Kevlar Shock Chord Strength

[lakeroadster]

I've been using 75lb Kevlar for smaller rockets (LPR) and 100lb Kevlar for my bigger stuff that has bolts for nose cone ballast (MPR).

What do you use?

Seems like there should be some rule of thumb, something like a 100:1 ratio? Got a 1 pound rocket, 100:1, use 100 lb Kevlar.

I'm looking at all the time and effort I've put into my Lifting Rocket... and that 100lb Kevlar to lash down the 30" parachute just looks so itty bitty...

 

[jmasterj]

Knots reduce the strength, ejection charges will degrade it over time, especially the really thin kind, etc. I've always, therefore, over-specified my kevlar. I use 100 or 150lb for LPR, but anything that's using an F or larger gets either 500lb or 1000lb depending on the weight. Small high power gets 1500lb.

 

[mh9162013]

I only build LPR stuff and I try to use at least 100 pound Kevlar. It's not that I need that much strength, but the added stength means it should withstand ejection charges longer.

I'm currently building an Estes Yankee and I'm using an ejection baffle plus 150 pound Kevlar shock cord. I went overkill here because I can't replace the Kevlar shock cord in this rocket.

I'm willing to go to 75 pound Kevlar line, but only if it's replaceable after installation.

 

[shockie]

I only build LPR stuff and I try to use at least 100 pound Kevlar. It's not that I need that much strength, but the added stength means it should withstand ejection charges longer.

I'm currently building an Estes Yankee and I'm using an ejection baffle plus 150 pound Kevlar shock cord. I went overkill here because I can't replace the Kevlar shock cord in this rocket.

I'm willing to go to 75 pound Kevlar line, but only if it's replaceable after installation.

Me 2
But do you use yellow or black?

 

[Handeman]

I only build HPR now so mostly used tubular Nylon shock cords, but smaller L1 sized rockets usually get 3mm 1000 lbs. braided Kevlar or the 3.5mm 1500 lbs braided Kevlar. You can get 300 ft of the 3 mm for about $0.19/foot or 3.5mm for $0.37/foot off ebay.

 

[Pem Tech]

We use 75lb yellow KEVLAR thread in all LPR kits and in the Bucky Jones. You have to wrap the shock cord at the lip of the BT to prevent zippering but I've only seen 75-100lb KEVLAR fail after a CATO. Elastic is a blight on the oily forehead of rocketry!

Edit: Forgot to add HPR choice.
We have used 9/16th tubular nylon and have moved to 9/16th nylon strapping. No complaints yet.

 

[lakeroadster]

I had some 75 lb fail, at a knot, on my Cygnus Probe... but it had a 29mm composite motor and no shock chord and a lot of nose cone ballast.

There's a lot of variables to take into consideration to determine the actual load the Kevlar will experience.

 

[jmasterj]

I know @BEC has had small kevlar burn through in BT-50 rockets many times. I've had 150lb kevlar burn through in a (BT-50) BMS school rocket after ~6 flights. Not sure heavier kevlar would be the solution; I repaired with an Estes-style teabag mount.

 

[BEC]

Yes, I've had kevlar tied to a motor mount burn through many times. The fewest flights was two (on a 24mm minimum diameter model with 3D printed parts widely discussed here). In general it happens at around two dozen flights give or take a half-dozen. My first BMS School Rocket got to the low 30s of flights before its kevlar burnt through. This was ~10 years ago. So these days I just don't build the model that way. I have an Apogee Research Express (BT-50-based, 18mm motor) on the near term build queue and I won't be doing it the way the instructions say (make a loop around the motor tube, feed the kevlar up beneath the upper centering ring.

Instead I use the Estes tri-fold to hold the kevlar, usually a foot or so of it, then tie the kit's shock cord (if rubber or elastic) or a replacement made from FAI Model Supply sport tan rubber, to that. I also have started putting a half-inch or so of 1/16 heatshrink over the kevlar, located so the kevlar doesn't wear through on the top of the body tube. This has been almost entirely successful - I have had one failure of the kevlar just above the heat-shrink after I started adding this bit. But still this was after ~50 flights on the model.

