# Shock cord material for lpr/mpr

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#### AtomicStorm

##### Well-Known Member
I feal like a total noob asking this but why does everyone recommend kevlar shock cords? I know they are strong but isnt that too much shock? Doesnt it need to bungee like the elastic or nylon cordage? Please explain what is best because i need the best on my rockets asap lol

#### David Schwantz

##### Well-Known Member
TRF Supporter
Elastic, rubber, or bungee can cause the NC to slam back into the booster or payload section as it "snaps back"Kevlar has small chance of that. Is heat resistant, strong and is a cool color
You need to have the length correct as then there is no shock as the force has diminished before it hits the end of the cord.

##### Well-Known Member
TRF Supporter
I feal like a total noob asking this but why does everyone recommend kevlar shock cords?
That one is easy - Kevlar is strong, fire resistant, and has become pretty cheap.
Like this source:

I know they are strong but isnt that too much shock? Doesnt it need to bungee like the elastic or nylon cordage?
It's not "too much" shock if it doesn't break the shock cord, nor the anchors to which said cord is tied. Braided Kevlar satisfies these requirements in spades.
Nylon cords are, unfortunately, lack all the positive characteristics of Kevlar: they burn, then can break from over-stretching.

Case in point - I made the mistake of flying my first Estes 1:200 Saturn V with the stock daisy-chain of Kevlar and nylon. One the second ascent with a Quest motor, the nylon broke, and nose cone drifted into the next zip code. The bulk of the airframe came down ballistic, narrowly missing a parked car.
You can bet I had gone back and 3x checked all my Low-power rockets to purge them of any remnants of nylon or stretch cords.

HTH,
a

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#### AtomicStorm

##### Well-Known Member
Finally i understand, thankyou! So how long of a piece do i need from the nc to the top of the bt? Like 6ft?

#### teepot

##### Well-Known Member
TRF Supporter
Finally i understand, thankyou! So how long of a piece do i need from the nc to the top of the bt? Like 6ft?
That's going to depend on how big the rocket is and what motor you use. I fly MPR and HPR and my shock cords are 15" to 20" and a lot of others would say that's to short. If it's a HPR dual deploy that's 40' of shock cord. In general it should be long enough so the parts don't hit each other and the nose cone runs out of steam before the cord is tight.

#### AtomicStorm

##### Well-Known Member
That one is easy - Kevlar is strong, fire resistant, and has become pretty cheap.
Like this source:

It's not "too much" shock if it doesn't break the shock cord, nor the anchors to which said cord is tied. Braided Kevlar satisfies these requirements in spades.
Nylon cords are, unfortunately, lack all the positive characteristics of Kevlar: they burn, then can break from over-stretching.

Case in point - I made the mistake of flying my first Estes 1:200 Saturn V with the stock daisy-chain of Kevlar and nylon. One the second ascent with a Quest motor, the nylon broke, and nose cone drifted into the next zip code. The bulk of the airframe came down ballistic, narrowly missing a parked car.
You can bet I had gone back and 3x check all my Low-power rockets to purge them of any remnants of nylon or stretch cords.

HTH,
a
That really sucks, so can i make the lines for my 1.1 ripstop chute out of the same kevlar cordage? Which dia would you go with?

#### AtomicStorm

##### Well-Known Member
That's going to depend on how big the rocket is and what motor you use. I fly MPR and HPR and my shock cords are 15" to 20" and a lot of others would say that's to short. If it's a HPR dual deploy that's 40' of shock cord. In general it should be long enough so the parts don't hit each other and the nose cone runs out of steam before the cord is tight.
They are all lpr to low mpr.
I have a stock built mean machine 24mm.

#### AtomicStorm

##### Well-Known Member
I need to know what size kevlar cord to get. 100lb., 150lb, 300, 400,? Like i said the rockets are lpr to low mpr and if i can get 1 spool to make my parachute lines and shock cord that would be great because im far from wealthy lol

