fin attachment

I had been toying with the idea of attaching my fins on my Black Brant to allow them to be removed .
Sooo I put my idea to paper . Now toying with solid fins made out of Alumilite casting resin ?

Opinions are welcome

Bobby

I take it you remove the two pins (or screws) from the aft end to take the fins out. Biggest issue I can see is that there may be some movement left and right on each fin allowing them to flutter. I think you will need to have 2 of your CRs made of Aliuminum to prevent the holes from enlarging. Also may want to make sure you have internal fillets to wedge the fin tabs hard into.

One approach that I have seen used:

https://vatsaas.org/rtv/construction/removable/fins.aspx

Terry

That is were I got the idea from

Mat

The screws are all thread threaded rods that goes through all of the tabs .
The bottom of each tab is form fitted to the engine tube .
running a line of epoxy putty on each side of the tabs would help in preventing side to side movement .aluminum centering rings would be another plus.
The airframe is g12 but I would like to go with a carbon fiber airframe

SpaceManMat said:

I take it you remove the two pins (or screws) from the aft end to take the fins out. Biggest issue I can see is that there may be some movement left and right on each fin allowing them to flutter. I think you will need to have 2 of your CRs made of Aliuminum to prevent the holes from enlarging. Also may want to make sure you have internal fillets to wedge the fin tabs hard into.

Click to expand...

bobby_hamill said:

Mat

The screws are all thread threaded rods that goes through all of the tabs .
The bottom of each tab is form fitted to the engine tube .
running a line of epoxy putty on each side of the tabs would help in preventing side to side movement .aluminum centering rings would be another plus.
The airframe is g12 but I would like to go with a carbon fiber airframe

Click to expand...

perhaps wooden fillets like these
https://www.ausrocketry.com/forum/viewtopic.php?f=6&t=1782&hilit=velociraptor&start=14

Mat

That looks like a winner to me !

Or better yet(maybe), make the fin, mmt, and CRs a permanent unit. Leave the forward most CR mounted to the airframe and use the long screws to go through the fin/mmt unit into the forward CR (probably best made from aluminum for better threading). Then you just have to slide everything into place put 4 screws in and your good to fly, i'm sure a method to make the fin and mmt removable is doable.

If the fins are cast, why not make the root end (the base) a 'T' form, rounded of course to match the MMT & BT circumferences. That way, two screws (rods); one each thru the tabs of the 'T' to limit fin rotation (flutter as mentioned). You also get more surface area against the MMT to hold them upright..

Paul

The bottom part of the tab is curved to fit next to the motor mount airframe but the outer airframe of the rocket tab joint is not.

( at least not yet )

Can't see why it wouldn't work. Just make sure you compensate for that heavy aft-end with some nose weight.

bobby_hamill said:

Paul

The bottom part of the tab is curved to fit next to the motor mount airframe but the outer airframe of the rocket tab joint is not.

( at least not yet )

Click to expand...

sorry, I thought it was understood, like this:

(the upper rounded part matches the BT, the bottom, lower rounded part matches / mates with the MMT.. And two screw holes, so you don't get the fin pivoting around one screw..)

dr wogz said:

sorry, I thought it was understood, like this:

(the upper rounded part matches the BT, the bottom, lower rounded part matches / mates with the MMT.. And two screw holes, so you don't get the fin pivoting around one screw..)

Click to expand...

So this is like having the internal fillets integral to the fin, which would be nice and stable, but...

How do you get the tabs through the slots in the airframe?

how about this new idea ?

separate vertical section at red line have "B" section a tad larger than "A" section .

That way "B" section would be fixed to the motor tube

and "A" section would go through the airframe and into "B" section and be held together by 2 sections of all thread ( yellow dots )

?

the attached 2 photos may help in the details

photo 1 shows how the bottom of the fin tabs have a wedge shaped taper to match the fin contour

Photo 2 shows aluminum ring 1 in front that will mate up beside the fixed aluminum ring inside the airframe and aluminum ring 2 that will be on the rear of the engine mount to apply stiffness to the assembly and to bolt the engine retainer to

bobby_hamill said:

I had been toying with the idea of attaching my fins on my Black Brant to allow them to be removed .
Sooo I put my idea to paper . Now toying with solid fins made out of Alumilite casting resin ?

Opinions are welcome

Bobby

Click to expand...

I Like this idea, but I do have two questions, a comment, and a suggestion.

Questions:

• Why so many centering rings? One or two intermediate rings are needed if you use my suggestions below, but why more than that?
• Why leave the gaps in the casting between some of the centering rings?

Comment:

• The permanent bosses sandwiching the root of the fins is probably a good idea, but I may have a better one.

