Protecting the shock cord when using Nomex parachute protector

[overklock]

Hey RF, quick question.

I'm rebuilding my Mega Der Red Max, and purchased a Nomex chute protector for the first time. I'm still using an elastic shock cord. Ideally, I don't want to have to use dog barf or recovery wadding.

How would one go about protecting the shock cord in this scenario? Is the nomex chute protector also enough to protect the shock cord?

 

[waltr]

If the chute is wrapped in the Nomex Burrito style then no cord protection.

You could add a Nomex cord sleeve or Kevlar from anchor to just out of BT with another piece of Nomex as wadding.
I use the second method if BT has enough space.

 

[ThirstyBarbarian]

You can use a nomex cord protector sleeve, like the ones at the bottom of this page: http://topflightrecoveryllc.homestead.com/page3.html

Lots of sources for those kinds of things, so check around.

Realistically though, that elastic cord is not going to last, even with protection. Ideally, you’d find a way to replace it with a nylon or kevlar one. You can protect a nylon cord with the nomex sleeve I mentioned before, and it will last a long time.

And BTW, a lot of people still use a bit of dog barf even with protective blankets and sleeves. Nothing wrong with using both.

 

[sriegel]

And BTW, a lot of people still use a bit of dog barf even with protective blankets and sleeves. Nothing wrong with using both.

Second this! A hot ejection charge can burn through the wrap. Using some dog barf to soften that hit greatly extends the life of the blanket.

Think of it as using both seat belt and airbag in a car.

 

[overklock]

You can use a nomex cord protector sleeve, like the ones at the bottom of this page: http://topflightrecoveryllc.homestead.com/page3.html

Lots of sources for those kinds of things, so check around.

Realistically though, that elastic cord is not going to last, even with protection. Ideally, you’d find a way to replace it with a nylon or kevlar one. You can protect a nylon cord with the nomex sleeve I mentioned before, and it will last a long time.

And BTW, a lot of people still use a bit of dog barf even with protective blankets and sleeves. Nothing wrong with using both.

Any worry about a half inch nylon bridle zippering a cardboard tube?

 

[ThirstyBarbarian]

Second this! A hot ejection charge can burn through the wrap. Using some dog barf to soften that hit greatly extends the life of the blanket.

Think of it as using both seat belt and airbag in a car.

Yep. The blanket helps, but they are susceptible to blast just blowing holes right through them. The really hefty kevlar ones aren’t as susceptible, but a handful of dog barf does the trick too.

When my old blankets get a bit tattered, I give them a second life as a kind of primary blast protector. I cut them down to about the airframe diameter and poke a hole in them so the hole will fit down over the eyebolt I use as the chord anchor. Then I attach my cord in its sleeve to the eyebolt. When it’s time to fly, I position that blast protector piece of nomex over the bulk plate in the chute compartment so it covers the motor tube hole where the ejection charge blast will come from. Then I toss in a handful of dog barf on top of that. Next I z-fold the cord and set that in on top of the dog barf, followed by the chute burrito in its blanket. On top of that is the cord leader to the nosecone.

The first blanket and the dog barf on top provide a nice protection for everything above it.

 

[ThirstyBarbarian]

Any worry about a half inch nylon bridle zippering a cardboard tube?

Nope. I’ve never had it happen. I’d be concerned about a small diameter kevlar cord, but not 1/2” nylon. Of course it the delay is way off, anything can happen, but within reason, the nylon should be fine.

 

[overklock]

Nope. I’ve never had it happen. I’d be concerned about a small diameter kevlar cord, but not 1/2” nylon. Of course it the delay is way off, anything can happen, but within reason, the nylon should be fine.

Thanks! I think I may replace my elastic with Nylon. Luckily I paid attention this time and used a quick link. The Mega Der Red Max instructions have you permanently mounting the shock cord through the forward centering ring which i've come to realize is a huge no-go.

 

[waltr]

Only tube zipper I've had with 1/2 nylon happened when pulling the rocket out of a tree. We did snap the nylon cord at one point in the retrieval and left 1/4 of the chute in the tree. There were three of us hanging on the 40 foot lineman's pole.
We also had a Zephre in a tree (an L1 cert flight) with 1/2" nylon and zipped tube all to way to the motor mount pulling it down.

