Is kevlar really THAT much better than nylon?

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Arsenal78

Well-Known Member
Joined
May 7, 2018
Messages
746
Reaction score
227
Working on a 5.5" LOC Goblin and everyone I've talked to swears up and down that tubular nylon is the worst possible thing to use in high power as a shock cord and to use kevlar. If it's THAT bad, why do companies still put nylon in the kits and not kevlar? I'm using 5/8" climbing grade tubular nylon. Eventually I'll end up switching to kevlar but my understanding is that because kevlar is thinner material in diameter, it has a higher chance of creating a zipper if something goes wrong compared to a thick flat nylon. So how long does tubular nylon really last? I'm guessing a few flights, provided there's enough cellulose and nomex in there.
 
Nylon stretches. Kevlar does not so you need to use more kevlar. I still use both, depends on what I have on hand. Have to replace nylon more frequently due to burns. Have also had a few zippers from kevlar.
 
No necessarily BETTER, but certainly DIFFERENT. BOTH have their uses.

Part of the problem is that in LP and MP we call it a 'shock cord' because we can't really control the ejection with BP and many smaller SU motors to effect a tailored ejection of the recovery gear. We use a stretchable material to help manage the added energy imparted to the nose cone since adding more recovery line is often not possible or advantageous due to weight and cargo space available.

But with nearly every HP motor, we can tailor said ejection force to 'just enough plus some' as well as dual deploy ejection charges....thus, in most HP birds we should be calling it a 'recovery harness' since we try to avoid any 'shock' that might compromise the airframe or mounting points.

With nylon, you MUST protect it from the blast. Even repeated heat cycles can cause weak spots. Nomex sleeves, barf, and nomex burritos all simply slow down the degradation. It's NOT recommended for perma-installs like gluing to the motor tube. The advantage is that it's cheap, easy to get, easy to work with, and has plenty of 'stretch' as a fudge factor. I have nylon harnesses that have flown 20+ times and appear perfect, and I have ones that have flown twice and been burned through/nearly melted through regardless of nomex wrap.

Kevlar is mostly blast/heat resistant, but is subject to degradation. It's lighter and stronger than nylon for a given length, and has a smaller packing density. A wise rocketeer will still protect the kevlar from the blast with nomex, barf, and the nomex burrito to slow down the eventual degradation. It's more expensive, has zero 'stretch' compared to nylon, and tends to stress the mouths of tubes and contribute to zippers. I have kevlar harnesses that have flown 20+ times that are perma-mounted to the motor tube, and some are showing some degradation and wear. The removable ones I have all get washed after each flight, and are in perfect condition after multiple flights in multiple rockets.

Bottom line is, know the strengths and weaknesses of each, and choose based on your budget, availability, and overall needs.

Personally, at this point I think that manufacturers including things like fasteners, hardware, and (all too often all too short) lengths of tubular nylon is a waste of time to a large portion of rocketeers and is done more because it's 'expected' and 'convenient' based on marketing a 'complete kit' than because you want that stuff that they've chosen(lowest common denominator cheap hardware). I wish that there was a click box to decline that stuff, since sourcing stronger and more corrosion resistant hardware is only a few $$.
 
Last edited:
Neither are "better" Each have their applications. Nylon is cheaper and has stretch and so will absorb more of the ejection charge forces but it isn't as fire resistant as Kevlar. Kevlar is more expensive and doesn't have any stretch so you'll need more of it as compared to nylon but it is fire resistant so it should last longer. In a 5.5 rocket, you can easily get to the attachment points and replace the nylon as it gets flame damaged but odds are the rocket will fail (lost, damaged, etc.) long before the nylon needs to be replaced. 4" and larger I go exclusively nylon. 3" and narrower, I use at least enough Kevlar to go from the MMT to a foot outside of the break point. LPR is 100% Kevlar cord since I got 500' of 100# for $20 a long time ago.
 
Side question, when using a Y harness, how far out should the V part come out of the tube or if it even should come out? The upper centering ring in my Goblin is just under 2ft from the lip of the body tube and the V part comes out about 3ft above that (so total about 5ft) and then the rest of the nylon is attached to the top of it. I would most definitely use kevlar but I've noticed in the past it tends to twist up easily.
 
