Kevlar Shockcord and Kevlar Sock

Hi All,

I was't sure where to put this post in the forum, but I figured that if anyone in the forum has used these products, it is probably you guys in the HP section.

In my next project, I plan to use some kind of reinforcement on the airframe. Originally I planned on just 2-3 layers of 6 oz. glass. However, after seeing the Kevlar sock from giant leap, I have to wonder about using it. Has anyone used this before? Is it as strong as regular Kevlar, about as strong as what thickness as the 'glass equivalent (8 oz., 10oz., etc...)? Is it really that easy to use? (If I use the kevlar, I will have two layers of 4 oz. 'glass over it)

Also, I am planning on using a Kevlar shockcord for the section that takes the brunt of the BP charge. Is 5/8 " tubular strong enough for a HP rocket, as strong as 3/4" strap nylon? Also, does it glue to MMT's as strong as tublualr nylon does, like in PML kits? Finally, does anyone know of anywhere to get this stuff cheaper than Magnum's cost of $1 per foot?

Thanks for any information you can provide.

David

I used the Kevlar sock once. I queried Giant Leap at the time about the strength and didn't get a definitive answer. Just that it would take many layers of glass to be equivalent. It seems *very* tough, but I luckily haven't had to test it It is also *very* easy to use, a lot easier than glassing. However, be aware that it sucks up epoxy like nothing I've ever seen, and hence gets quite heavy. And, you will need one layer of light glass to provide an outer covering since you can't sand the Kevlar.

I think the 5/8" tubular Kevlar is stronger than 3/4" tubular nylon but can't cite a reference. I don't know if the HPR strength of materials test site has tested Kevlar or not. It wicks epoxy very well and attaches nicely to glass and phonolic, but I haven't tried it on Quantum Tube. Whether is will work of course depends on the weight of the rocket. Giant Leap used to have weight recommendations on their site.

Since you mentioned Giant Leap - have you priced their tubular Kevlar? They are also now selling glass sock IIRC.

Just remembered, John Coker has a good description of the Kevlar sock.

Thanks Rstaff3, that information is quite helpful.

Based on that, I am going to use the 5/8" tubular kevlar to replace the strap nylon in this rocket. I wasn't so much concerned about how strong it was as I was concerned about how well it would bond the phenolic, but you have put that risk aside.

As far as the kevlar, it sounds like I will use one layer of the sock for the six feet of tubing that I will have. Best I can tell, it should be more than enough strength on a 3" tube. Because it is probably stronger than two layers of 6 oz. 'glass, I will only use one layer of 4 oz. 'glass as a veil over it. Do you agree that only one layer of 'glass is good enough?

David

BTW- The rocket is a heavily modified PML sudden rush. Lengthened airframes, fin can and MMT to accomodate K185, K550 and K700. Also, fin can filled with foam and reinforced with 'glass. RRC2 for recovery with trans. beeper and smoke canister. Plan to fly to 10K.

Sounds cool!

Some additional points. First, the 5/8" Kevlar is quite stiff. Make sure you can pack it into the airframe before you bond it to your rocket. All the normal guidance on length still holds, so make sure all that stuff will fit nicely. There is no problem in a 4" airframe, but I haven't tried it in a 3". Also, although it adheres to phenolic, I still prefer to the MMT attachment for my shock tethers. Finally, the Kevlar will tend to zipper, however your airframe should be real tough.

I'm not much on remembering the weights of glass, but I used a VERY thin veil. Not for strength, only for finishing.

Best of luck!

David,

I would definitely use one layer of Kevlar sock, covered by 2oz cloth for finish only...it will be extremely strong!

As far as Tublar Kevlar goes, Ross has it the cheapest at $1.00/ft as you noted, but it is well worth it! I have used the same 5/8" Tubular Kevlar for my L3 projects...this stuff is rated at 6000lb test...damn strong!!! Since it is only 5/8", it is much lighter than the 2" Equivelant strength Nylon Strap. I do suggest you use a Nomex sleeve as Kevlar is fire resistant but NOT fireproof.

Also, you would do much better getting into the practice of not gluing the shock strap to the motor mount as PML does...I have several PML kits and have since swayed from that practice. I install a U-Bolt instead and the shock cord is attached with a quiklink...this way you can remove it if necessary and replace it if worn.

Something else to consider...Ross also sells Kevlar Strap...about 1/2" wide for less than half the price of the Tubular stuff. I have used this strap with all my L2 birds and others rockets up to 17lbs and it works well. Just use the Nomex sleeve and ALWAYS go with 5 times the total rocket length for shock cord length...PML usually only gives you the minimum of 3 times.

Good Luck,

Carl

Hmm...you have given me plenty to think about.

First, thanks for the heads-up on the stiffness on the kevlar. The fore and aft airframes are each lengthened by at least 4 inches. The pistons will be removed and I will only be using 5 feet of the kevlar out each end with nomex protectors over it and nomex parachute protectors at the ends of where the nomex shockcord protectors end. Then, I'll have a quick link and about 20-25 feet of 9/16" TN. So, its not like having 25 feet of kevlar...a lot more room needed for that.

