Shock cord protection - RTV silicone?

[SolarYellow]

In planning my next complex build, I end up with a part of the end of the shock cord exposed to the ejection charge. I've looked into Nomex and other heat resistant wraps, but they add a lot of bulk and complexity to the installation. I had a brainstorm today, related to the thread that's still just a few posts below this one, of using high-temp RTV silicone from the auto parts store on the Kevlar. It's rated for intermittent exposure up to 700 degrees, far higher than any of the glues we normally use for rockets. Silicone dries flexible, so it shouldn't lead to the concentration of stress that other glue saturation strategies generate. It should definitely keep ends from fraying and knots from ever untying.

It will add stiffness/bulk to the line, and some stickiness, so it will require attention to how it's laid into the rocket so it pays out without tangling. But I'm only looking to use it on the end loop and the first few inches of shock cord that I can't figure out a better way to protect, and which is going to be adjacent to the glove tip when the charge goes off.

Could also be good for the threaded/vented forward closures where the charge comes blasting up past the motor retention hardware, if that's also going to expose the shock cord.

Has anyone tried this approach? Know or can think of any reason to expect problems that I haven't thought of?

 

[tfish]

What's your shock cord material and size.?
You can thin silicone with Coleman white gas. I'd try impregnating some silicone that's..way thinned down 'into' the shock cord, cardboard or whatever you're trying to protect.

Tony

 

[SolarYellow]

Looking at Kevlar, not sure on the weight. At least 300 lb, but have been contemplating doing it massive overkill at 750 lb due to not having a good protection method in place. It's for a ~26-in. BT-60 rocket with 18-in chute and DD.

 

[jmasterj]

Sounds like you're already overkilling it with this idea, lol.

 

[AZ_Ron]

I've been using 2500lb kevlar for my recovery leashes since 1997. It's thin and lightweight and supremely overkill. It's also very fire resistant. Since Kevlar has no give whatsoever, you need to fold up the leash into small bundles and use masking tape bands to secure them. Breaking the tape bands dissipates the deployment energy. You could obviously go with a much lower rated kevlar, but same principles apply. I tend to use 30-50ft long leashes, lots of tape bands and very, very rarely to I find all of the bands broken. The only use I have for Nomex is deployment bags or chute burritos.

Ron

 

[Al_delaIglesia]

In planning my next complex build, I end up with a part of the end of the shock cord exposed to the ejection charge. I've looked into Nomex and other heat resistant wraps, but they add a lot of bulk and complexity to the installation. I had a brainstorm today, related to the thread that's still just a few posts below this one, of using high-temp RTV silicone from the auto parts store on the Kevlar. It's rated for intermittent exposure up to 700 degrees, far higher than any of the glues we normally use for rockets. Silicone dries flexible, so it shouldn't lead to the concentration of stress that other glue saturation strategies generate. It should definitely keep ends from fraying and knots from ever untying.

It will add stiffness/bulk to the line, and some stickiness, so it will require attention to how it's laid into the rocket so it pays out without tangling. But I'm only looking to use it on the end loop and the first few inches of shock cord that I can't figure out a better way to protect, and which is going to be adjacent to the glove tip when the charge goes off.

Could also be good for the threaded/vented forward closures where the charge comes blasting up past the motor retention hardware, if that's also going to expose the shock cord.

Has anyone tried this approach? Know or can think of any reason to expect problems that I haven't thought of?

I use a piece of heat shrink tubing near the motor end on my kevlar shock lines.....never had an issue

 

[cbrarick]

kevlar. Because it just works and isn't complicated.

 

[manixFan]

I also use heat shrink tubing to protect Kevlar. It’s still reasonably flexible and very inexpensive when bought in longer lengths. If it’s not hard to reach, I will also just wrap a long piece of blue masking tape lengthwise along the section that needs it.

Kevlar is not immune to damage from ejection charges. It makes sense to protect it whenever possible.


Tony

 

[cwbullet]

Interesting idea. Please let us know how it goes.

 

[G_T]

RTV silicone or other coatings with or without solvents - look up the chemical compatibility of the material before doing it. You may inadvertently change the load and/or the shock rating by a lot and not realize it.

RTV silicone depending on type emits acetic acid.

BTW, with Kevlar you want to rate by the shock rating not the load rating.