Side question, when using a Y harness, how far out should the V part come out of the tube or if it even should come out? The upper centering ring in my Goblin is just under 2ft from the lip of the body tube and the V part comes out about 3ft above that (so total about 5ft) and then the rest of the nylon is attached to the top of it. I would most definitely use kevlar but I've noticed in the past it tends to twist up easily.

There are no rules, whatever you prefer.
 
Personally I make sure to go past the lip of the tube by a foot or more. If the cord goes over the lip in 2 places, it puts 1/2 the stress on the lip (really spreads the same stress over a wider area) and so less of a chance for a zipper. As Tom says above, whatever you prefer.
 
Kevlar is heat-resistant, but not heat-proof. I managed to melt a 9/16" Kevlar harness a few weeks ago... a J800 went blowtorch on me. It melted through the Kevlar, put a nice big hole in the chute protector, turned the chute into Swiss cheese... and didn't damage the FG airframe at all.
 
The only time I had nylon fail was due to repeated ejection charge gas exposure and it got brittle - I pushed my luck on one more flight before changing it and it didn't holr. I've had kevlar fail twice (1,500 pound test stuff in small airframes) fail far below rated strength.

Steve is right...mix it up as necessary. Nylon is great for shock absorption, provided it isn't getting exposed to ejection gas.
 
On another note, there seems to be no real industry standard for the load rating of kevlar. I know that they each "spec" or advertise a given rating......but I look at two seemingly identical weaves on two different pieces....and one is 5X as bulky as the other....but the same rating?

All in all, seems that most TN is used for load rated stuff like cargo or climbing, leading to somewhat of a standard. If there's a standard in braided/tubular weave/flat weave kevlar, I have yet to find someone (even folks who sell the stuff) able to explain it in a manner that doesn't sound like some 3rd grade anal gymnastics.
 
No necessarily BETTER, but certainly DIFFERENT. BOTH have their uses.

Part of the problem is that in LP and MP we call it a 'shock cord' because we can't really control the ejection with BP and many smaller SU motors to effect a tailored ejection of the recovery gear. We use a stretchable material to help manage the added energy imparted to the nose cone since adding more recovery line is often not possible or advantageous due to weight and cargo space available.

But with nearly every HP motor, we can tailor said ejection force to 'just enough plus some' as well as dual deploy ejection charges....thus, in most HP birds we should be calling it a 'recovery harness' since we try to avoid any 'shock' that might compromise the airframe or mounting points.

With nylon, you MUST protect it from the blast. Even repeated heat cycles can cause weak spots. Nomex sleeves, barf, and nomex burritos all simply slow down the degradation. It's NOT recommended for perma-installs like gluing to the motor tube. The advantage is that it's cheap, easy to get, easy to work with, and has plenty of 'stretch' as a fudge factor. I have nylon harnesses that have flown 20+ times and appear perfect, and I have ones that have flown twice and been burned through/nearly melted through regardless of nomex wrap.

Kevlar is mostly blast/heat resistant, but is subject to degradation. It's lighter and stronger than nylon for a given length, and has a smaller packing density. A wise rocketeer will still protect the kevlar from the blast with nomex, barf, and the nomex burrito to slow down the eventual degradation. It's more expensive, has zero 'stretch' compared to nylon, and tends to stress the mouths of tubes and contribute to zippers. I have kevlar harnesses that have flown 20+ times that are perma-mounted to the motor tube, and some are showing some degradation and wear. The removable ones I have all get washed after each flight, and are in perfect condition after multiple flights in multiple rockets.

Bottom line is, know the strengths and weaknesses of each, and choose based on your budget, availability, and overall needs.

Personally, at this point I think that manufacturers including things like fasteners, hardware, and (all too often all too short) lengths of tubular nylon is a waste of time to a large portion of rocketeers and is done more because it's 'expected' and 'convenient' based on marketing a 'complete kit' than because you want that stuff that they've chosen(lowest common denominator cheap hardware). I wish that there was a click box to decline that stuff, since sourcing stronger and more corrosion resistant hardware is only a few $$.
Great post, Tom!
 
