Epoxy fillets ...

[mtnmanak]

@JimJarvis50 I watched your video & I have a question. What exactly is a z-fold? I've Googled it but didn't find anything useful. Does anyone know of a video that explains how to do a z-fold?

It is a method of folding your shock cord, shroud lines, bridle, etc - anything that is a cord or strap - using a series of folds that fold back on themselves and form a series of "Z"s. This allows force to be applied to one end of the cord and it will deploy in an orderly manner. In the photos below, you can see a shock cord laid out in a loose series of "z-folds" then what it looks like when you push them all together neatly. Imagine the end with the blue heat shrink tubing attached to your nosecone, coupler, bridle, etc and the loop with the yellow heat shrink tape attached to a parachute. When the parachute pulls on the cord, it neatly unfolds towards the hardpoint attachment and doesn't tangle. This method also ensures you can pack the cord in the tube in the most efficient manner. For long cords, multiple z-folds along the cord are usually required. For packing a deployment bag, you must use this method. I am not aware of any other folding method to get the cords properly folded into the d-bag stays.

For a smaller rocket, you would probably not want to use a strong rubber band to hold the z-fold together - you may not want to use a rubber band at all just z-fold the cord and neatly place it in the tube. There needs to be enough weight to overcome the ability of the rubber band to hold the stack together. It all depends on what kind of rubber band you use. I have some very lightweight dental rubber bands that work great for lighter rockets.

 

[JimJarvis50]

Great job on the video and i love the techniques you referred to.

Are you going to produce more informational videos?

Again, real nice presentation. I found it very easy to watch.


Tom

If you're commenting on my video, I will probably do another for NARCON next year. They are actually quite a bit of work.

Jim

 

[JimJarvis50]

I know this is an epoxy thread and don't want to derail it too much, but did you document that wiring diagram with the shunt and resistors in any more detail someplace? Would love to see a picture and description of the resistors used, shunt design, wifi switch, etc. My rocket building skills are okay, but my electrical engineering skills are subpar and would like to learn more. May be worth another thread, unless there already is one.

I haven't really documented this anywhere in more detail, although I have commented about shunts quite a bit here. I like them, but they have to be designed and then tested. Generally, they should be a short as possible to minimize resistance. I design them so that they can be pulled off from the ground.

In combination with a shunt, a resistor can make the design safer (less current through the ematch if the altimeter fires when it's not supposed to). I use shunts for staging. The need for resistors for deployment charges depends on the altimeter and how it's powered. The idea is to protect the altimeter from damage if the igniter shorts out. For some altimeters, this is not necessary. However, there are some common altimeters that I use (older Ravens and Stratologgers) where a resistor might help if the altimeter is powered by Lipo's. For ematches, I shoot for a 1A firing current. I choose a resistor wattage that at least approaches the wattage that might be expected, but I am not really sure what is actually necessary.

Jim

 

[Greg Furtman]

It is a method of folding your shock cord, shroud lines, bridle, etc - anything that is a cord or strap - using a series of folds that fold back on themselves and form a series of "Z"s. This allows force to be applied to one end of the cord and it will deploy in an orderly manner. In the photos below, you can see a shock cord laid out in a loose series of "z-folds" then what it looks like when you push them all together neatly. Imagine the end with the blue heat shrink tubing attached to your nosecone, coupler, bridle, etc and the loop with the yellow heat shrink tape attached to a parachute. When the parachute pulls on the cord, it neatly unfolds towards the hardpoint attachment and doesn't tangle. This method also ensures you can pack the cord in the tube in the most efficient manner. For long cords, multiple z-folds along the cord are usually required. For packing a deployment bag, you must use this method. I am not aware of any other folding method to get the cords properly folded into the d-bag stays.

For a smaller rocket, you would probably not want to use a strong rubber band to hold the z-fold together - you may not want to use a rubber band at all just z-fold the cord and neatly place it in the tube. There needs to be enough weight to overcome the ability of the rubber band to hold the stack together. It all depends on what kind of rubber band you use. I have some very lightweight dental rubber bands that work great for lighter rockets.

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Thanks!

 

[Nytrunner]

Either way, you are in rarified air... literally - both Cleese and Jackson are over 6'5"!

If you've met Jim in person, he's not exactly short either.

 

[dr wogz]

My preferred tool is a hotel room key card. Usually free. And, epoxy don't stick to them! You can cut the corner to what-ever radius you want.

If you hold it just right, it'll scrape clean a fin when you pull the fillet.. The flat edge can also be used to 'nudge' or scrape off the stray blob here & there.. Holding it at a consistent angle is key..

The biggest "mess" I see from people pulling fillets (is that the right term? 'Draw' a fillet maybe?!) is that they don't wipe / clean their tool after each pull..

