Papering fins with vacuum, thinned epoxy

SolarYellow

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I'm going to be building an Estes Goblin for dry-lake launches with AT 24/40, maybe 24/60 power. Probably F35 or F39 as the highest average impulse, as I'm more interested in altitude than speed with this model. Will have an Eggfinder Mini in the nose. I want durability on the hard desert. Have already decided to go with 1/16 bass and TTW construction for the fins. Thinking about papering them.

Also leaning toward a hot-wire chute release run off a Quark for motor-eject dual deploy with a 15-in Estes chute. Which should get it slowed down enough that just painting the basswood and TTW with good fillets might be more than good.

I have a Harbor Freight vacuum pump. Thinking maybe BSI 30-minute epoxy thinned with isopropyl alcohol and some nice smooth bond office paper bagged would give me the smoothest finish with greatest strength.

Is this massive overkill? Any recommendations or pointers? I've done a little searching, but found limited results on bagging fins for papering them.

Monokote is also an option, and would probably be lighter and easier. Would just have to paint the yellow ones, as I only have red, black and white. I guess white MK under yellow paint and black MK for black would probably give best cosmetic results. Do the Estes water transfer decals stick to Monokote?

Should I invest in the BMS foil-lined MMT for the AT cases? (Guess I should pick up some centering rings to go with the OD greater than Estes BT-50, too...)

Any other thoughts?
 

MidOH

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Do they make a supersized Goblin or something? Mine rips on E's.

I'd use a longer bt55 tube and stuff it with couplers. Consider dual deployment with an Apogee kit and chute.
 

Neutronium95

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I feel like if you're going to be doing composites with resin infusion, you should probably just bite the bullet and get some thin fiberglass.

That said, if I was concerned about fin durability, I'd probably just use some good quality aircraft plywood and call it a day.
 

MidOH

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An easier option ($42):
 

David_Stack

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...Thinking maybe BSI 30-minute epoxy thinned with isopropyl alcohol and some nice smooth bond office paper bagged would give me the smoothest finish with greatest strength.
Admittedly you are not using the epoxy for a significant structural bond but... Thinning epoxy with alcohol significantly decreases the strength of the epoxy bond. See: https://www.rocketryforum.com/threads/904-ceramic-adhesive.140275/#post-1941678

You may want to consider investing in laminating resin instead of structural adhesive epoxy (Finish Cure is the BSI product)
 

SolarYellow

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This isn't about, "What rocket to build?" It's about packing a bunch of coolness into the rocket kit I've already decided I'm going to build. Part of the challenge and interest is figuring out how to make all the stuff fit and work in a small rocket. Sure, if it was longer or a bigger tube, it would be easier to fit more stuff in it. Like with a Cherokee-E.

I want to mess with the Goblin. Because.

I'm purposely staying away from transonic in order to stick with, and work within the constraints of, the original airframe.

Understood that thinning epoxy weakens it, and have already read up on the Finish Cure stuff. But I have 30-minute and isopropyl alcohol on hand, and for laminating paper on fins, I reckon it will be fine.

Kinda just wondering if anyone else has done this and has useful pointers.
 

Neutronium95

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I'm purposely staying away from transonic in order to stick with, and work within the constraints of, the original airframe.
Estes tubing is surprisingly tough. I have a rocket that uses a BT-60 body tube that I fly pretty regularly on Cesaroni 24mm G motors. The only damage that it has sustained all comes from recovery damage.

If you want to just build a solid flier, either traditionally papered fins or plywood will be perfectly fine, assuming they're attached through the wall. If you want to push the limits of your buulding skills, do whatever you want.
 

SolarYellow

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Pretty sure BT-60 is going to be well below transonic on a G, given my sims with Gs in 24mm MD stuff. BT-60 is ~2.7x the cross sectional area and the 6-grain G only gets a 24mm built to handle it up to Mach ~1.3 in the sim. So I wouldn't expect any problems. As I don't expect problems keeping a BT-55 below ~Mach 0.8.
 

rharshberger

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So, one of my favorite fliers is an Estes Nike Smoke, with a 24mm mmt, TTW papered (traditional glue) balsa fins, and a JLCR, hits right around 2010' plus or minus 20' usually. Motor retainer is a Estes plastic one. It is not double tubed/sleeved.
 