On another note, there seems to be no real industry standard for the load rating of kevlar. I know that they each "spec" or advertise a given rating......but I look at two seemingly identical weaves on two different pieces....and one is 5X as bulky as the other....but the same rating?
Aramid fibers are likely made to all sorts of different qualities, depending on the process and raw materials I guess. Hench the variation in the bulk of the final woven yarns.

Don't forget too that Kevlar is a trademark of DuPont. The more generic name, for whenever other manufacturers make it, is an aramid fiber.
https://en.wikipedia.org/wiki/Aramid
FWIW I normally fly nylon but aramid is much more compact so it gets used in tight chute bays.
 
Last edited:
Nylon has about 10% stretch at break. Kevlar about 3%. In shock loading, assuming the same length and size of harness, the maximum load on the Kevlar will be about 3 times that of nylon. Here is where you lose the advantage of Kevlar's higher strength. You also need stronger harness attachment points. In shock loading, the 2 materials are close to the same strength. Knots will weaken Kevlar far more than nylon.
Kevlar does have better heat resistance than nylon. Kevlar will zipper fiberglass airframes, nylon breaks. Both will zipper cardboard or Blue Tube airframes.
My harness mounts are either eye bolts in the top centering ring or Kevlar strap epoxied to the motor mount. This strap goes just beyond the end of the booster tube. My main harness is almost always nylon, Kevlar only used if space is tight in the rocket. I use quick links at both ends of the main harness for easy replacement.
 
Last edited:
First,
Nylon is fine, many love it, it just has to be protected from the e charge heat far more than Kevlar does..
As far as stretch I have never been one to accept that helps much or at all..
Neither material stretches very much, and then only when you really load them..
They're all quite strong, if you're putting enough load on either material to stretch then something is wrong with your set up and something is going to eventually fail..
Don't forget, generally speaking, these harnesses are stronger then everything else they're attached to..
If the harness isn't damaged, something else will fail before the harness does..
Personally, I have found that when you're done satisfactorily protecting Tubular Nylon from the heat of the e charge..
it can become bulky enough to be a problem fitting everything in the cargo space you have for the recovery equipment..

The one place there is a gain with Nylon is if you're knotting the material..
Here the difference in the stretch properties of the two materials makes enough of a difference to speak about, ( still not a drastic difference though )..
The reason Tubular Kevlar doesn't do well with knots is it's lack of elasticity.
If you stress Tubular Kevlar to failure and there's a knot in it it will fail every time at the knot.
As the knot wraps tightly around itself one side of the material comes to tension before the other side,
this can be difficult to explain, when the knot wraps tightly like a turn around a race track, there is a longer way around and a shorter inside track around the turn.
The fibers that take the long way around come to tension first and then fail first. If the whole piece of material isn't coming to tension at the same time
and doesn't fail at ( near ) the same time then you don't get the full capacity of the whole width of the material..
This is the reason that a knot in Tubular Kevlar will make the whole length of material fail at an average of 1/2 the fail of the material elsewhere in the length.
So by not knotting the material you can use material of an average of half the weight and heft and have the same fail capacity..
All of this speak for me is neither here nor there.
If you're putting this kind of tension on your harness then you have a problem with your set up..
The answer is not a stronger harness as something else will fail.
The answer is to figure out why you're stressing your harness this way and correct the problem..


Kevlar weight ratings-
Tom said it,
I've seen Tubular Kevlar from different manufacturers that are similar in size and far off each other in weight ratings..
For that reason I don't put as much stock in the # a manufacturer specs as much as the heft of the material you can see and feel..
( Caveat -- the type of weave used in manufacturer does make a difference in overall strength )..


Any attachment point is built into the rocket, it must be.
It is not generally serviceable or replaceable ( unless you have the room on the top centering ring for links of course )..
It should not reach out of the booster…
Something is going to wear against the sharp edge of
the booster,, that can be the built into the rocket “Y” attachment point
that is very difficult to access/service,,
or what wears can be the very easy to access/service booster harness...
This is why I keep these attachments short enough so they
don’t reach out of the booster...

