Minimun Diameter fin mounting

[edwardw]

I am building a minimum diameter 54mm rocket. I had this idea for mounting the fins so that it would be a little stronger than surface mount. (I don't have a fin can so I can't do that either ) I was going to slot the airframe tube the width of the fin (3/16). I was then going to put the casing inside of the rocket. I was then going to put the fins in the slots butted up against the motor casing. Add a fillet of epoxy on the outside and that would be done. Does anyone else use this? What do you think of the idea?

Edward

 

[rocketkid88]

sounds like it would work to me, but i'd advise putting a piece of masking tape or something on the inside of each of the fin slots so that epoxy dosn't leak thru and glue your cassing in !
then just give the cassign a little 'slide' every now and then while the epoxy sets so that you can really be sure that it is'nt going to be permanintly set in there.
good luck, man!

 

[Ryan S.]

this might work, or you could only slot part way through the airframe this way you dont need to worry about getting epoxy on your casing, if you go all the way through epoxy will probably get on your casing and lots of sanding will be required which will be hard to do.

 

[edwardw]

The fin fit in the slot is extremely tight, and I put some vaseline on the casing then I was test fitting everything together. I was thinking about partially slotting, but it was just easier to slot the airframe all the way on the router. I will be moving the casing back and forth just to be sure. And Using a lot of Vaseline.

Edward

 

[midpwrguy]

Did you mean somthing like this Edwardw?

 

[edwardw]

That's what I'm envisoning...I've got them cut out and everything dry fitted with tons of vaseline...

How did you keep them from epoxying everying together?


I like the retainer

Edward

 

[midpwrguy]

I've got a post in the midpower threads. And the picture you seen here was done just like that one. just alittle masking tape to act as a stop just to keep the fins from going into the motor tube.

 

[powderburner]

Before you go slathering the vaseline all over, you should do some testing on scrap material to make sure your epoxy system will still bond to parts that have a thin vaseline covering.

That stuff can get in the way of a good glue/epoxy joint if you let it get onto the wrong surfaces. Probably best to skip the vaseline. If you are worried about accidentally epoxying your motor casing into the rear of the rocket, wrap your motor first in wax paper or something.

 

[edwardw]

The fin joint there isn't any place for epoxy to get through, the joints are very tight. I put a thin film on the casing (put some on my hands, got my hands greasy and then wiped down the casing) then I put a little on the inside of the tube where the fin is...just in case any gets through. I got it all epoxied up and it's curing right now. I think it's a little cool in my apt for the 20 min cure time so I'll leave it overnight.


Edward

 

[powderburner]

Vaseline is a petroleum-derived product, and contains some nasty oils and liquids that can wick into fin and airframe materials. Once these oils get in there, they spread out and stay.

These oils and chemicals can interfere with the chemistry of curing epoxy. I don't know if a very thin layer of Vaseline will be enough of a problem to worry about, so maybe everything will turn out just fine. OTOH, if the whole point of your post was how to get the best root attachment in a marginal situation, why go looking for trouble? I think you should use a wax paper wrap over the motor casing next time.

 

[edwardw]

The two fins that cured overnight are on there. But hey, I don't mind building two rockets so I will try the no vaseline - wax paper method today. I hve my smash lab thursday so I'll do some testing. Then I'll build a third rocket


Edward

 

[hokkyokusei]

Originally posted by edwardw
I was going to slot the airframe tube the width of the fin (3/16). I was then going to put the casing inside of the rocket. I was then going to put the fins in the slots butted up against the motor casing. Add a fillet of epoxy on the outside and that would be done. Does anyone else use this? What do you think of the idea?

Edward

Haven't done this with conventional rear end fins, but have done it successfully with canards.

 

[wyldbill]

Originally posted by powderburner
I think you should use a wax paper wrap over the motor casing next time.

I'll second that one. I've used wax paper a lot as a release when laminating fins. Epoxy CAN soak through it under pressure, but it shouldn't be a big deal here. Use a couple of wraps.

-bill

 

[wyldbill]

While we're on the subject, doing a plain surface mount and fillet, and then glassing tip to tip (or a least a few inches up the fin) will produce a very strong fin can. Try to keep your span small to minimize flexing/flutter (better for alt. anyway) and use a medium weight glass.

FWIW,
-bill