Kevlar Shock Cord Damage

[cornbeef007]

Hey everyone,

I attempted my L2 with a 2.6" fiberglass Madcow Tomach and it didn't go as planned. I had a drag separation of the lower fuselage right at burnout. Luckly, the drogue sacrificed itself and the rest of the rocket was mostly OK, so I will get to use it to try again. I've come up with some fixes for the drag separation issue but I have some damage that I could use some opinions on:

I had a little zippering, which damaged the kevlar shock cord. The cord is mounted permanently around the motor mount, so repair is kind of difficult. A fix recommended at the launch was to cut the line below the damaged section. Then knot the line and add a replacement line above that knot. While I'm ok with this fix, my concern is for the force that the entire line has sustained. It was enough force to slightly mangle the quicklink and eyebolt it was attached too.

I'm thinking the best plan at this point is to use a threaded forward closure on motor casing and just use a plugged motor. Attach the shock cord to the forward closure and that way it can be replaced easily when needed. Is this what the threading is intended for? I'm using Aerotech 54mm hardware. Is this type of setup considered normal?

Any other ideas?

Thanks much!

 

[Art Upton]

On even smaller rockets I drill holes in the centering rings, feed the shock cord to the aft of the rocket with a feed stiff wire and loop the cord around the motor tube; Lariat Loop. If it breaks I can replace it. Maybe you could do something like that?

A build using a loop:

https://www.rocketryforum.com/threads/pro-series-ii-e2x-power-build.132757/#post-1561505

 

[spigalau]

a) Use a shear pin (or three)
b) apply some heat shrink to the kevlar where the cord exits the tube (reduces abrasion)
c) Remove the cord, attach new cord to forward enclosure

 

[RokitFlyr]

Cut it, tie it. It'll be fine.

Edit: to make it easier, pull the shock cord out the aft end. Don't have to try and reach inside.

 

[rharshberger]

Cut it, tie it. It'll be fine.

Edit: to make it easier, pull the shock cord out the aft end. Don't have to try and reach inside.

Good tip Mark, I forgot to point that out to the OP when he brought the rocket back to the RSO table after the flight. Pulling the shock cord out the rear will allow easier shortening of the recovery harness.

To the OP an eyebolt in the forward closure (a plugged threaded FC) is a totally acceptable method of securing the shockcord.

 

[manixFan]

I've often posted how kevlar has to be protected over any kind of edge. It does not like any kind of sharp bend, and it's worse when the edge is abrasive. One easy solution (as mentioned by @spigalau) is several layers of heat shrink tubing long enough to completely protect the kevlar where it meets the body tube. I generally use about 4" lengths that I will layer over each other. Even then I often have to replace it after a few flights, especially if it's in a heavy fin can. I've used other methods to protect the kevlar, including threading it through a cork, but it's hard to keep that place correctly.

And keep in mind that most knots knock about 30-50% off the strength of kevlar – used splices or a wrapped knot when possible.


Tony

 

[MarsLander]

wrapped knot

What is this ^^^ ?

 

[kramer714]

dumb question, did you have a vent hole in your body tube?

Just asking....

 

[Art Upton]

Use a Water Knot ?

https://www.animatedknots.com/water-knot

 

[manixFan]

What is this ^^^ ?

check out this post:

https://www.rocketryforum.com/threads/tying-knots-in-kevlar-cord.169359/#post-2202248

Tony

 

[cornbeef007]

Hey thanks for the great replies!

I did have a vent hole and I’m going to make it slightly larger. Along with two shear pins, I’m pretty confident the drag separation won’t happen again.

I was able to pull the cord aft, so it’s pretty easy to work with. The cord is in good shape, so I’m just going to use the knot and attach a new line there.

I like the idea of using the heat shrink to protect the line.

I was going to ask about which knot to use but that was answered.

 

[cornbeef007]

Good tip Mark, I forgot to point that out to the OP when he brought the rocket back to the RSO table after the flight. Pulling the shock cord out the rear will allow easier shortening of the recovery harness.

