Building A Motor Mount Outside The Body Tube: I Have A Question

[o1d_dude]

Okay, I'm clear on the principle of building a motor mount with fins on the bench outside the body tube. I even understand the reason for doing so and how to install the completed structure inside the body tube.

A rocket kit I'm working on has three centering rings. I see no problem in applying the epoxy/glue inside the body tube for the uppermost centering ring and of course the gluing the rearmost centering ring in place is a simple matter as well.

Question: How do I get the epoxy/glue in place for the middle centering ring? Should I wait until the motor mount is installed and then add the fins (with no internal fillets on the motor tube) as I have done before? Sometimes I drilled holes in the body tube and injected epoxy fillets...my Mad Cow Frenzy comes to mind.

The kit is an 3" upscale Cherokee-H by T. VanderBurn and the scaled up Cherokee fins are rather large as you can imagine.

Seems weird building a rocket kit from a kid/guy I remember from when he was about 8 years old...I used to fly AMA Free Flight airplanes with his dad.

 

[David Schwantz]

Doing one myself at the moment. Outside of the CRs, no problem as you stated. But on the bottom of the top CR and the top of the middle CR I drill holes and drip glue down them onto the top CR and then rotate it is the glues runs around the joint. Kind of the same for for the top of the middle CR, just lay it on its side and squeeze glue past the holes onto the top of the CR and again hold at an angle and just let it run around the joint. Can look through holes with flash light to watch it run.

 

[o1d_dude]

Doing one myself at the moment. Outside of the CRs, no problem as you stated. But on the bottom of the top CR and the top of the middle CR I drill holes and drip glue down them onto the top CR and then rotate it is the glues runs around the joint. Kind of the same for for the top of the middle CR, just lay it on its side and squeeze glue past the holes onto the top of the CR and again hold at an angle and just let it run around the joint. Can look through holes with flash light to watch it run.

Sometimes the solution to a problem is so amazingly simple that it can not easily be seen.

Thank you, sir.

 

[David Schwantz]

That'll be 12 bucks

 

[David Schwantz]

Or maybe 2 rolls of TP!

 

[o1d_dude]

I've pondered your solution and have come up with perhaps a slight variation or maybe just one of those "everybody knows that" things...

The body tube is quite long which would make injecting glue into the "top compartment" of the motor mount from the front of the body tube problematic. What to do?

Well, most of us don't button up the motor mount by glueing the rear centering ring into place until we've applied internal fillets so...

I will drill the injection holes into the middle centering ring, insert the motor mount into the body tube and glue it in by the top centering ring, The middle and rear centering rings will keep the assembly in proper alignment.
When that has dried, I will inject glue into the "top compartment" to create internal fillets to the body tube.
When that has dried, I'll do internal fillets on the bottom/rear side of the middle centering ring to the body tube.
When that has dried, I will glue the rear centering ring into place.
Finally, I will JB Weld the motor retainer to the motor tube.

Definitely a multi-step process.

If I've missed anything, let me know.

Thanks again.

 

[David Schwantz]

Sounds like a lot of steps. I just use a long PVC pipe through one of the lightning holes in the middle CR and let the glue run down that onto the CR.

 

[Nilo]

Doing one myself at the moment. Outside of the CRs, no problem as you stated. But on the bottom of the top CR and the top of the middle CR I drill holes and drip glue down them onto the top CR and then rotate it is the glues runs around the joint. Kind of the same for for the top of the middle CR, just lay it on its side and squeeze glue past the holes onto the top of the CR and again hold at an angle and just let it run around the joint. Can look through holes with flash light to watch it run.

David:

Read your post and the only negative I see is that you reduce the strength of the Centering Rings (CR) by drilling those big holes.

I pondered how to do it by just having 4, 1/4" holes on the centering CR. Please see the attached diagrams and give me your opinion.

 

[ether]

what is the motor mount tube size? if the space between the airframe and the MMT is 1/2" or less, you can do epoxy dams that can also cover the mid centering ring. The fin slot is wide enough to use a syringe to inject the epoxy to fill the dams that will glue the fins and the mid centering ring.

However, I would venture to say that you really don't need to epoxy the mid centering ring, the top and bottom rings and the fins through the airframe should provide more than enough of an area to transfer the motor thrust to the airframe. The mid centering ring is helpful for keep the slotted airframe shape.

 

[ether]

other technique is when you insert the MMT when the mid centering ring is about to be inserted, place epoxy on the motor mount tube near the mid centering ring and quickly insert the MMT all the way in, then spin the airframe to force the epoxy to the walls of the airframe and the mid centering ring.

