true strength of shock cord ???


Gary Slater
 

I am building two small diameter rockets, both scratch.  One is 2.56 ID the other 2.14 ID, both have 29 mm motor tubes.  So there is little room for eye bolts down there.  I have seen lots of build threads on TRF where Kevlar cords are epoxied to the motor tube, which is what I am considering for both.  Some make these just long enough to come to top of air frame where you can reach it to attach a shock cord like nylon.  I have found some braided Kevlar cord on ebay rated at 2000 #.  It's only 3.5 mm.  But there is also a note that states "NOTE! Safe Working Load: 20% of original strength".  Both rockets are going to be in the 2-4 pound range.  The 2000 # rating seems quite adequate but that 20% is bothersome.  Are all cords really only that good?  Would any of you use this product?  


Wes Cravens
 

I think that the primary reason that they don't go all the way out of the body tube is that it's really easy for kevlar to cut a line in your rocket like cheese wire.  Usually because they are so thin.  3.5mm Kevlar is probably overkill for sub 4"-5" rockets.  I could be wrong, but my instinct is that you can get away with much smaller.  Of course, it it was my rocket, I'd be doing math too.

Working load is probably the weight that the cord can carry day in day out forever.  So 20% of 2000# forever is a bunch more than 2-4 lbs.  I've not done much research into the impulse strength of kevlar.  Probably not as good as dyneema but still probably better than just about any other material that you're likely to put into a mid sized rocket.

So yes, I would trust the product you are describing, but I also think that you are at least an order of magnitude (or two) over engineered.

$0.02,

Wes 

On Thu, Apr 30, 2020 at 5:19 PM Gary Slater <gws77@...> wrote:
I am building two small diameter rockets, both scratch.  One is 2.56 ID the other 2.14 ID, both have 29 mm motor tubes.  So there is little room for eye bolts down there.  I have seen lots of build threads on TRF where Kevlar cords are epoxied to the motor tube, which is what I am considering for both.  Some make these just long enough to come to top of air frame where you can reach it to attach a shock cord like nylon.  I have found some braided Kevlar cord on ebay rated at 2000 #.  It's only 3.5 mm.  But there is also a note that states "NOTE! Safe Working Load: 20% of original strength".  Both rockets are going to be in the 2-4 pound range.  The 2000 # rating seems quite adequate but that 20% is bothersome.  Are all cords really only that good?  Would any of you use this product?  


Daniel Greuel
 

I’ve read it should be 50times the static weight of the rocket. So 200# for 4# rocket. What you got should be plenty. I run 1/4 tubular on my 38mm and it is overkill and takes up valuable space. 1/8” would be plenty in a 2# rocket. I like to Brummell splice my ends and 1/4” is about as small as I can handle splicing loops. I also use electric shrink wrap to hold the Brummell splice and to protect Kevlar where it rubs on the end of a tube. 

Dan


On Apr 30, 2020, at 5:33 PM, Wes Cravens <wcravens@...> wrote:


I think that the primary reason that they don't go all the way out of the body tube is that it's really easy for kevlar to cut a line in your rocket like cheese wire.  Usually because they are so thin.  3.5mm Kevlar is probably overkill for sub 4"-5" rockets.  I could be wrong, but my instinct is that you can get away with much smaller.  Of course, it it was my rocket, I'd be doing math too.

Working load is probably the weight that the cord can carry day in day out forever.  So 20% of 2000# forever is a bunch more than 2-4 lbs.  I've not done much research into the impulse strength of kevlar.  Probably not as good as dyneema but still probably better than just about any other material that you're likely to put into a mid sized rocket.

So yes, I would trust the product you are describing, but I also think that you are at least an order of magnitude (or two) over engineered.

$0.02,

Wes 

On Thu, Apr 30, 2020 at 5:19 PM Gary Slater <gws77@...> wrote:
I am building two small diameter rockets, both scratch.  One is 2.56 ID the other 2.14 ID, both have 29 mm motor tubes.  So there is little room for eye bolts down there.  I have seen lots of build threads on TRF where Kevlar cords are epoxied to the motor tube, which is what I am considering for both.  Some make these just long enough to come to top of air frame where you can reach it to attach a shock cord like nylon.  I have found some braided Kevlar cord on ebay rated at 2000 #.  It's only 3.5 mm.  But there is also a note that states "NOTE! Safe Working Load: 20% of original strength".  Both rockets are going to be in the 2-4 pound range.  The 2000 # rating seems quite adequate but that 20% is bothersome.  Are all cords really only that good?  Would any of you use this product?  


