Quantcast

Using a long slip knot to control shock cord length?

The Rocketry Forum

Help Support The Rocketry Forum:

SoCalChris

Well-Known Member
Joined
Dec 9, 2015
Messages
91
Reaction score
0
So I've got about 15' of shock cord. If I tie the entire thing into one long slip knot, it's cut down to around 18", which is obviously much much easier to manage within the tube and keep from getting tangled.

I've done some preliminary testing, and it appears to be working properly, and working better than zig-zagging the cord back and forth. The knot comes undone very quickly, and with just the weight of the shock cord pulling to untie it. During a separation event, I suspect that the cord would deploy as quickly, if not quicker than using a more traditional method. The only obvious caveat that I see is that it's absolutely critical that the initial loop in the knot doesn't get wrapped around something. But that's really not so different from having to ensure that the cord doesn't get tangled using a traditional method.

So my question is this. Has anyone tried this? Am I missing any blazingly obvious flaws to this idea? Like I said, I will definitely ground test this once I get to that point in the build in a few days, but I'm just looking to see if anyone has any experience with this?

IMG_20170119_233447.jpgIMG_20170119_233858.jpg

[YOUTUBE]ojCr8ysm1TU[/YOUTUBE]
 

dhbarr

Amateur Professional
Joined
Jan 30, 2016
Messages
6,985
Reaction score
1,436
I've seen folks usually refer to this as crocheting the line. The other one I liked the looks of is spooling it up on a smaller tube, sliding it in, then pulling the tube. Oh and z-folding either w/ blue tape or small elastic bands.

But yeah, this way should work fine, and it looks pretty cool. Fat crochet hook (N size?) is supposed to make it go very quickly once you get the knack.

As usual, Apogee has some decent video : https://youtu.be/ETiB_V-_SGs
 
Last edited:

SoCalChris

Well-Known Member
Joined
Dec 9, 2015
Messages
91
Reaction score
0
Thanks guys for the confirmation that this should work. I tried searching, but didn't know what to call it, so my search results weren't coming back with anything. Crocheting is actually where I got the idea for this, my youngest son used to love nothing more than to run up to my ex wife as she was crocheting, grab the end of her yarn, and run off laughing like a maniac while she was screaming at him to stop. I'll definitely use this going forward.
 

gpoehlein

Well-Known Member
Joined
Jan 18, 2009
Messages
3,564
Reaction score
9
Actually, Tim Van Milligan at Apogee has a video on this very subject - he suggests using it when your shock cord has no elastic and is just a long kevlar thread.
 

Banzai88

Lvl 1,Wallet....Destroyed
TRF Supporter
Joined
Jul 15, 2015
Messages
2,561
Reaction score
726
There ARE issues with this, in my experience. I used the crochet method for nearly 50 flights last year. I favor it in small fiberglass 29mm powered rockets like the Formula 38 or 54, et al.

Looking at your pictures and your video, your loops are WAY too small and tight for the size cord that you're using. I used to do it like the Tim VanM/Apogee video, but the smaller/tighter loops lead to (at best) highly inconsistent deployment. One of the old salts at the field got ahold of me after struggling for a few flights and did it for me by hand with about 1/3 of the loops that I had used and VERY loose by the video standards. It worked 100% of the time that he did it, so I adjusted my methods. His advice, "If you can't put your hand through a loop, it's too tight, and no loop should be any smaller than the inner diameter of a coupler".

Remember, the cord will spit out straight FIRST as a string of loops, then start to pull out loops. Because by that time the force is roughly equal along the length, the smallest loop will close first as they start to pull out from the open end. If the smallest loop closes fast enough and tight enough, it WILL knot, wherever it is.

Regardless of loop size of the body of the pack, if the first and last loop isn't left VERY loose, it will often knot up under a short, sudden jerk provided by initial deployment. This can also happen anywhere along the chain under deployment (one notable instance is when the nose cone has reached full deceleration and the chute opens hard before full string deployment). If it happens, you can get nose cone snap back, which can foul the chute and/or cause all sorts of other strange things. The thinner and more limber the cord, the more critical this becomes to proper and full deployment. Solution = larger loops.

Also, this method generates a lot of friction. Inspect your cord after every deployment. Different weaves of flat and tubular respond differently to the deployment stresses and friction. I have some MadCow stuff and some from Chris that is the same spec, but behaves VERY differently when crocheted and packed and deployed.

Another thing is to make sure that there is enough weight on both ends to fully deploy the cord. This method works WAY better on heavier rockets than it does on lightweight ones, due to thinner cord knotting more easily and less weight under canopy to pull out the loops.

Because often about half of the loops will open under the weight of the dangling rocket under the parachute, it will often ground test differently than it will fly. If you knot a cord mid-deployment on the ground during testing, your rocket may very well flip over and cartwheel over the fence into the neighbors yard (ask me how I know :( )

Just my personal insight after crocheting extensively in LP/MP/HP rockets over the last 2 years or so. If I had to learn it all over again, I would start with larger loops, and then gradually increase the number of loops as you decrease the size of each loop for each successive flight. You'll likely reach a point of consistent safe/complete deployment of much few loops of much larger size than you have there. At least that's how I teach others how to do it now.

YMMV
 
Last edited:

MikeyDSlagle

Well-Known Member
Joined
Dec 17, 2013
Messages
2,432
Reaction score
424
I've always called it daisy chaining. I use it to roll my extension cords. Start in the middle and loop, both ends will be side by side and you only stretch out how much you need. It won't come undone and won't tangled when tossed in a gang box. Been doing it that way for 15+ years.

Never tried using it on shock cord but seems like it should work. I wonder if you could get a nomex sleeve and slide down over the entire thing, or even a nylon sleeve, just to keep the loops in order and pointing in the right direction. A sleeve with velcro would be easier to work with. Lay your daisy chained harness on the nomex and wrap it up. Leave it all loose of course. Hmmm.

Mikey D
 
Last edited:

dhbarr

Amateur Professional
Joined
Jan 30, 2016
Messages
6,985
Reaction score
1,436
That's how a gear bag is attached below a paratroop's harness IIUC. Z-folded, wrapped, & velcro.

Bama recovery makes 'em, need to send in a chute & harness to have fitted that way.
 

rms

Well-Known Member
Joined
Feb 3, 2009
Messages
353
Reaction score
0
I have used the daisy chain method on a number of occasions without a problem but did have an issue once. The daisy chain only allows the cord to pull out in one direction and if something interferes with the slip knot sliding out it will lock up and shorten your available recovery harness. In my case a gps tracking antenna some how ended up through one of the slip loops about a foot from the avbay attachment point stopping the slip loops from opening up any further. As a result my recovery harness ended up being only about 6 feet long. No damage to the rocket, it was the apogee drogue less event of a DD flight.
Although I sure this is rare, be aware it can happen
 

Latest posts

Top