Braided steel shock cord?

[Rktman]

I'm strictly an LPR person so my experience using a D engine surprised me; on my model's 2nd D engine flight the ejection charge totally fried through the 1200lb kevlar shock cord right above the engine tube. To say I wasn't expecting this would be an understatement, and luckily the cord parted ways with my chute and nose cone only 10 feet above the ground. Granted a D or E black powder motor isn't designed to heft a heavier fiberglass/plywood/composite rocket so I wouldn't expect the ejection charge to be so violent.

Does anyone here in the HPR group use braided steel cord, like the flexible stuff used to hang pictures on the wall? Or just heavier gauge kevlar cord/ribbon? Thought I'd ask the folks with the most experience dealing with large motor issues. Your feedback/suggestions would be greatly appreciated, as I'm about to scratch build a model that will use D or E motors.

 

[kuririn]

I'm strictly an LPR person so my experience using a D engine surprised me; on my model's 2nd D engine flight the ejection charge totally fried through the 1200lb kevlar shock cord right above the engine tube. To say I wasn't expecting this would be an understatement, and luckily the cord parted ways with my chute and nose cone only 10 feet above the ground. Granted a D or E black powder motor isn't designed to heft a heavier fiberglass/plywood/composite rocket so I wouldn't expect the ejection charge to be so violent.

Does anyone here in the HPR group use braided steel cord, like the flexible stuff used to hang pictures on the wall? Or just heavier gauge kevlar cord/ribbon? Thought I'd ask the folks with the most experience dealing with large motor issues. Your feedback/suggestions would be greatly appreciated, as I'm about to scratch build a model that will use D or E motors.

A couple of my ASP mid power kits, the Aerobee 100 Junior and the D Region Tomahawk, use a thin wire cable attached to a thick elastic shock cord. I'm thinking it should last the life of the rocket.



P.S. The attachment point at the centering ring is two holes with the cable looped through them and secured with a crimped ferrule, like the other end, as seen in the picture.
I would take precautions against zippering.

 

[Rktman]

A couple of my ASP mid power kits, the Aerobee 100 Junior and the D Region Tomahawk, use a thin wire cable attached to a thick elastic shock cord. I'm thinking it should last the life of the rocket.

View attachment 339536

P.S. The attachment point at the centering ring is two holes with the cable looped through them and secured with a crimped ferrule, like the other end, as seen in the picture.
I would take precautions against zippering.

That definitely won't have any burn-through problems. Is it the twisted wire picture-hanger variety or a more specialized braided and more flexible steel cable?

 

[kuririn]

It's a braided thin wire cable, about 1/32" thick. Appears to have a plastic coating over it, maybe to prevent rust. In the instructions part list, all it says is "steel cable". I don't see why picture wire cable wouldn't work also.

 

[Wallace]

I've used this on several different projects and have yet to find it to be the weak link. Something else has always failed first. This nose cone/av bay setup was involved in a shred @ near mach, the chute didn't hold together but the leader wire and crimps survived..

 

[mccordmw]

I've always used kevlar. Never had any problems. I suspect the positioning of your shock cord was more of an issue?

 

[kuririn]

I've used this on several different projects and have yet to find it to be the weak link. Something else has always failed first. This nose cone/av bay setup was involved in a shred @ near mach, the chute didn't hold together but the leader wire and crimps survived..

Right! A fishing supply store should have what you need, Eric.

 

[RocketFeller]

I have used thin cable as a shock cord attachment a couple of times. I made it so that the loop of cable was just below the end of the airframe to prevent zippering.

 

[rharshberger]

I have used thin cable as a shock cord attachment a couple of times. I made it so that the loop of cable was just below the end of the airframe to prevent zippering.

Just don't use the pre-made steel fishing leaders they seem to fail after a few flights, most of the ones I used failed inside of 10 flights. The heavier material especially in stainless would be much better, but IMO Kevlar is hard to beat and it weighs less.

 

[FredA]

Not really needed and cuts like a hot knife through butter given the opportunity to zipper, so you better nail your recovery timing.

 

[bobby_hamill]

RKTman

Have you tried this method ?
https://www.rocketryforum.com/attachment.php?attachmentid=330015&d=1507958877

I used it on a motor tube of BT55 inside an BT60 airframe . The BT55 insulates the Kevlar
to a degree

Bobby

 

[blackwing94]

I used to paddle offshore and catch sharks out of a kayak. I have about a hundred yards of braided steel cable left I used to make shark leaders out of. I've used it on a few mid power rockets (29mm and 38mm) as the connection from the eye bolt (above the motor) to just shy of the end of the body tube. I used two crimps for each looped end and covered the crimp in a rain drop of epoxy to make a smooth surface over the crimp. Then I used either nylon or kevlar for the recovery harness. I've never had one fail.