SolarYellow

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Current state of the sim for this build is

~3300 ft on F35 (don't have the 24/60 case yet)
~3100 ft on F24
Mid-1900 ft on Estes E9 or E12
All four motors have available delays within a few tenths of the optimum calculated by OR.

Fastest speed is Mach 0.73 on either the F35 or an F44 SU. I figure the standard Estes BT-55 is more than adequate. I'm sure painted basswood TTW would be fine for the flight, it's just the landing I'm thinking about beefing them up for.

With all the electronics in the nose (EF Mini, Open Log, Quark, two batteries, plus a sled), it's pretty close to optimum weight and plenty stable even with the big case. The real accomplishment on this build will be getting the sled packaged to fit all that in the nose cone. Adding nose weight increases apogee, but very slowly, so I still have room to build and finish without OCDing too much on weight.

Still thinking I'll just hang the MMT out the back of the BT a little and use tape retention. But if I order more parts from somewhere before I get that far, maybe I'll add some Estes motor retainers.
 
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Like Tim said, this is your project, so you should do it the way you want to. I think you should just go ahead and use fiberglass fins,No finishing, super strong, not that much difference it will make in altitude, and will save you lots of the precious time you have in your life. When it hits the desert floor they won't break.
 

bjphoenix

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I do a lot of fin papering- balsa and Titebond II and printer paper. I don't think you gain anything by using epoxy. If you want more strength then I would suggest some form of cardstock instead of printer paper. But as other people have said you could go with plywood. And then unless you are dedicated to the design of the Goblin you could reshape the fins so they don't project back behind the airframe and that way they won't get hit so hard on landing. I think the bond between body tube and fin is also a weak point so it is worthwhile to build up some good sized fillets.
 

rharshberger

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This is...was...a SBR Lil' Fusion "hot rodded" aka red shifted to use 24mm AT G55W and G110T SU motors as well as 6 grain 24mm CTI's. First picture is on a AT G55W to ~3000' (altimeter section lost due to separation of 3D printed anchor point). Second picture is prior to its final flight to ~3500' (I have the altimeter max alt just cant remember exact altitude), that flight ended in a ballistic recovery but the altimeter survived. Other pictures are post flight carnage and preflight altimeter bay setup. Nosecone covers a 180mah 2s LiPo, and Missleworks screw switch, inside the Blue Tube 2.0 29mm coupler is an Eggtimer Quark configure for apogee only deploy (resistor across main deployment outputs as recommended by Cris). Recovery was supposed to be via a 1.5"x 144" mylar streamer, for whatever reason deployment charge didnt fire (I might have broken a wire inserting the bay into the airframe). The rockets name was...Vanishing Act IV, its name started out as just Vanishing Act but each post flight repair added a Roman numeral.
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flyingeagle

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I have tried everything under the sun to seal balsa fins. Glue slurry, CWF, Deft sealer that the fumes are unbearable. The only method that was easy and successful for me was papering the fins with running a little thin CA around the edges to seal the paper. My problem with paper fins is when I prime the fins it lifted the paper fibers for the exception of the CA coated edge. I then sand them smooth with 400 grit sand paper gently but on the second coat of primer the paper fibers lifted again. I found using a q-tip and painted the remaining paper surface with CA. When dried and sanded I took CWF diluted with a few drops of water, consistency of yogurt then spread it with my fingers very very thin, translucent then sanded again. The fins are like glass before priming. Dose anyone else have any ideas of sealing the paper without adding extra weight.
 

