Do I need internal fillets?

I'm building a BSD 3" Horizon which has been sitting in my closet for the last year. I tacked the fins to the motor mount with 30 minute expoxy, then glassed the exterior of the fins tip to tip with 1.5 oz Sig cloth and West epoxy. I'm planning to foam the interior of the fin can, then seal it up with the aft centering ring, then apply exterior fillets. There will be no internal fillets. Should the fin can as described be strong enough to handle an H128 and H180?
Thanks for your thoughts.
Murrill

I personally would do fillets but if you foam that is fine, just make sure that aft ring has glue on the fin tab area so it increases strength. If it were a 54mm motor I would be more worried, even the glass isnt needed... on your motors...

My answer is YES. I recently built a rocket w/o interal fillets, and it tore apart on an H73, It was not pretty. IF you have small diameters to work with, go to WalMart and get their epoxy that comes with a 4" mixer, aplicator tube, it can inbetween a 54mm and 38mm tubes and also 3" to 54mm, It is 5 min poxy so it sets quick. IF you dont have a WallyWorld try a local hardware store.

always fillet, with glass and fillets, it should handle a nice J motor no prob... my 3" scratch build, has minimal glass fillets only and is quantem tube, I fly it on the j350, j400, j420 all the time.

its worth the extra time.

I've got the BSD 3" Horizon. I love this on an H165R! It really moves! For mine, I did internal fillets composed of 30 minute epoxy mixed with milled fiber. I'd do the internal fillets rather than the foam. Both wouldn't hurt, but the internal fillets are going to be stronger than foaming the fin can. I was curious also, why tip-tip fiberglass? On the motors this bird can fly on, it doesn't need it, IMO. Also, whenever I glass I do the external fillets then glass over the fillet. I think this provides a stronger bond and smoother finish. External fillets with milled fiber (plus the internal fillets...both at the motor mount and at the inside fin to body tube junction) work just fine for this rocket. That's assuming you're uisng the standard 29mm mount that came with the kit. Now, if customizing to a 38mm mount, then the glassing would be appropriate and unquestionably do all the fillets mentioned.

Supposedly you don't need the internal fillets iof you will be foaming, but the only bad it can do would be to add a little extra weight. I would to small internal fillets with some strengthener like milled fibers just for insurance. No need for large fillets if you will be foaming though.
Reed

Always try to fillet the fins to body tube on both sides, inside and outside and fins to motor tube. You do not want to see it shred in regard to the time and money spent on it.
Mark G.

The 3" thor has a 29mm? I remember it being stock with a 38mm and they were selling a special 54mm version.

Do I need internal fillets?

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Only a licensed doctor can tell you for sure.

I'm pretty sure that with an H128 or an H180, you'd be fine without the fillets, but I'll bet you wind up wanting to try something larger/faster.

Originally posted by jraice
The 3" thor has a 29mm? I remember it being stock with a 38mm and they were selling a special 54mm version.

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Read the original post again. He's building a 3" Horizon which comes stock with a 29mm MMT, NOT a Thor. Yes, the 3" Thor does come with a 38mm mount.

Wow, bad mistake... nice catch, I was just sanding the fins on my thor and running rocksim files, guess I was thinking about the thor to much

Trust me from experience- if you foam the fin can, the fins are going nowhere. One tip if you are concerned- drill 3 or 4 1/4 inch holes in the fin tab, glue it, and then foam it. I have done this- had a rocket lawn dart, and the fins were the ONLY thing in tact. You should be fine.

I can only tell you miy experience, I have a Binder Stealth Jr (2.6" wide) and did my L1 with it. It flew straight up and then landed perfect by the way. I launched it on the H128W. When I built mine, I didnt do the internal fillets thing. All I did was put epoxy on the motor mount tube, then used superglue gell to hold the fin in place while the epoxy cured. Then I did the whole fillets on the outside using 30min epoxy. It is very strong. As for bigger motors, cant tell you, but it worked well for me. Went up around 2200ft... Hope this helps

I'd say you've done plenty to hold the fins on with the epoxy to the motor mount and fillets on the outside of the tube. Foam will add some weight, and will make the fins really hard to get out if you ever want to repair it (it can be done with a DADO blade and table saw, though). I would say your good for the motor choices you will have available.

Edward

Highest thrust 29mm motor (besides EX or kosdon motors) is probably the EM H275 and I flew a rocket with 1/8" fins (2.5" diameter) and the fins just had glue applied to the motor tube to tack the fin it (probably created small fillet) and they were fine. With a 29mm MMT anything will work.

The fin can would be strong enough without all of the extra work you have already put into it. Some simple 5 minute epoxy to glue the fins through the wall and then some 5 minute epoxy for fillets is really all you need. It's how I built my 3" PML Bull Pup, 4" Pterodactyl Jr, 4" Thor, 5.5" Honest John and 6" upscale Pterodactyl Jr. All have flown on motors ranging from H through L with absolutely no problems. I did no tip to tip glassing and did no foaming of the fin can.

As a side note, foam is going to add a lot more weight to the part of the rocket that you don't need it. Plus, you already glassed the fins tip to tip, and they aren't going to flutter on those motors anyhow, so anything more than that is truly just over kill.

Internal fillets are fine, but I wouldn't foam it.