Chain Stitching Shroud Lines and Shock Cords

[BobCox]

In the Feb 2009 issue of ROCKETS magazine , Dan Michael describes using a "chain stitch" to braid the shroud lines on a parachute. It was also mentioned in coverage of NARCON 2009 in the latest Sport Rocketry magazine.

The technique involves making a loop in the shroud lines, reaching through the loop to grab the lines above it, and pulling them back through the loop to create a new loop. This process is repeated until all the entire length is bundled.

When the chute is deployed, each loop unravels in reverse order, in a very orderly fashion. This braiding is supposed to keep the shroud lines from getting tangled, and unravels gradually to keep the chute from snapping open abruptly.

Has anybody else tried this technique?

Would it also work on non-elastic shock cords, like Kevlar or tubular nylon? It seems that the friction required to pull the loops apart would help to spread out the ejection energy and lessen the jerk at the end of the cord.

[Chrisn]

I was confused when you said on the shroud lines, as I have seen it been used/mentioned/photos of it being used on kevlar/nylon shockcords (on the internet, try the trf archive or rocketryplanet) I have yet to try it myself.

[dedleytedley]

I used the chain stitch method for the power wire in my nike-ajax two-stage. I did it mostly to prevent tangling of the wire and shockcord and it worked very well. I learned it while working in construction, cords never tangle when tied this way. Ted

[hardinlw]

In the Feb 2009 issue of ROCKETS magazine , Dan Michael describes using a "chain stitch" to braid the shroud lines on a parachute. It was also mentioned in coverage of NARCON 2009 in the latest Sport Rocketry magazine.

...

Would it also work on non-elastic shock cords, like Kevlar or tubular nylon? It seems that the friction required to pull the loops apart would help to spread out the ejection energy and lessen the jerk at the end of the cord.

I was at NARCONN and attended Dan's session. He uses it with non-elastic shock cords as that is the only thing you would use in rockets of the size he builds. I assume he is capable of building a small rocket, but I've never seen one. As I understand it, the idea is that the shroud lines and shock cord will be completely extended before the parachute starts to open and the time it takes to open the chute smooths out the jerk.

[chanstevens]

Well now I know there's a name for it--I've been doing that with my Kevlar-only shock cord lines on competition models for a little over a year now. Basically a long series of slip knots or braids, goes a fantastic job of "shock absorbtion" plus prevents pyro knots in the Kevlar. Takes maybe 90-120 seconds longer to prep, but very worth it.

[talkin Monkey]

Kinda like this?



The chute actually fouled a bit but I'm pretty sure it was because I had the nosecone and parachute locations reversed on the shock cord. I think the nosecone hanging from a few feet of cord kinda spun around at deployment and caused the problem



If you tie the loops tight enough, I think this setup takes some of the shock out of the parachute inflating.

...However...

The use of small rubber bands and even hair "scrunchies" has been recommended as well. IMHO, I think these ideas would work even better to disperse deployment shock energies if needed.

EDIT: 1st pic originally too big and data rude in size, more on the subject here http://www.rocketryforum.com/showthread.php?t=3425

[troj]

The use of small rubber bands and even hair "scrunchies" has been recommended as well. IMHO, I think these ideas would work even better to disperse deployment shock energies if needed.

Tape or rubber bands is actually better for one primary reason -- friction.

As your shock cord (or shroud lines) "unstitch" themselves, they have to slip past one another, and if it's happening at any speed at all (there's more there than you think), it's creating friction, and especially in the case of nylon, friction = bad.

Use a little masking tape, or light rubber bands. It still keeps things neat & tidy, but it reduces friction.

Of course, if you avoid 3 miles of recovery harness, it's not so hard to manage in the first place....

-kevin

[Micromeister]

In the Feb 2009 issue of ROCKETS magazine , Dan Michael describes using a "chain stitch" to braid the shroud lines on a parachute. It was also mentioned in coverage of NARCON 2009 in the latest Sport Rocketry magazine.

The technique involves making a loop in the shroud lines, reaching through the loop to grab the lines above it, and pulling them back through the loop to create a new loop. This process is repeated until all the entire length is bundled.

When the chute is deployed, each loop unravels in reverse order, in a very orderly fashion. This braiding is supposed to keep the shroud lines from getting tangled, and unravels gradually to keep the chute from snapping open abruptly.