I would like to attach the shock cord via a u-bolt, which I have done previously on larger diameters, but because I am using CPR, I doubt there is enough room on a 3-1.5 centering ring. However, my whole point of using kevlar instead of TN is to make sure the shock cord can stand many flights without replacing.

I doubt the tube will zipper with the 5/8 kevlar against kevlar reinforced airframe but if I am really in doubt I will downgrade to 1" strap nylon (wider to reduce zipper chance) and just make sure the nomex is secured really well.

Also, I think I will use either 2 or 4 oz. 'glass. 2 oz. would be preferred from what I have heard but I may already have 4 oz. and so I may use it just so I don't need to buy more...price for project already getting up there...

David

One other thing...anyone know where I can get a small cold smoke canister?

Thanks

David

Rethinking it, with a long airframe the stiffness of the Kevlar shouldn't be a problem. Worst case it takes just a tad longer to prep the rocket as you have to coil the Kevlar into the airframe. And I doubt zippering will be an issue.

The thicker cloth will be fine. Its just that thicker cloth may require more finishing effort...unless you are like me and start with the intent of having a perfect finish and half way thru decide that if it looks good from 10 feet away, then that is good enough

After seeing the damage to some of the shock cords from rockets which have been around the block a couple times I wanted to make sure mine lasted. What I do now is I have bought a sheet of nomex and expoxied it into a tube. This tube of nomex is slid down over the shock cord where the cord meets the mmt. I noticed from multiply delay charge burns many shock cords would fail deep inside the rocket. Now with the nomex acting as a sheath over the shock cord the cord will not fail as easily, I just replace the nomex. And with using quick links on everything, after recovery I can slide the sheath off the shock cord and use it on another rocket. No more shock cord attachment failures.

After seeing the damage to some of the shock cords from rockets which have been around the block a couple times I wanted to make sure mine lasted. What I do now is I have bought a sheet of nomex and expoxied it into a tube. This tube of nomex is slid down over the shock cord where the cord meets the mmt. I noticed from multiply delay charge burns many shock cords would fail deep inside the rocket. Now with the nomex acting as a sheath over the shock cord the cord will not fail as easily, I just replace the nomex. And with using quick links on everything, after recovery I can slide the sheath off the shock cord and use it on another rocket. No more shock cord attachment failures.

Yahoooo!
My modified kit from PML and reinforcing materials from Giant Leap are on their way and should be here today or tomorrow according to Jim at DHC.

So, because the first step in the build is applying the kevlar over the tubes, I have a few questions;

At what point in the curing of the epoxy should I cut the remaining kevlar off the ends of the tubes?

Should I wait until it is fully cured or not?

Also, anyone know what the best tool to cut it off with is? I have a fiberglass reinforced dremel cutting disc but since kevlar is so strong, will I need more...diamond disc?

Thanks all.

David

Mav,

Depending on the epoxy you use for the time...ya might just have to keep an eye on it...trim it off while it is still "green" .

That means when it is no longer tacky (sticky) but still kinda soft. A nice sharp disposable knife works great. It'll cut real easy at this point.

If you wait till it is cured, it gets really hard AND SHARP!...the edges can cut ya real good!

Use a disposable knife, the $.50 kind, 'cause it might be a little gooey yet and ya don't wanna gunk up a nice Exacto knife.

sandman

Sandman's suggestion agrees with my experience. If you get any 'hairs' after trimming, soak them in CA and sand lightly.

Alright, I am using 20 min. finishing resin which actually takes about an hour to get to the point of softness that you mentioned (if I remember how it behaved last time I did some 'glassing). So, at that point, I will use a cheap blade in the utility knife to trim it off. Then once fully cured, sand the edges smooth.

Sounds good.

I am covering the kevlar in 5 oz. 'glass (that's what I have), so, what would you recommend for finishing; just sand the 'glass smooth or apply a filler first and then sand?

David

Definitely use a filler, dont sand into your layer of 'glass!


rocwizard

I hang with a bunch of old codgers that build competition rockets that usually place well in European Cup and World Cup competitive events. They can put a sub-one ounce rocket in the air with an A motor and keep it up for five minutes.

Their unanimous opinion is that we HPR guys never see the real performace our rockets are capable of because we way over-build them. I tend to agree.

I have this Kevlar cord in my tool box that is less a quarter of inch in diameter and has a test strength of 1000 pounds and a breaking strength of 1500 pounds. Can somebody tell me why they would need more than that kind of restraint?

OK, you are playing to the grandstands so you deploy at the bottom of the ballistic arc and use a metal mesh reinforced fighter plane drogue 100 feet off the deck... then use >two< quarter inch Kevlar lines in that case ;-)

Or do you guys use a whole lot more BP in your ejection charges than we do? ;-)

Same deal with the reinforced tubes. I've flown supersonic unreinforced rocket tubes but I grant you that the fins were very well glued on and strengthened with GRP. If you build a rocket with a Kevlar-reinforced tube do you stop worrying about non-deployment? ;-)

Paul