 

[mccordmw]

Not really needed and cuts like a hot knife through butter given the opportunity to zipper, so you better nail your recovery timing.

+1 to this one. I think you're just adding another variable that can cause failure (via zippering), so you'll have to add complexity to protect against that.

Keep it simple. Talk to Teddy at Onebadhawk. He will help you select the proper kevlar shock cord.

 

[samb]

The braided steel leader is used in North Coast Rocketry midpower kits. Some nice pictures in the instructions for their Bounty Hunter model.

https://cdn.shopify.com/s/files/1/1762/3593/files/Bounty_Hunter_Instr_121916.pdf?9826273292200455441

Apparently a favorite technique of the designer from way back:

https://www.ninfinger.org/rockets/catalogs/NCR1989CAT/NCR1989.pdf

 

[Rktman]

I've always used kevlar. Never had any problems. I suspect the positioning of your shock cord was more of an issue?

Anchored in the CR as usual. I don't know if the D had a faulty ejection charge or what.

 

[Rktman]

+1 to this one. I think you're just adding another variable that can cause failure (via zippering), so you'll have to add complexity to protect against that.

Keep it simple. Talk to Teddy at Onebadhawk. He will help you select the proper kevlar shock cord.

I used a 1200lb braided kevlar cord which I thought would provide extra insurance against this kind of thing, since that size is usually used in MPR models and up.

 

[RocketFeller]

The braided steel leader is used in North Coast Rocketry midpower kits. Some nice pictures in the instructions for their Bounty Hunter model.

https://cdn.shopify.com/s/files/1/1762/3593/files/Bounty_Hunter_Instr_121916.pdf?9826273292200455441

Apparently a favorite technique of the designer from way back:

https://www.ninfinger.org/rockets/catalogs/NCR1989CAT/NCR1989.pdf


View attachment 339593View attachment 339594

The two rockets I did this to were my NCR Eliminator and Interceptor G. Just following instructions...

 

[T-Rex]

I have a Cluster-R Standard ARM that came with a steel cable for the first section of the shock cord. I obviously did something wrong because the rest of the cord and the 'burrito' got stuck in the tube on it's first flight. I don't plan to use one again.

That being said, I do not believe it was the sheathed/plastic coated type. I may go check the fishing section at the local big box store and see what is available.

 

[samb]

In my experience ejection charge strength in black powder motors can vary somewhat. But I don't think you had a "bad" one. Kevlar will burn through, sooner or later. I experienced this with a few Quest kits years ago. Quest uses a kevlar leader tied around the motor tube and tied to an elastic tread going to the nose cone. That put the kevlar close to the fire in the confined space of a 3/4 or 1 or 1 1/2 inch LPR airframe. They all burned through eventually. Those I recovered got a good old Estes teabag mount right below the nose cone shoulder for the new shock cord.

[edit] I think the HPR guys use those big honkin' 1/2 inch or 5/8, sown loop onebadhawk harnesses in much bigger spaces, airframe diameter-wise. And nomex sleeves to protect and preserve the bit closest to the fire.

 

[kuririn]

I just remembered some NewWay kits use heat shrink tubing over the section of Kevlar thread closest to the motor tube. I suppose it offers some added protection in low power birds.

 

[jdbectec]

That definitely won't have any burn-through problems. Is it the twisted wire picture-hanger variety or a more specialized braided and more flexible steel cable?

Don't use the picture wire it will fray and break in no time. Trust me I know from experience.

 

[kuririn]

Don't use the picture wire it will fray and break in no time. Trust me I know from experience.

Yeah, I'm gonna backtrack on the picture wire being O.K. I just remembered that black powder residue is corrosive to bare metal. That's why you see gun enthusiasts seemingly obsessively cleaning their firearms: it's to keep the internal parts from pitting and corroding.
Also picture wire will break after repeated bending, like a paper clip that is bent back and forth. Use a high tensile clear coated steel cable instead.

 

[dr wogz]

Cosmodrome kits come with a steel wire (or they did!) I'm not a big fan.

I have to ask Rktman: what kit was this? Could it be, that you managed to pack the recovery with the cord right over the motor eject? Was it truly Kevlar or maybe just yellow nylon cord? Did the ejection eject the wadding as well??

I don't' mean any disrespect or mean to come across as condescending or the like. I'm asking to see if there might be another reason for the burn-thru. I had one kit, that after only a few launched the elastic burnt thru. The wadding never really ejected on ejection (got as high as the Estes triple fold attachment), so I assumed the flaming hot gasses were kept in the tube, and burn / ignited the weakest link; the cord. I've also seen some pack & pack wadding into the tube, so it never escapes / has a chance to vent / cool down.. And, some will stuff the cord in, then the wadding, then the chute & NC. So, ALL the cord is in the 'hot area' below the wadding. I know some people insist 'Mega Blocks' is the same as Lego. They aren't. The same with 'yellow cord' and Kevlar. They may look similar, but differ greatly..

