Help: Cut Kevlar During Descent!

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edwinshap1

edwinshap1

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This is the second time this has happened to a rocket I've flown.

Nose eject a drogue at apogee, and all I end up finding is the GPS since the body cut the Kevlar during descent and caused separation.

What can be done to fix this, swivels, thicker harness, rounding the inside of the airframe, not using carbon fiber, other?

I've attached a pic of the Kevlar found on the nosecone.ImageUploadedByRocketry Forum1412620935.400379.jpg
 
Maybe try a zipper bubble where the cord exits the body tube?
 
It would help to know some more information such as rocket kit, materials, size, length, weight, shock cord diameter, etc.

I wrap a couple layers of duct tape around my shock cords to prevent this. I also use swivels on my parachutes to attempt to lessen the amount of spinning around of the booster and payload during descent.
 
Heat shrink tube where the Kevlar passes over the edge. For larger rockets try to find horse harness swivels, very heavy duty as you can imagine.
 
DuctTapeandRocketFuel said:
It would help to know some more information such as rocket kit, materials, size, length, weight, shock cord diameter, etc.

I wrap a couple layers of duct tape around my shock cords to prevent this. I also use swivels on my parachutes to attempt to lessen the amount of spinning around of the booster and payload during descent.
Click to expand...

3" CF minimum diameter, around 10 lb after burnout. 11" drogue at apogee, was supposed to pop at 60" at 1000 ft.

Shock cord was 1/4" 1500 lb test kevlar.

All these options sound good. I know GLR has an option for a safety ball to go on the shock cord. Any links on a horse harness swivel that has a load rating?
 
Wow Edwin,
I am truly impressed..
I usually don't respond to threads like this for fear of looking like I'm trying to drum up business..
I am not... I don't need to...
First,,
It's hard to get a feel for the size of that Tubular Kevlar from the pic you posted..
But I'd say, for sure,, On a 3" rocket, even best, lightest case, 1500 lb material is too light..
11 lbs on the main chute is a bit heavier then the best case scenario...
That really is very strong material,, I am impressed that you were able to snap that like that...
The only time I've ever seen a break like that is when the material is severely under rated for the job at hand...
You surly must have enough room in a 3" airframe for heavier material then that...

If you'd like,, I would be happy to help...
I have some material that would be perfect for that application...

Teddy
 
I had a rocket stuck in a tree once - hanging and blowing and spinning in the wind from the top of the tree.

It took about a month for the thin - BT80 cardboard tube to cut the #750 Kevlar shock cord. Very little damage to the edge of the tube, the frayed Kevlar looked just like your picture.

I'm guessing the edge of your carbon tube is acting like a saw blade.

I would try a combination of things:
- Sanding, rounding, smoothing the edge of the tube.
- swivels
- heavier Kevlar, or a different supplier
- heat shrink or something to protect the Kevlar.
 
I always use a small piece of heat shrink tubing on the kevlar right where it exits the body tube on my fiberglass and carbon fiber builds. I have never had a problem since I started doing this.
 
Kludgy as it sounds, duct tape at the exit point on the cord spreads the load to help prevent zippers and sawing action. I nake about a 1" square 'pad' flattened out around the line 1/2" above and below the lip of the b/t. I like the shrink tubing idea a lot also.
 
onebadhawk said:
Wow Edwin,
I am truly impressed..
I usually don't respond to threads like this for fear of looking like I'm trying to drum up business..
I am not... I don't need to...
First,,
It's hard to get a feel for the size of that Tubular Kevlar from the pic you posted..
But I'd say, for sure,, On a 3" rocket, even best, lightest case, 1500 lb material is too light..
11 lbs on the main chute is a bit heavier then the best case scenario...
That really is very strong material,, I am impressed that you were able to snap that like that...
The only time I've ever seen a break like that is when the material is severely under rated for the job at hand...
You surly must have enough room in a 3" airframe for heavier material then that...

If you'd like,, I would be happy to help...
I have some material that would be perfect for that application...

Teddy
Click to expand...

I had a 100G shock load and was still below rated load. And the cuts are way too smooth to be a snap.

I do like everybody's suggestions though, I'll have to incorporate them into the next rocket the club makes.

And to those who have multiple options that you use, do you find that softer/more malleable materials are better, or just that building up material over the important location is what does the trick?

Also I should note the wind was less than 3 mph and I had a 1000 ft main eject altitude. The nosecone landed in the middle of a lakebed, so if it failed at main deploy it wouldn't have been more than 1000 ft away, and we could see over a mile. No parachute billowing in the wind, no orange nomex, so it must have separated at a higher altitude. Apogee was >13500 at a 30+ degree angle.
 
Last edited:
edwinshap1 said:
I had a 100G shock load and was still below rated load. And the cuts are way too smooth to be a snap.

I do like everybody's suggestions though, I'll have to incorporate them into the next rocket the club makes.

And to those who have multiple options that you use, do you find that softer/more malleable materials are better, or just that building up material over the important location is what does the trick?

Also I should note the wind was less than 3 mph and I had a 1000 ft main eject altitude. The nosecone landed in the middle of a lakebed, so if it failed at main deploy it wouldn't have been more than 1000 ft away, and we could see over a mile. No parachute billowing in the wind, no orange nomex, so it must have separated at a higher altitude. Apogee was >13500 at a 30+ degree angle.
Click to expand...

From everything you've said I don't see a doubt that the failure occurred at apogee separation,, as opposed to the main event...
In my experience,, the material was too light..
The material I would suggest is the 11/32" 400 lb. Tubular Kevlar you'll find on my site..
And even that,, under these circumstances I'd sleeve just where it exits the airframe...
Anything you can do to spread out the load,, so it's not on such a sharp point...
3" or 4" of 3/4" Tubular Kevlar would do the trick quite nicely...

I'm sorry,,
I have to go watch dancing with the stars now or else I'm going to be seeing stars very shortly......

Teddy
 
How embarrassing...............
She was calling and calling...............
I woulda got beat up............

Teddy
 
How long was you Kevlar?
Are you sure it was cut after apogee?

If not, try threading it through some TN as a protector?


JD

edwinshap1 said:
3" CF minimum diameter, around 10 lb after burnout. 11" drogue at apogee, was supposed to pop at 60" at 1000 ft.

Shock cord was 1/4" 1500 lb test kevlar.

All these options sound good. I know GLR has an option for a safety ball to go on the shock cord. Any links on a horse harness swivel that has a load rating?
Click to expand...
 
Duct tape is your friend! Carbon tube will cut Kevlar like nothing. Had a main come out at apogee and it almost cut through a 1/2 Kevlar harness from 10 thousand feet. Put some tape on it and no more problems again.

Jason
 
Likely your drogue is too small or your recovery harness is too long, or both.

Your fins are likely generating more drag than your drogue is, causing the fin can to be at about the same altitude as the drogue. When the main deploys, it's stopping dead while the fin can likely continues to fall until it hits the end of the harness with a pretty solid jerk.

Even with a carbon fiber tube, it takes a fair amount of force to cut through 1/8" Kevlar like that.

Reduce the load you're getting when the airframe hits the end of the harness and you'll solve the problem.

-Kevin
 
By the picture it looks to be 1/8" Kevlar. I use a small section tubular nylon and slip it over the Kevlar where it comes in contact with body tube. glue it or just sew it by hand. it doesn't take much to hold it in place.
 

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