7BB4A691-C498-414B-9073-A1DC03AD2597_1_105_c.jpeg



820F783A-31E4-4915-AE14-ECE73DD1598F_1_105_c.jpeg

FF2778FE-F160-45FC-B37D-5DDBAF0FDF23_1_105_c.jpeg

This is a 4" airframe with a 75mm motor mount..
First drill the centering ring 180 degrees apart on either side..
Then mount the long shank eyes..
Then grind the inside of the washers so the ring will go back on the motor tube..
Then epoxy the ring in place..
Then grind the outside of the washers so the motor tube assembly will go back into the airframe tube..
Then continue your build as you would ..
I also bury this in epoxy with a syringe taped to a dowel after the motor mount is in the airframe..

Teddy
 
Knots are always the weak spot in a line, we use knots in rescue on both ropes and webbing and are always aware that depending on the knot you lose 20 (figure 8) to 50 (overhand/water knot) percent of rated strength.

Doesn't matter if the material is dynamic or static rope or tubular nylon, the knot is the weak link.
 
Just remember. When installing a harness, if you don’t expect the rocket to last very long go ahead and epoxy it in behind the last motor forward centering ring like a lot of kit instructions tell you. If you have the fortune of flying the rocket for years, the harness is going to eventually fail no matter what. (If it’s not replaceable.)
If one can get their arm down the tube, an eyebolt on the forward centering ring allows one to use a quicklink and replace the harness. (Might want to use a thicker forward centering ring in this case)
The problem comes if one does a long necked rocket or a small diameter one and can’t get their arm or tools down the bodytube. With a 4 inch diameter project, I used two eyebolts and wire cabling that is swaged on the eyebolts to form a loop that is long enough to get to to use a quicklink to reach down and attach the harness to the wire loop. So far little wear on the loop.
Smaller rockets, one could try a smaller single eyebolt or it would be just easier to epoxy a cable in just behind the last forward centering ring. Swage a loop on the end to have something to attach the harness to and one will have a robust system with a ”cable/wire” leader line. The cabling might corrode over time but I think it would last for quite a few years. If the harness shows wear, easy to replace since it isn’t epoxied in.
If the rocket is completely fiberglass, heck I’ve been known to hit the inside with mild soap, long brushes and a garden hose. That would be reasonable as long as the centering rings are not plywood. That could get the corrosive products off the leader cable but of course can’t be used for cardboard.
Anyone have other strategies for replaceable harnessing in various diameters of rockets, post ‘em as I’d like to see and learn from them. Kurt Savegnago
 
Years ago I bought a lot of Mil-Spec surplus aramid cord, in sizes from 200# to 2000#. I've used it extensively since then. But I also have a fair number of rockets with nylon strap, mostly PML kits I build very early on. I have at least one PML kit where the strap is glued in as instructed that has over 30 flights on it and the strap is probably good fro another 30. But it's big strap in a Tiny Pterodactyl, so not a lot of load. I've never had a nylon strap break, but I have broken aramid a couple of times.

As I've posted before I don't use any knots with aramid, only the 'fingertrap' method. Its easy to do and undo and avoids the use of Quicklinks and the like. The main reason I use aramid is for space - I fly a lot of MD where space is a concern. However when I have the room, especially in a big rocket, I'll use nylon strap since I have a number of nicely made harnesses. With big ejection charges I do use a protector over the strap near the charge wells. To avoid zipper from aramid I slide a cork down the cord and position it at the top of the airframe. I've never zippered a rocket using this method.

Both work and each has its pros and cons. It really can be a matter of personal preference as long as you take the peculiarities of each material into account.


Tony
 
This is for u Kurt. can't believe u haven't seen it as many times I have posted this technique over the years.
How to replace installed damaged cords.

Simply use few inches of coupler and another small section coupler with slice removed so it fits inside.