Has anybody else tried this technique?

Would it also work on non-elastic shock cords, like Kevlar or tubular nylon? It seems that the friction required to pull the loops apart would help to spread out the ejection energy and lessen the jerk at the end of the cord.

Bob:
Haven't used Chain Stitching on Shroud or Shocklines but have used it extensively back in the day as a Beach Lifeguard on many different diameter and types of lines. about the smallest stuff I've used was 1/8" nylon squib line. some as long as 300yds. As long as the chained line is coiled and laid carefully it should work pretty well for most rocket shroud lines as well. It will most certainly slow down the payout from the tube. I'd think the larger the Rocket the better it would work.
Great idea!

[BobCox]

I've been doing that with my Kevlar-only shock cord lines on competition models for a little over a year now. Basically a long series of slip knots or braids, goes a fantastic job of "shock absorbtion" plus prevents pyro knots in the Kevlar.

That's good to know. I was concerned that the Kevlar might seize during a rapid deployment.

BTW, what is a "pyro knot"?

Tape or rubber bands is actually better for one primary reason -- friction.

As your shock cord (or shroud lines) "unstitch" themselves, they have to slip past one another, and if it's happening at any speed at all (there's more there than you think), it's creating friction, and especially in the case of nylon, friction = bad.

Yeah, I was wondering about the frictional heating on tubular nylon if it tried unraveling too quickly. Have you actually seen it happen to a braided nylon cord, or is your concern based on other incidents?

[chanstevens]

That's good to know. I was concerned that the Kevlar might seize during a rapid deployment.

BTW, what is a "pyro knot"?

That's what you get when a 3-foot length of Kevlar gets tangled up on the way out and is effectively only about 6" worth of knots and mess, yanked nice and tight by the overstressed rest of the recovery system.

[troj]

Yeah, I was wondering about the frictional heating on tubular nylon if it tried unraveling too quickly. Have you actually seen it happen to a braided nylon cord, or is your concern based on other incidents?

I've never seen anyone use this method in rocketry, and I'd recommend against it, as tape works just as well, without the risk of friction.

Yes, I've seen damage to nylon (tubular and ripstop) from friction. I've even had it demonstrated that without much effort, you can cut one piece of tubular nylon using another piece -- friction induced damage.

We ruined a 28 foot man-rated parachute on a large rocket from friction. Not from deployment, but from it bouncing off the ground as the wind dragged the rocket.

Nylon + friction = bad. Spectra + friction = worse. You don't see a gob of Spectra in rocketry, but I have seen a little bit of it.

-Kevin

[MachineGuy]

I found this link that explains how this is "tied".

http://www.animatedknots.com/chainsinnet/index.php

[PemTech's Squeeze]

LOL! And thus crocheting enters rocketry!

And if crocheting them is a bad idea (really, THAT much friction?) then stay away from my scrunchies and hair rubberbands, and go for the tape. Less junk distributed over the flightline and field that way. Which means happier owner of field, which means he lets you keep coming. Masking tape is biodegradable, and cleans off stuff easier than duct tape. And, afterall, you just want it to hold a little bit, not have the cord rue the day it ever contemplated holding it's own hands, so I humbly suggest it's the better option.

And keep your hands and eyes away from my hair skunchies!

Trudy
PemTech's Art Department and owner of Long Wild Tresses

[cjl]

I agree with Kevin, and I personally have used masking tape bundles with great success.

[Diosces]

Tape or rubber bands is actually better for one primary reason -- friction.

As your shock cord (or shroud lines) "unstitch" themselves, they have to slip past one another, and if it's happening at any speed at all (there's more there than you think), it's creating friction, and especially in the case of nylon, friction = bad.

Use a little masking tape, or light rubber bands. It still keeps things neat & tidy, but it reduces friction.

Of course, if you avoid 3 miles of recovery harness, it's not so hard to manage in the first place....

-kevin

Kevin, Glad you mentioned rubber bands. I was having some recovery difficulties with my L2 cert flight attempts on carbon fiber laminated 2.6 dia Jaguar.

David W of MDRA keyed me onto using small orthodontic rubber bands to secure my recovery harnesses. Worked perfectly, and helps keep everything in order when assembling the rocket the rocket.

