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Chuck Mcgill

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I've just started launching model rockets with my nephew. The problem we are having is that the shock cord always seems to burn through after a few launches, just when he's getting into it. I used the suggested number of sheets of wadding and attached the cord to the body per the instructions. Am I doing something wrong? The kit we were using was the Estes Hi-Flier.
 

Karl

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Nope you are doing nothing wrong , estes shockcords are always to short , or burn through after a few flights . The best thing to do is to get your self some kevlar , replace the shockcord with it , put one end into the bodytube like normal , at the other end get some replacement shockcord and tie it to the kevlar , then tie the free end of the shock cord to the nosecone :) So the kevlar will take any heat that the wadding doesent take care of , and the shock cord will take the pressure of the nosecone being shot off.
-Karl
 

Johnnie

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Chuck,

Welcome to the Forum...

Estes shockcords?? Were to start?? Oh yeah, throw them in the trash when you open the kit. I am of the opinion that elastic belongs in your underwear.

Walmart has a small hardware section near lighting and plumbing that has a few varieties of nylon string. In different colors too...I picked flouescent Red nylon string. It is roughly 3/32nds" in dia, and you get a coupla hundred feet for a few bucks. I have outfitted rockets from Estes, Questes, Fliskits, Semroc, etc. etc and I still have half a roll left. (Bought in 1998) This method allows you to make nice long shockcords, that do not take up alot of room, or add alot of weight. Unless I set fire to the rocket ( I have been known to do that) the shockcords do not burn through.

Mounting is different:

1) You can affix/glue the nylon directly to the top centering ring, or

2) use 5 minute epoxy and glue it to the side wall in the same location you would the Estes shockcord mount. The folded paper method of mounting usually does not work with this string, as it is to thick.

Once again welcome, the best bunch of rocketeers ever can be found right here at the Forum...
 

jflis

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Originally posted by Chuck Mcgill
I've just started launching model rockets with my nephew. The problem we are having is that the shock cord always seems to burn through after a few launches, just when he's getting into it. I used the suggested number of sheets of wadding and attached the cord to the body per the instructions. Am I doing something wrong? The kit we were using was the Estes Hi-Flier.
Welcome to rocketry and welcome to TRF. This is, bar none, the best place to get started (and to continue in) rocketry, on the web.

The elastic cords found in most model rocket kits (FlisKits included) have their limits. The smaller the diameter of the body tube the quicker they will get damaged during flight.

Kevlar thread is the best suggestion for replacement. Kevlar can be purchased in a number of places, mostly online. FlisKits carries a limited selection and other sport rocketry vendors have selections as well. It's a good idea to always have some on hand.

I will also add... ...if you're a *beginner* at this and you're keeping your rockets *long* enough to burn through the shock cord, you're already ahead of the pack in this hobby! :)
 

pdooley

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Doesn't Thrustline have a deal going for Kevlar?
Hopefully its not all gone, I wanted to get some more.
 

flying_silverad

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Originally posted by pdooley
Doesn't Thrustline have a deal going for Kevlar?
Hopefully its not all gone, I wanted to get some more.
I do have some .035 in stock. PM me for a quote.
 

rocketkid88

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welcome to TRF, and to the hobby:)
what john sais is good... get some nylon string and replace the elastic. make sure you use plentty of it... you probobly want at least 3 times the lenght of the elastic cord that came with the kit. i've found the easiest way to attach this type of cord is to take a rectangle of paper and puch three wholes in it... see the attached for a better explenation... grafics speak so much better than words;)
 

kenobi65

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Originally posted by rocketkid88
i've found the easiest way to attach this type of cord is to take a rectangle of paper and puch three wholes in it... see the attached for a better explenation... grafics speak so much better than words;)
That looks mightily familiar...a whole lot like the self-sticking shock-cord mount that Centuri used to make. :)
 

johnnwwa

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I've been back in model rocketry about eight months now, the first rocket I built was an Estes Alpha 111 flew it for the first time last weekend on a C6-5 nice flight but shock cord let go. I used a combonation of wadding and dog barf. point being I WILL NEVER use elastic shock cord again. I just wish Estes would get up to speed on this . Never did find body tube and fins all I managed to get back was parachute and nose cone. All of my rockets except this one have kevlar installed mounted and attached to the centering ring on motor mount.
Live and learn and to save you time and money you have come to the right place GREAT bunch of people here BAR NONE:D


