Shock Cords

[AKPilot]

I originally posted this under Techniques, but didn't get a lot of response there so I thought I might try here . . .

Yesterday, I had a NC/body tube separate at apogee on one of my Maxi Alpha 3s. It was my fault really, I upgraded it to an E and didn't take into consideration the greater thrust. Consequently the stock Estes shock cord just snapped. The nose cone floated down quite nicely, but the body tube can screaming in. Easily enough fixed, though.

But, this leads me to a question. I've searched the forum on how people have improved on their cords. While I've read various ways of doing it, I'm under the impression that a shock cord should roughly be 3x-4x the length of the body tube, give or take. I've also seen that people tend to anchor them to the a centering ring and use kevlar-to-an-elastic transition.

Now here's my question, is there any sort of kevlar/elastic combination out there, for LPR & MPR, that has an elastic cord wound by kevlar - thereby having both properties, the fireproofing and elasticity? If so where do you get it from, and what's a preferred size, based on the weight of the rocket (e.g. < 8oz., 9-16 ozs, >1lb, etc.).

Thanks

 

[Steward]

I haven't heard of anything like what your asking. Others may know different.

I use a steel leader attached to the upper motor centering ring, that comes up just short of the upper body tube lip (preventing zipper effects). From there I simply tie normal elastic to the leader. I use different sizes and widths for different size rockets, but always use at least 3 times the rockets length.

Never had a serious problem with this method. As long as the right amount of wadding (or other protection) is used, it's very unlikely that anything burns.

Anyone else please chip in... There may be a material like what you're asking...


Good Luck anyway....

 

[shreadvector]

1) Thrust or total impulse of the motor will not make your cord brreak. Ejection charge force plus the mass of the two parts being blown apart will affect the breakage of the cord.

2) Did you Google "elastic kevlar cord" ??? Plenty of sources.

Originally posted by akpilot
I originally posted this under Techniques, but didn't get a lot of response there so I thought I might try here . . .

Yesterday, I had a NC/body tube separate at apogee on one of my Maxi Alpha 3s. It was my fault really, I upgraded it to an E and didn't take into consideration the greater thrust. Consequently the stock Estes shock cord just snapped. The nose cone floated down quite nicely, but the body tube can screaming in. Easily enough fixed, though.

But, this leads me to a question. I've searched the forum on how people have improved on their cords. While I've read various ways of doing it, I'm under the impression that a shock cord should roughly be 3x-4x the length of the body tube, give or take. I've also seen that people tend to anchor them to the a centering ring and use kevlar-to-an-elastic transition.

Now here's my question, is there any sort of kevlar/elastic combination out there, for LPR & MPR, that has an elastic cord wound by kevlar - thereby having both properties, the fireproofing and elasticity? If so where do you get it from, and what's a preferred size, based on the weight of the rocket (e.g. < 8oz., 9-16 ozs, >1lb, etc.).

Thanks

 

[loopy]

On some of the kit mods I've done (up-engines and such) I've just used a kevlar leader out the top (wrap masking tape around it where it hits the body tube for some zipper insurance) and tie a loop at the top, the elastic attaches to the nose cone and I attach the two with a quick link at prep time. This allows me to use a nomex heat shield when I fly.

 

[SwingWing]

Pratt hobbies has a Shock Cord sheath available here:
https://www.pratthobbies.com/proddetail.asp?prod=HSSS-3Y

Giant Leap Rocketry has a "Kevlar Bungie" material avaialble as well as shock cord protectors
https://www.giantleaprocketry.com/hpdefault.asp

 

[AKPilot]

So I take it many people are using a combination of kevlar & elastic then?

I've also been told that you can just use straight kevlar thread, giving it proper length.

Any comments or ideas on this?

 

[llickteig1]

I don't understand why you want any elastic at all. If the cord is short enough and the ejection forces are strong enough to stretch the elastic out, what will happen to all the rocket pieces when the elastic snaps back? Can't be anything good.

Use really long "shock" cords of braided kevler or tubular nylon and ditch the elastic.

--Lance.

 

[shreadvector]

Kevlar does not stretch like elastic cord. If you make it reallllly reallly loooong, the nose section will eventually slow down from air drag, but if it blows off with shotgun like force you may not have enough length.

