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Plywood fin tips for an MPR build

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Off Grid Gecko

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Hey there, looking for some ideas here.

I finished airfoiling some fins for another 1.64" rocket from my Mean Machine kit-bash. I'm filling the tube spirals and sanding at my leasure as I finish off another rocket made from the kit.

If it matters, the ply is Lauon leftover from making a bookshelf. It's 3 plys of really thin birch sandwiched with two layers of Philippine wood that is softer and thicker. The wood itself seems fine and I've done my first round of filling and sanding. I'm using a conventional fin design where the fins extend beyond the rocket body and come to a point, which is the tip in question.

I don't want to do a tip-to-tip fiberglass as I'm not sure of my skills, though I have the materials to do so. I just had so much trouble trying to build a jig and then figure out how to make the edges come out straight and uniform (destroyed 3 sets of fins before running the jig through my table saw for good results) that I'd rather not ruin the fins on a bad fiberglass job. I haven't glassed anything in years, just bought some fiberglass kit on a whim in case I need it.

I thought about papering, as this likely won't be a Mach I rocket and even if it is, it won't go far past that. Without doing the calculation though, I can look at the fins and they are quite tall from the rocket body...I'm rambling.
The main concern is descent. I'm trying to avoid chipped and broken fin tips if at all possible. Would papering help with that?
I'm also considering an epoxy soak, where I smear it on fast and thin over everything in the hope that it will penetrate a bit and harden the wood like CA. The stuff I have sets a little fast, like 15-20min working time, so I might need to do them all one side at a time (ie two batches of four sides). And all for naught if it fails.

Wood glue is my go-to filler for tiny rockets, but I haven't tested for any strengthening power. I have some leftover balsa I could do a test with (glue coating vs naked) to see if it makes the fin any stronger or more impact resistant.

Any ideas? The fins came out really pretty, but this guy wants to be flown and I won't have a layabout sculpture rocket in my home. Rockets are made to fly.
 

BABAR

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Picture would help.

If you already used plywood, not sure papering would help much, but it wouldn’t hurt. If you paper for STRENGTH, use Glue, self adhesive adds little if any strength.

Using rear ejection OR using and external shock cord attachment (removable for display purposes) that allows rocket to hang fins up, impacting on the body tube (which may need a bit of reinforcment if you go this route, like an internal coupler, but that may cause problems with chute deployment.....)

Big chute, light winds, and soft grass certainly would help.
 

Off Grid Gecko

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Picture would help.

If you already used plywood, not sure papering would help much, but it wouldn’t hurt. If you paper for STRENGTH, use Glue, self adhesive adds little if any strength.

Using rear ejection OR using and external shock cord attachment (removable for display purposes) that allows rocket to hang fins up, impacting on the body tube (which may need a bit of reinforcment if you go this route, like an internal coupler, but that may cause problems with chute deployment.....)

Big chute, light winds, and soft grass certainly would help.
Screenshot from 2020-07-10 07-17-15-fins.png

Pretty typical fins, this was before I filled them with plastic wood, obviously.
Bringing the whole thing down on the head sounds like a cool idea. If I had thought of that, but the body tube is pretty skinny and I plan on running a long motor tube (29mm) to anchor the paper BT as far forward as possible to something sturdy. I might do a build thread on it. The whole concept of the airframe is beefing up and bracing without resorting to fiberglass or other heavy materials.
I suppose the fins are more than strong enough, but I haven't seen the field I'll be flying it at yet (hopefully next weekend I'll be up there). I suspect there is plenty of tall grass though as the farmers around me finished their first hay and seed harvests just weeks ago.
I am also planning on setting it up for dual deploy, so maybe just be sure to pack a big chute at like 500ft... 200ft if it's a nice flat area? lol.

I have a bunch of leftover fins that didn't make the cut. Maybe I'll try some different glues and do some testing this weekend. I'll post pics and results if anything interesting comes from it.
 

BABAR

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Looking at those, I think papering with glue might help a bit.

Also, mounting position relative to the farthest posterior non-fin part of the rocket has an effect,

Your classic rocket has swept back fins that often extend caudal or downward past the nozzle end of the engine casing of the motor retainer. Think Estes Alpha. This means when the rocket comes down, dangling on the shock cord, then first thing to hit is the fin. Ouch. If you mount the fins forward and/or sweep the fins forward, the first point of impact is the motor casing or retainer. The downside is that you lose a little bit of stabilizing power of the fins moving them forward, but most long skinny rockets tend to be a bit overstable, so you have some room to work with.
 

Nytrunner

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am also planning on setting it up for dual deploy, so maybe just be sure to pack a big chute at like 500ft... 200ft if it's a nice flat area? lol.
Thats the way to do it. Unless you're adding a composite, there's not much to do to increase plywood fin strength
 

Off Grid Gecko

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Looking at those, I think papering with glue might help a bit.

