Possible causes of shock cord break?

[billdz]

See attached photo, any thoughts on what might have caused this shock cord to break during flight? I know it is not Kevlar, it's the stock cord for my Estes Mammoth, I added a payload section at the top. I've launched it twice before with no issues, used an Estes F15-6 motor for all 3 flights. No burn marks on the cord. To my knowledge, the cord was not frayed prior to the flight.

The chute was connected about 1.5" feet from the payload end of the cord, so once the cord broke the payload was in free fall for over 1000'. There was a Posh Micro phone inside, and remarkably it survived, albeit with a crack in the glass. After this, I'm wondering if it is safer to connect the chute directly to the payload, not to the shock cord.

 

[Buckeye]

Elastic is for underpants.

 

[blackwing94]

From the way it's frayed, it looks like it just pulled apart. Could it have been damaged by rubbing the rim of the booster tube?

Just another data point.... For rockets in the E, F, G range, I use 250 or 550 parachord from Hobby Lobby or Michaels. Easy to pack, tie a loop knot in, or replace.

 

[billdz]

Thanks for the replies and the paracord tip. I don't think the cord was rubbing on the rim but I suppose anything is possible. Do you attach the chute to the NC or payload or do you attach to a loop on the cord? I've heard it is better to attach to the cord but perhaps more dangerous.

 

[afadeev]

See attached photo, any thoughts on what might have caused this shock cord to break during flight? I know it is not Kevlar,

That's part of the problem.
Leave the elastic for underwear duty (or for low-powered rockets without payload), and stick with nylon braided or kevlar cord for shock-cord duty.
I bought a 100-foot spool of kevlar cord to use as a replacement for elastic cords included in all low-power or mid-power kits:
https://www.amazon.com/gp/product/B01850OST0/?tag=skimlinks_replacement-20

On HP rockets, I add an additional kevlar sleeve to the shock cords in the areas exposed to ejection charge heat.

it's the stock cord for my Estes Mammoth, I added a payload section at the top. I've launched it twice before with no issues, used an Estes F15-6 motor for all 3 flights. No burn marks on the cord. To my knowledge, the cord was not frayed prior to the flight.

Based on the picture, it looks like it tore in flight from either snagging on something sharp (fins?) or just that the combined weight of the nose cone with your heavier payload was too much for that size elastic cord.

After this, I'm wondering if it is safer to connect the chute directly to the payload, not to the shock cord.

Please don't do that - you need to give the chute time to gradually unfold and inflate. Otherwise, the chute may shred/rip/tangle up from the force of the ejection charge!
Longer airframe bodies also generate a fair amount of aerodynamic drag, and descent semi-horizontally. You do NOT want to attempt to deploy a chute in dirty air right above the highly draggy object. The airframe will disrupt the airflow in unpredictable ways that you can't model. The chute may not deploy fully, it may deploy, than collapse in dirty air, may flutter, and otherwise not function as expected.

For all of these reasons, you want to have a shock cord about 3x the length of the rocket, and ideally, have chute tied 1/3 of the way from the nose cone to the main airframe.

More relevant reading materials here:
https://www.google.com/url?sa=t&rct...etter348.pdf&usg=AOvVaw2CcfZaPuU0qzipbG_JY7FB

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[Handeman]

That seems to be a classic case of too large of ejection charge. The charge forced the nosecone/payload off the rocket at a high speed and the weight and inertia of the nosecone/payload caused the elastic to extend past its breaking point.

Unless you've had an Estes motor eject on the ground, it's hard to appreciate how strong those ejection charges are. Of course you have no control over the size or force of the charge so all you can do is control the externals. If the rocket came with a plastic nose cone and that elastic it was probably designed to work with that weight on the end of the elastic during ejection. You added weight to the nose cone by adding a payload section, that is probably why it broke. The elastic can't handle the heavier nose cone. All rocket design is compromise. In this case you added weight to the ejected part and didn't compensate with longer elastic, or stronger shock cord.

 

[ksaves2]

Old age and too much force! Kurt

 

[billdz]

The rocket had the same payload and motor on the earlier flights, but I see your point. I'll head to Michaels tomorrow.

 

[billdz]

OK folks, bought some paracord, but I'm now wondering how safe it is to remove the original shock cord. As the attached pic shows, the cord is glued to a black plastic piece that is glued to the airframe. If I attempt to remove it, the airframe will likely be damaged. My first thought was to tie the paracord to the elastic cord, but a chain is only as strong as its weakest link, so maybe that's not a good idea. Second thought was to fold the elastic cord, glue the free end to the airframe between the black plastic piece and the top, thereby producing a double strip of elastic cord to tie to the paracord. Any better ideas?