Place new Y-harness between the 2 and glue.
when cured slide down fincan to top CR and epoxy into place.
U can drill small hole above assembly and inject glue or Attach syringe to dowel slide down ...use another dowel to push plunger. etc. etc

To remove any remaining damaged original cordage. push it through MM out the back. Use X-acto, razor blade etc. to cut off flush at edge of tube, where its bending into mm tube. I attach blade to my angle iron used to mark lines on airframes, fits right into angle. easy peasy!:cool:

DSCN5005.jpgDSCN5007.jpgDSCN5013.jpgDSCN5011.jpg
 
Last edited:
For smaller diameters use an eyebolt at the top centering ring, then put a loop of kevlar shock cord that is long enough to reach past the airframe. Attach the shock cord to the top of the kevlar loop, when its time to replace the kevlar loop, cut it and use it as a leader to pull through a new kevlar loop in place, easy to stitch kevlar. use tublar kevlar and weave the ends into each other then use kevlar thread to hold it in place - think finger trap.

20201113_121447.jpg
 
The only complicated part is when assembling the rocket to make sure the kevlar loop near the eyebolt does not get any epoxy when gluing in the motor mount/centering rings into the airframe. Use tape to protect it and be careful in how the epoxy is placed.
 
Working on a 5.5" LOC Goblin and everyone I've talked to swears up and down that tubular nylon is the worst possible thing to use in high power as a shock cord and to use kevlar. If it's THAT bad, why do companies still put nylon in the kits and not kevlar? I'm using 5/8" climbing grade tubular nylon. Eventually I'll end up switching to kevlar but my understanding is that because kevlar is thinner material in diameter, it has a higher chance of creating a zipper if something goes wrong compared to a thick flat nylon. So how long does tubular nylon really last? I'm guessing a few flights, provided there's enough cellulose and nomex in there.
I recently purchased some 1/2" nylon webbing from Apogee. I've used tubular nylon webbing for climbing and remember it being much more limber. So going with the climbing nylon was a good move. I'm wondering how the limberness of Kevlar compares to nylon of equal strength. Nylon is certainly much cheaper and easier to cut.
 
Thanks for the suggestions. I remember that Jim had posted on this before but I don’t remember the pictures. Looks really clear now. Gotta get my shops cleaned up and start building again. Domestic chores take me longer than I thought they would but my deceased wife would be proud I’m keeping her kitchen spotless and uncluttered. Kurt
 
be careful in how the epoxy is placed.
Just went thru this....wet epoxy on top CR, then dropped the kevlar leader...sucked up a lot of epoxy in a split second....had to replace a never flown section of kevlar in a 2.25" body tube. At least it went to a eyebolt.
Don't remember why I didn't just have it pulled back thru the MMT. But lesson learned.
 
(dead thread resurrection)

Mkay......gota chime in, cuz, well, ya know...ahem.... A buddy was looking for a new snatch- saw this, (some how this topic popped up!!!) then pinged me. There's some good info and....well....WRONG info. Let's dive in. Kill me if this has been done before but....

I don't use the term shock cord. I call it a strap, it's what it is. Regardless of how many you use. Or whatever, it's just a STRAP and nothing special. Get over it.

Shock cords are made from training bras. Cuz they are shockingly cool (if yer under 18).

Lets just call them all "straps", for now. It's simply what they are. No need to get fancy. (I like the term "leash", cuz I have a dog, and my dog is stronger than a rocket.)

So....the Straps: Nylon or Polyester or Kevlar or, or ?

AFTER, inspecting/replacing your stuff if it shows ANY wear.......

Depends.

High heat/abrasion? Pick "Kevlar" (or ARAMID as the term applies today). Really that simple. Then put some tape around it at the fore end to "reduce" chaffing/abrading of everything. Move on, have a nice flight. Rinse/Repeat. Flames/Sparks? Yeh- fibers lose, every time (simply don't matter WHAT kind). Only solution is to CLEAN AND REPLACE AS NEC, ON INSPECTION.

I mean comon, they cost how much? NOTHING.

Keep reading if you disagree!

Stretch? Depends on the WEAVE- NOT the Plastic/Yield/FP characteristics of said materials. If the material stretches due to yield? It must be replaced.

Everything can stretch to a degree. Even steel. Till it "strains", then necks, then parts ways. Weaving steel is not an option. So what to do?