I'll post some pix this weekend.

[Diosces]

Pictures as promised. The oral rubber bands work well on the shock cord harness,

I think the rubber bands would equally well on chute shroud lines, will try ground test and subsequent flight

[cjl]

I do something similar to that but with wraps of masking tape (on occasion I have also used electrical tape, when I really wanted to take the energy out of the deployment on a heavy rocket). The tape breaks when it deploys, keeping everything organized and also helping to slow down the components before they hit the end of the shock cord.

[rocketsmith]

Chain stitching is a great way to keep a chute opening in an orderly fashion. The chutes I get from Aerocon typically come with chain stitched lines as is from de-mil'ed sources. I use the little rubber bands to keep the harness in order and chain stitch the chute lines. The idea of friction damaging lines on a chute seem a bit specious to me. The chute opens more slowly and I have never had a chute foul because of chain stitching. I would add a couple hundred feet to the deployment height as the chute take 3-5 seconds to open, at least the 5 and 10 footers I have used. Also, leave about 6-10 inches of shroud line "unstitched" as this allows air into the bottom of the chute so it will pull itself open. I also use a drogue (24-36") at the main harness to pull the main out. I have begun designing all my rockets with rear deploy of all chutes to prevent zippers.
Hope this helps, David B. Smith (tra 11892)

[Porthos II]

Chain stitching would be great for when you retrieve your rocket and you want to do something with the lines to keep them from getting tangled, but should never be used for deployment. It creates friction that can melt your lines. They also can get jammed if pulled too tight before releasing or the lines scrunch up wrong. Use tape or rubber bands to bundle your lines. A professional rigger or a skydiver would cringe at that idea, and never trust their lives on it!

[talkin Monkey]

My boots rarely catch fire when I untie them, but then again, I don't often pull 30' of laces out tight in a 1/10th of second either.

I used the chain stitched shock cord thing (mainly for organisational purposes) for both of my successful L1 and L2 flights.

My L2 main chute deployed at 700' and remained a (moderatly) tightly packed chute even when the redundant charge fired at 500'...It finally inflated somewheres around the 350~450' neighborhood.

Just speculating here, but I wonder if all the little happy loops in my shock cord turned it into a nice and draggy pseudo-streamer before (a lucky/low alt. /heart-stopper) chute inflation?

Kevin and Eric, Thanks for the good advise on this subject. You guys have any archived shots of well organised / banded-up-taped-up shock cord installs that could be posted here that could be used as a good example?

Scott Broderick

[talkin Monkey]

I do something similar to that but with wraps of masking tape (on occasion I have also used electrical tape, when I really wanted to take the energy out of the deployment on a heavy rocket). The tape breaks when it deploys, keeping everything organized and also helping to slow down the components before they hit the end of the shock cord.

I wonder also, if taping sticky side out helps things a bit by not having anything still adhered to the line as it straightens out.

[dixontj93060]

David W of MDRA keyed me onto using small orthodontic rubber bands to secure my recovery harnesses..

I have also found some very high quality small sized rubber bands at Beauty Supply stores (they must be used for some hair prep/procedure) that work great. They come in bags of 100 typically black in color.

[cjl]

I wonder also, if taping sticky side out helps things a bit by not having anything still adhered to the line as it straightens out.

You could try it, but I've never had any trouble with sticky-side in, and mine always come back with the tape gone. As for pictures, I don't have any at the moment, but they look almost exactly like Diosces's picture except with wraps of masking tape instead of rubber bands.

[rocketsmith]

Chain stitching would be great for when you retrieve your rocket and you want to do something with the lines to keep them from getting tangled, but should never be used for deployment. It creates friction that can melt your lines. They also can get jammed if pulled too tight before releasing or the lines scrunch up wrong. Use tape or rubber bands to bundle your lines. A professional rigger or a skydiver would cringe at that idea, and never trust their lives on it!