 

[PatB]

Numerous options- the simplest would be to simply clean the inside of the body tube and epoxy your new cord in place. Paracord is rather thick, and if you went with that method then I'd only use the outer sheath of the cord by removing the inside. That would allow it to lay flat for gluing purposes.

For additional support you could slide a short coupler down the tube that has a slice out of it so that it clears that piece of plastic. Your new shock cord could be attached to the inside of that new coupler, or wedged in between the coupler and outer airframe.

I prefer more strength and would probably drill a hole through any centering rings you have attached to your motor tube and run your new shock cord up through the bottom of the rocket. Tie a knot for support, and use epoxy to also secure it. If you were able to remove the bottom centering ring then you could also wrap your shock cord around the motor tube and epoxy it in place, then replace the centering ring with a new one. It only takes a few minutes with a Dremel tool and Exacto knife to get the old one out.

 

[afadeev]

OK folks, bought some paracord, but I'm now wondering how safe it is to remove the original shock cord. As the attached pic shows, the cord is glued to a black plastic piece that is glued to the airframe. If I attempt to remove it, the airframe will likely be damaged.

I would not touch the old mount. Cut the elastic off at the mount, and leave the mount alone.

Then re-mount the new cord using a method similar to the one described in the LOC instruction sheet below, see "Shock Cord Mount Instructions" section:
https://www.google.com/url?sa=t&rct...on-Sheet.pdf&usg=AOvVaw3QpEqBQyQCWU_QrKbZ4h8m


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[billdz]

Guess no one thinks it would be OK to use a portion of the old elastic cord doubled over? Your methods make sense, and I've successfully used the LOC method on other rockets. My only hesitation is that the plastic piece makes a significant indentation on the inside of the airframe, and a second mount would make it even tighter, possibly inhibiting the laundry from ejecting. Attaching the cord to the motor mount would be ideal but it's in deep and it won't be easy.

 

[afadeev]

Guess no one thinks it would be OK to use a portion of the old elastic cord doubled over? Your methods make sense, and I've successfully used the LOC method on other rockets. My only hesitation is that the plastic piece makes a significant indentation on the inside of the airframe, and a second mount would make it even tighter, possibly inhibiting the laundry from ejecting. Attaching the cord to the motor mount would be ideal but it's in deep and it won't be easy.

If the old plastic mount did not interfere with chute deployment before, it won't start doing so after you return it from active duty. If it can be safely and easily removed - do it. But no point in destroying the airframe if it was epoxied in place well.

LOC-style loop has minimal profile, and when attachment point is properly covered with epoxy, both reinforces the tube and cures into a smooth and slippery surface that does not interfere with laundry.

Motor mount shock cord attachment is ideal, but is virtually impossible once MMT has been glued in place.

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[Handeman]

All this is good, but I would highly recommend buying a few feet of 1/4" or 3/8" tubular Kevlar and use that in the BT. You can tie anything you want to it.

Para-cord is very highly susceptible to heat, Kevlar is not. If you don't loose the rocket when the para-cord separates and it will separate, you will be going through this again. I would put a piece of Kevlar in the BT and it should last longer then the rocket. If not Kevlar, a 1" wide thick nylon strap like PML uses on the motor side of their pistons would last a long time.

 

[billdz]

Guess you can't beat kevlar.

 

[Handeman]

Guess you can't beat kevlar.

Depends on who you talk to and what their criteria is for why it's used. I think it's hard to beat, but other like the softer shock provided by the more elastic tubular nylon. It's like everything else in this hobby, you have to make your own choice and live with it. Change it if it doesn't work the way you want.

 

[billdz]

Is there any local source for kevlar cord? I'm not seeing it at Home Depot, Lowes, or Michaels.

 

[tim cubbedge]

Wildman, Missileworks, Giant Leap are great on-line sources for Kevlar! Bernie sells it at NEFAR & SRA.

 

[Onebadhawk]

Is there any local source for kevlar cord? I'm not seeing it at Home Depot, Lowes, or Michaels.

Try here--

https://onebadhawk.com/light-kevlar.html

Super inexpensive and you'll damage the rocket before you damage the harness..

Teddy

 

[blackjack2564]

You'll be hard pressed to find a better deal than Teddy has going. get the 1/8 TUBULAR, you won't regret it & his shipping is downright cheap.

You'll wear out the rocket before the cord & can move to a different one.

His goods are of the finest quality....ALL KEVLAR IS NOT THE SAME!