Greg Smith
 

I have used 1500 pound test Kevlar in rockets up to about 4” diameter with no problems whatsoever.

 

I would rethink the idea of not using a U-bolt, however. It will make it nearly impossible to replace the Kevlar cord if it does wear out. Both of the two video camera rockets of mine that you have seen frequently are 2.14” diameter with a 29mm motor mount, and have a U-bolt in the front centering ring. The trick is to find a small enough U-bolt – and a great source for these is to disassemble a small (1/8” or ¼”) wire rope clip. Check them out at the hardware store and see if you can find one that will work. Lots of epoxy is recommended, and a time or two, I have had to slightly grind down the edge of one of the nuts where it protruded outside the edge of the centering ring, but it can be made to fit.

 

- GDS

 

 

 

From: cia-rocketry@groups.io [mailto:cia-rocketry@groups.io] On Behalf Of Gary Slater
Sent: Thursday, April 30, 2020 5:20 PM
To: cia-rocketry@groups.io
Subject: [cia-rocketry] true strength of shock cord ???

 

I am building two small diameter rockets, both scratch.  One is 2.56 ID the other 2.14 ID, both have 29 mm motor tubes.  So there is little room for eye bolts down there.  I have seen lots of build threads on TRF where Kevlar cords are epoxied to the motor tube, which is what I am considering for both.  Some make these just long enough to come to top of air frame where you can reach it to attach a shock cord like nylon.  I have found some braided Kevlar cord on ebay rated at 2000 #.  It's only 3.5 mm.  But there is also a note that states "NOTE! Safe Working Load: 20% of original strength".  Both rockets are going to be in the 2-4 pound range.  The 2000 # rating seems quite adequate but that 20% is bothersome.  Are all cords really only that good?  Would any of you use this product?  


Gary Slater
 

I agree about the U-bolt Greg. I found this (https://giantleaprocketry.com/products/components_recovery.aspx) at Giant Leap and am considering it.  It's only about $7-8.  Have to use JB Weld.  I would also put a small ring above it.  I don't know the weight but I can also move it as far forward as allowable for the chute and shock cord.  A U-bolt for a 54mm tube will be problematic with a 29mm MMT.  A U- bolt for the  2.56 tube might be easier to find.  


On April 30, 2020 at 5:54 PM Greg Smith <gregs@...> wrote:

I have used 1500 pound test Kevlar in rockets up to about 4” diameter with no problems whatsoever.

 

I would rethink the idea of not using a U-bolt, however. It will make it nearly impossible to replace the Kevlar cord if it does wear out. Both of the two video camera rockets of mine that you have seen frequently are 2.14” diameter with a 29mm motor mount, and have a U-bolt in the front centering ring. The trick is to find a small enough U-bolt – and a great source for these is to disassemble a small (1/8” or ¼”) wire rope clip. Check them out at the hardware store and see if you can find one that will work. Lots of epoxy is recommended, and a time or two, I have had to slightly grind down the edge of one of the nuts where it protruded outside the edge of the centering ring, but it can be made to fit.

 

- GDS

 

 

 

From: cia-rocketry@groups.io [mailto:cia-rocketry@groups.io] On Behalf Of Gary Slater
Sent: Thursday, April 30, 2020 5:20 PM
To: cia-rocketry@groups.io
Subject: [cia-rocketry] true strength of shock cord ???

 

I am building two small diameter rockets, both scratch.  One is 2.56 ID the other 2.14 ID, both have 29 mm motor tubes.  So there is little room for eye bolts down there.  I have seen lots of build threads on TRF where Kevlar cords are epoxied to the motor tube, which is what I am considering for both.  Some make these just long enough to come to top of air frame where you can reach it to attach a shock cord like nylon.  I have found some braided Kevlar cord on ebay rated at 2000 #.  It's only 3.5 mm.  But there is also a note that states "NOTE! Safe Working Load: 20% of original strength".  Both rockets are going to be in the 2-4 pound range.  The 2000 # rating seems quite adequate but that 20% is bothersome.  Are all cords really only that good?  Would any of you use this product?