Centering Ring Reinforcement Concept

[eugenefl]

I'm not sure if this is new or not, but I thought I'd share a "breakthrough" I had while building my MEGA-Gyroc MMT.

Purpose: Increase the "blockage" area ahead of the centering ring to better distribute the forces of liftoff thrust on the motor mount. To prevent the MMT from breaking loose inside the airframe and flying through the airframe.

Please see attached drawing. I'm curious if this has been done before or has ever been given any thought. Typically, "fin cans" or instances where the fins are incorporated into the MMT take the place of this design. In essence though, I kinda see it as extra insurance.

Comments welcomed!

 

[Bill]

Originally posted by eugenefl
Purpose: Increase the "blockage" area ahead of the centering ring to better distribute the forces of liftoff thrust on the motor mount. To prevent the MMT from breaking loose inside the airframe and flying through the airframe.

One word: gussets. Cut three or four pieces of scrap balsa wood as wide as the distance from the outside of the motor-mount tube to the inside of the outer body tube and as long as the distance between your centering rings. Glue them onto the MMT as you would fins. Apply fillets where the gussets meet the MMT and centering rings. Adds lots of strength with little added weight unless you go crazy with the glue. Balsa sheeting is also cheaper and more available than a coupler.

If yours is a long motor-mount/stuffer tube, then cut triangular pieces instead of rectangles to form braces between the MMT and centering ring.


Bill

 

[DynaSoar]

Originally posted by eugenefl
I'm not sure if this is new or not, but I thought I'd share a "breakthrough" I had while building my MEGA-Gyroc MMT.

Purpose: Increase the "blockage" area ahead of the centering ring to better distribute the forces of liftoff thrust on the motor mount. To prevent the MMT from breaking loose inside the airframe and flying through the airframe.

Please see attached drawing. I'm curious if this has been done before or has ever been given any thought. Typically, "fin cans" or instances where the fins are incorporated into the MMT take the place of this design. In essence though, I kinda see it as extra insurance.

Comments welcomed!

Some older Estes kits had motor mounts which used a piece of coupler tube the whole length between the centering rings. That much contact would distrubute far more power than engines had in those days.

To make that design, or yours, even stroonger, as Bill says, "gussets": pieces of balsa cut at an angle and glued between the centering ring and the tubes. All my 18mm to 24mm conversions get them. It may be overbuilding, but better that than the opposite.

 

[Stewart32]

can't get much more surface area than that?

 

[Stewart32]

can't get much more surface area than that?

 

[Micromeister]

Good Ideas Eugene, but not at all new. The en-ga-neer has to agree with Bill: quick and easy reinforcements are to add gussets to your centering rings-MMt joint. That is by the way why TTW fins seem to work better then surface mounted fins. the second attachment point creates a gusset plate between the MMT and airframe.

 

[eugenefl]

Originally posted by Micromister
Good Ideas Eugene, but not at all new. The en-ga-neer has to agree with Bill: quick and easy reinforcements are to add gussets to your centering rings-MMt joint. That is by the way why TTW fins seem to work better then surface mounted fins. the second attachment point creates a gusset plate between the MMT and airframe.

Haha...I went to EMRR to look up "gussett".

Gusset - A roughly triangular shaped piece of material added to reinforce a joint, often in a corner.

This whole time I was picturing something to the effect of spokes on a CR or something like a wagon wheel.

 

[Micromeister]

Traditionally a gusset is a series of triangular pieces welded or mounted perpendicular to a base plate and its 90 degree projecting member. Guessets do not have to be triangles, any profile can be used. an example would be a piece of 2" x 1/8" rolled scroll steel connecting a horizontal projecting arm member to a vertical support post acting as a reinforcing gusset for a cantilevered sign.
Yes your mental picture of a wagon wheel is accurate for this MMT/centering ring application. The "gussets" are the spokes, and would be connected to the MMT, centering ring (base plate) and the outer ring or Airframe. Minimum in this example would be 4 pieces, but any number could be used.

Honeycomb construction is another form of greatly increasing the strength of a spanning member (centering ring) without dramatically increasing the overall weight or thickness of the original part. ie two cardstock disc (centering rings) with the same cardstock thickness used for the gusset walls 3/16" to 1/4" high (combined thickness of the completed honeycomb part) will be stronger than say a 1/8" plywood ring alone and maybe 1/3 the weight.

See Yes-at-day I couldnt speel En-GA-neer...now I are one

 

[limd21]

For those of us who cut our own centering rings, the nice thing about using gussets is that the rings themselves can be built out of relatively light and, most importantly, easy to cut material (e.g. card stock). It's just plain easier to manually cut accurate rings with light material than it is with heavy, thick material. (You guys using fly cutters are cheating ;-)

The gussets take the structural load, the centering rings do the, well, centering.

