Preventing parachute and shock cord entanglement

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Dave S.

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I had a rocket crash yesterday caused by the parachute getting entangled with the shock cord. Very disappointing I worked on it for a long time.

This was a high power H motor launch with a 30 inch parachute and about a 12 foot length of thin Kevlar shock cord.

I have a nomex blanket tied to the shot cord about a foot away from the open end of the body tube. I get the parachute folded up into a burrito bundle in the blanket. Usually I put the remaining length of shock cord loosely in the end of the tube on top of the nomex blanket bundle, put the nose cone on and it’s ready to go. This has worked for me until yesterday.

Any suggestions for preventing the parachute from getting entangled in the shock cord when they are ejected out of the tube?

Is it better for the remaining length of shock cord to go into the tube under the parachute burrito bundle instead of on top of it?
 
First in should be last out. Definitely put the shock cord in the tube first with the parachute bundle above it. If you don't already, I would also recommend "organizing" the cord in the tube by z-folding it. If the nose has sufficient weight, you can even use a small bit of painters tape keep the cord bundle together. The momentum of the nose cone at ejection should be enough to tear the tape.
 
Also, am I understanding correctly that the nomex blanket is attached to the shock cord closer to the airframe rather than closer to the nose cone? Where along the shock cord is the parachute attached? At the nose cone eye? If so, I would recommend attaching the Nomex blanket close to the parachute: maybe 12" away. This way, the chute/Nomex bundle is ejected together away from the airframe and the cord. The chute is less likely to start unfurling/inflating close to the airframe and all the unwinding cord. It will start inflating farther away with the cord fully extended.
 
Was this flight at Red Glare? If so then I saw that flight.

I keep the Nomex close to the chute attachment point.
Then either Braid the cords or do taped bundles (as mentioned in above post). This keep the cords from becoming a tangle and also help absorb energy from ejection and chute opening producting a smooth depolyment.
 
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I've experimented with a lot of ways to do this. For my L1 rocket I just stuffed everything in there- chute protector and chute first, then shock cord. It worked fine for many launches.

I started rejuvenating many of my old LPR and converted them to 10' kevlar shock cords. I tried different systems to manage that much cord in a small rocket. If I just stuff it all in there it it rarely worked. I've tried wrapping in a figure 8, that didn't work for me.

I tried the braiding technique, I don't know the correct name, and that works reasonably well but it hard to do at the launch site. Basically you take 10' or cord and get it down to about 2' or less of braid. I lay out the parachute folded in half, lay half the braid on top of the chute and fold it in half again, flip the other half over on top of the parachute and fold again, then put the parachute in the tube and go.

I have another thread on it- I made a small "spool" and wind the cord around it. This doesn't work for anything smaller than BT-60. I found that if I put the wadding in first, then the spool, then the parachute, and it is fairly reliable. The HPR analog to this would be using flat nylon or kevlar cord, folding it up like an accordian, and taping it. I finally realized that at ejection the nose cone comes off with the parachute connected fairly close to it and the cord stringing out behind it. Whether a spool or taped bundle, the rocket body is pulling on it one direction, the nose cone and parachute are pulling on it the other direction and it can then unroll/unfold clear of the parachute.
 
I had a rocket crash yesterday caused by the parachute getting entangled with the shock cord. Very disappointing I worked on it for a long time.

This was a high power H motor launch with a 30 inch parachute and about a 12 foot length of thin Kevlar shock cord.

I have a nomex blanket tied to the shot cord about a foot away from the open end of the body tube. I get the parachute folded up into a burrito bundle in the blanket. Usually I put the remaining length of shock cord loosely in the end of the tube on top of the nomex blanket bundle, put the nose cone on and it’s ready to go. This has worked for me until yesterday.

Any suggestions for preventing the parachute from getting entangled in the shock cord when they are ejected out of the tube?

Is it better for the remaining length of shock cord to go into the tube under the parachute burrito bundle instead of on top of it?
Hi Dave,

There is of course no guarantee of anything in rocketry, but along with a FILO configuration mentioned by @Kane, z-coiling the harness is the best way anyone has found to stow it. Other that optimizing space in the airframe, the progressive release of harness results in no extra harness to become tangled. The easiest way to form these coils is using a device invented by a student for the purpose. More information about the “Harness Winder” and the sizes available can be found @ https://www.facebook.com/ReliableRocket. Here are some photos and video that you may find informative.
 

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Also, am I understanding correctly that the nomex blanket is attached to the shock cord closer to the airframe rather than closer to the nose cone? Where along the shock cord is the parachute attached? At the nose cone eye? If so, I would recommend attaching the Nomex blanket close to the parachute: maybe 12" away. This way, the chute/Nomex bundle is ejected together away from the airframe and the cord. The chute is less likely to start unfurling/inflating close to the airframe and all the unwinding cord. It will start inflating farther away with the cord fully extended.
Had a rocketeer show me where he had done exactly that, blanket attached about 18" from chute attachment point. The blanket was still neatly, but loosely, wrapped around the chute when it landed.
 