Suggestion:

• At the intermediate centering rings, notch the rings and fins so that they interlock like the dividers in a wine case. This will give angular stability without adding weight or complexity.

bobby_hamill said:

how about this new idea ?

separate vertical section at red line have "B" section a tad larger than "A" section .

That way "B" section would be fixed to the motor tube

and "A" section would go through the airframe and into "B" section and be held together by 2 sections of all thread ( yellow dots )

Click to expand...

Well, that would let the root edge go through the slot, but it seems excessively complex and heavy to me. I like the original idea much better.

I'm on my third attempt of a papercraft Black Brant II. The first one landed in a ditch on it's first flight at NSL 2014 and was destroyed as a result. The second got it's fins banged up. The lastest one printed out the best and was to get fins with balsa end plates, but going with sheet balsa fins this time.

jqavins said:

So this is like having the internal fillets integral to the fin, which would be nice and stable, but...

How do you get the tabs through the slots in the airframe?

Click to expand...

The slot into which the fin is mounted would be notched to clear the wider tabs. The slot wouldn't be a nice straight slot but one with notches cut as needed.. (yes, ugly..)

bobby_hamill said:

how about this new idea ?

separate vertical section at red line have "B" section a tad larger than "A" section .

That way "B" section would be fixed to the motor tube

and "A" section would go through the airframe and into "B" section and be held together by 2 sections of all thread ( yellow dots )

?

Click to expand...

Bobby, that's essentially what you originally proposed. What I'm proposing has the retaining rods farther apart, and the surface held against the MMT is larger. in doing these two things, you gain a mechanical advantage: the retaining screws are farther apart, so there is less stress to rotate around the axis of the fin (less of a rotational moment): a longer lever if you would.. [and the greater the distance between the yellow dots, the less accurate you need to be on your hole placements], and you have more bearing surface against the MMT (and you're capturing more of the 'round' of the MMT and are then also gaining more of the curvature to help hold the fin upright). The rotation around one of the rods at the yellow dots is decreased due to the larger surface area against the MMT...

And yes, your slot is more of a notched slot to allow the tabs to pass thru..

Of course, the more I think about it, this is entirely what I would do, to have swappable fins: (Lets' see how well I can explain it...)
The tabs I've illustrated above, would be larger, that is, they would be longer. They would be a 1/4 circle with the lower diameter the MMT diameter, the larger diameter the BT diameter. The end faces would be 90° to each other, 45° to the fin, like the 1/4 of a pie... And the holes (yellow dots) far apart as originally illustrated. this would make the 4 fins, when installed, a full circle, a solid the volume of the space between the MMt & the BT. Also, then you only need a BT that ends on the first, upper most CR, and a very short one that straddles the inner CRs, and so on.. No slots needed, just a set of rings as it were.. mind you, you would loose the strength the outer BT offers.. But you have the Motor adding a bunch of strength here..

(Drill out the middle of these tabs to reduce weight.. Drill out most of the fin / litter the fin with holes and sheet with 1/32" or 1/16" ply to further reduce weight..)

Something that I don't see discussed in rockets like this is that you've lost the strengthening power of the fillets. When we cut slots for our fins, we create a huge weakness in the airframe. When filleting the airframe to the fins, we overcome that to some extent, or perhaps completely. A couple years ago I went to a lot of trouble designing and building my "glue-less rocket". It had four centering rings in the motor mount section connected with aluminium threaded rod and an Acme fin unit was sandwiched between two of the rings. The only physical connection between the airframe and the rings/rods/fins was a thrust plate at the bottom and a number of screws from the side that went through the bottom of the airframe into the thrust plate.

On its maiden flight, I launched it on about the mildest 38mm motor I could find, an H123. The rocket came off the rail, immediately started sky-writing, slamming into the ground right after ejection. The failure? The (3" Blue Tube) airframe had fractured from the top of one fin slot to part way down the adjacent slot, the motor mount and fins were no longer parallel to the airframe, and away it went. Maybe if I would have also screwed the topmost centering ring to the airframe, the stresses would have been distributed better, but it also would have created another weak spot in the airframe.

I think we often underestimate the benefits of having a seamless airframe on a rocket with all of the centering rings and fins bonded together and TO THE AIRFRAME. It is incredibly strong.

Dugway

I am glad you bought that point up.
I was going to have the fin slots open at the end of the airframe but that would leave a week section of airframe .
Instead I allowed for a small section of airframe to go all the around to strengthen the airframe at the end .

In the attached photo you can see the "steps" #1 on the fin root to allow the airframe to contact the fin root
#2 is where the fin tab actually goes through the airframe .

Bobby