neil_w

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I have tried everything under the sun to seal balsa fins. Glue slurry, CWF, Deft sealer that the fumes are unbearable. The only method that was easy and successful for me was papering the fins with running a little thin CA around the edges to seal the paper. My problem with paper fins is when I prime the fins it lifted the paper fibers for the exception of the CA coated edge. I then sand them smooth with 400 grit sand paper gently but on the second coat of primer the paper fibers lifted again. I found using a q-tip and painted the remaining paper surface with CA. When dried and sanded I took CWF diluted with a few drops of water, consistency of yogurt then spread it with my fingers very very thin, translucent then sanded again. The fins are like glass before priming. Dose anyone else have any ideas of sealing the paper without adding extra weight.
Label paper takes primer smoothly... but only 800 grit or higher should ever be allowed to touch its surface, otherwise the surface gets roughed up and there is no easy recovery. In other words, you must sand the primer gently and carefully.
 

bjphoenix

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My problem with paper fins is when I prime the fins it lifted the paper fibers for the exception of the CA coated edge. I then sand them smooth with 400 grit sand paper gently but on the second coat of primer the paper fibers lifted again
I have not noticed that problem. I put some elmers wood filler on the fin edges and sand them, this fuzzes the paper edges a bit. I spray on Rustoleum primer then sand off the rough stuff and spray on the color. If you have fuzzies after 2 coats of primer I would say sand them off and spray color. If you want really good surface finish then I don't think papering is the way to go.

Before I got into papering I would smear on a thin layer of elmers wood filler on the whole fin then sand it off carefully. I've gotten some really smooth fins that way, but these days I care more about strength than quality.
 

flyingeagle

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When I sprayed Rust-Oleum 2x gray primer over the papered fins is when the paper fibers lifted. I thought for sure after sanding the primer smooth without going into the paper I was good to go. I primed again and when dried it did it again. I don’t want to keep priming and priming or I’m going to have fins as heavy as the spray paint can itself. The only reason I paper fins is for strength. I tried label paper but not much luck on a good adhesion. The label lifted off to easy. Only a small amount stuck to the balsa. I am now trying an experiment with a q-tip and thin CA painted over the paper, sanded then apply a thin see through coating of CWF and then sanded smooth again. I will try priming that tomorrow. Fingers crossed.
 

neil_w

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I tried label paper but not much luck on a good adhesion. The label lifted off to easy. Only a small amount stuck to the balsa.
Sand balsa smooth. Use soft brush to remove as much dust as possible. Then use blue tape to lift off the remaining dust. After this, adhesion is very good.

That said, label papering will inherently not provide as much strength as glued paper.
 

flyingeagle

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I stopped at the UPS Store and the guy there gave me a few labels. That’s what I tried and didn’t care for it. I stopped at Staples and looked at label paper, wow a lot of money. What do you guys think about what I said in coating the paper fin with CA? The fins are already glued to the body tube with Tite Bond ll. The only part of the fins that are smooth under the primer is where I did the edges with CA after papering
 

Back_at_it

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I did this with a Der Red Max. Same basic idea as the Goblin with one less fin.

BT50H for the motor tube.
Plywood forward ring. Only needed as it was an anchor point for my Kevlar.
Full length coupler minus room for the nose cone.
Basswood fins TTW.
Estes Motor Retainer.
Kevlar Leader.
About 6ft of 1/4" Elastic.
1oz of Nose weight.
3/16" lug.

Coated each fin with BSI 30 min epoxy. Just warm it up and it flows and levels perfect. Let it dry for and day then flipped and did the other side. Fair amount of sanding on the edges as the epoxy flows overs. Attached fins to the body with more 30 min epoxy.

Has survived numerous E30's and three F44's. I've recorded 1486ft on an E30 and it SIM's at 74 mph off the rod :)
 

CalebJ

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I have tried everything under the sun to seal balsa fins. Glue slurry, CWF, Deft sealer that the fumes are unbearable.
Have you tried finishing epoxy (Zpoxy as one example)? A thin coat didn't even show up as a measurable weight increase for me, but the finish is superb.
 

Back_at_it

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I have tried everything under the sun to seal balsa fins. Glue slurry, CWF, Deft sealer that the fumes are unbearable.

If all you are looking to do is seal the fins then any of the wood fillers on the market would be your first step. I use the DAP brand. Follow that up with Minwax sanding sealer then sand it smooth with 400 git and call it done. Neither of these items have a strong smell.
 
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