Most people build like the Quest method: Kevlar cord for the anchor with elastic cord for shock absorbing. That way your shock cord acts like a shock cord. If it did not need to absorb the shock, it would be called a "connecting cord".

Originally posted by akpilot
So I take it many people are using a combination of kevlar & elastic then?

I've also been told that you can just use straight kevlar thread, giving it proper length.

Any comments or ideas on this?

 

[AKPilot]

Originally posted by shreadvector

Most people build like the Quest method: Kevlar cord for the anchor with elastic cord for shock absorbing.

I've heard this method being used a number of times. But doesn't this method still expose the elastic to the same ejection charge? Thereby contributing to wear and fatigue?

 

[shreadvector]

That's why we use wadding.

The wadding is blown to the bottom of the body tube and rests against the top of the motor mount. It fills a length of tube equal to 2 to 3 (preferably 3) body tube diameters. (i.e. a 1 inch tube should be filled about 3 inches with fluffy wadding). When you blow it into the tube, it goes "THUNK" when it hits the motor mount. This indicates you have a good gas seal and the wadding blob is acting like a piston.

If using cellulose insulation for wadding, it's a good idea to use one square of estes or Quest wadding (I cut my Quest wadding into quarter sheets since the sheets are HUGE) as a 'cup" and insert it first. This prevents the cellulose from blowing out the bottom of the rocket.

Naturally your motor is not installed yet.

The Kevlar cord is attached to the thrust ring or to one of the centering rings. it is heat resistant, not "fireproof". it will survive MANY ejection charges and delay afterburns. Delay afterburn is the killer. Do a web search.

The elastic cord is way at the top of the Kevlar cord. It is nowhere near the hot ejection gasses (IF you used the correct amount of wadding).

All of this info is useless for stupidly short rockets where you cannot get enough wadding inside to protect the recovery system (cords and parachute). Examples of stupidly short rockets: Estes Alpha, plastic Estes "mini-mites" - that's why they had to switch from elastic back to rubber band shock cord so they would survive more than one flight.

Nice long models have few problems. Examples include: Estes Generic E2X, any Quest rocket, Mean Machine (ever see a burned shock cord in one of those???), etc.

Originally posted by akpilot
I've heard this method being used a number of times. But doesn't this method still expose the elastic to the same ejection charge? Thereby contributing to wear and fatigue?

 

[loopy]

Originally posted by akpilot
I've heard this method being used a number of times. But doesn't this method still expose the elastic to the same ejection charge? Thereby contributing to wear and fatigue?

Use wadding or a heat shield (Refer to my earlier post)

 

[jflis]

Originally posted by akpilot
I've heard this method being used a number of times. But doesn't this method still expose the elastic to the same ejection charge? Thereby contributing to wear and fatigue?

Yes and no

Yes it is still exposed to some of the ejection charge resulting in some wear and fatigue. The *feature* is that it is *less* exposed to these items so that the wear and fatigue are lessened. the kevlar takes the brunt of it and is better suited to do so.

My philosophy is to use a combination, with the kevlar cord anchoring the whole thing in the rocket body. the elastic cord is tied to this then to the nose cone. The connection is outside the body so that when the elastic cord is showing wear and fatigue I can replace it. The elastic cord is there to do the job of providing "shock absorbancy" and is an important part of the whole recovery setup (imho). I also look at the shock cord like I do breaks on my car... A general maintenance item that has to be replaced every so often. Having it tied to a length of kevlar makes changing it a breeze.

 

[AKPilot]

Thanks for your guys' opinions on all of this. That's whats greats about this forum, everyone is willing to help and contribute whenever necessary.

What everyone has said makes a great deal of sense and I'm going to adopt some of these methods mentioned.

Also for fun, I think I'm going to give this issue to a team of aeronautical engineers at work (those guys are starving to play with things like this) and see what they come up with.

 

[BobCox]

AKPilot,

I used Kevlar-wrapped elastic in a beefed-up Executioner and I love it. Like you said, it's got the best properties of Kevlar cord and elastic. It's very strong and durable. The extra bulk and softness of the elastic core helps reduce the chance of zippering tubes that is so common with thin Kevlar cord.