Also, mounting position relative to the farthest posterior non-fin part of the rocket has an effect,

Your classic rocket has swept back fins that often extend caudal or downward past the nozzle end of the engine casing of the motor retainer. Think Estes Alpha. This means when the rocket comes down, dangling on the shock cord, then first thing to hit is the fin. Ouch. If you mount the fins forward and/or sweep the fins forward, the first point of impact is the motor casing or retainer. The downside is that you lose a little bit of stabilizing power of the fins moving them forward, but most long skinny rockets tend to be a bit overstable, so you have some room to work with.
Its little sister is done like this, where the motor hits first (in theory). I also like being able to stand them up in the house without a dedicated rocket stand. This may be the only rocket I make with the trailing fin edges. I've pretty well tuned the body (without e-bay) so that it will run any 29mm motor with plenty of stability (1.7-2.7 cal) while keeping it shorter so the tube is a little stronger from the design aspect. Short spans are tougher. Going with 24 inches of BT, optional 7-8 inch electronics bay, and about a 6 inch nose.
My other designs are almost always clipped delta wings to avoid this problem in particular, and end up with a longer aspect ratio (I shoot for a minimum of 12:1).
 

Off Grid Gecko

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Fin Test.JPG

Alright the results are in.
Bear with me as this was not incredibly scientific, but a good qualitative comparison, imo to make some decisions.
The test setup was simple. Imagine a 2ft long 2x4 pivoting on one side from flat against the ground to vertical. It is raised and dropped like a hinge. About 5 inches from the end of its reach is placed the fin is such a way that the tip is pointed into the 2x4 at the weakest point.
For each test I raised and lowered the 2x4 to a 45deg angle and dropped it directly on the fin.

The treatments:
One fin was not modified in any way.
Two recieved different amounts of thin CA to the trailing edge and tip
Two recieved epoxed layed thin along the trailing edge and tip
One was papered with thing manilla and TiteBond III
The epoxy was 30min Zap Z-poxy

The untreated fin was snapped off in the first drop and can be seen to the top right.
All other fins were hit 3 times. I may have bashed the papered one a fourth time.

Results:
Epoxy did nearly nothing to strengthen the tips. Surface coat without a re-enforcing fiber matrix seems to be pretty pointless for this test. One of them did well on the first hit but failed on the second. These are the fins to the left of the image.

The papered fin (bottom right) was crushed but didn't come apart. It can be pushed back to straight and still looks good enough to fly again, though sooner or later the tip will get flimsy, and I'm certain that performance will suffer.

A CA soak did surprisingly well, and they both (middle fins) took three solid hits to fail. One snapped and the other crushed. Note: the one that snapped broke behind the epoxy line that I had laid down.

Analysis:
CA definitely increases the longevity of porous wooden fins, which we already knew. This test wasn't for strength in flight, as the ply is strong enough for that, but in the landing. Just like stiffening a tube end, some strength and crush resistance is added as well.
Papering does seem to help as well for the thin spots on these tapered fin profiles.
Best part, both of these methods are incredibly easy and offer some peace of mind to the flier (me). For those going plywood fins, it would seem that the application, at least of CA on the airfoiled edges, would be a best-practice. It adds very little weight and increases the stability of the material.

Again, this wasn't super scientific and I'm not claiming such, but it was a chance to take some fin pieces that didn't turn out quite right and do some destructive testing before they go in the burn pile.

For this MPR build, I now plan to at least apply a coating of CA to the fin edges. I'm not sure if I will paper them tip-to-tip on top of this, but it doesn't look like a bad idea. I will likely do both, as until I order an L2 kit, this is the last rocket on the assembly line at the moment. Doesn't hurt to take my time with it.

Unless...
Anyone know if the CA will interfere with wood glue to paper over the top? Negative effects of either papering or CA over time when exposed to paint and environmental conditions? In short, any reason NOT to do this with plywood fins if the materials are on hand?
 

Nytrunner

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Anyone know if the CA will interfere with wood glue to paper over the top?
Wood glue seeps into a porous wood surface. Superglue seals off the porous surface and qont let the wood glue wick in as much.

You could always superglue the paper after its bonded to the fin
 

dhbarr

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Which epoxy did you use? A laminating ( thin ) epoxy should wick into the surface much like the thin CA.
 

Off Grid Gecko

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Which epoxy did you use? A laminating ( thin ) epoxy should wick into the surface much like the thin CA.
30 minute Z-poxy. It's kinda thick. I have an unopened fiberglass resin but it has the little tube of hardener with it and I was using so little that it hardly seemed worth it to break that out for this project. If anyone wants to give that a go and see if it strengthens up the fins that would be awesome, but I don't have any of the 50/50 mix stuff and I'm not sure how thick my laminating resin actually is.
 

