HPR L2 Fillets

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davdue

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Am getting ready to start on my scratch built L2 rocket. My question is which fillets on a TTW fin provide the strength? From what I have read fillers like chopped fiberglass and colloidal silica make the fillets stronger. I am not in the position right now to purchase West Systems or Aeropoxy so I will be using hobby epoxy. The airframe will be quantum tube so no tip to tip. With all that said what filler should I use?

Thanks
 
I wouldnt use a filler with a hobby epoxy. All of my experiences with fillers have been bad. It's time to spend $40 and buy a nicer epoxy IMHO.


Alex
 
Why do you feel you need a filler for the epoxy? All I've ever used for fillets is the Loctite 5 min epoxy (in the twin bottles). I've never used filler and never had a fillet fail, even at 80Gs or just over Mach.

For an L1, or even most L2, the only reason I would use a filler would be to thicken the epoxy to make it easier to handle and spread, not for what it does for the joint.
 
+1 on the bad experience with hobby epoxy. I did my L1 with it, worked fine. After some fast descents with landing on hard salt, all the epoxy was cracked so I tore off the fins and guides and rebuilt using aeropoxy. Aeropoxy structural is a 1:1 by volume or weight so it's easy to use. I really like the Proline thick but you need an accurate scale to at least .1g for small amounts. I still use tons of the hobby epoxy for all sorts of non-critical fastening applications and odds and ends around the shop/home. I have probably 6 types of epoxy, all for a purpose. If you get a laminating epoxy such as USComposites, you can add various fillers to use it for fillets, or whatever. As for your original question regarding the filler, chopped fiberglass (small snips with the scissors) works grand but can really make a mess of trying to make nice fillets. Fillers such as corn starch, glass bubbles, microspheres, and such will make for nice and easy filling but can reduce the strength to possibly an unacceptable level. (experimentation here?) Good luck, there are some extremely knowledgeable people on this forum.
 
After some fast descents with landing on hard salt, all the epoxy was cracked so I tore off the fins and guides and rebuilt using aeropoxy.

If it were my rockets I would be re-evaluating my recovery techniques rather than concentrating on building with the more expensive epoxy.
 
Am getting ready to start on my scratch built L2 rocket. My question is which fillets on a TTW fin provide the strength? From what I have read fillers like chopped fiberglass and colloidal silica make the fillets stronger. I am not in the position right now to purchase West Systems or Aeropoxy so I will be using hobby epoxy. The airframe will be quantum tube so no tip to tip. With all that said what filler should I use?

Thanks

I'm assuming you meant which "FILLERS" provide the strength and not "FILLETS"?? My L1 Mad Cow Phoenix was built exclusively with DevCon 5 and 30 minute epoxies, no filler. With JB Weld on the Aeropack retainer of course.

I just finished my L2 LOC Bruiser, which will be flying Saturday for the first time. Its build with West Systems epoxy. From experience, building the fillets with epoxy thickened with colloidal silica was a wonderful experience. No more catching drips while the 30 minute epoxy set up. And it is STRONG. Honestly, once you start using fillers, you won't want to go back.

When I've been in a hurry, I've been able to mix the West colloidal silica with 30 minute hobby epoxy, there isn't any compatibility issues..... the only thing I noticed is that it seems to shorten the cure time. And it works better than straight epoxy for fillets. For bonding parts you can mix in some West high density filler for max strength.

Lets be honest, this is NOT a cheap hobby. If you want to stick with hobby epoxy, just pick up a container of colloidal silica and a container of high density filler. They cost about $14 each. Which together is just a few dollars more than the cost of one "H" motor. Mixed with the right fillers, even hobby epoxy will be stronger than the parts you are bonding......
 
The airframe will be quantum tube... Thanks

The PML site has recommendations on epoxy. They have an adhesiveve FAQ and they even sell epoxy. Not the expensive kind... NHP brand. This is the typical generic Hobby store stuff. So if you are working with Quantum tube, per PML, cheap epoxy without chopped carbon fiber - should be fine.