BAR
John
 

rocketkid88

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Originally posted by kenobi65
That looks mightily familiar...a whole lot like the self-sticking shock-cord mount that Centuri used to make. :)
actually, i first did that after reading Stines "Handbook of Model Rocketry", which, Chuck, i highly recoment getting. your local library probobly has it, if you just want to read it, but after reading it you will probobly go and buy one as well:D
 

DynaSoar

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Originally posted by Chuck Mcgill
I've just started launching model rockets with my nephew. The problem we are having is that the shock cord always seems to burn through after a few launches, just when he's getting into it. I used the suggested number of sheets of wadding and attached the cord to the body per the instructions. Am I doing something wrong? The kit we were using was the Estes Hi-Flier.
As they're all saying, get some kevlar thread. But don't throw those shock cords away. You can still use them.

Use a loop of kevlar going from the engine mount (under the hook, where it goes through the tube works well) and almost long enough to reach the top of the tube. But only almost (so obviously you take these steps before finishing building). Tie a loop in the end of the kevlar. Tie the shock cord in a loop. Run the shock cord through the loop in the kevlar, and run the other end of the shock cord loop through the other end, to form a slip knot. Oull it tight. Now finish building.

When you're done and you pull on the cord, the kevlar will reach almost to the end of the tube, and the rest of the cord will stretch. The reason you don't want the kevlar to go all the way to the end is called "zippering". If the chute catches the cord hard, it could yank the cord against the tube. If the cord at the edge of the tube were thin kevlar, it could rip right down the side of the tube and ruin the rocket. This way, that can't happen because the flat elastic (two layers of it) will be pulled against the tube, plus you still have some springy elastic to cushion the shock.

Just make sure you're pulling on the cord when you go to pack the recovery wadding in, so everything other than the very bottom end of the kevlar is protected from the ejection charge.
 

JeromeK

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As long as you have enough wadding and you are mounting your shock cords in the Estes style up towards the top of the tube you should be ok. Just don't pack your wadding too much. I just started using crepe paper. You know the streamer stuff for parties. You can get a good sized roll at Walmart or your local 99 cents store. It's fire resistant (even says it on the package). It's easy to use, you can tear off the amount you want and you're all set to go.

Another thing is that if you use the Estes style wedge mount and white glue/wood glue, you can easily pry it off without damaging the tube after several launches and put a new cord on.

I use a double length of shock cord or rather double the length of my rocket tube. This cuts down on the nosecone sling shotting back and dinging your model.

Be careful using kevlar and the motor mount centering ring style. It's very easy to have your cord "Zipper" the tube. This can happen on the first launch! If you aren't familiar with this phenomenon, it's when your Kevlar cord acts as a saw and literally rips your tube!


Just my thoughts from my experiences.


Jerome

:)
 

adrian

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Am I the only person who actually likes elastic shock cords? :D

True, in one recent launch, the cord was snapped by a particularly vicious ejection blast from a D12. On the other hand, another rocket had a Kevlar shock cord attached to a ring which was fixed to the motor mount. In a previous launch, a D12 from the same pack blew the whole shock cord and mount out of the rocket, and also blew a hole in the heat shield (which at least did its job and protected the parachute from damage). Due to the length of the rocket, I couldn't glue the Kevlar shock cord mount back where it had been. So I fitted the rocket with a standard Estes-style folded paper mount and a long elastic cord. The rocket does not appear to have suffered after a more recent launch. :)
 

surdumil

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No, you're not the only one.

Kevlar twine is not easily available where I live. I've been experimenting with various elastic and rubber shock cords.

Flat elastics with a high percentage of polyester are pretty bad. After a single flight they get brittle and fragile.

Flat elastics with cotton instead of polyester, however, are very durable. Locally, the smallest width that can be found is 1/4", which is too large for the smaller models.

Round elastics, however, are around 50% polyester and 50% rubber. I'm finding that these work quite well. Quest has been using these as the elastic part of their kevlar/elastic shock cord system.

