Engineer response requested on Kevlar cord

[rdrown]

I am building a 7 pound fiberglass rocket and am planning on gluing a short section of Kevlar along the sides of the motor mount.( up one side and back to the other side). I have some Emma Kites 700 lb. MBS Kevlar cord and was wondering if I could braid three pieces together to obtain 2100 lb MBS? The WLL of the single cord is 140 lb.
Would braiding the cord add the strength of each individual cord?
Let me know please.
I am in a hurry and don’t want to wait for a shipment of 3/8th inch tubular Kevlar, 2000 lb.

 

[OzHybrid]

When you hand braid, one strand will always be shortest and take the most load, one will be longest and take the least, the rest will share - ish. Also your braid will use the existing kevlar at a weave angle which will further reduce the strength.
The Emma Kites spec is a break spec not a working load spec.
The main thing with kevlar is to take the shock out. Taped bundles, rubber bands etc.
I've recovered 10 kg rocket on 1000 lb Emna Kites kevlar. 3 braids of 700lb souuld be fine for a 3.5Kg rocket. Keep it neat and not too tight so when you get to the end you can check the tension in each strand and make it as even as possible.
Avoid shock loading. That's your enemy and will zipper fiberglass tube.
Good luck with the flight.

 

[rdrown]

When you hand braid, one strand will always be shortest and take the most load, one will be longest and take the least, the rest will share - ish. Also your braid will use the existing kevlar at a weave angle which will further reduce the strength.
The Emma Kites spec is a break spec not a working load spec.
The main thing with kevlar is to take the shock out. Taped bundles, rubber bands etc.
I've recovered 10 kg rocket on 1000 lb Emna Kites kevlar. 3 braids of 700lb souuld be fine for a 3.5Kg rocket. Keep it neat and not too tight so when you get to the end you can check the tension in each strand and make it as even as possible.
Avoid shock loading. That's your enemy and will zipper fiberglass tube.
Good luck with the flight.

Thank you for your input. This is why I post on this forum, excellent advice.

 

[OzHybrid]

Thank you for your input. This is why I post on this forum, excellent advice.

The other thing with the Emma Kites kevlar is the variation in the density of the weave. I think the 700lb is ok. Unfortunately, there is no information on the site on the weave. Some of the higher strength ones have a very tight weave. When you bundle it into say 5 loop bundles with tape/ bands whatever, you can get material coming out at the kink. It won't go back in or even up and produces a reduced strength spot. So avoid the really tightly woven cords. Great for kites, bad for rocketry.
Good news is it's real Kevlar (generic). Not like some Kevlar parachute cord I bought, that burned with a lovely black sooty flame. If it burns, it ain't kevlar.

 

[OverTheTop]

As Norm mentioned, avoiding snap loading is the key. Kevlar typically stretches to about 7% extra length, then abruptly snaps.

For a comparison nylon stretches much further, but of course is not rated to anywhere near the same force for similar-sized cord.

 

[manixFan]

As Norm mentioned, avoiding snap loading is the key. Kevlar typically stretches to about 7% extra length, then abruptly snaps.

For a comparison nylon stretches much further, but of course is not rated to anywhere near the same force for similar-sized cord.

Hmmm, that elongation figure seems high. According to DuPont:

https://www.dupont.com/content/dam/.../documents/en/Kevlar_Technical_Guide_0319.pdf
The elongation is under 4%, depending on which version of Kevlar. A handy table from that PDF is clipped below.


Tony



for additional varieties, from https://www.chem.uwec.edu/chem405_s01/malenirf/project.html

 

[Tractionengines]

Hmmm, that elongation figure seems high. According to DuPont: ...

The 4% is for strand /thread. When braided, especially into hollow core braid you get "extra" stretch from the braid tightening up. This same tightening up can make the transitions a smaller radius, and larger stress concentration. Depending on braid pattern.

 

[manixFan]

The 4% is for strand /thread. When braided, especially into hollow core braid you get "extra" stretch from the braid tightening up. This same tightening up can make the transitions a smaller radius, and larger stress concentration. Depending on braid pattern.

Ahh, excellent point, and I figured I must have missed something, based on @RocketScientistAustralia previous posts.

In my research, splicing a loop into the end of a Kevlar harness to form connection point rather than using a knot provides a considerable improvement in strength. (Basically following the smaller radius issue raised above.) How do you terminate braided strands without losing strength to a knot? It seems prudent to take into account the effective loss of strength if you have to use a knot to connect the braided cord.


Tony

 

[OzHybrid]

You've done it now........ The knot police are on the way. I can hear the sirens. Someone is going to have to distract them. I'll take one for you kids..... Now run.
Which glue should I use for sticking a non rubber sole back onto a synthetic upper.