Gerald

 

[SolarYellow]

From DuPont
Kevlar_Technical_Guide_0319.pdf

EFFECT OF CHEMICAL AGENTS ON KEVLAR®
Kevlar® is chemically stable under a wide variety of exposure conditions;
however, certain strong aqueous acids, bases and sodium hypochlorite
can cause degradation, particularly over long periods of time and at
elevated temperatures. Table II-3 summarizes the effect of chemical
agents on the breaking strength of Kevlar®.

How much aqueousness is there coming out of the cured RTV silicone?

 

[cbrarick]

hmm, it's much easier to make the Kevlar replaceable then try what you're doing. if you run a kevlar loop then you can always cut it and tie a new piece in at one end, then pull it through. Vola! new kevlar. Of course I generally loose the rocket or have some other problem before I wreck the kevlar.

 

[SolarYellow]

My whole deployment scheme and recovery arrangement are different than you're assuming. If I'm understanding what you're saying correctly (something I've seen many times before and even plan to do where applicable), it's not relevant to this design.

 

[lr64]

Isn't Kevlar supposed to be good up to 800 degrees?

 

[cbrarick]

please share your design.

Personally I tend to use welded eye bolts or u bolts. either way the change is a snap.
Although I've broken a bunch of cord for various reasons (almost all my fault), I've only burnt thru one - a drago with hundreds of flights,on a cheap piece of the kevlar ribbon (not the best design) quite literately.

plus I don't get it. RTV will not only attack the kevlar, but make it quite stiff and difficult to pack, not to mention bulky. It melts below that of the kevlar so at best it's a sacrificial element. Seems to me like all it does is add weight, slow your rocket and make things more difficult. Perhaps I'm wrong, however

 

[SolarYellow]

It's BT-60, one-compartment zipperless. Ebay in the nose cone with the ejection charge at the base of the nose cone, blowing everything downward, with a chute tender. I'm going to have 2D of dog barf between the charge and everything else, but there still has to be a Kevlar loop attaching to the nose cone (well, first an anchor, then a quick link, then the Kevlar), adjacent to the ejection charge. The Kevlar harness is going to use the "fingertrap" technique with stainless "thimbles" to form the loops and maintain a nice, large radius. I have silicone-impregnated braided fiberglass sleeve (1, 2, 3, and 5mm ID on hand) for the first few inches of the harness, but I don't even want to try making the fingertrap loop with that wrapping around the thimble and cord. So I'm thinking about coating just the loop with the silicone to protect it from the fire. BTW, charges will be small, with an ejection charge calculator spreadsheet I downloaded indicating <0.2 gram FFFFG.

I understand the whole thing is overkill for this little rocket, but I'm laying the groundwork of skills and experience for bigger stuff.

 

[lr64]

Can you use silicone impregnated fiberglass cord for the first few inches? Or for the whole thing? Or the sleeve, since you already have it?

Maybe instead of heavy metal hardware at the ends, you could just use a slightly larger Kevlar cord, which would also probably be more fire resistant.

If it was me, and the ejection charge was separate from the motor, I might do a ground test. I'd put some Kevlar, under tension, at the appropriate distance from the charge and see what happens to it. You may be borrowing trouble.

If you can save weight or leave out things you don't actually need, performance goes up. In the latter case, the expense goes down, too.

I can see using silicone to prevent knots untying.

Above is not based on experience with high power rocketry, but with experience in other parts of the real world.

 

[cbrarick]

You are unnecessarily complicating a very simple manner. There is no problem, as evidenced by the number of rocketry people who fly quite happily without problems using standard kevlar. In fact, I'd suggest that kevlar is the standard in high powered rocketry. Yes, I know some people still use nylon but those people are vastly in the minority. if kevlar melting was such a problem we'd all be using nomex sleeves, which is the standard for protecting nylon. If you're really worried, try a nylon sleeve like the rest of the world and leave the RTV alone. Thimbles are also solving a nonexistent problem. Not needed. I routinely put quick links through kevlar sewn loops on rockets of a scale you haven't. If you're really worried about your kevlar, look at the type of knot you use. I use figure 8's with follow through. This knot is routinely used in sailing and rock climbing.

 

[SolarYellow]

Can you use silicone impregnated fiberglass cord for the first few inches? Or for the whole thing? Or the sleeve, since you already have it?

