Wondering "Why Kevlar?"

I use Kevlar because of it's resistance to heat. The shock loads imposed on it are easy to manage within the diameters used. Quarter inch Kevlar cord will take a much higher load than anything I'll ever subject it to. The same is true with eight and sixteenth inch material. I can imagine that stopping a 180lb man falling down a cliff face would require some stout line. However, mod rockets are relative feather weights. For me, the charring and burn through issues are paramount and Kevlar solves them right now.

I've been selling Kevlar for shock cords for eight years now, and using Kevlar yarn in my own kits. In that time I've had two reports of cords breaking. One I was never able to track down, the other was the edge of a fiberglass body tube that managed to saw through the Kevlar on the way down. So I don't think strength is a major issue. My kid brother, who runs the engineering department at a college in Pennsylvania, decided to find the breaking strength of my large Tubular Kevlar a few years ago. It let go at 10,000 pounds, but he doesn't consider it a valid test because it broke at the knot.

Heat resistance is far more important. Nylon cords are strong, but heat weakens them, and the only way to tell that you had a cord with a heat-weakened spot is that's where it broke.

I'm not saying your spelunking tests are irelevant or disrespecting your work in any way. I just want to point out that there are different grades and compositions of Kevlar cord. Even the difference between Kevlar thread, yarn and braid is substantial. You were testing specific grades of cord for a specific purpose. We've done a lot of testing of our cords, not to mention adjusting of specs...that's why we buy Kevlar cords from three different manufacturers.

G. Harry Stine was the one who convinced me that stretchy shock cords were not a wonderful idea. Yes, they absorb energy...and they use it to pull your nosecone back toward the body, sometimes smacking into it. IIRC, in small models Harry used carpet thread for shock cord. Bill Stine's Quest rocket kits may well be the ideal compromise: he gives you a length of Kevlar to secure into the rocket, which you tie to a length of elastic to complete the shock cord. Best of both worlds.

Rope and cord technology has developed to an amazing degree in the last few years. Some of the literature I get from suppliers is amazing. I can get cords with unthinkable tensile strength, heat resistance, you name it...for ten bucks a foot! NASA can afford to make Mars lander parachute shrouds out of that stuff, but somehow I don't think rocket modelers will rush right out and buy it.

Sorry to blather; I hope this contributes to the discussion. Doug Pratt dad-at-pratthobbies-dot-com

Its high heat resistence makes it good near the motor ejection and overkill on rating seems to be the procedure to overcome brittleness. There is a cottage industry in sewing loops in Kevlar ribbon.

Jerry

Strength is a relative term. Kevlar is about 10 times stronger than

6061-T6 aluminum and 5 times stronger than 302 stainless steel, but is more brittle than both. As a VERY general rule of thumb, as ultimate tensile strength increases, ductility decreases. And for fibers, flexibility and durability depended on the diameters used. As diameter decreases, flexibility (as measured by bend radius) increases according to d^-4.

So for some applications, kevlar is great... bullet proof vests only have to work once, and a lot of the fibers will be destroyed. But for other applications, it's not too hot. For rock climbing, you'd probably want something that had a very prolonged ductile failure mode... something that could undergo a partial failure, but still bear lots of weight. That way you'd catch in-progress failures with inspections. By comparison, kelvar doesn't fail ductily, but undergoes brittle failure (ie all at once).

Dave

Kev> Hi folks,

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As others have said, heat resistance and thus durability when repeatedly exposed to ejection gases is the main reason. Kevlar is normally used as a 'leader' (as is steel wire sometimes) for an elastic (for model rockets) or nylon (on larger rockets) shock cord, which will act to absorb some of the energy of deployment. I don't think many people use kevlar alone for shock cords, but even so 1mm kevlar thread rated at 100kg is unlikely to encounter that sort of force in a model rocket weighing a few ounces and hopefully deploying at a low speed.

Kevlar fabric is also sometimes used for body tube reinforcement, sometimes in combination with CF.

As others have allready mentioned the heat resistance properties, however its normal practice to also attach a long (as much as 4-7 times the length of the rocket) shock cord of various stretch materials, this helps absorb the energy of the deployment charge on the rocket engine. this is just a basic explaination of the system.

Yes, knots are important. But the whole point of shock cords is to absorb the shock loads. Transferring 10,000 pounds (or whatever) of force through a MR/HPR shock cord would tear the parachute to shreds, and/or rip the anchor out of the airframe.

Even icons can be wrong.

Much better!

So is there a new type of really elastic Kevlar fiber available now?

Alan

I would say the whole point of (misnamed) shock cords are to keep the rocket together, not act as shock absorbers. Asking them to serve as bandaids for poorly implemented recovery systems, and/or to maintain integrity on intentionally ridiculous ballistic flight paths, is why they fail.

May I recommend the Knacke bible:

Properly designed and deployed, the only "load" on the Kevlar is the weight of the rocket, dangling under the canopy. There should be no real "shock" to the cord, otherwise you risk damging not only the cord, but the attachment points, as well.