 

[rharshberger]

Ahh, excellent point, and I figured I must have missed something, based on @RocketScientistAustralia previous posts.

In my research, splicing a loop into the end of a Kevlar harness to form connection point rather than using a knot provides a considerable improvement in strength. (Basically following the smaller radius issue raised above.) How do you terminate braided strands without losing strength to a knot? It seems prudent to take into account the effective loss of strength if you have to use a knot to connect the braided cord.


Tony

Fingertrap method on tubular braid is fairly common and easy to do, similar to what you might do with the sheath of 550 cord, a couple of stitches is all it takes to lock the fingertrap for additional safety margin.

 

[Steele Nickle]

Ahh, excellent point, and I figured I must have missed something, based on @RocketScientistAustralia previous posts.

In my research, splicing a loop into the end of a Kevlar harness to form connection point rather than using a knot provides a considerable improvement in strength. (Basically following the smaller radius issue raised above.) How do you terminate braided strands without losing strength to a knot? It seems prudent to take into account the effective loss of strength if you have to use a knot to connect the braided cord.

backrooms​

Tony

A few sutures are sufficient to secure the fingertrap, so providing an increased level of protection.

 

[OzHybrid]

A lot is written on the knot subject. Knots are usually created for specific purposes.
Fishing knots- are usually tested on weirdly, fishing line. This is predominantly a monofilament or a small diameter woven line.
Sailing knots-these are of some use. A monkeys fist is particularly useful for making sure people don't do the same thing again. And we get the bowline from it. A knot that regardless of how tight it gets can always be released. There are variations on how much strength you lose, but lets say 50% ish
Climbing knots- there are lots of good ones. The modern rope that climbers and yachting people use is more of a rope system. The inner is a high strength fiber usually linear covered with a protective woven exterior. The interior slides inside the exterior sheath. Not much, but enough to make a difference on whether a knot bites or slips open.
So we've established that the general testing results will be similar but not the same as the cord, woven flat or small diameter that we use in rocketry.

What do I use?
If your cord is suitable, the fingertrap method works well. But it does require to be implemented over a good length. If I had a 100 lb rocket, I'd be using rope of a large enough dia to be able to do that. The tubular kevlar is big enough but 2000lb Emma kites kevlar is barely big enough to do that. I have no idea why you would need 2000lb kevlar on a 7lb rocket. Are you planning to tow some people out at Black rock?

If you terminate to a small dia welded ring like the ones that are fitted to the large ball bearing fishing swivels, I'd put heatshrink on the cord, wrap twice through the loop and secure back with a few half hitches . Pull the tail back trough the heatshrink. Push heatshrink up covering half hitches and some/all of tail and shrink the heatshrink.

Or use a bowline with a half hitch and HS over the knot/tail and live with the strength reduction.
The knot police will arrive shortly......


Removing cord shock is much more important than increasing the strength of your cord. A single 700 lb cord would be more than enough strength wise but might not provide enough physical protection from rubbing at the body tube contact point or others. Although kevlar is strong, there's not a lot of it needed for that strength so losing small amounts from chafing can result in significant strength reduction.

.Good luck with the flight.

 

[Tractionengines]

I finger-trap the 700# from Emma Kites all the time.... it works great with the 1 MAJOR NOTE and 1 minor note:

MAJOR: KEVLAR is slick and when used in repeated loading (like rocket shock cords) it MUST have the locking stiches done. LOTS of people just do the finger-trap and don't do the finishing process. If you don't do the finishing stiches, it will "creep" over time and eventually come loose. The stiches prevent the creep.

Minor: make sure to taper the embedded tail end. (A little is all that needed on our line sizes.) If you have full size right to the end, you get a stress concentration where the diameter changes as it passes the end of the tail.

 

[OzHybrid]

I finger-trap the 700# from Emma Kites all the time.... it works great with the 1 MAJOR NOTE and 1 minor note:

MAJOR: KEVLAR is slick and when used in repeated loading (like rocket shock cords) it MUST have the locking stiches done. LOTS of people just do the finger-trap and don't do the finishing process. If you don't do the finishing stiches, it will "creep" over time and eventually come loose. The stiches prevent the creep.

Minor: make sure to taper the embedded tail end. (A little is all that needed on our line sizes.) If you have full size right to the end, you get a stress concentration where the diameter changes as it passes the end of the tail.

There we go, I'm wrong already.......

 

[Tractionengines]

There we go, I'm wrong already.......

Sorry...