I got mine from Mile High Rockets: https://www.milehighrockets.com/kevlar_elastic.html They offer it two ways:
1) KEVLAR Elastic Shock Cord Kit - $3.00 for kit, includes 14" of Kevlar-wrapped Elastic Shock Cord and 24" of 120 lb. Kevlar Filament Cord

2) Kevlar-wrapped elastic only, shipped uncut. Price is $7.00 per 10' of elastic. (FREE SHIPPING)

I like the second one. It's possible to make a continuous piece from the motor mount to the nose cone with no knots to get snagged or weaken the cord. To make it easier to fasten the ends, I stretch the elastic a few inches out of the end of the Kevlar sleeve and snip it off so that the end retracts inside the sleeve. This leaves a short section of thin sleeve-only that is easy to tie, without compromising the strength of the Kevlar cord. The tension on the sleeve holds the elastic in place like a Chinese finger puzzle.

 

[shreadvector]

Make sure you have a CCN for them or we'll all end up in another full day of "training".

And don't leave your laptop laying around....

Originally posted by akpilot
Thanks for your guys' opinions on all of this. That's whats greats about this forum, everyone is willing to help and contribute whenever necessary.

What everyone has said makes a great deal of sense and I'm going to adopt some of these methods mentioned.

Also for fun, I think I'm going to give this issue to a team of aeronautical engineers at work (those guys are starving to play with things like this) and see what they come up with.

 

[Steward]

I love the ability to learn something new everyday...!!!

 

[Mad Rocketeer]

Originally posted by Steward

I use a steel leader attached to the upper motor centering ring, that comes up just short of the upper body tube lip (preventing zipper effects). From there I simply tie normal elastic to the leader. I use different sizes and widths for different size rockets, but always use at least 3 times the rockets length.

Ever since I first heard of this technique, I've looked off and on in sporting goods stores and Wal-Mart, etc., for steel leaders, but I've never seen one. Fishing section, right? Where can these be found?

 

[adrian]

Originally posted by shreadvector
All of this info is useless for stupidly short rockets where you cannot get enough wadding inside to protect the recovery system (cords and parachute). Examples of stupidly short rockets: Estes Alpha, plastic Estes "mini-mites" - that's why they had to switch from elastic back to rubber band shock cord so they would survive more than one flight.

If the rocket has a plastic nose cone, bore a hole in the back of the cone, or there already is a hole, enlarge it. As with Quest models, use kevlar for the lower part of the shock cord and elastic for the upper part. Push as much of the elastic as possible into the nose cone.

I have two "stupidly short" rockets, both fitted with kevlar leaders tied to fabric elastic; my A-9 and Feuerlilie F-25. Both have very short body tubes by my own design. There is barely enough room in there for a small parachute and wadding, and that's only because Quest blue wadding is thinner than Estes toilet paper. Using the above trick with the shock cord means the elastic is out of the way of the ejection charge and also leaves a little more room in the body tube for the parachute.

 

[AKPilot]

Originally posted by Mad Rocketeer
Ever since I first heard of this technique, I've looked off and on in sporting goods stores and Wal-Mart, etc., for steel leaders, but I've never seen one. Fishing section, right? Where can these be found?

Yes, it's a fishing thing . . .

I think it must be a regional thing. We had them in the Wal-marts up in Alaska. Of course, there we were fishing for 50 lbs salmon and 300 pound halibut. In central Texas, I'm not sure if you'd have the same need.

Check Bass Pro Shop if there's one nearby, or a specialty store. If not go on-line.

 

[ben]

one ting I was going to try is taking a rubber band and "banning" a small loop together so when it ejects it will pull the loop out and act as a small bumper. BTW this is with 1/8" Kevlar Strap.

Thanx, Ben

 

[BAR_Daddy]

I like attaching kevlar to the motor mount and stopping it just short of the top of the body tube, with a loop on the end.

I then attach some elastic that I purchased from the sewing department at Walmart. About the length of the body tube.

To that I attach more kevlar, about twice the length of the body tube.

Since I have started doing this, I do not get zippers, or the dreaded "Estes bounce-back".
I don't seem to have any problem with scorching either.

Even after a dozen or so flights.

Just my three cents (inflation you know).