Maybe instead of heavy metal hardware at the ends, you could just use a slightly larger Kevlar cord, which would also probably be more fire resistant.

If it was me, and the ejection charge was separate from the motor, I might do a ground test. I'd put some Kevlar, under tension, at the appropriate distance from the charge and see what happens to it. You may be borrowing trouble.

If you can save weight or leave out things you don't actually need, performance goes up. In the latter case, the expense goes down, too.

I can see using silicone to prevent knots untying.

Above is not based on experience with high power rocketry, but with experience in other parts of the real world.

I haven't tested it, but the silicone sleeve is not sold for structural use. There isn't a good (or any) body of literature on how to make loops and knots in it. It's sold for thermal protection of wires, which is close enough to how I'm going to use it.

The steel thimble in the Kevlar loop makes the fingertrap loop work better as well as holding a minimum radius that should limit the strength reduction of the Kevlar due to wrapping the small-diameter metal loops. The quick link is so the Kevlar shock cord will be replaceable and so it doesn't have to be fabricated already attached to the rocket and so the whole assembly is more serviceable. The finished rocket design as it sits is well below optimum weight for motors that matter, so I'm not very concerned about the weight of a 1/8-in. quick link or two.

The system will absolutely be ground tested before being flown, and it will first fly in a beater LPR to shake everything down and validate the design at altitudes where I can watch everything happen. This particular beater has streamer-chuted in on a lake bed and flown again a few minutes later, so it's a good mule for this kind of thing.

Kevlar does eventually have problems when exposed directly to ejection charges. I got where I am on this topic by looking at Nomex sleeves. I found nothing of an appropriate size and didn't like the cost of materials to make my own. The silicone-impregnated sleeving is dirt cheap in comparison to anything Nomex, and is available in sizes appropriate for small LPR rockets. I'm just looking at what's left.

 

[Hobie1dog]

I'm interested to see how this turns out

 

[lr64]

I think if you tested the sleeve, you'd probably find it was very strong for its size. It might be stronger than Kevlar, even, though not stronger for the weight. The question would be how much mechanical abuse it could take, and maybe the silicone would help with that.

There's something called fiberglass lacing tape that people DO tie knots in. But I'd be surprised if a figure 8 wasn't enough for the sleeving.

 

[SolarYellow]

Lacing tape is for harnessing wires and attaching the harnesses to structures. It's not a load-bearing element, except to the degree needed to keep wires from flopping around.

(I have a big 'ol roll of lacing tape and will also be using that where appropriate, instead of zip-ties.)

 

[lr64]

I ran across some listings where they specified what the strength was.

 

[bjphoenix]

I've read about people using stainless steel fishing leaders for the first part of the shock cord, the part that would be closest to the ejection charge. Of course this wouldn't apply to you but you could do a similar thing with stainless steel leader attached to the nose cone.

 

[John Kemker]

I've read about people using stainless steel fishing leaders for the first part of the shock cord, the part that would be closest to the ejection charge. Of course this wouldn't apply to you but you could do a similar thing with stainless steel leader attached to the nose cone.

NCR used to have that in their HPR kits. It was nicknamed "shock line saw."

 

[lr64]

You can get plastic coated cable.

Or you could put heat shrink tubing on the cable.

 

[kramer714]

The issue with shock cords is usually just where it attaches to the rocket and where it hits the edge of the tube. Shock cord protectors take up a lot of room in the body tube.

One simple solution is to use thin galvanized or stainless cable. On some small rockets i use Amazon 1/16 steel rope and make it JUST longer than the body tube, and add a few layers of shrink tube wrap right where it reaches the end of the body tube to reduce zippering.

For the attachment to the rocket, If it is big enough for a U-bolt, I just make a loop and pass the loop through the u bolt (long fishing hemostats are really helpful here) through the loop. That way I can replace it in the future if I need. If it is near minimum diameter, for example a 38mm motor mount in a 54mm tube, I drill a hole through the aft AND forward centering ring, run the cable through both, and swedge a ball to the cable to keep it from pulling through.

Tip for U bolts. I prefer U bolts to eye bolts for connections, a great source for small u bolts is cable clamps like 1/8 cable clamps

 

[kramer714]

looks like..