 

[Nilo]

what is the motor mount tube size? if the space between the airframe and the MMT is 1/2" or less, you can do epoxy dams that can also cover the mid centering ring. The fin slot is wide enough to use a syringe to inject the epoxy to fill the dams that will glue the fins and the mid centering ring.

However, I would venture to say that you really don't need to epoxy the mid centering ring, the top and bottom rings and the fins through the airframe should provide more than enough of an area to transfer the motor thrust to the airframe. The mid centering ring is helpful for keep the slotted airframe shape.

This is for 54mm motor mount tube. With this technique I epoxy the front and back of the forward and middle CR's to the body tube.

 

[David Schwantz]

Nilo, I am not sure what you are asking. Do you want to know how to glue the CR in place? Or are you asking how to make them lighter without the holes?
the one I was talking about is 16" DIA. So the lightning holes are a great work area. You can reach the whole rocket interior through them. You could use an acid brush on a stick to get fown inside yours. Glue can be wood glue, epoxy, resin and glass cloth. All will work. I am in the truck right now and gone from the shop. When I get home, I can send some pics if you'd like.

 

[David Schwantz]

nilo, what rocket? You can build MMT and then install into tube by cutting slits in the cardboard and then slide in place. when you do your fillet, that will cover the slits.

 

[Nilo]

nilo, what rocket? You can build MMT and then install into tube by cutting slits in the cardboard and then slide in place. when you do your fillet, that will cover the slits.

David:

This is my first Level 1/2 rocket, a LOC EZI-65. Since I want a strong assembly to withstand a Level 1 and 2 flight, I would like to ensure the CR are glued to the airframe ,as well as the engine motor mount. Your solution solves the problem, but I don't want to drill big holes on the CR.

 

[David Schwantz]

Nilo, the holes that I drilled in this last one were 4.5". I would not do that to yours..
The LOC it I would build the MMT and CRs all outside of the tube. Cut the slits, and spread them open to install built MMT. I leaave the rear CR with no glue and use it to center everything. After the first glue is dry and everything is secure, pull out the rear CR and then you can glue the back of the middle CR and then glue the rear CR in place.

 

[David Schwantz]

Great rocket and good luck.

 

[Nilo]

I've pondered your solution and have come up with perhaps a slight variation or maybe just one of those "everybody knows that" things...

The body tube is quite long which would make injecting glue into the "top compartment" of the motor mount from the front of the body tube problematic. What to do?

Well, most of us don't button up the motor mount by glueing the rear centering ring into place until we've applied internal fillets so...

I will drill the injection holes into the middle centering ring, insert the motor mount into the body tube and glue it in by the top centering ring, The middle and rear centering rings will keep the assembly in proper alignment.
When that has dried, I will inject glue into the "top compartment" to create internal fillets to the body tube.
When that has dried, I'll do internal fillets on the bottom/rear side of the middle centering ring to the body tube.
When that has dried, I will glue the rear centering ring into place.
Finally, I will JB Weld the motor retainer to the motor tube.

Definitely a multi-step process.

If I've missed anything, let me know.

Thanks again.

Please see post #8- I think this is a variation of your solution.

 

[Nilo]

Great rocket and good luck.

David:

Once I get the camera , altimeter, complete the building of the rocket, complete the launch pad from a Rockwell Jawstand, and build my controller, I'll launch it with some G's , and then I'll have to wait for a sponsored launch, so I could get certified for my Level 1. I'll keep you posted of my progress.

 

[Gramparocket]

Put enough epoxy in the bt at the top cr so it's will create a filet when you push it and turn.
Slide the mmt fin assy in but leave the center Middle ring out, use a stick of other device to apply a bead of epoxy at the location for the middle ring slide that in just short of the rear ring repeat epoxy application for bottom ring shove it home.
Lastly do your bottom filet, and done.
No drilling no extra steps and you get your middle ring glued.
On a side note I'm not sure that ring even need glue. The fin fillets and fore and aft rings will hold it just fine. I have many HPR with only 2 cr's

 

[Gramparocket]

Oops my mistake you can't turn it.
Go back in the top with a popsicle stick taped to a dowel and smooth the top filet out key is getting enough epoxy in for that filet to form

 

[jimzcatz]

you dont need it on the center ring. enough will squeege by to attach it. Ya'll are definitely overthinking this. It doesn't have to be so complicated.

 

[David Schwantz]

Please do, and we all love LOTS of pics.