If I get a chance I will post a few photos of my splices in Emma Kites 700# kevlar, LOC Precision 1/8" tubular kevlar, and LOC Precision 3/16" Tubular Nylon. (Always willing to let others critique my work, and "usully" open to suggestions.)

 

[SolarYellow]

I finger-trap the 700# from Emma Kites all the time.... it works great with the 1 MAJOR NOTE and 1 minor note:

MAJOR: KEVLAR is slick and when used in repeated loading (like rocket shock cords) it MUST have the locking stiches done. LOTS of people just do the finger-trap and don't do the finishing process. If you don't do the finishing stiches, it will "creep" over time and eventually come loose. The stiches prevent the creep.

Do you stitch through it, or just do the whipping? I have a couple sizes of Nomex thread I can use, either way.

ETA: I'm thinking the greater elasticity of the Nomex should avoid concentrating stresses on specific threads of the Kevlar. Flame retardance is similar. And a rounded point needle to avoid damaging the Kevlar fibers.

 

[Tractionengines]

Do you stitch through it, or just do the whipping? I have a couple sizes of Nomex thread I can use, either way.

About 1/2 way down the "trapped" section make 3 stitches down thru the core, then a 90 degrees stitch, them make 3 stiches back up (at right angle to the first set). Make another 90 degree stitch, back to the starting point. Tie the two ends together and burry the knot inside.

 

[Tractionengines]

ETA: I'm thinking the greater elasticity of the Nomex should avoid concentrating stresses on specific threads of the Kevlar. Flame retardance is similar. And a rounded point needle to avoid damaging the Kevlar fibers.

There is very little stress difference between the outer layer, and the inner core at the middle of the fingertrap. The stiches are to prevent "creep" as it goes through cyclical loading. If you do a static loading 1time and hold the load, the stiches aren't needed. (Theoretically)

I use whatever thread I have. (I shouldnt say this, but that's often polyester upholstery thread. If the outside melts a little oh well. The stich thru the core doesn't.)

I use a sharp needle for the stitches. You want it to separate the fibers, not push/tear them.

I do use a large rounded point needle for passing down through the core when making the finger-trap, for cords too small for a normal fid. (ie 700# Emma Kites Kevlar) With a sharp needle it is too easy to get out of the center of the core, and that will loose efficiency of the splice.

 

[manixFan]

I am building a 7 pound fiberglass rocket and am planning on gluing a short section of Kevlar along the sides of the motor mount.( up one side and back to the other side). I have some Emma Kites 700 lb. MBS Kevlar cord and was wondering if I could braid three pieces together to obtain 2100 lb MBS? The WLL of the single cord is 140 lb.
Would braiding the cord add the strength of each individual cord?
Let me know please.
I am in a hurry and don’t want to wait for a shipment of 3/8th inch tubular Kevlar, 2000 lb.

I guess I wasn't clear in my question earlier - I've been using the fingertrap method successfully for many years, with all sizes of kevlar. But I don't understand how that would work if you have a braid of three strands of tubular kevlar woven together.


Tony

 

[rdrown]

I finished braiding the Kevlar shockcord. I used three lengths of Emma kites cord (see pic) to make a braid like a hair braid.
I attached the braided cord to the sides of the motor mount and looped it to the upper end of the booster. No knots used.
note that the working load limit is 140 lb per cord, hopefully about 420 lb WLL.
There is a surprising amount of stretch in the braid. I guess because it is hand braided instead of machine.
I will be using 3/4inch tubular nylon shock cord for the rest of the shock cord runs. This is a dual deploy, Wildman 3inch Shredder.

 

[Tractionengines]

No knots used.
note that the working load limit is 140 lb per cord, hopefully about 420 lb WLL.
There is a surprising amount of stretch in the braid. I guess because it is hand braided instead of machine.

You did do AMAZINGLY CONSISTENT WORK. It is very even, with no loose/tight spots.

If you were referencing the breaking strength as 3x700# and expecting this to be 2100#, I would say your not going to be there... but since you are working with WLL of 140# (5:1) and thinking it combines at 420# I think you are OK. BUT I would expect it to fail before 2100# ultimate strain, so your true safety ratio MAY BE closer to 4:1 to 4.5:1...as a guess. (Could make another and test, but a 1-off test wouldn't be great data either.)

In any case, your observation that it has an amazing amount of stretch is a good thing as that will absorb the "shock" so your peak loading is reduced and hopefully well below any failure point.

Make sure you protect it and the edge of the body tube. Both ways. The cord can zipper the tube, AND the edge of the tube will cut the cord.

 

[manixFan]

Nice looking braid, thanks for sharing your technique with us, always good to see a new way of doing things.