To the OP an eyebolt in the forward closure (a plugged threaded FC) is a totally acceptable method of securing the shockcord.

Thanks again for the help yesterday.

I think the knot is going to be the way to go. The remaining line below the cut is in great shape.

I’ve switched out the 1515s for the 1010s and added the shear pins and enlarged the vent. I think I’ll be good for the next launch.

 

[Zman1961]

I've often posted how kevlar has to be protected over any kind of edge. It does not like any kind of sharp bend, and it's worse when the edge is abrasive. One easy solution (as mentioned by @spigalau) is several layers of heat shrink tubing long enough to completely protect the kevlar where it meets the body tube. I generally use about 4" lengths that I will layer over each other. Even then I often have to replace it after a few flights, especially if it's in a heavy fin can. I've used other methods to protect the kevlar, including threading it through a cork, but it's hard to keep that place correctly.

And keep in mind that most knots knock about 30-50% off the strength of kevlar – used splices or a wrapped knot when possible.


Tony

I use several wraps of bright orange duct tape as the sacrificial material placed on the shock cord where it would rub on the airframe. It stays in place and is relatively easy to remove when it's worn out.

 

[thzero]

I use several wraps of bright orange duct tape as the sacrificial material placed on the shock cord where it would rub on the airframe. It stays in place and is relatively easy to remove when it's worn out.

I like to use high temp fabric tape for same purpose.

 

[thzero]

Hey everyone,

I attempted my L2 with a 2.6" fiberglass Madcow Tomach and it didn't go as planned. I had a drag separation of the lower fuselage right at burnout. Luckly, the drogue sacrificed itself and the rest of the rocket was mostly OK, so I will get to use it to try again. I've come up with some fixes for the drag separation issue but I have some damage that I could use some opinions on:

I had a little zippering, which damaged the kevlar shock cord. The cord is mounted permanently around the motor mount, so repair is kind of difficult. A fix recommended at the launch was to cut the line below the damaged section. Then knot the line and add a replacement line above that knot. While I'm ok with this fix, my concern is for the force that the entire line has sustained. It was enough force to slightly mangle the quicklink and eyebolt it was attached too.

I'm thinking the best plan at this point is to use a threaded forward closure on motor casing and just use a plugged motor. Attach the shock cord to the forward closure and that way it can be replaced easily when needed. Is this what the threading is intended for? I'm using Aerotech 54mm hardware. Is this type of setup considered normal?

Any other ideas?

Thanks much!

Or leave it? Doesn't look that bad.. fraying on the outside layers, but probably overkill on a 2.6". Heat shrink and/or tape to protect it against further rubbing.

 

[cornbeef007]

Or leave it? Doesn't look that bad.. fraying on the outside layers, but probably overkill on a 2.6". Heat shrink and/or tape to protect it against further rubbing.

I ended up cutting it below the fray and attaching a new line. I have incorporated tape to protect the line from here on out.

 

[Buckeye]

Hey thanks for the great replies!

I did have a vent hole and I’m going to make it slightly larger. Along with two shear pins, I’m pretty confident the drag separation won’t happen again.

I am curious about the drag separation, as that is a very rare occurrence. The Tomach is a pretty average rocket with smallish fins. Did you have an extreme amount of mass in the upper section? Was the coupler fit ridiculously loose? Could this have been a motor failure, instead?

 

[rharshberger]

I am curious about the drag separation, as that is a very rare occurrence. The Tomach is a pretty average rocket with smallish fins. Did you have an extreme amount of mass in the upper section? Was the coupler fit ridiculously loose? Could this have been a motor failure, instead?

I visually observed the separation via excellent quality binoculars and inspected the rocket upon recovery. Motor was fine and there were no signs of abnormalities, separation occured almost exactly at motor burnout. Coupler was proper fitted but not shear pinned, as I also RSO'd the rocket pre-flight and requested that the flier tighten the fit up more as I felt it was too loose, which the flier did. IMO shear pins are a must in this type of rocket in both section for consistency reasons, friction fit is just too variable and may or may not be the "right" fit.

Drag separations are not rare, they happen quite often.