The Research Express' shock cord is all kevlar. After I see how long it is, I'll decide if I need to add an elastic element or not.

in LPR builds/rebuilds I use 100 lb. kevlar (sold by a fellow who was calling himself The Kevlar Store here on TRF) generally. That company doesn't look like it's still around. Oh well, I got enough 100 lb. stuff to probably last me the rest of my life already.

 

[sl98]

Yes, I've had kevlar tied to a motor mount burn through many times

My solution is to put a length of heat shrink on the kevlar. Since going this route I have not had burn through of kevlar tied to a motor mount.

 

[lakeroadster]

External Mount?

 

[BEC]

My solution is to put a length of heat shrink on the kevlar. Since going this route I have not had burn through of kevlar tied to a motor mount.

As I mentioned in my post, I do this now. But the kevlar can still fail from abrasion on the end of the body tube, especially if it has been hardened with thin CA.

Oh - you mean down at where it ties to the motor mount? Hmmmmmmm....maybe I'll try that on that Research Express. I could also try it on the Super Orbital Transport I'm building as I haven't built the motor mount yet (still trying to decide whether to put in a longer tube/hook so I could use an E12 in it).

 

[sl98]

Oh - you mean down at where it ties to the motor mount?

Exactly! I use a few inches or so.

 

[BEC]

Exactly! I use a few inches or so.

OK - I’ll give that a try on a near-term future build.

 

[teepot]

I use only Kevlar. Kevlar of any stated strength only has 20% of that as a working load limit. That is why I use 500lb in LPR and 1500lb as the rockets get heaver. For the heavy ones, 5 to 10lbs, I use 2000lb. I use 16' to 20' or more of the Kevlar. Plenty long enough so there is less shock when the nose cone or rocket section reaches the end of the slack. In rockets that will have many flights or takes a large motor, I put a Nomex sleeve around the shock cord. I have several and can move them from rocket if need be. I have used tubular nylon a few times. I stopped using it because it takes up more space than Kevlar.

 

[manixFan]

Knots reduce the strength, ejection charges will degrade it over time, especially the really thin kind, etc. I've always, therefore, over-specified my kevlar. I use 100 or 150lb for LPR, but anything that's using an F or larger gets either 500lb or 1000lb depending on the weight. Small high power gets 1500lb.

I bought a lot of military surplus Kevlar years ago (many weights, 1000's of feet). After some failures with it I started doing a lot of research and came to the conclusion it really isn't very well suited for our purposes! Exactly for the reasons listed by many in this thread - the load limit is far less than the breaking limit, poor resistance to abrasion, it's not a fireproof as most believe, etc. The other big issue is that most knots weaken aramid based cord far more than traditional ropes - like nylon - and reduce the safety margin even more. The bowline knot for example can reduce the strength of kevlar by 50% or more, compared to about 30% for normal ropes. And according to a couple of studies I read, if kevlar is 'shock' loaded, it can suffer internal damage to the fibers that is very difficult to detect but can reduce the strength by a significant portion, leading to unexpected failure. Kevlar also hates any kind of a sharp bend - it will lose a significant portion of strength similar to a knot.

BUT, also as stated in this thread, it's very easy to get a 100:1 or greater margin with kevlar, and it's size and weight allow for easy packing and use in model rockets. Even 200lb kevlar easily fits in a model rocket that weighs less than a pound, and provides such a large margin of safety that a lot can go wrong before it will fail. So in that respect it works very well for rocketry. When using proper termination techniques and protecting the cord from direct flame and abrasion, kevlar can be a good choice for our use.

Unfortunately, most of us learn these lessons the hard way - there should be a 'everything you need to know about kevlar and rockets and don't want to learn the hard way' document that is required reading for all new rocketeers.