Curious: Was a JLCR involved? I've wondered if the chute release may help keep things from getting tangled, as the chute isn't trying to open while the shock cord is "doing it's thing". But this is purely conjecture on my part.

Hans.
 
I had a rocket crash yesterday caused by the parachute getting entangled with the shock cord. Very disappointing I worked on it for a long time.

This was a high power H motor launch with a 30 inch parachute and about a 12 foot length of thin Kevlar shock cord.

I have a nomex blanket tied to the shot cord about a foot away from the open end of the body tube. I get the parachute folded up into a burrito bundle in the blanket. Usually I put the remaining length of shock cord loosely in the end of the tube on top of the nomex blanket bundle, put the nose cone on and it’s ready to go. This has worked for me until yesterday.

Any suggestions for preventing the parachute from getting entangled in the shock cord when they are ejected out of the tube?

Is it better for the remaining length of shock cord to go into the tube under the parachute burrito bundle instead of on top of it?
I experienced this problem with my SDX3 rocket two years ago on the Salt Flats at LDRS.

But in his NARCON 2021 YouTube video, Jim Jarvis has presented some great ideas for preventing the parachute from getting tangled with the drogue and shock cord. I’m working to incorporate some of his methods in my L3 build.

 
First in should be last out. Definitely put the shock cord in the tube first with the parachute bundle above it. If you don't already, I would also recommend "organizing" the cord in the tube by z-folding it. If the nose has sufficient weight, you can even use a small bit of painters tape keep the cord bundle together. The momentum of the nose cone at ejection should be enough to tear the tape.
FWIW, I have had far fewer tangles since I learned about the Z fold method.
 
For reference, I'm using a 36" nylon parachute with twelve 54" shroud lines, and 15' of 350-pound Kevlar shock cord.

First, I flat-fold the parachute so the canopy is full length, with gores stacked. Then I s-fold the shroud lines so they're 1/3 the length of the canopy, securing each bundle end with medium-weight orthodontic rubber bands.

IMG_0644.jpg IMG_0649.jpg IMG_0652.jpg

Next, I z-fold the parachute with the shroud lines in between the bottom and second layers, then roll it loosely into a bundle, then burrito-fold the bundle in the blanket.

IMG_0653.jpg IMG_0655.jpg IMG_0657.jpg

Next, I secure the rolled bundle with a rubber band—TEMPORARILY—to keep it rolled while I s-fold the shock cord next to the bundle. Then I lay the s-folded cord alongside the bundle, remove the rubber band, then slip everything into the body tube, with the shock cord ALONGSIDE the parachute bundle.

IMG_0660.jpg IMG_0661.jpg IMG_0662.jpg IMG_0663.jpg

At ejection, the nose cone pulls the shock cord and shroud lines simultaneously out of the tube, with the blanket and canopy exiting last.

Notes: My blanket is attached to the shock cord 12" above the top of body tube. The parachute is attached to the shock cord 15" below the nose cone. The parachute is rolled loosely before being wrapped in the blanket, and the blanket is wrapped loosely around the parachute.

The only failure I've had is the one time I inadvertantly turned the s-folded shock cord bundle 180 degrees from its orientation during folding, which put a single twist in the cord lengths that extend above and below the s-folds. That twist prevented the cord from being pulled out of the tube one s-fold at a time.
 
There are a number of good techniques to avoid tangles and to achieve a consistent deployment, many already mentioned in this thread. But one thing not metioned is packing volume.

A rocket undergoes a fair number of positive G's, and some negative G's. It may experience lateral G's, and spinning. If the packing job has room for the chute and lines to move around, then they WILL move around!

I prefer a packing job that, when done, requires a little bit of compressing to push the nosecone in place. The shear pins hold it. That way NOTHING can move. So the way it comes out stays the reverse of how you put it in.

Lots of kits have way too much volume left for recovery packing. That is done so that if you use a chute and lines that take up a lot of volume you can still get the packing done. But by being sized for the worst-case, most times stuff can shake around.

Personally, I'm not one to leave a kit unmodified, and prefer doing my own designs anyway. So I'll shorten tubes as necessary to fit exactly what I want them to fit. Wasted volume increases the odds of something going wrong.

Now if poor packing techniques are used, then of course garbage-in, garbage-out. And on rare occasion on these threads I've read suggestions for rather poor techniques. Because they've been gotten away with. But at least if the packing cannot move in transport or flight then what happens will quite probably be reproducible - so you can learn to do it better.

For anyone who is not really sure on how to do a good packing job, study some rigger training videos etc. These folks have the century of experience of hard knocks to know how to make it work, as close to 100% of the time as possible. People flying high power rockets should take the same care. Because it is going to come down. And it can be quite lethal - even a small one - if recovery fails.

I'm not too concerned if you blow it up on the pad. That's what stand-off distances are for. And IMHO you are responsible for fixing or paying for the pad if you wreck it. Distance handles that safety issue.

But failing to deploy or having other safety issues happens way too often. Even once can be too often. If you know how, do it well. If you don't, study until you know how. There is a lot of information out there - and I think many rocket folks ignore it.