You WEAVE the lightweight, abrasion/flash "resistant" material- for a LOT of reasons.

Nylon: "Snatch Straps" for off-road: are made of Nylon (also by DuPont, '59) for RECOVERY- cuz it's butt tough (damn near the toughest stuff on earth) and with a special WEAVE "it" streeeeeetches- the WEAVE stretches, not the material. Nylon has good strain/abrasion properties per given application, all dim's considered and equal. My truck weighs 7000#. My snatch is 1/8" x 3". I can snatch some big stuff with it, because of the weave, but I won't TOW with it. Tough but low resistance to heat, heat-cycling and poor memory. It WILL fail (woven) when heated or stressed. I've done that. I've snapped big straps pulling peeps out of bogs. Sometimes ya gota givver.

Polyester: is used for (some flat dangerous, don't do it) towing and tie-downs using a different weave because it is a tad "softer" and lighter and has almost no give before failure in this app. (shock/strain/yield/failure). See above. Polyester simply evaporates near heat and has zero memory. Most PS have a snap-limit on them that prevents damage to other gear down the line. Good for logs, bad for bogs. Moving on.

Kevlar: was "discovered" by Ms. Kwolek and is made by DuPont. In a LOT of ways. I've used her fire suits/gloves. I've also used Kevlar vests. Moving on....

DuPont don't make hobby rocket parts for us.

Kevlar is a para-aramid fiber. Good stuff, decent abrade/heat resistance cycles (well below yield) . If it "stretches, like some say it does (hahahah...yeh, no.) for nose cone straps?
It's cuz of the WEAVE, not the materials.

Any compromise of any mission critical fiber type for any reason = mandatory replacement ASAP. Moving on....

"Kevlar": for straps: Unless your product has a DuPont label on it? It ain't Kevlar, but a Chicom copy of same in an "aramid" fiber form. Yes, the patent expired. But not the trademark. Aramid (aka: "KEVLAR") is what you want for a nosecone strap near the poop chute sending sparks....check it every flight. Cuz it's a fiber around heat. And it beats the hell outa what else is out there.

Zipper an air frame? You are going too fast when the laundry got tossed for the given BT mouth strength. Zat simple.
Zippers has abalootly ZERO to do with the strap you use. You could use P50 in the right length, it would be just fine (works for 300# GI's dropping out of Gooneybirds over the enemy). All this smoke about this and that? Zippers are caused by speed and BT mouth yield. Period.

Strap "failure" (aka: neglect/abuse)? : I got straps, of all kinds here, even a nice Brand new LOC 1/4" "Kevlar" 15' leash for my Gobbler project. I can hang from a tree with it like a monkey. I'm 5'7" / 165, and I aint (at ALL) weak. Strap? Might weave-flex a tick ( I mean a BITTY BIT), but It don't stretch an INCH. I can hook it to my 5th wheel and snatch it for all I'm worth (with welding gloves on), nothing. Not a scratch. It's butt solid. I could pull you out with it doubled up. Measure it horizontally on the ground, not an inch longer after stress test with 25# dumbbells on each end keeping it level/taught. It aint DuPont, but it won't strain-fail in any L1 hobby rocket. Same test with the flat-weave in the box Nylon strap. Nothing to note. Bank on that.

Random: I've also pulled small block Fords and Chevy's with an oil and grime covered 15 year old Ancra MC tie down. Didn't even faze it. THAT'S a LOT of (tension) stress going boing boing boing on the hook.
So- do the math....Tell me your shooter put's that much strain on YOUR straps, I'll buy you a beer.
I'll buy ya two. Maybe three. Oh, wait, I gota drive.

Two.

Leash/strap/whatever failures: that's on me (or you) cuz I/YOU aint checking my stuff. The gear is good to go, it's dirt cheap and it really is good to go. Pick a decent one, keep it CLEAN and away from fire. Never fail. Really that simple. Rocketry is complicated enough! Don't make it more so with the whole strap thing.

Look- BP is nasty, NASTY stuff, if you don't wash and INSPECT your laundry when you get home (or sooner!) it don't matter WHAT you use. BP will eat ANYTHING and your strap will break- SO much sooner than later. Straps vs/ BP? hahaha....right. Unless ya know? Ya don't with BP. Ever seen an old smoke pole bore?