Tony

 

[SolarYellow]

Make sure you protect it and the edge of the body tube. Both ways. The cord can zipper the tube, AND the edge of the tube will cut the cord.

Might also help to chamfer/round/soften at least the ID of the tube end. On the OD, much of the time we are trying to make the seam disappear, which requires a nice, sharp, square edge neatly mated to the opposing edge. But if that can be compromised, make sure both the ID and OD of the tube end are smooth and rounded-ish, and it may reduce any tendency for the edge to cut fibers.

 

[rdrown]

Nice looking braid, thanks for sharing your technique with us, always good to see a new way of doing things.


Tony

Thank you for the compliment. I learn from this forum also.

 

[rdrown]

Might also help to chamfer/round/soften at least the ID of the tube end. On the OD, much of the time we are trying to make the seam disappear, which requires a nice, sharp, square edge neatly mated to the opposing edge. But if that can be compromised, make sure both the ID and OD of the tube end are smooth and rounded-ish, and it may reduce any tendency for the edge to cut fibers.

Thanks for the suggestion. I hadn’t considered the sharp edge. I’ll have to do some thinking.

 

[manixFan]

Thanks for the suggestion. I hadn’t considered the sharp edge. I’ll have to do some thinking.

I learned the hard way, several times, how a sharp body tube, especially carbon fiber, can saw through a Kevlar shock cord. It’s a painful lesson. On smaller rockets I now shrink several layers of heat shrink tubing around the cord where it contacts the body tube. For larger rockets, I use a short section of vinyl tubing or something similar, with heat shrink on the ends to keep it in place.

Both times it was the drogue section that got cut, from flights over 15,000’ (one was 25,000’). Lots of time for a heavy fin can to saw through a cord.


Tony

Here's a close call: (that's an artificial wine cork, which are really too short to work well, and need to be restrained so they don't slide as this one did. This is the fin can of a CF 38mm Mongoose that went to about 12,000' IIRC)

 

[OzHybrid]

You could have chamfered the inside diameter edge. It survived.

Where are we going to get corks from. Penfolds Grange is now a screwtop. The end of cork and therfore cork substitutes is coming soon.....

 

[manixFan]

You could have chamfered the inside diameter edge. It survived.

Where are we going to get corks from. Penfolds Grange is now a screwtop. The end of cork and therfore cork substitutes is coming soon.....

You just need to start drinking more wine so you can build up a reserve while you can still get corks. Not a terrible task, and I’m sure you can find a lot of helpers.

But as I mentioned, I’ve found using multiple layers of heat shrink tubing has worked well in place of the cork. I don’t like beveling the edge because on the first rocket I tried it on I got a couple of chips in the edge where it had been thinned to create the bevel. I probably overdid it but after that I decided to just try and do a better job of protecting the cord.


Tony.

 

[bad_idea]

Here's a close call: (that's an artificial wine cork, which are really too short to work well, and need to be restrained so they don't slide as this one did. This is the fin can of a CF 38mm Mongoose that went to about 12,000' IIRC)

After experimenting a bit with the same, I found that putting a little shrinkwrap on the cord on either side of the cork kept the cork from moving, but it was at the expense of a great deal of guessing and assembly difficulty for cords that were permanantly mounted to a recovery anchor. Multiple sleeves of shrinkwrap straight on the cord seem like a much better idea.

You could have chamfered the inside diameter edge. It survived.

Where are we going to get corks from. Penfolds Grange is now a screwtop. The end of cork and therfore cork substitutes is coming soon.....

As a wine drinker who lived through the peak of the cork-taint and cellar-taint years, I say Hallelujah. As an occasional wine maker, I wish they'd go to crown caps instead, since the equipment to refill a screwtop bottle is prohibitively expensive. As a rocketeer, I like Tony's "shrinkwrap alone" solution.

 

[OzHybrid]

After experimenting a bit with the same, I found that putting a little shrinkwrap on the cord on either side of the cork kept the cork from moving, but it was at the expense of a great deal of guessing and assembly difficulty for cords that were permanantly mounted to a recovery anchor. Multiple sleeves of shrinkwrap straight on the cord seem like a much better idea.

As a wine drinker who lived through the peak of the cork-taint and cellar-taint years, I say Hallelujah. As an occasional wine maker, I wish they'd go to crown caps instead, since the equipment to refill a screwtop bottle is prohibitively expensive. As a rocketeer, I like Tony's "shrinkwrap alone" solution.

As someone who lived through the French Alps crown cap revolution of 78, the wine was plentiful and supplied in crates but not really fit to wash your socks in. Enough said.
Grab your corks while you can.