As mentioned, just trying to ask a few questions to see if, to think about if there might be a reason for the burn thru other than a violent / over ambitious ejection charge..

 

[Rktman]

Cosmodrome kits come with a steel wire (or they did!) I'm not a big fan.

I have to ask Rktman: what kit was this? Could it be, that you managed to pack the recovery with the cord right over the motor eject? Was it truly Kevlar or maybe just yellow nylon cord? Did the ejection eject the wadding as well??

I don't' mean any disrespect or mean to come across as condescending or the like. I'm asking to see if there might be another reason for the burn-thru. I had one kit, that after only a few launched the elastic burnt thru. The wadding never really ejected on ejection (got as high as the Estes triple fold attachment), so I assumed the flaming hot gasses were kept in the tube, and burn / ignited the weakest link; the cord. I've also seen some pack & pack wadding into the tube, so it never escapes / has a chance to vent / cool down.. And, some will stuff the cord in, then the wadding, then the chute & NC. So, ALL the cord is in the 'hot area' below the wadding. I know some people insist 'Mega Blocks' is the same as Lego. They aren't. The same with 'yellow cord' and Kevlar. They may look similar, but differ greatly..

As mentioned, just trying to ask a few questions to see if, to think about if there might be a reason for the burn thru other than a violent / over ambitious ejection charge..

Hi Paul, this incident happened with a scratchbuilt not a kit. It is indeed kevlar (from Emma Kites purchased through Amazon) and I anchored it in the usual way to the centering ring. My prep routine is to stuff in several sheets of flameproof wadding followed by generous amounts of dog barf, all loosely packed, followed by the cord and chute. As mentioned, the cord burned through right above the engine tube so I assumed it was an overly aggressive ejection charge since it's never happened with smaller motors I've used.

Given the feedback, I am now planning to use flexible steel cable from the motor mount up a distance of 6 - 12 inches and from there using kevlar (with some shrink tubing over where it passes over the lip of the BT's end.

 

[boatgeek]

It's stainless steel cable, not braided steel, but I used a wire for motor mounts in this build thread: https://www.rocketryforum.com/showthread.php?121507-Design-Comment-Build-Thread-Tuna-Rocket post #8. It's still kicking along great after a couple of dozen flights. I think the cable is 7x7.

 

[MaxQ]

Cosmodrome kits come with a steel wire (or they did!) I'm not a big fan.

I never used steel cable as a shock cord attachment until I built one of the original Cosmodrome Aerobee Hi two stage kits.
It came with what appears to be 1/16th inch diameter steel braided cable, the same kind I've since bought at Lowes by the foot.
Lowes has a steel braid cutter you can get a very clean cut with when you buy it...which is important if you want to avoid frayed wires.

The kit came with ferrules you slide over the looped terminal ends and compress with pliers or a crimping tool meant for this kind of thing.
Mine has never failed over the years, but then I can't reach the root end inside the sustainer to see what condition it is in.
We've all seen the oxidation and corrosion on metal parts from ejection charges - like links...so I can only guess what is going on down there at the attachment point near the motor.

A look at the recent kit instructions online appears to show they now use something different than steel.
That said - I had tubular nylon melt and snap when the nomex sheath slid/moved when I packed the recovery harness, leaving an exposed area near the ejection charge.

 

[Rktman]

It's stainless steel cable, not braided steel, but I used a wire for motor mounts in this build thread: https://www.rocketryforum.com/showthread.php?121507-Design-Comment-Build-Thread-Tuna-Rocket post #8. It's still kicking along great after a couple of dozen flights. I think the cable is 7x7.

I found what I believe is 1/16" twisted strand steel cable (individual strands twisted together then bundled and twisted together again to form the cable) in our household "miscellaneous" drawer. Purchased at Lowes to rig some heavy bird feeders on pulleys and tough enough that I had to struggle with tin snips to cut the stuff. Should be strong enough to use from the motor mount up 6 - 12 inches before transitioning to 1200lb kevlar no?

 

[Wallace]

Obviously not for every possible combination but set up properly it just plain works. Weight of an 8" to 10" piece of leader wire/crimps is a non issue. A coast lock swivel or quick connect, if used, could potentially be but at least it's on the "right end". A spool of stainless leader wire and bag of 100 crimp connectors is pretty much a lifetime supply and allows you to then use cheaper/more forgiving/readily available nylon for the rest of the shock cord.

 

[Andrew_ASC]

Not a fan of metal wire shock cords by metal fatigue.

 

[rharshberger]

Not a fan of metal wire shock cords by metal fatigue.

And you have a lot of experience with them....?