#### teepot

TRF Supporter
There is a thing called working load limit. Which is the maximum limit for safe use. Exceeding it can cause premature failure. For instance 550 pound kevlar has a WLL of 110 pounds. Your WLL is a function of the weight of the nose cone and the speed at which it's ejected. That's why a long shock cord is recommended. If your nose cone is a few grams you could probably use 100 pound kevlar cord. The thinner it is the more likely it is to burn through though. So overkill is better. I regularly use 1500 pound and 2000 pound kevlar with a shock cord protector made of Nomex and my rockets weigh between 2 1/2 and 6 pounds. The kevlar 100 lb cord is about a 32nd[1 mm] of an inch thick the 550 is about a 16th. 100 lb kevlar is $9 for 100 feet on Amazon. The 250 lb [1.5 mm] is$12 for 100 feet. I use a brand called Emma Kites. There is also kevlar thread. 100 lb thread is about $21 for 1000 feet. I'm sure others here will have different suggestions. #### bill_s ##### Well-Known Member The physics say the peak forces will always be FAR higher, all else equal, for a shock cord with almost no stretch. Real life may differ, and the trend has been against elastic, which does have a lot of failure modes. I guess a nasty zipper still beats a crash, and you can overbuild easiest with kevlar, strong and compact. I still use elastic and think it is good, but have had a couple of problems which may or may not have survived with only kevlar instead; it all benefits from experience. You should use backup in parallel with any elastic on larger, more dangerous rockets. I've also tended towards shorter, thicker elastic in series with extenders, so it doesn't just bottom out and has higher strength if it does. With LPR I have come to actually like the Estes (silicone) rubber bands, just add some string to extend it for less chance of dent. Most of my "Estes dents" have actually been CTI dents though, those Cesaroni ejection charges are double of AT's 29 mm Hobbyline. One thing is I wouldn't swap out the elastic in a larger kit without also swapping a plastic parachute also for something stronger.... If everything's strong enough, who needs elastic, just drag that chute through the air at high speed. You will want zipper protection and maybe fiberglass though. #### neil_w ##### Good at some things TRF Supporter I do Kevlar from MMT to just short of the end of the body tube, and then braided elastic from there to the nose cone. No problems. I'm not convinced that an all-Kevlar solution is objectively "better", more a matter of preference. #### AtomicStorm ##### Well-Known Member I think im going to go with the continuous kevlar cord with a 1 or 2ft section of nylon tied into it. Couldn't find any pics and its hard to describe but im sure you guys know what im talking about. Does anyone use this stuff w/ kevlar? #### Attachments • 55.3 KB Views: 23 #### AtomicStorm ##### Well-Known Member There is a thing called working load limit. Which is the maximum limit for safe use. Exceeding it can cause premature failure. For instance 550 pound kevlar has a WLL of 110 pounds. Your WLL is a function of the weight of the nose cone and the speed at which it's ejected. That's why a long shock cord is recommended. If your nose cone is a few grams you could probably use 100 pound kevlar cord. The thinner it is the more likely it is to burn through though. So overkill is better. I regularly use 1500 pound and 2000 pound kevlar with a shock cord protector made of Nomex and my rockets weigh between 2 1/2 and 6 pounds. The kevlar 100 lb cord is about a 32nd[1 mm] of an inch thick the 550 is about a 16th. 100 lb kevlar is$9 for 100 feet on Amazon. The 250 lb [1.5 mm] is $12 for 100 feet. I use a brand called Emma Kites. There is also kevlar thread. 100 lb thread is about$21 for 1000 feet. I'm sure others here will have different suggestions.
You want to go halvsies on 1000ft? Ill buy it all and you can paypal me and ill ship it to you. I think im going to go with the 200lb and i may even braid a few of them together. Going to do about 15ft. on each rocket.

##### Well-Known Member
TRF Supporter
Finally i understand, thankyou! So how long of a piece do i need from the nc to the top of the bt? Like 6ft?
The rule of thumb that I heard somewhere, and now follow, is to have shock-cord length be at least 3x that of the full airframe.
I usually go longer on heavier HP rockets.

I need to know what size kevlar cord to get. 100lb., 150lb, 300, 400,? Like i said the rockets are lpr to low mpr and if i can get 1 spool to make my parachute lines and shock cord that would be great because im far from wealthy lol
Different sizes for different airframes.
For low-power (<10 oz rockets), I go with a spool of 150lb braided Kevlar.
For mid-power (<2.5 lbs rockets), I go with a spool of 700lb braided Kevlar.
For high-power, I work off weight, and usually rely on Nylon or Kevlar harnesses of varying width, some tubular.

HTH,
a

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#### teepot

##### Well-Known Member
TRF Supporter
You want to go halvsies on 1000ft? Ill buy it all and you can paypal me and ill ship it to you. I think im going to go with the 200lb and i may even braid a few of them together. Going to do about 15ft. on each rocket.
I already have 1000 feet. I use it to sew shock cord protectors. I figure I have a life time supply.

#### AtomicStorm

##### Well-Known Member
I already have 1000 feet. I use it to sew shock cord protectors. I figure I have a life time supply.
Already bit the bullet and got a lifetime supply. Lol unless i go camping with my old eagle scout troop.