Other wise I braid the nylon cord so that when pulling apart the braid a lot of energy is absorbed.
Same rocket had a delay go way too long. Came in ballistic from 600 feet and ejected about 100 feet from the ground. Perfect deployment with no damage of any kind.

I do not believe in many of the 'zipper prevention' methods that have been posted.
I believe in controlling and dissipating the energy of deployment to prevent SHOCK to the recovery system.

 

[ThirstyBarbarian]

Thanks! I think I may replace my elastic with Nylon. Luckily I paid attention this time and used a quick link. The Mega Der Red Max instructions have you permanently mounting the shock cord through the forward centering ring which i've come to realize is a huge no-go.

Perfect. If you have an eyebolt and quick link setup, then I’d definitely upgrade to nylon. Here’s the super-deluxe nylon harness I have on my modified Mega Der Red Max. I kind of splurged because it was my L1 cert rocket, so it’s a bit of overkill. But it comes with the cord, sewn loops, a kevlar sleeve, and a nice quick link, all included and assembled together very nicely. If you get all the parts separately, it might add up to something similar. I’ve had mine for around 8 years. Plus Teddy AKA @Onebadhawk is one of the nicest people in rocketry, and makes great products.

https://onebadhawk.com/916-tubular-nylon-w-kevlar-sleeve--2-loop.html

 

[ThirstyBarbarian]

Only tube zipper I've had with 1/2 nylon happened when pulling the rocket out of a tree. We did snap the nylon cord at one point in the retrieval and left 1/4 of the chute in the tree. There were three of us hanging on the 40 foot lineman's pole.
We also had a Zephre in a tree (an L1 cert flight) with 1/2" nylon and zipped tube all to way to the motor mount pulling it down.

Other wise I braid the nylon cord so that when pulling apart the braid a lot of energy is absorbed.
Same rocket had a delay go way too long. Came in ballistic from 600 feet and ejected about 100 feet from the ground. Perfect deployment with no damage of any kind.

I do not believe in many of the 'zipper prevention' methods that have been posted.
I believe in controlling and dissipating the energy of deployment to prevent SHOCK to the recovery system.

My shock absorption strategy is to z-fold the nylon cord, and wrap a loop of masking tape around each folded handful. Typically you get 3-4 bunches, each with a single wrap of tape — not too much tape! The shock breaks each piece of tape, dissipating energy.

 

[overklock]

Only tube zipper I've had with 1/2 nylon happened when pulling the rocket out of a tree. We did snap the nylon cord at one point in the retrieval and left 1/4 of the chute in the tree. There were three of us hanging on the 40 foot lineman's pole.
We also had a Zephre in a tree (an L1 cert flight) with 1/2" nylon and zipped tube all to way to the motor mount pulling it down.

Other wise I braid the nylon cord so that when pulling apart the braid a lot of energy is absorbed.
Same rocket had a delay go way too long. Came in ballistic from 600 feet and ejected about 100 feet from the ground. Perfect deployment with no damage of any kind.

I do not believe in many of the 'zipper prevention' methods that have been posted.
I believe in controlling and dissipating the energy of deployment to prevent SHOCK to the recovery system.

Yeah I generally braid/crochet even my smallest rockets. Elastic shock cord or not

 

[overklock]

My shock absorption strategy is to z-fold the nylon cord, and wrap a loop of masking tape around each folded handful. Typically you get 3-4 bunches, each with a single wrap of tape — not too much tape! The shock breaks each piece of tape, dissipating energy.

Sounds quite a bit like rig packing when skydiving. I like it

 

[teepot]

I make my own shock cord protectors and blankets. I sew some of the Kevlar cloth into a sleeve and slid it down the cord. I made several and move them from rocket to rocket. I get the Kevlar aramid cloth from Amazon. I use only Kevlar shock cords. Get the delay right and no zippers. Here is how I load a tube in order from the bottom. Dog Barf, Z folded shock cord, chute burrito with Z folded chute, the rest of the shock cord.. That is whether it's LPR, MPR or HPR.

 

[waltr]

My shock absorption strategy is to z-fold the nylon cord, and wrap a loop of masking tape around each folded handful. Typically you get 3-4 bunches, each with a single wrap of tape — not too much tape! The shock breaks each piece of tape, dissipating energy.

Yes, that method also works well and is good in smaller BTs as the cord packs in less space than the braided cord.