See post #22 for my technique:
(shameless plug, I know.. )
https://www.rocketryforum.com/threads/madcow-super-dx3-patriot-a-build-thread.140215/

 

[jrap330]

You probably know it by a different name? I've heard it called an S fold and there are probably others. It's just folding a harness back and forth and then taping the bundle to keep the harness organized. I just do it with blue tape to keep things organized, but with stronger tape, it can also serve as a shock absorber upon deployment.

Jim

Accordion fold??

 

[Conway Stevens]

One thing to note that any fillers you use may give you benefit of making the epoxy hold shape, become lighter, be more easily sanded. But none of them will enhance strength. Even chopped glass or carbon fiber adds minimally. Using pure epoxy as a fillet gives maximum strength even West Systems notes this on their website that adding fillers cause a weaker bond. If your looking for strength with something added this is where tip to tip glassing using a woven cloth laminated to the structure gives the max strength to lightest weight.

My preference for epoxy for structural joints is Aeropoxy 6209. Nothing needs to be added as it holds its shape and has superior strength. It also has a long pot life. For laminating i like their laminating epoxy or the 3:1 us composites.

These are my observations and opinions. I have used lots of others that are good but each has their gives and takes. I felt the Aeropoxy gave the most benefit.

 

[Jmhepworth]

I’ve wondered about the utility of adding chopped carbon fibers to Aeropoxy 6209 when injecting internal fillets. The chopped carbon makes the epoxy a gooey mess. I’ve done it because I accepted the guidance that it makes the fillet stronger. It probably seals any small leaks when I don’t make the centering rings fit tightly enough against the fin tabs better than 6209 alone. But I’m thinking I’m going to just go back to straight 6209. It’s a lot less messy.

 

[Conway Stevens]

I’ve wondered about the utility of adding chopped carbon fibers to Aeropoxy 6209 when injecting internal fillets. The chopped carbon makes the epoxy a gooey mess. I’ve done it because I accepted the guidance that it makes the fillet stronger. It probably seals any small leaks when I don’t make the centering rings fit tightly enough against the fin tabs better than 6209 alone. But I’m thinking I’m going to just go back to straight 6209. It’s a lot less messy.

Think about it. Loose fibers with no direction no control over the way the lay over the joint. No structure. Just kinda floating in the epoxy? when you use these materials in a woven cloth is where you gain strength, gain structure. the epoxy is just what bonds it. But the strength and structure comes from the cloth lay out of woven fibers creating a directional load on a surface or joint. Even in filament wound fiberglass tube it has direction, structure. A group of wound fibers cross wound against each other and the epoxy just holds it there. Chopped fibers arent able to give that kind of strength or structure.

 

[Arpak]

I choose a resistor wattage that at least approaches the wattage that might be expected, but I am not really sure what is actually necessary.

That reminds me, I wanted to do some ground tests to see how long a lower wattage rated resistor lasts with a high wattage.

The rating is based on sustained current, and since we're only pushing current for a couple seconds and most I feel like it wouldn't heat up enough to do any damage. I tried to sift through some datasheets to see if there was any existing info on that but came up empty. I'm sure blowing up some resistors is more fun anyway.

 

[JimJarvis50]

One thing to note that any fillers you use may give you benefit of making the epoxy hold shape, become lighter, be more easily sanded. But none of them will enhance strength.

I'm OK giving up a little strength of the epoxy in exchange for control of properties like viscosity. I use that capability regularly and I would miss it, and I don't have a big problem with my rockets falling apart. Having said that, I do plan to try the Aeropoxy 6209 at some point since I have not used it in the past.

Jim

 

[prfesser]

I’ve wondered about the utility of adding chopped carbon fibers to Aeropoxy 6209 when injecting internal fillets. The chopped carbon makes the epoxy a gooey mess. I’ve done it because I accepted the guidance that it makes the fillet stronger. It probably seals any small leaks when I don’t make the centering rings fit tightly enough against the fin tabs better than 6209 alone. But I’m thinking I’m going to just go back to straight 6209. It’s a lot less messy.

The guidance I've seen on some composite supply vendors' web pages is that carbon or fiberglass fiber does increase the strength of epoxy at the expense of mess. That's consistent with what I've read in a couple of books on epoxies. But unless you're doing an extreme project it's hardly necessary, as epoxies alone are stronger than the stuff they're joining. Well, except when joining properly made cloth+epoxy joints.

For internal fillets in an extreme project where fiberglass cloth is inconvenient, perhaps the fiber additives are warranted.

