Epoxy fillet question

"Never used epoxy til rocketry" that will be printed on my tomb stone. So, needless to say, I'm a novice when it comes to epoxy. Well, last night I finished my last epoxy fillet for the launch lugs on my Mirage. They turned out ok considering my experience. Actually, I'm impressed. Thanks to Ken and Carl for showing me how to do them. But now I have a few questions.

1) When I poured the fillets they all came out relatively equal in shape, except one. (there always has to be one) It seems to be more "swept back" than the others. More aerodynamic I would think. The question is, does each fillet have to be perfectly equal and symetrical?

2) Now it is time for sanding. What grit of paper should I use? or, is there any technique that I should follow to finish the fillets?

3) I primed and sanded the body tube before I applied the epoxy. Did I screw up?

Thanks for your help...

1. If your bond before the fillets was good enough, it only matters to your eye what the fillets look like. For aesthetic reasons, you may want to have em all the same, but from a flight performance POV, it doesn't matter.

2. Sanding pure epoxy (what I assume you have used) is kinda hard. Wet sand with a large grit (120) to the shape you want to have it in and then finish wet sand (400).

3. You should have just sanded the body tube. Primer, since it's part of the finish, comes onto the epoxy. But if you're fins were on solid before, it doesn't matter. As long as there is no K motor in the Mirage nothing will come off...

I advise you to use microballoons in the epoxy next time (at a slightly melted ice cream consistency). Much easier to shape, doesn't run away, I mean off, and sands much easier.

Oliver

Cool, thanks. Oooo, MicroBalloons. Now that sounds interesting. Please tell me what the heck those are...

you should have put the fillets on before the primer, but it shouldn't be such a big deal on that rocket.. now HPR would be another area. You can use Elmers wood filler to the fillets to make them all the same... dry's fast and sands easy. This is all purly cosmetics... Which I do care quite abit about.. A good look'n rocket is alway's part of my goal. And Micro-Ballon's are a must for fillets.. they are awsome. They are tiny tiny glass beads, that comes in a plastic bottle. You mix it in with the epoxy after you have the epoxy equiled out.. mix it in good, a thick cream consitancy works great.. You'll be supprised with the results. Good luck with you projects.. post some pic's of you rocket when it's complete.

Well, what I'm doing is modifying the Mirage to take H - I engines so I think at least filleting the fins is a requirement. Tonight I will be going to pick up some Micro-Balloons. Can't wait to tell my friends that I used micro-balloons on my fins. They will look at me with awe and admiration.

I'm taking pics at every point of the process and I will definitely post the links as soon as I finish. Right now, I am trying to fill time. I have to wait until I get my motor retention system in the mail before I can finish the motor tube and such. I hate waiting...

Sounds great.. Yes, you defently want fillets on your fin/motor mounts going HPR. But next time, do it before you put the primer on or sand the primer down to raw material where the epoxy comes in contact at. What are you using for motor retention? Slim line? Aero-Pac? I use Aero-Pac on my nice rockets.. ie.. Upscale HeatSeeker.. 4" V-2...

Nor much to add that hasn't already been said on this one except that a combination of microbaloons and fiberglass mil will make the fillets strong and sandable.

Also, I should have noted that while pouring fillets, I use a small level to keep the body tube level; it will prevent the "swept back" look you had with one of your fillets and allow them to flow in the channel evenly.

Carl

Originally posted by PGerringer
Tonight I will be going to pick up some Micro-Balloons. Can't wait to tell my friends that I used micro-balloons on my fins. They will look at me with awe and admiration.

Click to expand...

We already look at you with awe. It's the admiration part that I'm struggling with!

Mark

Luckily the only thing that got filleted (is that a word?) after primer was the front launch lug. That is also the one with the uneven fillets. Carl, I did them just like you and Ken did but apparently the swept back masking tape was more swept back on one side. oooops... Well, for a first attempt it isn't too bad. Also, I did use a level and it came out real nice. (Thanks Ken)

When I fillet the fins, I will be using the microballoons and epoxy. Wish me luck.

For a motor retention system I bought the AeroPac: https://rocketsilo.safeshopper.com/95/838.htm?192 They shipped it yesterday but it isn't here yet. It is coming USPS and I dont have any faith that it will be here this year ( ) Anyway, I cannot put together the motor tube until it is here cause I don't know how far the tail end is supposed to stick out. So, I'm on hold for the fins and tube. Thus the reason I'm priming and filleting the upper tube. I'm anxious to do something...

if a dimention is all your wait'n on.. just leave 3/8" of the motor tube sticking out of the aft. center ring. This will be enuff for the Aero-Pac retain to hold on to.. also if you don't know.. get some JB Weld, this is what you want to put your retain on with. As it doesn't heat up like most hobby epoxy's and won't damage the thin aluminum. I've put together both the 29mm mount and a 38mm mount. This is a awsome motor retention set-up..

Thanks for all the good answers. I think I can start making the plywood center rings and dry fitting the motor tube. Couple more questions (cause that is the way I am)

The guy from Aero-Pac said 1/4" for Dr. Rocket and 3/8" for Aerotech. Does that mean that if I put it 3/8" from the ring, I cannot use Dr. Rocket reloads?

How far is your rear center ring from the end of the body tube? I had thought of having the Aero-Pac where it barely sticks out the end of the rocket, just enough to grab on to. But I also want to seal the rear center ring and motor tube with epoxy. (did that make sense?) In other words I wanted there to be a layer of epoxy between the Aero-Pac and center ring. Is that good or bad?

Oh, and BTW: I'm up to the 7th layer of primer. (but I've sanded 2 off) The goal, while I wait for the Aero-Pac is to have no, what do you call them, crease lines? Roll lines? Paper lines? Well, you know...

Well, if you put it at a 1/4" you won't be able to use Dr. Rocket cases.. Well you can but the nut will not go down as far as it should. That's why I said 3/8"... That way you can use both.
Yes, I know what you mean about the aft. center ring... I wanted this on my HeatSeeker... what I did was use 2 after center rings... One a 1/4" up from the bottom of the body tube and epoxy in, the next one I cut out the center hole a bit more so the flange part of the Aero-Pac retainer and the nut will fit into it.. (make sense?) Then this get's epoxyed in.. Now I installed the Aero-Pac retainer with JB Weld. And whe the nut is installed only about a 1/4" is exsposed. very clean finnish.
Primer, Primer, Primer... Most would say.. oh my.. not me.. if you were to really look at it.. you sand off most of your primer.. Now if you are using the primer as the only means of filling the "Spiral Groves".. you need to change your method.. Use Elmers Wood Filler, ( Mix this with a "SMALL" amount of water, untill you get a tooth paste consistance.. smear it in place, let dry and sand off. Spiral Groves sometimes take two aplications.) The more preperation you do prior to painting.. mean's.. Less paint.. and a much better finnish.

Hey, I just thougth of another question. These plywood center rings, do they need to be sealed/primed/coated??? Do they need protection from the heat?

no, no and no... lol... the only one that really needs anything is the aft. center ring... and I put a light coating of epoxy on it when I'm putting it in.. then sand it a little prime and paint with the rest of the rocket.

Actually, if your fillets are not only for cosmetic reasons, but also for structural stability, better use milled fiberglass or cotton. They have, like glass mat, a fiber structure that adds a lot of strength to the fillets. Microballoons are just for better handling and less weight (and were the first ones to come to my mind).

Oliver