 

[jahall4]

Unless weight is an upmost concern, the best/simple solution is always going to be, use the longest Kevlar harness you can. Kevlar for all practical purposes will not fail in a given launch due to the ejection charge (but should always be inspected). The length mitigates zippers. The saying… “If you have any room left in the airframe the harness is not long enough.”

This rocket has nearly 100 feet of harness:

 

[overklock]

Unless weight is an upmost concern, the best/simple solution is always going to be, use the longest Kevlar harness you can. Kevlar for all practical purposes will not fail in a given launch due to the ejection charge (but should always be inspected). The length mitigates zippers. The saying… “If you have any room left in the airframe the harness is not long enough.”

This rocket has nearly 100 feet of harness:

100 feet? That’s a bit extreme, no? Keep in mind the projected altitude of my current mega der red max is like, 600 feet

 

[jahall4]

100 feet? That’s a bit extreme, no? Keep in mind the projected altitude of my current mega der red max is like, 600 feet

Nope. Not for the rocket you see being tested. Deployment energy that needs to bleed off could be extreme. The point is use Kevlar, as much as you can. Harness length is the "great redeemer".

At the very least what is the tallest tree in your launch area?

 

[overklock]

Nope. Not for the rocket you see being tested. Deployment energy that needs to bleed off could be extreme. The point is use Kevlar, as much as you can. Harness length is the "great redeemer".

At the very least what is the tallest tree in your launch area?

About zero feet, as there are none over at Lucerne dry lake.

 

[jahall4]

About zero feet, as there are none over at Lucerne dry lake.

Then you are blessed. Might I suggest you fly higher than 600' In my region its difficult enough to find launch sites, much less w/o trees close by. The closest I've come is Argonia and even that site had a few small trees.

 

[overklock]

Then you are blessed. Might I suggest you fly higher than 600' In my region its difficult enough to find launch sites, much less w/o trees close by. The closest I've come is Argonia and even that site had a few small trees.

It's a double edged sword, actually getting there is quite the adventure to say the least and can be quite expensive.

Working on my Level 1 now

 

[Wrightme43]

Only tube zipper I've had with 1/2 nylon happened when pulling the rocket out of a tree. We did snap the nylon cord at one point in the retrieval and left 1/4 of the chute in the tree. There were three of us hanging on the 40 foot lineman's pole.
We also had a Zephre in a tree (an L1 cert flight) with 1/2" nylon and zipped tube all to way to the motor mount pulling it down.

Other wise I braid the nylon cord so that when pulling apart the braid a lot of energy is absorbed.
Same rocket had a delay go way too long. Came in ballistic from 600 feet and ejected about 100 feet from the ground. Perfect deployment with no damage of any kind.

I do not believe in many of the 'zipper prevention' methods that have been posted.
I believe in controlling and dissipating the energy of deployment to prevent SHOCK to the recovery system.

Sir, would you elaborate on how to braid the kevlar shock cord.
If you don't mind. I would like to understand more.

Steve

 

[mtnmanak]

Sir, would you elaborate on how to braid the kevlar shock cord.
If you don't mind. I would like to understand more.

Steve

Most people refer to it as "daisy chaining" the shock cord - if you search TRF for that, you will find a bunch of discussions and pictures. "Braiding" is something a bit different and some people do actually braid multiple strands of kevlar together to make a single shock cord. I have not done that, but the idea is that braiding multiple strands together gives it a little bit of stretch.

For daisy chaining, essentially, you just make a loop low on the shock cord, pull the shock cord above it a little way through the loop, to make another loop, and continue on up the shock cord.

Apogee has a video:

You don't need a crochet tool and you should make the "knots" looser than he does in the video. Just lightly snug the loop closed each time and you end up with a nicely daisy chained shock cord.

I use daisy chaining on my small and mid-size rockets. Once you move up into larger sized shock cords, I recommend moving to Z-Folding. I hold my Z-Folds together with dental bands. I use Z-Folds for rockets starting at about 10 pounds and everything above that weight.

Couple Z-fold pics (note the rubber band in the second photo is NOT what I would use for flight - it is just how I store the shock cord. For flight, I replace it with a dental rubber band.)

 

[tsmith1315]

Couple Z-fold picks (note the rubber band in the second photo is NOT what I would use for flight - it is just how I store the shock cord. For flight, I replace it with a dental rubber band.)