#### Ez2cDave

##### Well-Known Member
TRF Supporter
You will have to worry about Kevlar "Zippering" your airframe, if it is too short.

Dave F.

#### shockie

##### High Plains Drifter
My lpr/mpr shock cords are 1/2 100lb test braided kevlar
and 1/2 94lb test nylon bungee cord aka paracord

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#### AtomicStorm

##### Well-Known Member
You will have to worry about Kevlar "Zippering" your airframe, if it is too short.

Dave F.
Im going to try to build a baffle to attach the cord to, if not i will most likely do the ear plug method.

#### icyclops

##### Well-Known Member
I feal like a total noob asking this but why does everyone recommend kevlar shock cords? I know they are strong but isnt that too much shock? Doesnt it need to bungee like the elastic or nylon cordage? Please explain what is best because i need the best on my rockets asap lol
I actually use the thick elastic 1/4” - 3/8” you buy from a fabric store. As I find that there is less zippering and rip-out than kevlar....if you use enough length of elastic you will have no problem with cone blowback. This just works for me in LP.

#### jrap330

##### Retired Engineer, NAR # 76940
TRF Supporter
I do Kevlar from MMT to just short of the end of the body tube, and then braided elastic from there to the nose cone. No problems.

I'm not convinced that an all-Kevlar solution is objectively "better", more a matter of preference.
Concur, how could all kelvar be superior if it induces the problem of zippering which too many rocketeer s write about. I still just used sewing elastic ..yes I am cheap. But Medium power go for the combo of kevlar and elastic.

#### jrap330

##### Retired Engineer, NAR # 76940
TRF Supporter
Im going to try to build a baffle to attach the cord to, if not i will most likely do the ear plug method.
ear plug was a nice little idea, thanks to the original
poster a few weeks ago

#### Ez2cDave

##### Well-Known Member
TRF Supporter
Concur, how could all kelvar be superior if it induces the problem of zippering which too many rocketeer s write about. I still just used sewing elastic ..yes I am cheap. But Medium power go for the combo of kevlar and elastic.
The main advantage of Kevlar is that it doesn't burn . . .

Now, if "elastic Kevlar" were available, that would be the best of both worlds.

Dave F.

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#### AtomicStorm

##### Well-Known Member
I think that the steal fishing leader may hold up better then kevlar. Whenever i get my kevlar in the mail, i will do a side by side burn test. I may even throw some rubber and elastic in the test too.

#### jrap330

##### Retired Engineer, NAR # 76940
TRF Supporter
The main advantage of Kevlar is that it doesn't burn . . .

Dave F.
yes Dave I know but as an engineer you solve one problem and introduce another so that negates the idea and concerto that kevlar is the greatest thing and only a naive rocketeer would use anything else. Also makes a good bullet proof vest.

#### jrap330

##### Retired Engineer, NAR # 76940
TRF Supporter
yes Dave I know but as an engineer you solve one problem and introduce another so that negates the idea and concerto that kevlar is the greatest thing and only a naive rocketeer would use anything else. Also makes a good bullet proof vest.
"and concerpt"

#### K'Tesh

TRF Supporter

##### Well-Known Member
Kevlar from the MMT up to above the BT break point and then either nylon or elastic from there. Best of both worlds with the kevlar in the area where it'll be exposed to the direct ejection charge and will be long-lasting and the shock absorbing of the nylon or elastic up to the nose and parachute. Nylon/elastic is replaceable when it becomes old. You can also right-size the kevlar to a lower WLL since most of the shock will be absorbed by something other than kevlar which doesn't stretch but snaps.

I've used steel fishing leaders but they don't hold up in the corrosive BP residue as well as the kevlar.

#### Handeman

##### Well-Known Member
TRF Supporter
That really sucks, so can i make the lines for my 1.1 ripstop chute out of the same kevlar cordage? Which dia would you go with?
I wouldn't bother with the expense of Kevlar for shroud lines. You are going to protect the nylon from heat, just include the shroud lines and use Dacron or Nylon for the shrouds. Much cheaper than Kevlar and works just as well.

#### Sooner Boomer

##### Well-Known Member
I've got several rockets that have kevlar, and no elastic shock cord. One way to reduct the stresses of deployment is to lay out the kevlar cord in a series of "Z"s. You then put a wrap (or two, or three) of tape around each place the cord is doubled. When tension is applied to the cord, the tape loops begin to break. The more tape on each loop, the more force it takes to break the loop. The more loops there are, the longer the force is applied to the cord (an extended low tension is better than a short high tension). If I do mine exactly right, I have one tape loop left, unbroken.

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