Off Grid Gecko

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Wood glue seeps into a porous wood surface. Superglue seals off the porous surface and qont let the wood glue wick in as much.

You could always superglue the paper after its bonded to the fin
Worth a shot, I suppose. The paper did a lot to help the fin strength. I was just really impressed with the results of wicking some CA in there. I was expecting it to come out really brittle.
 

tsmith1315

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I have an unopened fiberglass resin but it has the little tube of hardener with it and I was using so little that it hardly seemed worth it to break that out for this project.
That's polyester resin, not epoxy. Not as strong in general, but it should wick in to the wood better than the Z-poxy. A layer of carbon veil with thin epoxy was my first thought.
 

Off Grid Gecko

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Been thinking more on this today. Kraft tubing (thin wall) is still impregnated with phenolic resin, is it not? I'm not sure but it seems so. I mean...rocket tubes are very unlike cores for toilet paper, paper towel, etc. I might be wrong and the glassine finish is throwing me off.
Anyway. Straight wrapped tubes are stronger than spiral wound. I'm toying with the idea of putting a wrap or two of Kraft paper and glue around the airframe and then sealing the whole outer skin with thin CA. Cheaper than fiberglassing it, and albeit not nearly as strong, it should add some toughness to the tube while also aiding in smoothing out surface imperfections (I say aiding here as I plan on filling all the spirals with plastic wood before proceeding, just as I've done with the fins for the rocket). Though I might have one line that needs some work when it's all said and done. Or perhaps just "papering" the tube and sealing about an inch or so of each end with CA. If nothing else more layers should be more better for a paper rocket body. Right?
 

Off Grid Gecko

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Following the suggestion by @Nytrunner, I decided to paper first and then touch with CA. These came out NICE!
I used standard Kraft paper and Titebond III. The paper was cut in one piece and then folded over the leading edge of the fin, trimmed, and left to dry for a bit. Then I ran a thin line of CA down the rear fin. I'm hoping the stiffness of the CA along the edge coupled with the added crush strength of the papering process will keep these guys looking good for a long time, even if they land in an unsavory spot.
Completed Fins.jpg

In fact, they came out so nice that I feel I've set the bar pretty high for this project. I mean, I hold these things and it's like looking at a work of art. Very proud of these little monsters. Not bad for my first "real" airfoil job.
I did do some minor hand sanding to the edges just to get them all as even as possible to each other before papering. Some were perhaps a point thicker than the others at the edge. Once I get my parts order in, I might start a build thread, as there are some other cool "engineering" concepts I want to employ on this bird.
 

wolsen

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FWIW, I've really become a fan of glassing the fins using the vacuum bagging technique. I can get a quality, durable finish fairly easily using this technique. It does add a bit more weight than the papering of the fins (which I do for LP), but to me the end result is just soo nice. If you haven't seen this technique, John Coker has a good instructional video at his jcrocket.com site. You may want to take one of the fins you "practiced" on and try this technique just to see what you think of it (especially if you already have the supplies). I've done both tip to tip and vacuum bagging, and vacuum bagging is definitely easier IMO.
 

Off Grid Gecko

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FWIW, I've really become a fan of glassing the fins using the vacuum bagging technique. I can get a quality, durable finish fairly easily using this technique. It does add a bit more weight than the papering of the fins (which I do for LP), but to me the end result is just soo nice. If you haven't seen this technique, John Coker has a good instructional video at his jcrocket.com site. You may want to take one of the fins you "practiced" on and try this technique just to see what you think of it (especially if you already have the supplies). I've done both tip to tip and vacuum bagging, and vacuum bagging is definitely easier IMO.
The term scares me, as I assume I'll have to buy something expensive to make it work, or I'll end up wrinkling everything up. I don't have epoxy for it yet, but I do have some regular fiberglass resin (vinyl I think). Might look at it again. I have a pretty strict power budget but I can run just about anything off my big generator when I need to. I don't suppose a pump would eat too much energy. I'll look into it. Just relaxing for the night anyway.
 

wolsen

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The term scares me, as I assume I'll have to buy something expensive to make it work, or I'll end up wrinkling everything up. I don't have epoxy for it yet, but I do have some regular fiberglass resin (vinyl I think). Might look at it again. I have a pretty strict power budget but I can run just about anything off my big generator when I need to. I don't suppose a pump would eat too much energy. I'll look into it. Just relaxing for the night anyway.
FWIW, I do mine with a $50 Ziplock brand foodsaver I bought from Walmart, TotalBoat 5:1 traditional epoxy, cotton batting from Michaels for my bleeder cloth, and some extra ripstop nylon from Joann fabric for the peel ply (though that leaves some surface grooves). Its not an expensive setup (as I wasn't sure and didn't want to make a large investment). I don't think it pulls much power (though I don't really have that concern).
 
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