and replaced.

I like to think it also provides some minimal zipper protection if you extend the yoke just past the tubing, so you essentially have two sections of shock cord to spread the load over.

 

[captbk]

Where did you get that?

 

[rfjustin]

Where did you get that?

Looks at Mcmaster-carr under "solid rod ends"

Example for AT 54mm forward closures: https://www.mcmaster.com/catalog/128/1391

Edit: https://www.mcmaster.com/2434K22/

 

[Jowayen]

Kevlar's break strength is usually over-sized for any intended application. Kevlar has a few drawbacks. Two of them are:
1.) Kevlar is subject to UV degradation. Don't let it sit for hours in the sun (think of a rocket lost with the line stretched out all day soaking up rays)
2.) Shock loads can damage Kevlar without showing any signs that it is compromised (think of a file - hard, but brittle)
Both of the drawbacks above would be why I'd assume you don't see Kevlar used on anchor lines or dock lines of boats, or for climbing.

In my high power rocket with a baffle, I have no Kevlar. It isn't needed. But I do use oversized Kevlar when near ejection charges (under my pistons) but above that, it's nylon web/strap. Nylon has some stretch/spring back that helps take the "shock' load off of the Kevlar. I never use knots which create stress concentrations. I always lock-braid the Kevlar to properly sized eyebolts. Just my method. YMMV

 

[ep29030]

Kevlar's break strength is usually over-sized for any intended application. Kevlar has a few drawbacks. Two of them are:
1.) Kevlar is subject to UV degradation. Don't let it sit for hours in the sun (think of a rocket lost with the line stretched out all day soaking up rays)
2.) Shock loads can damage Kevlar without showing any signs that it is compromised (think of a file - hard, but brittle)
Both of the drawbacks above would be why I'd assume you don't see Kevlar used on anchor lines or dock lines of boats, or for climbing.

In my high power rocket with a baffle, I have no Kevlar. It isn't needed. But I do use oversized Kevlar when near ejection charges (under my pistons) but above that, it's nylon web/strap. Nylon has some stretch/spring back that helps take the "shock' load off of the Kevlar. I never use knots which create stress concentrations. I always lock-braid the Kevlar to properly sized eyebolts. Just my method. YMMV

One way to avoid opening shock with Kevlar cord is to fold your shock cord and wrap it with rubber bands. The bands bleed off some of the opening shock as they break. Also helps to organize your cords.

 

[David Schwantz]

Hi heada, I have a MC Tomach. If it had an eye or u bolt, I would never get the 30' streamer and shock cord packed in it. She flies on an aerotech L1000. Several others just like that, a 29mm MD that I stuff an I205 in. But I did just think OP was asking if it could be done to anything. Thanks, Dave.

 

[Sooner Boomer]

I've got a (series of) 54mm rockets. I have an AV bay with a tracker, and an Eggtimer Apogee installed in the aft bulkhead. There's not really any room for solid hardware. This works (serious knotage above bulkhead)

 

[rcktnut]

I prefer going with a u bolt or forged eyebolt. There are instances though that going with Kevlar is just a more practical way to go. I used it in my Nike Smoke nose cone which serves as its main chute compartment.

 

[heada]

Hi heada, I have a MC Tomach. If it had an eye or u bolt, I would never get the 30' streamer and shock cord packed in it. She flies on an aerotech L1000. Several others just like that, a 29mm MD that I stuff an I205 in. But I did just think OP was asking if it could be done to anything. Thanks, Dave.

You're right Dave, u-bolts and metal hardware in general isn't always the best option. Sometimes kevlar is the only option.

In my option, given things like non-minimum diameter, my order of preference is elastic -> non-forged eye bolt -> kevlar loop -> u-bolt. But to each their own and if you have a solution that works for your situation, go for it.

 

[AHansom]

I went with these instead of the big eyes to save space and weight.

I use just the u-bolt part of a cable clamp. Throw the clamp part away and use just the u-bolt piece. Drill a fender washer to catch both sides for backing. Very small and compact.

 

[Off Grid Gecko]

Lots of terrific Ideas here guys, thanks for posting.

Yep, I was kind of considering this a one-size-fits-all solution but hadn't thought much about break strength. It can be mitigated as people have said as well.

And sorry for not doing a search on this. Just came to me and I plopped it in a thread.

I've started using kevlar string on all of my little LPR models just because I have a spool of it and it's more than adequate, like 500lb breaking strength. I guess part of the equation would be what kind of shocks are you seeing and is that enough to damage the cord. Then I have this spool of 1500lb test and it looks so small and light it's just begging to be used up as well. I think when great care is taken it can be a viable solution, just make sure to use enough, always inspect for any frays or loose strands etc. If you think you need 200 pounds SWL then prolly use 3000lbs equivalent, lol.

Maybe another check would be to scribble down the length of the cord and replace after any noticeable stretch. This would be similar in thinking to how a seat-belt works. Once it stretches from a single use (in a crash) it's pretty well done and can't be trusted a second time. At least that's what I've always heard. Seat belt details weren't part of my physics curriculum.

 

[heada]

For LP, I use 1.7mm (400lb) glued and wrapped around the MMT and passed through the forward CR. I use enough so that a large loop is made about 6 to 8 inches past the lip of the forward BT. I then use elastic from the loop to the NC. The loop helps to spread the force on the lip of the tube and reduce the chance of a zipper. The elastic absorbs the shock loads and is easily replaced.

For MP, same setup but 2.7mm (700lb) and instead of elastic I use 5/16" nylon. Where possible I use a baffle and then skip the kevlar and use a small u-bolt and the nylon.

HP is 15' to 25' of 5/8" nylon on either side of the av-bay and forged eyebolts or u-bolts.

Kevlar is good but does wear out. And if you use too much or not enough it can zipper your BT like a hot knife through warm butter. If it's somewhere I can get my hand to, I'll use something else.

 

[jahall4]

Lots of good advice and specifics in this thread. There are a few rules to live by, axioms if you will, that if followed will result in consistent successes…

1. Kevlar does abrade and hence weaken, so inspect it. If it goes through a bulkhead make ensure it can be replaced.
   
   
2. Never use glue on a Kevlar between then attachment point and the load, it makes it brittle and prone to breaking. This has happened to enough rockets that the occurrence seems more than anecdotal.
   
   
3. Knots generally weaken Kevlar. Stitch it instead. It does take some time, but IMO its sort of fun and affords you some attachment options you would not have otherwise.
   
   
   
   
4. Harness length is the great redeemer nothing more reliably reduces shock load than the longest harness you can get into the rocket.
   
   

 

[Sooner Boomer]

You *can* glue kevlar. You shouldn't use a glue that sets up hard. White glue (PVA Type 1) and silicone RTV are two that come to mind. I use white glue (Elmer's Craft Bond) to help secure knots (some are necessary). As with most things, understanding the limitations of materials and methods will help you avoid failures.

 

[jahall4]

You *can* glue kevlar. You shouldn't use a glue that sets up hard. White glue (PVA Type 1) and silicone RTV are two that come to mind. I use white glue (Elmer's Craft Bond) to help secure knots (some are necessary). As with most things, understanding the limitations of materials and methods will help you avoid failures.

Sure, you can, I use it, do it in the videos I posted. Again I said... "Never use glue on a Kevlar between the attachment point and the load ..." A "knot" or "tag end" those would not be, but Ideally you would not use knots you would need to secure.