3d printed fin pocket

[kramer714]

Lately I have been playing with smaller rockets. Many of the build techniques for larger ones just dont really scale down. For example internal fillets in TTW construction. The space between a 29mm motor tube and a a 38 mm body tube is just a few mm. On larger rockets I run the fin slots all the way to the back of the rocket and build the fin can 'cartridge style' healthy high quality bond of the fin to the motor tube all hidden between the motor tube and the body tube. Not really possible to do between a 29mm motor tube - and 38mm body tube.

What I came up with is a 3d printed 'fly away tooling' fin slot mold. The fin slot mold is bonded (just to hold it in place) to the motor tube along with the centering rings. The assembley is then bonded in place into the body tube (see pictures) just from the centering rings, the edges of the fin slot are flexible and form a seal once installed into the bodytube . The edge design works like a blade seal, preventing adhesive from running out of the guide. The assembly sequence is;

1. bond the motor tube assembly - all surface prepared for adhesive bonding, dry fit and deburred.
   1. bond the aft centering ring, in my pictures there is a machined retainer but works the same with a traditional aft centering ring
   2. bond the 3d printed fin pocket in place - just boded with few spots of adhesive away from the fin slot
   3. bond a forward centering ring that seals the top of the fin pocket
   4. OPTION, bond a centering ring in place to hold the shock cord - this is what is shown in the picture
2. Bond the motor tube assembly into the body tube, make sure the fin slot feature in the fin pocket lines up with the fin slot
3. Bond the fins
   1. fill the fin slot with adhesive until filled - use a coffee stir stick to help 'force ' it in
   2. wipe the bonding surface of the fin with adhesive
   3. insert the fin into the fin slot in the bodytube - fin pocket
   4. wipe of any excess adhesive that got forced out
   5. use a piece of tape to hold the fin on place until the adhesive sets.

Works Well, pictures of a filled fin pocket, one before filling and renderings to show the concept

Mike K

 

[kramer714]

A few more details, There is a notch in the aft side of the fin pocket that aligns the fin can assembly to the slot in the body tube and helps support the fin during bonding. Plus a cutaway showing what the adhesive looks like once cast in place (with the body tube and forward centering ring removed for clarity.
.

 

[OzHybrid]

If you are printing your motor mount tube it doesn't need to be a parallel tube like a cardboard mmt would be. It can just be a front and rear contact ring. This can save you weight and prevent heat soaking into the printed tube.

 

[Pepe Le Pew]

Now, that is a great idea! Keep up the good work!!!

 

[Tractionengines]

@kramer714 any chance you would share the design file. That looks great.

 

[kramer714]

I will dig out the file and post it

 

[kramer714]

STL

STL for fin pocket 29mm Motor mount, 38 mm body tube

 

[Rschub]

Darn it, this is giving me ideas...

 

[kramer714]

Updated - 4 fin pocket

building up a few FiberGoblin Sprite (54 mm scaled Polecat Goblin), decided to do a fin pocket for this, incorporating the centering rings into the 3d printed fin pocket, plus making a provision for bonding on a shock cord. This is a '2.0' revision of the fin pocket I had done previously. Details;

• 54 mm bodytube - 38 mm motor mount
• 4 fin
• 2 piece fin holder, base and top closeout
• Incorporates centering ring into 3d printed fin pocket
• 'Sleeve' formed in place for shock cord

Bond the lower (orange) and upper ( brown) fin holder to the motor tube (plus whatever retainer you plan on using), bond the shock cord to the fin holder, keeping the adhesive below the sides
Drill 4 holes in the bodytube using the drill jig (optional)
bond the finholder assembly into the body tube, adding adhesive over the shock cord, use the (4 holes) to align it (or do this by eye if you dont have the (4) holes.
Verify fit of the fins
One at a time, pout a little adhesive into the fin slot, rub it around with a stick, then fill the slot with adhesive. With the root of the fin with adhesive and push it into the slot, it is aligned by the fin holder. Wipe off any remaining adhesive. Repeat on remaining fins.

Use one of the holes in the tube / holder for the lower rail button, drill a hole into the upper fin holder (brown) for the upper rail button

Done

 

[kramer714]

STl files plus drill jig

 

[kramer714]

bonded...

 

[cwbullet]

Nice design!

 

[dpower]

This is really cool. I see a 3D printer in my future!