[Look at a boat construction video, or at the internal surface of a boat. Ordinary boats intended for consumers usually use fiberglass mat---random strands of glass. Presumably that's done for strength (and thick mat is certainly less expensive than an equivalent thickness of cloth).]

Best -- Terry

 

[G_T]

There is another reason to use mat or something like it. It prevents joint glue starvation and can be used to help give you a known glue line thickness. Mat gives almost no strength at all. It does help keep epoxy from running away. Think of it like filler in a sheet. Mat is slightly better than chopped fibers if the 2D orientation of the fibers is better for some reason in your application to 3D, for the same fiber loading. But in either case you are getting little from the fibers compared to what the case would be if they were aligned to the expected load.

Except for microballoons, nearly all the fillers have a greater density than epoxy. The reason your density goes down when not using microballoons is probably due to mixing in a bunch of air. Air of course has no structural properties and weakens the mix. But it does make it lighter!

Gerald

 

[g.pitts]

That reminds me, I wanted to do some ground tests to see how long a lower wattage rated resistor lasts with a high wattage.

The rating is based on sustained current, and since we're only pushing current for a couple seconds and most I feel like it wouldn't heat up enough to do any damage. I tried to sift through some datasheets to see if there was any existing info on that but came up empty. I'm sure blowing up some resistors is more fun anyway.

If I may make a suggestion... get a resistor with a suitable pulse withstanding rating. I chose a 1.6 Ω safety-rated resistor from TE Connectivity. The one I'm using is nominally rated at 3 Watts, but can handle greater power for brief duration (see attached datasheet). These resistors also have a fuse characteristic in the event you somehow end up with an external short on your e-match path.

 

[Conway Stevens]

I'm OK giving up a little strength of the epoxy in exchange for control of properties like viscosity. I use that capability regularly and I would miss it, and I don't have a big problem with my rockets falling apart. Having said that, I do plan to try the Aeropoxy 6209 at some point since I have not used it in the past.

Jim

But as you mentioned as well Jim you do tip to tip correct? Your strength comes in that manner with that addition.

These are the fillets i get with just 6209. With a wider tool to form and scrape would get wider. But was looking for smaller this time as its the fin. By itself it needs no fillers because of its abilities.

 

[Arpak]

If I may make a suggestion... get a resistor with a suitable pulse withstanding rating. I chose a 1.6 Ω safety-rated resistor from TE Connectivity. The one I'm using is nominally rated at 3 Watts, but can handle greater power for brief duration (see attached datasheet). These resistors also have a fuse characteristic in the event you somehow end up with an external short on your e-match path.

Ah that's perfect! Guess I didn't look too hard but that datasheet has the exact chart I was looking for. Very right on the fuse characteristic, I was even planning to double check if it fails open/closed (I figured open).

 

[g.pitts]

Ah that's perfect! Guess I didn't look too hard but that datasheet has the exact chart I was looking for. Very right on the fuse characteristic, I was even planning to double check if it fails open/closed (I figured open).

Correct - it will fail open to interrupt the path of current flow.

 

[JimJarvis50]

But as you mentioned as well Jim you do tip to tip correct? Your strength comes in that manner with that addition.

Yes, the video information was in the context of a tip to tip application, where the properties of interest are the shape and shear characteristics of the mixture. I just didn't want to subtract from your point, which I agree with.

At some point, I'll try the Aeropoxy structural. I just happen to be messing with metal rockets at the moment. I think I would need to see for myself the ability of it to hold shape. After all, you pour it out of a can, so I'm not quite sure how there is no sag. If you have some in a cup, and you can turn the cup upside down and the epoxy doesn't move, then I'm sold. I've always been curious, if there was a little sag, to see how much fumed silica would be necessary to stop it.

Jim

 

[Conway Stevens]

Yes, the video information was in the context of a tip to tip application, where the properties of interest are the shape and shear characteristics of the mixture. I just didn't want to subtract from your point, which I agree with.

At some point, I'll try the Aeropoxy structural. I just happen to be messing with metal rockets at the moment. I think I would need to see for myself the ability of it to hold shape. After all, you pour it out of a can, so I'm not quite sure how there is no sag. If you have some in a cup, and you can turn the cup upside down and the epoxy doesn't move, then I'm sold. I've always been curious, if there was a little sag, to see how much fumed silica would be necessary to stop it.

Jim

Its not really a pour very easy out of the can. I have to use a large tongue depressor to scoop out a bit put it to the cup until i get it to what i want. Do the same with the hardener. Then i mix. Put it under a vacuum for 20 minutes to pull out air then apply it. Once i spread it and shape the fillet i let it cure...could there be some sag? Yes. Can i physically see it nope. It just seems to stick and stay very well.


Metal rocket? Do tell. Love to hear more.