While I agree that a skydiver would never chain stitch his shroud lines, mostly because of the manner in which it is rigged in the pack. Cargo chutes are almost always chain stitched as well as the ring slot chutes used in our space program. I have never seen chain stitching cause any damage to a chute. Any friction caused by excessive opening speed would most likely result in a stripped chute anyway. Chain stitching offers a more controlled opening of the chute and therefore would seem to be a safer method than trusting that a bunch of loose shroud lines will sort themselves out during the violence of ejection. It is almost impossible for chain stitching to tighten against itself because as soon as the first loop is pulled it relaxes the next loop making it easier to pull out. I had an incident this weekend where a Pyrodex charge didn't burn completely enough to shear pins holding the airframe together. The main opened after a 1500 foot descent and I barely got a zipper. I beleive it was from the soft opening of the main due to chain stitched lines. I will therefore always chain stitch my shroud lines. Rubber bands and tape work great, but they don't control the opening of the chute so you get the aggressive "pop" when it opens. That is a good way to shed shroud lines. Ultimately we should all use the methods we feel comfortable using.

[cjl]

While I agree that a skydiver would never chain stitch his shroud lines, mostly because of the manner in which it is rigged in the pack. Cargo chutes are almost always chain stitched as well as the ring slot chutes used in our space program. I have never seen chain stitching cause any damage to a chute. Any friction caused by excessive opening speed would most likely result in a stripped chute anyway. Chain stitching offers a more controlled opening of the chute and therefore would seem to be a safer method than trusting that a bunch of loose shroud lines will sort themselves out during the violence of ejection. It is almost impossible for chain stitching to tighten against itself because as soon as the first loop is pulled it relaxes the next loop making it easier to pull out. I had an incident this weekend where a Pyrodex charge didn't burn completely enough to shear pins holding the airframe together. The main opened after a 1500 foot descent and I barely got a zipper. I beleive it was from the soft opening of the main due to chain stitched lines. I will therefore always chain stitch my shroud lines. Rubber bands and tape work great, but they don't control the opening of the chute so you get the aggressive "pop" when it opens. That is a good way to shed shroud lines. Ultimately we should all use the methods we feel comfortable using.

As far as I can tell, this is completely false. Cargo chutes and all chutes I could find a description of in the space program (including the massive chutes used to recover the shuttle SRBs) are all deployed using a deployment bag with the lines stowed carefully in line stows. I looked over several different packing guides for various chutes, and in every single case, it involved a deployment bag. In no case did they mention chain stitching, except as a convenient method to store the chute when it isn't being used.

[rocketsmith]

As far as I can tell, this is completely false. Cargo chutes and all chutes I could find a description of in the space program (including the massive chutes used to recover the shuttle SRBs) are all deployed using a deployment bag with the lines stowed carefully in line stows. I looked over several different packing guides for various chutes, and in every single case, it involved a deployment bag. In no case did they mention chain stitching, except as a convenient method to store the chute when it isn't being used.

Fair enough. When using a deployment bag one would obviously use the available line stows. In leiu of that, I will chain stitch. I have some photos of parachute rigging of to be recovered payloads in a couple of sounding rockets (Aerobee's) that seem to show the lines chain stitched with only the chute in a storage/ heater bag. Hey, if one doesn't want to chain stitch, then don't. I know it works and will continue to do so until it doesn't. As far as the melting of shroud lines during opening, I still protest that if you are melting nylon shroud lines you are probably going to pull the harness apart or yank an eye bolt from a bulkhead with that amount of force.

[troj]

Fair enough. When using a deployment bag one would obviously use the available line stows. In leiu of that, I will chain stitch. I have some photos of parachute rigging of to be recovered payloads in a couple of sounding rockets (Aerobee's) that seem to show the lines chain stitched with only the chute in a storage/ heater bag. Hey, if one doesn't want to chain stitch, then don't. I know it works and will continue to do so until it doesn't. As far as the melting of shroud lines during opening, I still protest that if you are melting nylon shroud lines you are probably going to pull the harness apart or yank an eye bolt from a bulkhead with that amount of force.

An possibly important distinction... Are the photos you're looking at of the rocket, in a flight-ready configuration? Or are the payloads ready to be loaded, with their recovery attached, and the lines are chain-stitched to keep them neat and orderly until such time as the system is made flight-ready?

Riggers often do use chain-stitching, but it's only to keep lines neat and tidy until the rig is inspected, packed and made flight-ready. A flight-ready rig that needs to have a long length of line outside the rig would likely have that length chain-stitched to keep it neat and orderly until such time as it was made flight-ready.

Something else to remember is that friction damage won't necessarily bite you the first time, or the fifth time, but it does do damage.

-Kevin