Kevlar Thread

[Launch Lug]

Was wondering if Kevlar thread can be used in a LPR as shock cord. I'm building a Baby Bertha and don't want to use the elastic cord that comes supplied with the kit. I have a long length of Kevlar thread (10') and thought if I use a long enough piece it would work for shock cord. Let me know your thoughts.

Thanks

[GregGleason]

Kevlar thread will work. It's a good idea to put some masking tape around the thread, at the point where the middle of the tape is halfway out of the tube. The extra surface area of the tape will help prevent zippers. Just place the chute above the taped part of the thread, so that the chute is not obstructed by it on the way out.

Greg

[Montaro]

I'm start to use only kevlar in my cardstock rocket and is working fine !

The last one I use elastic cord, but i didn't flight it yet.

Regards!

[Launch Lug]

Baby Bertha is just over 12.5" so would a 40" length (past the tube edge) be adequate? That's a little more than 3X rocket length.

[Rocketbuilder]

Baby Bertha is just over 12.5" so would a 40" length (past the tube edge) be adequate? That's a little more than 3X rocket length.

Based on my experience, that should be fine. If you have the elastic cord from the kit, but just don't want to use it, consider adding at least part of it on to the end of the kevlar cord. You probably don't need it, but it seems like it would save unnecessary stress on the rocket.

[Pem Tech]

All Pem-Tech LPR kits come with 5 feet of 150lb KEVLAR thread....
Only the best for our babies.
With sufficient length the only probable issue is zippering but Greg already covered that one. A wrap or three of tape where the thread contacts the BT and it is no longer an issue. As Rocketbuilder corrected stated, some people are concerned about the lack of elasticity and that is a valid concern. It has been our experience (like his) that with a sufficient length of KEVLAR in LPR rockets the lack of elasticity doesn't put any undo stress on the airframe. Personally, I detest elastic or rubberband shock cords and have been flying KEVLAR only since 2003. In that time I have never had KEVLAR or components fail due to recovery stresses. (Your milage may vary) In all that time only two KEVLAR shock cords have failed and both of those were due to motor CATO's. (Please, nobody tell Fred I used the "C" word.)
In short....
40" sounds like plenty, Go for it!

[3boydad]

i find that using the elastic on the end of of the kevlar prevents zippers. it give the stretch when it needs it and can be replaced. i have not tried usin masking tape around where the kevlar would touch the BT but i will now.

[JPVegh]

I like to use a 50/50 kevlar/elastic shock system. I usually attach the elastic to the kevlar through a barrel swivel. I like the strength and heat resistance of the kevlar and the stretch of elastic. On smaller rockets though (T motors and MMX) a pure kevlar shock cord is what I normally do, it just saves so much space in the tight airframes and the lighter rockets don't need the stretch so much.

I suppose this is one of those deals where individual rocketeers have developed a system that they like and works for them so they go with it.

I have seen some kevlar failures on some of my older Quest rockets. The failure point is always down by the motor so I have to believe that it is some how related to heat, corrosive residues or mechanical abrasions from ejection. I suppose twisting from the recovery system could have something to do with it as well. Perhaps it is a combination of all of the above combined with the age of the kevlar.

[Rocketbuilder]

What I've done to protect (braided nylon) shock cords that are attached to the motor mount, Quest style, is to take a fairly long piece of heat-shrink tubing, and put it around the shock cord down next to the motor mount. I use a hair dryer to shrink both ends, leaving the middle un-shrunk so the air inside can act as further insulation.
Harbor Freight sells a pack of a bunch of pieces of heat-shrink tubing in different sizes for a couple of bucks. One of these will supply you for quite a few rockets.

[Micromeister]

I've been building Micro, LPR & MPR models with Kevlar/elastic combinations since Kevlar became available to the public. Rarely do any of my LMR models up to 3.3lbs need any Kevlar line greater the 100lbs. I really perfer 70-90lb kevlar for straight kevlar shocklines in Micro models. Standard Model Rocs up to 1 to 1-1/2lbs I've found 36-48" of 90-100lb kevlar with another 36" of 1/8" Oval elastic makes the perfect shockline anchored around the motor mount or motor stop ring. LMR's up to 3.3lbs (1500g) still get 100-130lb braided kevlar 48 to 72" long with 3 or 4 feet of 1/4" flat elastic.

Since using these sizes and amounts I have had No shockcord failures or zippered body tubes using mostly Estes .013" and .021" wall thickness.
On Larger models or those that I feel might have the potential for Zippering a simple 1/2" x 3/4" doubled over piece of plain masking tape centered on the Kevlar just inside the forward body tube with the kevlar stretched tight prevent all zippering.

Heres a few examples...hope they helps.

[blackbrandt]

I go at least 15 feet on all of my D powered models. I can use a smaller chute and I can grab the rocket easier in the inevitable brier patch.

[luke strawwalker]

I like to use a 50/50 kevlar/elastic shock system. I usually attach the elastic to the kevlar through a barrel swivel. I like the strength and heat resistance of the kevlar and the stretch of elastic. On smaller rockets though (T motors and MMX) a pure kevlar shock cord is what I normally do, it just saves so much space in the tight airframes and the lighter rockets don't need the stretch so much.

I suppose this is one of those deals where individual rocketeers have developed a system that they like and works for them so they go with it.

Yep, that's what I do too when I don't use a teabag...