 

[Micromeister]

Wow! Sorry WAY off topic but I almost moved to boulder about 28 years ago...was going to open a sign shop there but got cold feet and stayed in this #4#$#$ forsaken part of the country
Is boulder still as clean and clear skied as I remember....memories....

 

[DJ Delorie]

In the diagram, you have both thrust rings on the same side of the centering ring. What I would have done (and almost did on my last model, but will on the next) is insert the coupler into the airframe first, put the MMT braces behind the centering ring, and then glue it all together. That way, the engine thrust is always pushing parts together rather than pulling them apart and relying only on glue strength.

My last model had balsa spars (i.e. rectangular sticks) glued ahead of the front CR just to give more glue area between the MMT and the body tube, and to add shear strength. The next one will have the spars between the two CRs, with a coupler ahead of it.

The other thing I do is put extra glue inside the BT. After inserting the MMT, I stand the rocket up and let the glue puddle in front of the CR. That adds a little extra strength and burn-proofs the CR.

 

[Micromeister]

Just so we keep this stuff in perspective. All these wonderful strength reinforcements we've been discussing here help make the motor mount centering rings stiffer, providing less flex in these bulkheads that would otherwise be transfered to the area actually keeping the motor mount in place. What is that you ask? those butt mating, slide-in glue joints at the motor tube and the airframe tube. That's to say all that holds our models together is the glues and a bit of friction. I think we sometimes really get beyond the most important issure, fitting of parts and overall weight to thrust ratios.
It's more the shock absorbing elasticity of the adhesives we use then all the strength we "think" we're building into our models that keeps them flying. All these super reinforced motor mounts will fly straight through the model if those tiny little glue joints fail. Try to keep in mindl elasticity and shear strength! Or come up with a mechanical fastening system for cardboard to wood.
Just to keep us humble.

 

[Austin]

Micro makes a good point and it is valid for small to medium rockets...especially on those relying on the bodytube to take the brunt of stress that is applied during flight. Some may be necessary to act as a buffer.

However, when it comes to the big guns, the best structural designs usually include an internal airframe that takes the total lifting stress of the model, while the body tube is only along for the ride. Most of the power is distributed through the airframe and the adhesives are selected and used for the strength they will add rather than the "give" they can provide. I don't see West Systems epoxy giving much under stress, but it's holding and streghtening qualities are right where they need to be...

Moreover, most of the thrust on large airframes is applied at the rear, and sometimes at the top if secured there. this thrust is distributed through the airframe rather than the body tube. The lip you use for a bottom CR/thrustplate will play much into distributing power at launch. I personally use aluminum bottom plates, which prevent the "fly through" effect and distribute the power evenly across the bottom with little flex ; it has worked well.

As mentioned, joint types, materials and adhesives play a major part in modeling and use much of the bodytube for power distribution, while larger airframes have the major components attached to the frame itself, while the bodytube goes along for the ride.

Carl

 

[Elapid]

Butt Joints

a butt joint will take huge amounts of force and does not depend on adhesives for strength, PLUS adhesives are strongest with tensile loads rather than shear forces (rem the super glue commercials with the guy hanging from a steel beam? twist with the same force and the glue would fail).

for the forward centering ring, make two pieces, one that slides over the end of the tube, the other butts against the end of the tube, having the same I.D. as it does. these rings are then glued together. for the rear CR, for ultimate strength, butt it against the body tube AND the MMT tube, and like the forward CR, do it with two rings, one that fits around the MMT tubing and inside the body tube, the other would butt against both the MMT and BT.

epoxies get soft with heat. faster-curing epoxies are worse than slow-cure. I prefer JB Weld atm for its hig heat tolerance and strength.

hope this helps

 

[powderburner]

I put in another vote for gussets . . . sort of.

I also agree with DJ Delorie's comments about modifying the stack of the components, and MicroMister's observation that a tight fit is more important than a gob of glue.

Back to CR's and reinforcing webs: The deal is, structural loads follow the stiffest path. If you have a CR that is infinitely strong and does not deflect under load (ridiculous example, I know, but I'm trying to make a point) then that CR will transmit ALL THE LOAD between the motormount and the BT, regardless of what else, how many, what orientation, what attachment, etc, is used for reinforcement.

Now your mother-of-all-centering rings mega-plywood creation is going to be pretty stiff, even when compared to typical peak-thrust levels of mid-power motors. Addition of plywood gussets would help distribute the load and would be structurally effective, but would also be quite heavy. Addition of cardboard (or other soft material like balsa) gussets will probably not add much in the way of structural reinforcement.

If you were to use a thick cardboard CR (a relatively softer material that would deflect under thrust loads) then the gussets would just about 'take over' as the primary load path. Mind you, the solid CR would still be needed to help react the internal structural loads, and to help hold things together. Using thick cardboard for all components, you could build significantly lighter structure this way---this is only a half-step away from the 'honeycomb' structure that someone else mentioned.