I've found a small diameter round elastic that I'll be trying out over the next while. It doesn't take up nearly as much space as a flat elastic in a BT-5. We'll see how it holds up.
 

shreadvector

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Here's what I do or have done:

I use elastic shock cord to absorb the shock of ejection on regular model rockets. (For Micromaxx I just use a decent length kevlar cord.)

First, the rocket has to be long enough so that the shock cord anchor is far away from the delay train "afterburn". The delay continues to burn a bit after ejection and it is like a road flare. It will incinerate an elastic cord if it is anchored nearby. (Short Rockets = BAD). For short rockets I would use a Kevlar anchor tied to the thrust ring (a la Quest) OR a "Cotton Cable Cord" (at Home Depot near the twine and often 'mostly cotton with a tiny amount of polyester'). Cotton cable cord is VERY durable and is used by folks who anchor their shock cords to a centering ring near the motor mount in short fat rockets. You then tie the elastic cord to the cotton cable cord anchor and the cable cord is long enough to keep the elastic away from the afterburn. For other normal lenght rockets I use either a rectangular shock cord anchor (card stock with two holes punched) or I use an external mount: polyester or Kevlar cord epoxied to a fin root with the elastic cord tied onto it. The external cord is long enough to run up the side and then inside below the nose shoulder. Then you can pack the recovery system and shock cord with a few inches of the external anchor cord to spare.

Originally posted by surdumil
No, you're not the only one.

Kevlar twine is not easily available where I live. I've been experimenting with various elastic and rubber shock cords.

Flat elastics with a high percentage of polyester are pretty bad. After a single flight they get brittle and fragile.

Flat elastics with cotton instead of polyester, however, are very durable. Locally, the smallest width that can be found is 1/4", which is too large for the smaller models.

Round elastics, however, are around 50% polyester and 50% rubber. I'm finding that these work quite well. Quest has been using these as the elastic part of their kevlar/elastic shock cord system.

I've found a small diameter round elastic that I'll be trying out over the next while. It doesn't take up nearly as much space as a flat elastic in a BT-5. We'll see how it holds up.
 

DJ Delorie

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How do you choose an appropriate size (i.e. strength) kevlar cord for a given size rocket?
 

Silverleaf

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John at Thrsutline Aerospace has some of the best deals for recovery items, and with Jim at Fliskits and his fleet of rockets and great items and Rockethead Rockets for quality mylar chutes you just can't go wrong ordering from the regulars here.

I'd be remiss to forget the Sandman, I say the because his quality custom nosecones and his recent Little Joe 2 1/38th scale kit is da bomb !

Check on Ebay for John's kevlar like these..

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=2567&item=3152551320

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=2567&item=3152550728

and his store :

http://stores.ebay.com/Thrustline-A...lZ2QQdirZQ2d1QQsclZallQQsotimedisplayZ2QQtZkm

As to what size, I use the .025 with all my low power designs, but some of the mid and high power guys can give better answers for their class of design.

Hope that helps,
 

shreadvector

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Well, for the Polyester cord, I used some trial and error. I started with 50 and 60 pound test braided fishing line and it worked great for everything I was building and flying (mini through Estes D) that used external shock cord anchors. I tried some lighter cord and it broke when there was a strong ejection charge, so it was back to 50 or 60 pound test cord.

With Kevlar, the cord that Quest uses (and that I've bought in huge bulk spools from a mill) is about 2 and a half times the strength (T400 which is 135 pounds test). I've never had one break on ANY Model Rocket up through the Model Rocket maximum weight and power limits. Since it's so small in diameter I see no need to use anything smaller/weaker. I even use it in MicroMaxx models.

If you are looking for advice on HPR, that should be asked in the other forum....

Originally posted by DJ Delorie
How do you choose an appropriate size (i.e. strength) kevlar cord for a given size rocket?
 

GlennW

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Well, maybe I am just lucky but I've used only Estes shock cords for years and have only had one or two times where it seperated in flight. I'm still flying a 25 yr old Alpha III with its original stock Estes cord. Haha, now that I said that I'm sure its next flight will be the one where it fails!

GL
 

DJ Delorie

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My original Solar Sailer core sampled because the shock cord (read "cheap rubber elastic") broke (hey, it was 20 years old). It was highly non-repairable, so now I'm a bit more paranoid about such things ;-)
 

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