And the zip-tied+heat shrink loop is for parachute attachment?

 

[mtnmanak]

And the zip-tied+heat shrink loop is for parachute attachment?

Yes, I usually sew my own loops into nylon cord (for Kevlar, I just get them from Teddy - sewing Kevlar is not fun!) and I generally use a 3-loop for my drogues. I do use Kevlar thread for the sewing part. I then put heat shrink around all the sewn portions. I found that the middle loop gets a lot of stress because it is being torn apart length-wise, especially when the main opens, so I started putting a zip tie at the low end of the middle loop and heat shrinking that on. I started doing that about 5 years ago and never had a problem since. This is also an older photo - these days, I also put a properly sized swivel in that middle loop since most of the smaller drogue sized parachutes don't have usually have a built in swivel.

Note, this is largely only a problem because I hand sew my loops. At some point, I just bought a few rolls of different sized nylon and like to make my own shock cords exactly to the size I want. It is probably a waste of my time vs. buying a commercial shock cord, but it is just one of those things I like about scratch building - making your own parts where I can

 

[tsmith1315]

I just bought a few rolls of different sized nylon and like to make my own shock cords exactly to the size I want

Completely understandable. And a nice reminder to buy more tubular nylon soon.

 

[mtnmanak]

Going all the way back to the OP's first post, may I suggest another way to think about the recovery system? Separate the "attach point" of the system from the "shock cords" of the system.

Essentially, when I build a rocket, the "attach point" is securely (and, for smaller rockets, permanently) attached to the motor mount. I like to ensure it comes away from the motor mount tube in a "Y" configuration (as opposed to being attached to one point at, or near, the centering ring) so the end loop of your attach point is centered in the middle of your booster tube.

Here is a photo from a 54mm MMT to illustrate:



This is also an older photo and I now terminate the end loop (where the purple heat shrink tube is) with a properly sized swivel. If you use a heavy Kevlar cord for this attach point (that is appropriate for the rocket size), it will last darn near forever. Note in the picture above, the thick Kevlar used is way, way overkill in terms of the rocket size/weight, but, since it will be the permanent attach point for the recovery system, I did not see a reason to skimp . This should allow the attach point to weather almost endless deployment charges.

I size the length of the attach point to end so the swivel generally is touching the edge of the body tube:



Ideally, the middle of the swivel ends up on the edge of the body tube, which helps to prevent zippering:



I use this method even with small LPRs. For really big rockets, the attach point is usually removeable and attached to large U-bolts. Here is a photo of a recovery system I made for a 100 pound 8" diameter rocket. Part number 47 is the attach point, which is formed from two pieces of 5000# rated sewn Kevlar and a 5000# rated swivel (all the Kevlar cords and the attach point in this picture are from Onebadhawk):



By separating out the attach point, you can ensure you get a very secure mounting in your rocket that will not fail, you know it is centered and has a built in swivel and then you can easily attach any shock cord you want in the future by quick-linking it to the swivel (for small rockets, you can even use a small zip-tie to secure the shock cord to the swivel since a quick link may be heavier than the whole rocket or use a small fishing swivel with an openable clip), which is up near the top of the body tube, instead of messing around with eye-bolts and quick links way down by the motor mount centering ring.

I am not suggesting this is the "right" way to do it, just an alternative method of engineering your booster recovery attachments that I have found to work well and is scalable to virtually any size rocket you may build. I recently built an LPR with my daughter and we used that method and I am also in the process of building a 12" diameter rocket that will end up being around 500 pounds on the pad and the same method works just fine.

Also, a simple way to protect cords that will be exposed to deployment charges is to wrap them in masking tape. A single layer of green painters tape lasts multiple flights and is easily replaced when it gets beat up from the deployment charges.

 

[Wrightme43]

You guys are geniuses. I really appreciate the different ideas and methods.

 

[waltr]

I see the question as to how to braid a cord has been answered. This works great of tubular nylon.

The other method I use for Kevlar is taped bundles. Three to four loops of cord with tape around the 'bundle' like was shown above with a rubber bad. This also absorbs energy as the bundles pull apart.

I also use the anchor methods posted above. I do like the double 'heavy' kevlar epoxied to the MMT mtnmanak showed. Then on bigger rockets a hard point (eyebolt) to attach cord.