Many of the fiberglass Wildman kits recommend chopped carbon fiber in the fillets, but those kits need to be built that way to honor the guarantee.
 
Any 1:1 mix ratio epoxy already has fillers in it -- adding more creates a chance of interfering with the cure, and isn't recommended. Epoxies such as West Systems, AeroPoxy, Fibreglast, etc, the companies selling it all recommend and endorse the use of fillers to change the properties of the epoxy when cured.

You didn't give any specs for the rocket you're building, which would help on recommendations.

That said, if your fins go all the way to the motor mount, epoxy them to the motor mount and apply fillets there. Then, apply fillets to both the inside and outside of the airframe.

-Kevin
 
Why not? Any other opinions? Which provides the strength, the internal or external fillets?

Generally, the TTW system with two fin attachment points as opposed to one surface mount point provides the strength. That said, its probably better to focus on the internal fillet strength. The chopped fibers are a good idea for increasing strength. There are composite supply places that sell chopped fiberglass and carbon fiber. I've had good success with colloidal silica, although I'm not sure how much strength it adds.

The only thing I would be worried about with using fillers with hobby epoxy is keeping the thickness under control. The fibers can create a "hairball" pretty quickly even with a thin epoxy (west systems), so add only bits at a time. The other thing that I've found is that colloidal silica can speed up the curing process considerably.
 
If you want a cheap, internal, really strong, fillet...

use a piece of wood dowel epoxied to the each side of the motor tube/fin joint and the body tube/ fin joint. (4 sections of dowel per fin)

ugly but strong and cheap.
 
If it were my rockets I would be re-evaluating my recovery techniques rather than concentrating on building with the more expensive epoxy.

Descent was about 20fps, yeah a bit fast for hard ground. I used 30min epoxy. The fin fillets were just cracked. To disassemble the rocket, I just tapped parts with the hammer and the epoxy just broke. No flex whatsoever. I've heard 5 min is better. What can I say, I like the fancy epoxy now.;)
 
Am getting ready to start on my scratch built L2 rocket. My question is which fillets on a TTW fin provide the strength? From what I have read fillers like chopped fiberglass and colloidal silica make the fillets stronger. I am not in the position right now to purchase West Systems or Aeropoxy so I will be using hobby epoxy. The airframe will be quantum tube so no tip to tip. With all that said what filler should I use?

Thanks
Even though you're scratch building, follow the PML construction tips. You have to really rough-up the bonding areas of the QT. If the fins are G-10/fiberglass, you have to rough those up too. Epoxy doesn't soak in those materials like it does cardboard, phenolic, or wood. A good joint between the motor tube and fins is very important.

Early in my rocketry life, I built several rockets with hobby epoxy and most all are still flight-worthy and none experienced epoxy failure that I can recall. I would recommend at least 15 minute to give you plenty of working time. Once you gain experience, 5-minute will work, but you have to plan ahead and be quick. I don't use hobby epoxy any more - the small bottles aren't really cheaper than a quart or larger sized quality epoxy like Aeropoxy or West Systems. I've built several large rockets that would probably take an entire set of hobby epoxy bottles and I like working with the much less viscious epoxies like the high quality products.

As others have said, if you use hobby epoxy, don't use fillers. If using one of the high quality products, chopped fiberglass or carbon fiber will strengthen the joint. Those products with fillers are a bugger to sand, so plan ahead and shape the fillets as precisely as you can. Learn to use alcohol to smooth and remove epoxy. System 3 has a very good resource called "The Epoxy Book" that describes the usage of the various fillers. It is worth reading.

Good Luck, --Lance.
 