I have seen some kevlar failures on some of my older Quest rockets. The failure point is always down by the motor so I have to believe that it is some how related to heat, corrosive residues or mechanical abrasions from ejection. I suppose twisting from the recovery system could have something to do with it as well. Perhaps it is a combination of all of the above combined with the age of the kevlar.

That, and probably glue impregnation causing a "weak spot" where the cord comes through the ring area... I was watching one of Tim's videos the other day on the Apogee site and he was running his kevlar cord right up past the end of the motor tube... I always run mine through a notch in the centering ring out by the BODY TUBE WALL rather than right beside the end of the motor tube-- less abrasion from the blast of clay bits and also less heat out there, being further from the motor tube end where the hot ejection gases are blasting out... just seems like a no-brainer to me...

Later! OL JR

[Launch Lug]

Thanks for the replies everyone. I feel I need to further elaborate on the thread that I actually have in my possession. I purchased it from Apogee and it is the red thread that is often used for suspension lines on chutes in place of the heavy cotton string. I bought some to make parachutes and had some left over. So again this is the Kevlar Thread NOT Kevlar Cord. I already attached it to my MM so I hope its still good to use. If not no big deal...

[Micromeister]

If you bought it from Apogee; ask them what the break strength is on your "thread". Most of the Kevlar lines I use are Twisted Thread not braided lines from www.thethreadexchange.com with most of my braided kevlar coming from Pratt Hobbies.
Twisted thread works just as well "break wise" but is a good bit stiffer until worked in some. Braided will out-last Twisted due to the lessening of decomposition that occurs each time ANY kevlar is subjected to temperatures at or above 482°C-900°F which our ejection charges easily do.
Remember Kevlar is Heat resistant...not flameProof. While it does not burn..parts or sections of the line exposed to these temps do decompose at 900°f and above.

Whenever using kevlar keep the thread or line as far removed from the heat source as practical. That's way Apogee's shockcord video is so wrong. NEVER let kevlar line come though the forward Centering ring at the Motor tube forward end. Always take the shockline out to the outer edge if the centering ring at the outer airframe/ring joint. Even on minimum diameter models with just a motor block this little extra space helps extend the shockcord life.

This is exactly why I've started using a Stainless Steel Leader, small Stainless avaition cable or multi stranded Stainless Beading wire as Shockcord Anchors on many models. Just a 6" or 8" stainless anchor with end loop further removes the Kevlar line from most of the ejection heat without interferring with laundry ejection.

[Launch Lug]

Well I found my answer in real life testing... 40" of kevlar thread (past the end of BT) was not long enough . I took the Baby Bertha out last night with my wife and 19 month old daughter looking on and on the second flight the thread broke at ejection leaving the main body free falling back to earth and the nosecone being carried away by the still attached chute. Everything was retrieved and in tact. The first launch was flawless, on the second launch I watched for the ejection to see how far the nosecone blew from the main body. I was curious to see if the full length of the shock cord was used... it was. It broke about 1" below the top of the body tube. I'm guessing the break strength of the thread was not enough to see the full force of the ejection. I'm sure if I had used a longer length of thread it would have been just fine. Oh well, it was still fun to launch and I learned a lesson here. Now to figure out how to replace the shock cord with something a bit heavier.

[luke strawwalker]

Well I found my answer in real life testing... 40" of kevlar thread (past the end of BT) was not long enough . I took the Baby Bertha out last night with my wife and 19 month old daughter looking on and on the second flight the thread broke at ejection leaving the main body free falling back to earth and the nosecone being carried away by the still attached chute. Everything was retrieved and in tact. The first launch was flawless, on the second launch I watched for the ejection to see how far the nosecone blew from the main body. I was curious to see if the full length of the shock cord was used... it was. It broke about 1" below the top of the body tube. I'm guessing the break strength of the thread was not enough to see the full force of the ejection. I'm sure if I had used a longer length of thread it would have been just fine. Oh well, it was still fun to launch and I learned a lesson here. Now to figure out how to replace the shock cord with something a bit heavier.

Think you'd do much better just adding some elastic in there somewhere... I know a lot of folks just use straight kevlar, but it has ZERO elasticity, and when the shock hits, it will at some point find the weakest spot and break it. Elastic "gives" some and absorbs some of the energy, minimizing shock on the system and the cord itself.

I guess if you're willing or want to use rediculously long lengths of kevlar string through, that's a personal choice... whatever floats yer boat I guess...

Good luck on your repairs! OL JR

[Micromeister]

JR is correct twisted Kevlar thread has nearly 0% stretch. that's why most of use use 36-48" of 70-100lb kevlar with another 30-36" of 1/8" Oval elastic attached to the forward end. 'this gives the NC enough lenght and stretch to slow to a stop while the body traveling along behind catches up. Since switching to these shock cord lengths I cant remember the last time I've had a shockcord seperation.

Others use rubbler bands to bunge the kevlar ling which causes more drag on the line as it extends slowing the NC.

[JPVegh]

Hmmm a thread about kevlar thread or a kevlar thread thread. I think a portal just opened to the 8th dimension, beware the Lectroids.

[Montaro]

I'm with JR too, I just try my Zenit 2M last saturday the shock cord was 2 time the lenght of the body tube with a 8 Inch of estes elastic cord and works great, the elastic cord is in the body tube using the tri-fold the rocket flew on 4- D12.

Body length: 34"
Nose Cone: 13"
body Width: 3 1/3 "



My 2 Cents !!!

[DizWolf]

If you want to increase break strength, take 3 lines of the thread and braid them together. I did that for the leader on my onyx, and then a few feet of elastic