Tony

Unfortunately some of the links I used for kevlar comparisons are no longer working, but here's a post that shows how knots behave in two different kinds of ropes. In the chart linked to below, the graph shows the percentage of listed load that the know failed at - you'll see that Dyneema, which is similar to kevlar, simply does not like knots. With a bowline knot, it failed at 35% of the listed load limit! It also shows that a double fisherman's loop breaks at about 40% in dyneema, but is far stronger with marlowbraid, surviving until 82%. I realize neither cord listed is kevlar, but it's still instructive on how knots affect cord strength:

https://www.rocketryforum.com/threads/tying-knots-in-kevlar-cord.169359/page-2#post-2228022

 

[cls]

The bowline knot for example can reduce the strength of kevlar by 50% or more, compared to about 30% for normal ropes.

Bowline is entirely the wrong knot for rocketry. They are meant to be tied and untied in any condition, dry or wet, under strain or not.

It's a good reference for knotted strength, though, so I appreciate your comment about that.

My favorite is doubled figure 8, both in nylon webbing, and kevlar. Theoretically keeps 80% of line strength, is untieable with a little effort, and never unties itself.

 

[neil_w]

Does an extended motor mount tube help protect the Kevlar, or does it just move the danger point to the end of the tube? In otherwords, does simply having the Kevlar exposed to the ejection charge a few inches further from the motor help?

 

[G_T]

Yep, I use the figure-8 variant mostly. I trust it more. Boline was what I was originally taught. Now I don't use it for much.

--- Ejection charge sprays hot gas and sparks. Sparks damage Kevlar. Hot gasses might as well, depending on temperature and duration.

 

[mh9162013]

In otherwords, does simply having the Kevlar exposed to the ejection charge a few inches further from the motor help?

It always helps to move anything (Kevlar, baffle, parachute, etc.) further away from the ejection charge...at least from the point of view of fire/ejection protection.

 

[mh9162013]

Here's a Youtube clip that shows a C BP Estes engine's ejection charge in slow motion. I'm posting it here to help illustrate what an ejection charge can potentially be like inside a model rocket.

 

[manixFan]

Bowline is entirely the wrong knot for rocketry. They are meant to be tied and untied in any condition, dry or wet, under strain or not.

It's a good reference for knotted strength, though, so I appreciate your comment about that.

My favorite is doubled figure 8, both in nylon webbing, and kevlar. Theoretically keeps 80% of line strength, is untieable with a little effort, and never unties itself.

Exactly, but in many past threads about knots it comes up as the 'go-to' knot for loops, which is why I point out as often as I can how much it reduces strength. I've had plenty of folks tell me how they've been using it successfully for years so 'it must not be that bad'. No, they've just always had so much extra strength margin that it didn't matter, but physics is physics.

Speaking of which, I've never seen a knot in kevlar that tests to 80%, most fail below 50%. In the chart below, Technora, which is comparable to Kevlar (but with improved wear resistance) fails at 40% of rated load with a figure 8 knot.


Tony

https://user.xmission.com/~tmoyer/testing/High_Strength_Cord.pdf
the light blue-green bar is rated strenght, the maroon bar is failure weight. You can see a huge variation in how the same knot performs across different materials, with Technora (improved kevlar) performing the worst:

 

[mh9162013]

So the primary drawback of the bowline knot is reduced strength?

I'm more concerned about the strength of the line in terms of withstanding abrasion wear and the ejection charge heat. Oh, and the Kevlar knot staying tied.

 

[cls]

So the primary drawback of the bowline knot is reduced strength?

I'm more concerned about the strength of the line in terms of withstanding abrasion wear and the ejection charge heat. Oh, and the Kevlar knot staying tied.

Personally I have had more trouble with bowlines untying themselves than with ultimate strength of any knots... In rocketry, sailing, climbing, etc.

Come to think of it, I also never had any trouble with unwelded eye bolts opening up. (Ducks for cover)