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Descent was about 20fps, yeah a bit fast for hard ground. I used 30min epoxy. The fin fillets were just cracked. To disassemble the rocket, I just tapped parts with the hammer and the epoxy just broke. No flex whatsoever. I've heard 5 min is better. What can I say, I like the fancy epoxy now.;)

I re-kitted a wildman 4" vindicator at Red Glare this spring. built with 5 min epoxy. It landed hard and every fillet, attachment shattered. Fins, motor mount, fillets, etc. Tim recommended a laminating epoxy (I use US composites) and I bought some chopped carbon and collidal silica from Tim.

5 minute worked fine for me with cardboard and wood. Not so much with fiberglass.
 
I had a similar happening at the March MDRA launch this year.
The main charge wasn't strong enough to push out the chute and it came in from 5,122ft (reported by Adept22)
All fillets were done with Bob Smith brand epoxy and milled glass additive.
2 of the 3 fins on on the rocket (PR Intimidator3.0") broke away from their fillets.
I just glued them back on yesterday with Pro-Set (tm).

JD

I re-kitted a wildman 4" vindicator at Red Glare this spring. built with 5 min epoxy. It landed hard and every fillet, attachment shattered. Fins, motor mount, fillets, etc. Tim recommended a laminating epoxy (I use US composites) and I bought some chopped carbon and collidal silica from Tim.

5 minute worked fine for me with cardboard and wood. Not so much with fiberglass.
 
Should o' mentioned, my first L2 attempt was foiled by a chute failure to deploy but NC came off and rocket came down from about 5000' on the salt. Only damage was a cracked NC and 1 cracked fillet which was fixed with 5 minute epoxy and I was able to re-fly and certify! I had a Mad Cow something or other John with TTW fins. Bless the strong joint that makes!
 
Descent was about 20fps, yeah a bit fast for hard ground. I used 30min epoxy. The fin fillets were just cracked. To disassemble the rocket, I just tapped parts with the hammer and the epoxy just broke. No flex whatsoever. I've heard 5 min is better. What can I say, I like the fancy epoxy now.;)

Something isn't right with that picture. Sounds to me like either you didn't have enough epoxy or it wasn't mixed right. I am in the process reusing fins from a destroyed rocket that was originally built in 2006. There was no banging those fins off with a hammer. Not even close. Layers of tube that it was glued to came off with the epoxy and then it was a crapload of grinding and sanding to get the epoxy fillets off the fins. Thank God for a bench belt sander.

And that was back when I didn't quite know what I was doing and didn't have all the glassine coating sanded off.

-Dave
 
You didn't give any specs for the rocket you're building, which would help on recommendations.

That said, if your fins go all the way to the motor mount, epoxy them to the motor mount and apply fillets there. Then, apply fillets to both the inside and outside of the airframe.

-Kevin

Sorry I was posting earlier with my iPhone and forgot to add a link to the build thread on this rocket. It is an 3X upscale of the Semroc Lazer-X. https://www.rocketryforum.com/showthread.php?33922-L2-Semroc-Laser-X-Upscale. That is the first time I have heard about fillets on the inside of the airframe too. I would guess that would make things even stronger. Seems like a little bit of overkill for a rocket that won't go super sonic. OR tells me .8 mach on a K550 which would most likely be the biggest motor I would fly it on.

Even though you're scratch building, follow the PML construction tips. You have to really rough-up the bonding areas of the QT. If the fins are G-10/fiberglass, you have to rough those up too. Epoxy doesn't soak in those materials like it does cardboard, phenolic, or wood. A good joint between the motor tube and fins is very important.

Early in my rocketry life, I built several rockets with hobby epoxy and most all are still flight-worthy and none experienced epoxy failure that I can recall. I would recommend at least 15 minute to give you plenty of working time. Once you gain experience, 5-minute will work, but you have to plan ahead and be quick. I don't use hobby epoxy any more - the small bottles aren't really cheaper than a quart or larger sized quality epoxy like Aeropoxy or West Systems. I've built several large rockets that would probably take an entire set of hobby epoxy bottles and I like working with the much less viscious epoxies like the high quality products.