Some folks pay 100$ a motor. But neglect the straps?

How much that shooter cost again?




So- use shoestrings if ya want, it really, simply don't matter with HPR hobby rockets if yer tossing it too late or your gear aint shielded/ pristine.

And zippers aint caused by the strap. They were made BY the strap. Because.........
 
(dead thread resurrection)

Mkay......gota chime in, cuz, well, ya know...ahem.... A buddy was looking for a new snatch- saw this, (some how this topic popped up!!!) then pinged me. There's some good info and....well....WRONG info. Let's dive in. Kill me if this has been done before but....

I don't use the term shock cord. I call it a strap, it's what it is. Regardless of how many you use. Or whatever, it's just a STRAP and nothing special. Get over it.

Shock cords are made from training bras. Cuz they are shockingly cool (if yer under 18).

Lets just call them all "straps", for now. It's simply what they are. No need to get fancy. (I like the term "leash", cuz I have a dog, and my dog is stronger than a rocket.)

So....the Straps: Nylon or Polyester or Kevlar or, or ?

AFTER, inspecting/replacing your stuff if it shows ANY wear.......

Depends.

High heat/abrasion? Pick "Kevlar" (or ARAMID as the term applies today). Really that simple. Then put some tape around it at the fore end to "reduce" chaffing/abrading of everything. Move on, have a nice flight. Rinse/Repeat. Flames/Sparks? Yeh- fibers lose, every time (simply don't matter WHAT kind). Only solution is to CLEAN AND REPLACE AS NEC, ON INSPECTION.

I mean comon, they cost how much? NOTHING.

Keep reading if you disagree!

Stretch? Depends on the WEAVE- NOT the Plastic/Yield/FP characteristics of said materials. If the material stretches due to yield? It must be replaced.

Everything can stretch to a degree. Even steel. Till it "strains", then necks, then parts ways. Weaving steel is not an option. So what to do?

You WEAVE the lightweight, abrasion/flash "resistant" material- for a LOT of reasons.

Nylon: "Snatch Straps" for off-road: are made of Nylon (also by DuPont, '59) for RECOVERY- cuz it's butt tough (damn near the toughest stuff on earth) and with a special WEAVE "it" streeeeeetches- the WEAVE stretches, not the material. Nylon has good strain/abrasion properties per given application, all dim's considered and equal. My truck weighs 7000#. My snatch is 1/8" x 3". I can snatch some big stuff with it, because of the weave, but I won't TOW with it. Tough but low resistance to heat, heat-cycling and poor memory. It WILL fail (woven) when heated or stressed. I've done that. I've snapped big straps pulling peeps out of bogs. Sometimes ya gota givver.

Polyester: is used for (some flat dangerous, don't do it) towing and tie-downs using a different weave because it is a tad "softer" and lighter and has almost no give before failure in this app. (shock/strain/yield/failure). See above. Polyester simply evaporates near heat and has zero memory. Most PS have a snap-limit on them that prevents damage to other gear down the line. Good for logs, bad for bogs. Moving on.

Kevlar: was "discovered" by Ms. Kwolek and is made by DuPont. In a LOT of ways. I've used her fire suits/gloves. I've also used Kevlar vests. Moving on....

DuPont don't make hobby rocket parts for us.

Kevlar is a para-aramid fiber. Good stuff, decent abrade/heat resistance cycles (well below yield) . If it "stretches, like some say it does (hahahah...yeh, no.) for nose cone straps?
It's cuz of the WEAVE, not the materials.

Any compromise of any mission critical fiber type for any reason = mandatory replacement ASAP. Moving on....

"Kevlar": for straps: Unless your product has a DuPont label on it? It ain't Kevlar, but a Chicom copy of same in an "aramid" fiber form. Yes, the patent expired. But not the trademark. Aramid (aka: "KEVLAR") is what you want for a nosecone strap near the poop chute sending sparks....check it every flight. Cuz it's a fiber around heat. And it beats the hell outa what else is out there.

