Assembly Question

[Wrightme43]

I have seen a few rockets where the motor mount, centering rings, and fins are assembled, filleted, then slid into the body tube after extending the fin slots to the end of the body tube.

I haven't seen any closeups or discussions on the process.

I assume a short piece of tube coupler, or body tube with the thickness of the tube cut out and reassembled is used behind the aft centering ring to restore rigidity.

Does anyone have experience with this? It seems like it would make for perfect motor mount to fin internal fillets, at the cost of internal body tube to fin filets, but that could be covered by strong external body tube to fin filets.

I just wondered what people thought about it.

 

[kswing]

I've used this technique on a few rockets and it works well and does allow for very good fin to MMT fillets.

Typically I'll save the bit of tube that I cut out from below the fins and then glue it back to fill the gap. The lower CR helps support the lower body tube, so, the rigidity at the bottom isn't really compromised.

Depending on the placement of the lower centering ring, you may need to use a bit of fiberglass or extra cardboard to help hold the cutouts in place.

Here's a build thread where I used this technique: https://www.rocketryforum.com/threads/loc-precision-bruiser-exp-build.175318/page-2

 

[bjphoenix]

I have seen a few rockets where the motor mount, centering rings, and fins are assembled, filleted, then slid into the body tube after extending the fin slots to the end of the body tube.

I've done this on a number of rockets. I mark the tube for the slots after I've built the fin can unit. After final assembly I still put liberal fillets from fins to outer body tube, and inside the rear of the tube from rear centering ring to inside surface of the outer body tube. I usually have the rear centering ring relatively close to the back of the outer tube so there isn't any tube hanging out there that needs to be strengthened, and the outer tube is fully glued to fins and centering rings so it all becomes a monolithic unit again, I don't think any tube reinforcing is required. I stand the rocket up vertically, drip some epoxy in from the top end of the tube, and spin the tube a little bit to distribute it around the inside perimeter between the front centering ring and the inside of the outer tube. I'll do this about 3 times and each time will distribute the glue around about 1/3 of the circumference. I figure the motor mount is very well connected to the outer tube through fillets so it doesn't require a lot of glue around the front centering ring.

 

[pathtouch]

I just make a slit with a hacksaw in fiberglass or with a razor knife in cardboard. The back of the airframe flexes out enough to slide everything in. Then l tourniquet the airframe to the aft most centering ring and fill the kerf behind the fin with epoxy.

See around 4:00 into this video for a good example.

 

[Rocketclar]

What I've done is to attach the forward and mid CRs onto the MMT and then glue (epoxy) it in place while leaving the aft CR on, but it's not glued to the MMT. It's just used as an alignment guide to keep the MMT in correct position. Further, I had put a couple of screws thru the aft CR so I can pull it off after the forward and mid CRs epoxy cures. I then epoxy the fins thru the walls to the MMT. I tape off the inside aft positions so after I apply fillets to the fins, MMT, and AF I can then epoxy in the aft CR. Picture shows the aft centering ring removed with tape to protect surfaces as I pour epoxy in from the end. There are 16 internal fillets (8 to the MMT and 8 to the inside AF). Once those are cured, I remove the tape and epoxy in the aft CR. That's made for very rugged fin cans.

 

[kramer714]

Works well

The way I do it,

1. Bond the aft centering ring to the motor tube, leave the fwd on loose
2. mark each fin slot, using 5 minute epoxy bond the fin to the motor tube only, make sure to 'snap' off any adhesive that may be on the body tube / fin, bond all the fins this way
3. before taking the fincan out, put a piece of tape on each side of the fins next to the body tube (this way you can make sure you dont get adhesive on the fin when you internally fillet them
4. remove the fincan carefully
5. slide the forward centering ring don to the tops of the fins temporarily tape in place
6. With the fin can horizontal, pour in the internal fillets making sure there is no adhesive near the tape filet the fwd centering ring top and bottom
7. repeat with all the fins.
8. Dry fit to make sure the fincan goes back in, you may need to use 'stir sticks' as shoehorns to get the body tube over the aft centering ring, sand if it doesn't fit.
9. OPTIION A, cut a small piece of coupler tube to fit in the aft of the bodytube, drill (4) small holes halfway between each of the fin slots for a pop rivet (I use 3/32 countersunk rivets), YOU WILL NEED TO CLAMP THIS, the body tube will want to curl up if you dont. NOTE THE PICTURE SHOWS A MACHINED END CLOSEOUT - DIFFERNET METHOD
10. Apply adhesive on the ID of the body tube just past the fin slots, slide the fincan in partway and apply adhesive to the ID on the bottom, slide the fincan all the way in, apply adhesive to the small coupler tube piece and while the adhesive is still wet pull the rivets, this pulls the bodytube round and tight to the coupler,
11. After the adhesive sets, add fillets to the Outer Diameter
    

 

[Trenton Baker]

I have seen a few rockets where the motor mount, centering rings, and fins are assembled, filleted, then slid into the body tube after extending the fin slots to the end of the body tube.

I haven't seen any closeups or discussions on the process.

I assume a short piece of tube coupler, or body tube with the thickness of the tube cut out and reassembled is used behind the aft centering ring to restore rigidity.

Does anyone have experience with this? It seems like it would make for perfect motor mount to fin internal fillets, at the cost of internal body tube to fin filets, but that could be covered by strong external body tube to fin filets.

I just wondered what people thought about it.

I dry fit everything together, take a coupler and slide it up against the aft ring and mark where I need to cut it. I slide the coupler into the primary tub to the line where it needs to be cut and use it as a guide to keep the cut straight with a razor blade. It provides a nice cleaned up appearance and also provides a connection for the lower body tube to adhere to before I apply external fillets.

I also use the same process for bulkheads and baffles, I place one above and below the plywood to add a bit more strength, especially an attachment point like the baffle pictured.

 

[Wrightme43]

I dry fit everything together, take a coupler and slide it up against the aft ring and mark where I need to cut it. I slide the coupler into the primary tub to the line where it needs to be cut and use it as a guide to keep the cut straight with a razor blade. It provides a nice cleaned up appearance and also provides a connection for the lower body tube to adhere to before I apply external fillets.

I also use the same process for bulkheads and baffles, I place one above and below the plywood to add a bit more strength, especially an attachment point like the baffle pictured.

I like that!

 

[jimzcatz]

This method is how I build every rocket I have except Estes. In my opinion its how everyone should build but everyone has their own methods.