As others have said, if you use hobby epoxy, don't use fillers. If using one of the high quality products, chopped fiberglass or carbon fiber will strengthen the joint. Those products with fillers are a bugger to sand, so plan ahead and shape the fillets as precisely as you can. Learn to use alcohol to smooth and remove epoxy. System 3 has a very good resource called "The Epoxy Book" that describes the usage of the various fillers. It is worth reading.

Thanks for the reminder on the PML FAQ's. I have read them on the QT but didn't realize they had an epoxy FAQ. I will check it out. I won't use any fillers for this then. I have so far only used 5 minute epoxy. Like you said you have to be on your toes and be ready. But I can do two apposing fillets at a time wait 2-3 minutes and do the next set and have all of the external fillets done in 30-45 minutes. I use a plastic spoon to shape them and therefore have very little sanding to do later. I only mix enough epoxy for two fillets at a time as well. For my next rocket I will buy some quality epoxy. I will probably build a fiberglass rocket or at least do tip to tip on it.

My goal is to have this rocket done by Sweaty Balls IV and fly it on an I motor to practice the DD and then do the cert flight at AirFest. I might even have my 2-stage HPR ready too (Have to rebuild the booster with a 38mm MMT first). It will use my L1 scratch built rocket as the sustainer. Thank you everyone for your comments. I continually learn a lot that saves me from a lot of mistakes from all you guys on TRF.
 
Sorry I was posting earlier with my iPhone and forgot to add a link to the build thread on this rocket. It is an 3X upscale of the Semroc Lazer-X. https://www.rocketryforum.com/showthread.php?33922-L2-Semroc-Laser-X-Upscale. That is the first time I have heard about fillets on the inside of the airframe too. I would guess that would make things even stronger. Seems like a little bit of overkill for a rocket that won't go super sonic. OR tells me .8 mach on a K550 which would most likely be the biggest motor I would fly it on.

Landing/transport is a cause for damage, as well. Looking at the original, the fins are going to be relatively large, which means there's good leverage at the tip. You'd be surprised how easily you can smack a tip and pop it loose at the motor mount, if it's not well attached there. When that happens, the body tube joint just becomes a pivot point. Fillets to the motor mount will help prevent that, and the nice thing is they're hidden and don't have to be pretty. :)

Thanks for the reminder on the PML FAQ's. I have read them on the QT but didn't realize they had an epoxy FAQ. I will check it out. I won't use any fillers for this then. I have so far only used 5 minute epoxy. Like you said you have to be on your toes and be ready. But I can do two apposing fillets at a time wait 2-3 minutes and do the next set and have all of the external fillets done in 30-45 minutes. I use a plastic spoon to shape them and therefore have very little sanding to do later. I only mix enough epoxy for two fillets at a time as well. For my next rocket I will buy some quality epoxy. I will probably build a fiberglass rocket or at least do tip to tip on it.

If you're using QT, Lance is spot-on about following PML's tips. And when they say to rough up the tube, they mean rough up the tube -- if you're not using 60 or 80 grit sandpaper, you're being too gentle.

-Kevin
 
If you're using QT, Lance is spot-on about following PML's tips. And when they say to rough up the tube, they mean rough up the tube -- if you're not using 60 or 80 grit sandpaper, you're being too gentle.

I had read the QT FAQ before and re-read it again. I also read the adhesives FAQ. They say 12 min or 30 min epoxy is stronger. Should I use one of those for the internal fillets and then the faster stuff on the outer for quick curing time? I just might see if Mark at the Science Education Center has some West or Aeropoxy and save on the shipping. I know he carries the hobby stuff.
 
I had read the QT FAQ before and re-read it again. I also read the adhesives FAQ. They say 12 min or 30 min epoxy is stronger. Should I use one of those for the internal fillets and then the faster stuff on the outer for quick curing time? I just might see if Mark at the Science Education Center has some West or Aeropoxy and save on the shipping. I know he carries the hobby stuff.