Zipper an air frame? You are going too fast when the laundry got tossed for the given BT mouth strength. Zat simple.
Zippers has abalootly ZERO to do with the strap you use. You could use P50 in the right length, it would be just fine (works for 300# GI's dropping out of Gooneybirds over the enemy). All this smoke about this and that? Zippers are caused by speed and BT mouth yield. Period.

Strap "failure" (aka: neglect/abuse)? : I got straps, of all kinds here, even a nice Brand new LOC 1/4" "Kevlar" 15' leash for my Gobbler project. I can hang from a tree with it like a monkey. I'm 5'7" / 165, and I aint (at ALL) weak. Strap? Might weave-flex a tick ( I mean a BITTY BIT), but It don't stretch an INCH. I can hook it to my 5th wheel and snatch it for all I'm worth (with welding gloves on), nothing. Not a scratch. It's butt solid. I could pull you out with it doubled up. Measure it horizontally on the ground, not an inch longer after stress test with 25# dumbbells on each end keeping it level/taught. It aint DuPont, but it won't strain-fail in any L1 hobby rocket. Same test with the flat-weave in the box Nylon strap. Nothing to note. Bank on that.

Random: I've also pulled small block Fords and Chevy's with an oil and grime covered 15 year old Ancra MC tie down. Didn't even faze it. THAT'S a LOT of (tension) stress going boing boing boing on the hook.
So- do the math....Tell me your shooter put's that much strain on YOUR straps, I'll buy you a beer.
I'll buy ya two. Maybe three. Oh, wait, I gota drive.

Two.

Leash/strap/whatever failures: that's on me (or you) cuz I/YOU aint checking my stuff. The gear is good to go, it's dirt cheap and it really is good to go. Pick a decent one, keep it CLEAN and away from fire. Never fail. Really that simple. Rocketry is complicated enough! Don't make it more so with the whole strap thing.

Look- BP is nasty, NASTY stuff, if you don't wash and INSPECT your laundry when you get home (or sooner!) it don't matter WHAT you use. BP will eat ANYTHING and your strap will break- SO much sooner than later. Straps vs/ BP? hahaha....right. Unless ya know? Ya don't with BP. Ever seen an old smoke pole bore?

Some folks pay 100$ a motor. But neglect the straps?

How much that shooter cost again?




So- use shoestrings if ya want, it really, simply don't matter with HPR hobby rockets if yer tossing it too late or your gear aint shielded/ pristine.

And zippers aint caused by the strap. They were made BY the strap. Because.........
Most of use don't use Kevlar because of any of what you listed...we use it because it outlasts nylon and is more heat resistant to ejection charges, it also for a given rating is more compact than nylon. Shock Cord IS the term used in our hobby and its unlikely to change. As for the lack of "stretch", we use other methods to bleed off that energy to prevent the proper term is "shock loading" the shock cord/recovery tether (the terms may be used interchangeably depending on who is writing the post).
 
Shock cords, bridles, straps, whatever. Not trying to change the lingo, made no effort to that effect. I condensed terms. Pretty obvious there. Look- they are just straps, call them what ya like. But shock-cord is not correct for my application.

The straps ("Nylon" and "Kevlar") for my HPR are not shock loaded, they are stressed under tension and are 10x stronger than any part of this pretty stout rocket. Remove the heat/corrosion factor and either would serve equally well until in need of replacement on a hobby rocket.

And I stated twice "Kevlar" (aramid fiber) straps are better in a given ....oh skip it. Read my points again on the Kevlar choice and construction for rockets.

As far as yield per mass/bulk, etc., that would need to be done in a lab under controlled conditions as Kevlar and Nylon are trademarks. Depending on the formula/type of destructive test, "Kevlar" is "stronger" than "Nylon".

I ran way long on this one, way too many points, apologies to the forum for the SECOND time this week, and switching back to DECAFFE.
 
And, w/o all the blather on my part, to answer the OP's question.

Aramid? Yes, yes it is better for our environments/usage. Zippers? That's up for debate, but a MUCH thinner, stronger, object may have a higher propensity to cut into another object under certain circumstances.

I should have just said that in the first place!
 
Back
Top