Kevlar's benefits are heat resistance and durability. Nylon works, but has to be replaced any time it's exposed to heat, as it weakens it dramatically.

-Kevin

I'm sure that it was 10,000 psi, not pounds force. (just clarifying.) I tested some of your kevlar and found an average of about 235 lbs-force when knotted, and about 518 lbs-force without a knot. (This was your "TKM" Kevlar thread.

I submit that the test was valid, in that most people use knots to secure their kevlar thread/string. It shows what an actual real world strength application would be.

Doc

I've always wondered what would happen is you were to braid Kelvar thread around a rubber core. Sort of a Kelvar bungee cord. The rubber will likely degrade, but the Kelvar will probably not let it break.

TRUE kelvar fibers (the kind used to reinforce composite materials) are rated to about 3.5 GPa, which is about 500,000 psi (yes, stronger than steel). For giggles, assuming it was a kevlar cord that held to 10,000 lbs and broke due to tensile failure rather than a stress concentration at the knot, then the cord only had to be .16 inches in diameter.

Yes, Kevlar(tm)'s resistance to heat and the ejection charge is the primary reason I use it. The technique I use is one Doug Pratt mentioned in another reply: I use enough Kevlar to get a loop that goes to the top of the body tube, and I use some cheaper material (typicly nylon) for the rest of the shock cord.

The way I use it, the kevlar is not the weak point in the recovery system.

The last time I went looking for sources of Kevlar, I found a kite shop in Colorado (in to the wind?) that advertised not only kevlar but sheathes for the kevlar. I got the impression that they always put a sheath around the kevlar where it contacts other items and where there are knots. Kevlar can cut other material, and itself. I suspect this is part of why the knots fail. I use a knot (if I recall correctly, a figure 8 knot) that I learned from a HPR magazine article from a few years ago.

The inside end is epoxied to the motor tube, and I usuall make sure there is something holding the kevlar against that tube. If using wood centering rings, I just rely on that. The kevlar-to-mmt bond holds well when the forces are longitudinal (along the motor tube) but the tube usually tears if there is much lateral force applied to it.

One alternative I've tried was stainless steel fishing leader. I anchored the leader to the motor mount centering rings and bent a loop near the top of the tube. While that has proven more durable than nylon, it's very inconvenient to pack a parachute, parachute protector and shock cord around it. I'm suprised (or maybe just lucky) that I haven't had any snags.

Glen Overby, KC0IYT Twin Cities, MN

I've had fishing leader burn, and break from kinks. Kevlar is a better choice.

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!

That is interesting. I think you first put me on to using the steel fishing leader. I only use the nylon coated steel leaders. I have not had to replace one yet. They do take a little more care in packing. To be fair, I don't fly much big (MR) stuff, and I don't install many leaders. I also don't think that I have used them with motors with pronounced "afterburn" effect. I'd guess both work well enough. Maybe the good Doc should do some testing and see how many hot flash/yank cycles each is good for.

Alan

Have a look at my rocket materials website at

and have a look at the shock cord testing page.

I'll put that on the list of tests to do. I'm backed up right now, so it's getting pretty long!

I couldn't help but to post a reply on this thread... yes - I read these too.

Before I had ever heard of the company "Quest Aerospace" I was buying Kevlar (some off ebay and some from Doug) to replace my student's elastic cords...

While the technical reasons have been well examined here my reason for using Kevlar so supplement the elastic was a bit more simple and basic: I was tired of seeing little kids who had worked REALLY hard on their model rocket projects get to the 3rd launch of the season and *pop* - the nosecone and parachute flies south (or whatever direction the wind was THAT day) and the fusilage "prangs" into the earth....

And - while I believe rubber is a great improvement and I compliment E on changing over - I cannot help but believe that the kevlar/elastic method is better in the long run. Present affiliation aside of course.

Matt Costabile Quest Aerospace

A few points:

1) The Estes rubber cords are indeed more durable than the elastic they were using, BUT they could have used a better elastic and longer elastic.

2) The rubber cords are the reason we have had the "Estes style shock cord mount" originally made from card stock and now made from thin paper in the instructions. Rubber will not 'glue' well, so it must be encapsulated into the folded over mount. This creates a great big recovery system clogger since it is glued inside the body tube near the top.

3) The Quest method is superior. It is insanely easy to attach/anchor the Kevlar cord (i.e. you tie a knot). The elastic then adds more length and does absorb some of the shock of ejection. (I have not opened my Courier, but I see that it uses dual chute recovery to solve the problem of absorbing HUGE shock loads in the elastic.)

4) The Kevlar will not deteriorate much over time and use. If the elastic ever wears out, you can tie on a new piece. Again: superior.

5) You mis-spelled "MicroMaxx" in the 2FEB Qblog. 2 x's.

6) On page 8 of the PDF Quest Educator catalog, the distiller seems to have eaten the "

" and spit it out with the last w sitting on top of the first dot.