Unless you are in a real hurry to build a rocket (like in a day), the 12 or 30 minute epoxy should be fine for the outer fillets. If by chance you do pick up the West or Aeropoxy along with fillers, it is possible to do all the exterior fillets at once. If the epoxy is thick enough, it won't sag while it cures.
 
5 minute is handy for tacking, such as the initial bond of the fins to the motor mount. Then use something with a longer cure time to do the actual fillets.

-Kevin
 
I had read the QT FAQ before and re-read it again. I also read the adhesives FAQ. They say 12 min or 30 min epoxy is stronger. Should I use one of those for the internal fillets and then the faster stuff on the outer for quick curing time? I just might see if Mark at the Science Education Center has some West or Aeropoxy and save on the shipping. I know he carries the hobby stuff.
Mark only has the hobby epoxy. You can get West Systems at Action Marine just east of Andover. Long way from home for you, though.

--Lance.
 
My L2 was built with 5 minute epoxy and 15 minute Bob Smith epoxy. The filler definitely makes the epoxy easier to work with since it doesn't run. The L2 rockety flew many times over 5 years until a CATO finished it. Save up the money and get a good epoxy. I like US Composites because it works great and the cost is reasonable. Aeropoxy is great for composites and I like their Aeropoxy light product for laying on the top of a fillet, sands very easily. A gallon of US Composites even with shipping will save you a ton of money compared to purchasing small tubes of epoxy. My local Lowes has Devcon .85 oz for $2.88 and I can get 4.5 oz of Bob Smith for around $6.50. When I figured out the per gallon cost of each of these which is over $180 for Bob Smith and over $400 for Devcon it made Aeropoxy and US Composites the cheap option.
 
Ok you guys have talked me into buying some West Systems epoxy. Now I have some more questions.
1. What do I get 205 or 206 hardener?
2. Do I get some filler Nd what kind?
3. Can you vary the cure time by adjusting the mix ratio?
4. Can I use this for fiberglassing on later rockets?

Thanks

Sent from my iPhone using Forum Runner
 
davdue said:
Ok you guys have talked me into buying some West Systems epoxy. Now I have some more questions.
1. What do I get 205 or 206 hardener?
2. Do I get some filler Nd what kind?
3. Can you vary the cure time by adjusting the mix ratio?
4. Can I use this for fiberglassing on later rockets?

Thanks

Sent from my iPhone using Forum Runner

1. I would get the 206 hardener. You will thank yourself when it's a hundred degrees out.

2. I would get the 406 filler for fillets. Mix until consistency somewhere between mayo and peanut butter.

3. No you cannot vary mixing ratios. Buy the pumps and then you won't have to measure out a batch.

4. Yes this epoxy w/o fillers is meant for composite layups.
 
1. I would get the 206 hardener. You will thank yourself when it's a hundred degrees out.

Agreed! You can always buy a small can of the other, for when you're working inside and want something that sets faster, too. Just don't ever mix the two hardeners in a single batch.

2. I would get the 406 filler for fillets. Mix until consistency somewhere between mayo and peanut butter.

Agreed! Great stuff! One note -- it's tough to sand, so you want to get the shape close, if you use much of it.

-Kevin
 
L2 covers a lot of ground when it comes to the power of the motors inclusive to that level.

I’ve seen Loc Precision kits built using Elmer’s Wood Glue with foamed fin roots survive Mid-Range “K” motors.

If you are expecting to go Mach then you’ll probably need to re-think your tubing choice as PML doesn’t recommend their Quantum Tubing for Mach attainable rockets.

I’ve used silica filler with hobby epoxies and so far I haven’t had any fillet failures.
Admittedly I haven’t had a lot of launches on these rockets yet and they have all been with “G” and low-range “H” motors.
 
Agreed! Great stuff! One note -- it's tough to sand, so you want to get the shape close, if you use much of it.

-Kevin

I bought some micro balloons at the hobby store the other day. I am thinking of using those for the outer fillets to make them easier to sand. What do you guys think about adding those to the West Systems?
 
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