I consider recovery failure the third most dangerous type of failure, following behind inadvertent ignition and landshark. These are the failures that can kill.

The OP is doing a good thing asking for advice. It is a start.

Dave, do you have any photos documenting your packing job for the rocket? When I pack I try to take a series of photos as a record of what was done. If it works well, I have a record. If for some reason it doesn't, well, then I can study the pictures to try to figure out what I did wrong. Here are some examples of some minimal documentation for two of my flights: https://www.rocketryforum.com/threads/sprite-6-and-a-baby-o.37382/page-4#post-456460 https://www.rocketryforum.com/threads/thrp-1.172951/page-3#post-2289468 Note the second of these had apogee main deployment, traced with less than 100% certainty to a poor screw switch to an altimeter. One altimeter was off by the time the rocket was on the ground.

Gerald
 
Hi Dave,

There is of course no guarantee of anything in rocketry, but along with a FILO configuration mentioned by @Kane, z-coiling the harness is the best way anyone has found to stow it. Other that optimizing space in the airframe, the progressive release of harness results in no extra harness to become tangled. The easiest way to form these coils is using a device invented by a student for the purpose. More information about the “Harness Winder” and the sizes available can be found @ https://www.facebook.com/ReliableRocket. Here are some photos and video that you may find informative.


I actually use your cord winder for everything 54mm and up. Keeps everything organized and compact in the airframe, minimizes errant cord snarling the chute, and provides some cushion by decelerating the airframe/nosecone and chute bundle.
 
I was not using a JLCR when this crash happened.

If I understand it correctly, what is needed for a variety of shock cord thicknesses for coils up to almost 4" in diameter is both the 98mm spacer set and the 98mm winder that goes with them - right?
 
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I was not using a JLCR when this crash happened.

If I understand it correctly, what is needed for a variety of shock cord thicknesses for coils up to almost 4" in diameter is both the 98mm spacer set and the 98mm winder that goes with them - right?

Generally yes, if you mean width instead of "thickness". However, if the harness is wide enough a spacer is not necessary, but does result in a flatter neater coil.

IMG_7352.jpg
 
Was this flight at Red Glare? If so then I saw that flight.

I keep the Nomex close to the chute attachment point.
Then either Braid the cords or do taped bundles (as mentioned in above post). This keep the cords from becoming a tangle and also help absorb energy from ejection and chute opening producting a smooth depolyment.
I agree. I attach my Nomex blanket at the chute shroud line link, too. Never had a tangle. Additionally, I usually fold excess cord into the blanket wrap.
 
I had a rocket crash yesterday caused by the parachute getting entangled with the shock cord. Very disappointing I worked on it for a long time.

This was a high power H motor launch with a 30 inch parachute and about a 12 foot length of thin Kevlar shock cord.

I have a nomex blanket tied to the shot cord about a foot away from the open end of the body tube. I get the parachute folded up into a burrito bundle in the blanket. Usually I put the remaining length of shock cord loosely in the end of the tube on top of the nomex blanket bundle, put the nose cone on and it’s ready to go. This has worked for me until yesterday.

Any suggestions for preventing the parachute from getting entangled in the shock cord when they are ejected out of the tube?

Is it better for the remaining length of shock cord to go into the tube under the parachute burrito bundle instead of on top of it?
Depending on the length it might benefit to use little bundles along the length of the cord. Loop the cord back onto itself and make 4-6 loops in each bundle for large shock cord and many more loops per bundle for Kevlar cordage. Then I hold the little bundles together with two tiny rubber bands each bundle top and bottom of the bundles. Depending on the length you could have several bundles along the length of the shock cord.
This helps keeping everything neat and tidy. The bundles will pay out with ejection, softening and slowing down the shock the cordage might experience.
 
First in should be last out. Definitely put the shock cord in the tube first with the parachute bundle above it. If you don't already, I would also recommend "organizing" the cord in the tube by z-folding it. If the nose has sufficient weight, you can even use a small bit of painters tape keep the cord bundle together. The momentum of the nose cone at ejection should be enough to tear the tape.
The suggestion is good but I have never liked the idea of using tape. I use rubber bands to hold the z folds instead of tape and I have never had a failure using them.
 
The suggestion is good but I have never liked the idea of using tape. I use rubber bands to hold the z folds instead of tape and I have never had a failure using them.

Interesting. I assume these are pretty small rubber bands? Do they break or does the cord just pull out of them?
I use super thin painters tape (about 3/16" wide). It tears easily and I've not had an issue yet.
 
Interesting. I assume these are pretty small rubber bands? Do they break or does the cord just pull out of them?
I use super thin painters tape (about 3/16" wide). It tears easily and I've not had an issue yet.
Yep, blue painter tape works just fine, but the pink stuff can be stretchy. Either will work.
 
Possibly both but it really doesn’t matter. If you put rubber bands on your Z folds and pull on your shock chord the z folds slide out without too much problem. I use the thin ones and wrap them a couple of times so they hold the shock chord well. I use one on each end of the folds. With an HPR rocket, the parachute opening will pull your shock chord out without any problem.
 
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