centering ring details

I overbuild to the point where none of this matters for low power, but as I start in on scratch mid and high power builds I had more questions about centering rings.

Most kits I have built come with centering rings rather tight and require sanding to even get them into place. If I am going to epoxy multiple layers and fillet, does the tight fit matter in any way? With scratch builds I am more likely to be using a centering ring from a different vendor or made of a different material that will not be that perfect, super tight fit but I would think strong glue would more than make up for it.

When using through wall fins, which is most of the time, I usually put the fore centering ring right at the leading edge of the fin tab to turn the whole area into a fin box. As my motors get longer and longer should I be moving it more toward the top of the motor mount, or even adding a third centering ring?

In general, gap-filling with epoxy is not recommended -- tight fits are best.

Internal CR's don't take any loading - don't need multiple layers and huge filets.

One CR tight to the fins, fore and aft, is what I do - and is all you really need.

CR's further forward do little other than provide a more-forward attach point for your recovery and help eliminate some volume that your recovery charge needs to pressurize.

That sounds good but how do internal rings not take any loading?

I think it depends upon the MMT material. I have seen "super tight" centering rings on cardboard or phenolic MMTs that constrict the MMT so that it can be difficult to insert and/or remove the motors. Tight to the point of having to exert a lot of force to get them on is not necessary. Snug fit is good. I question that if the centering rings are so tight that you have to force them on the MMT - is there even adhesive in there?

In the case of cardboard or phenolic MMTs, I suggest dry fitting the centering rings then slide a motor case in the MMT. It should go in and out easily.

This constriction is not uncommon on modrocs. A really tight fit plus shrinkage of water-based adhesive (white glue or carpenters/wood glue) can cause enough constriction that it becomes difficult or impossible to insert and remove your motors.

I think it is less likely to get centering rings too tight on fiberglass MMTs, but it may be possible. Again, dry fit and push a motor in the MMT. You do have to be careful with slotted minimum diameter builds because the spans between fins can be out of round and make motor insertion VERY difficult. In that case, I use a release material like waxed paper wrapped around a motor case to support the BT and fins. Be very careful to think this through so you don't glue your motor inside the rocket. But this is unrelated to your original question.

As far as number of centering rings, I would always sandwich the fins with your centering rings. If there is a couple inches or more of unsupported MMT above the top centering ring when you sandwich the fins with a pair of centering rings, adding a 3rd one might be a good idea. If nothing else, it eliminates the gap between the MMT and the BT in which recovery internals can get hung up and will effectively reduce the amount of your ejection charges.

One final thought - a Dremel type tool is indispensable for cleaning up centering rings. You can clean them up a bit in less than 30 seconds vs. several minutes of sanding and test fitting, sanding and test fitting, ad nauseum.

HTH, --Lance.

watermelonman said:

That sounds good but how do internal rings not take any loading?

Click to expand...

FredA is suggesting that the loads are transferred from the MMT to the airframe via the fins. The CRs just hold everything in place. I agree, although some loading via the CRs will take place if the whole thing is constructed in an integrated "fin box" as you suggest.

Yep -- CR's keep the motor aligned axially, but take ZERO load unless you are skywriting ;-)

Just the trustplate (aft CR) takes the load and transfers it to the fins and BT....which is where the drag occurs and (in general) are the bulk of the mass.

Oh! I read internal as in relation to diameter, while this clearly meant internal in relation to length. Sorry about that.

So while I am not trying to make big ugly gaps and then fill them, sometimes it happens, especially when I get careless with a dremmel in hand. If the epoxy is supposed to be as strong as the ring, or even stronger, what is wrong with having gaps? I suppose it could merely represent shoddy craftsmanship rather than a weakness in construction.

watermelonman said:

So while I am not trying to make big ugly gaps and then fill them, sometimes it happens, especially when I get careless with a dremmel in hand. If the epoxy is supposed to be as strong as the ring, or even stronger, what is wrong with having gaps? I suppose it could merely represent shoddy craftsmanship rather than a weakness in construction.

Click to expand...

The best example to answer your question here, is that yes it affects the strength of the entire structure. You only want as few molecule chains as possible, but enough that it is consistant.
Composite plate is comprised of layers of fabric and resin... you wouldn't want big voids of resin, or large areas of un-soaked fabric because at that point the strength of the composite plate is unpredictable, same with a rockets construction. So they infuse the resin with a vacuum and compress it with with heat and pressure.

So it is a weakness, not just unappealing.

The best answer to a non-perfect world, tight centering rings and un-round holes, and not perfectly square edges is adheasive modifiers. While epoxy and woodglue both have really high strengths, they are brittle. So, the strength is really irrelevent when it shatters under impact. adding sawdust to woodglue, or milled fiberglass, kevlar or carbon fiber, even aluminum oxide to epoxy can help with making these "gaps" non-issues.

Heres some epoxy I like to make, its light but modulous like concrete.
https://www.rocketryforum.com/showthread.php?58709-quot-BOOF-quot-Post-L3-Build-Thread-Mad-Dog-DD&p=603717#post603717

ClayD said:

The best example to answer your question here, is that yes it affects the strength of the entire structure. You only want as few molecule chains as possible, but enough that it is consistant.
Composite plate is comprised of layers of fabric and resin... you wouldn't want big voids of resin, or large areas of un-soaked fabric because at that point the strength of the composite plate is unpredictable, same with a rockets construction. So they infuse the resin with a vacuum and compress it with with heat and pressure.

So it is a weakness, not just unappealing.

The best answer to a non-perfect world, tight centering rings and un-round holes, and not perfectly square edges is adheasive modifiers. While epoxy and woodglue both have really high strengths, they are brittle. So, the strength is really irrelevent when it shatters under impact. adding sawdust to woodglue, or milled fiberglass, kevlar or carbon fiber, even aluminum oxide to epoxy can help with making these "gaps" non-issues.

Heres some epoxy I like to make, its light but modulous like concrete.
https://www.rocketryforum.com/showthread.php?58709-quot-BOOF-quot-Post-L3-Build-Thread-Mad-Dog-DD&p=603717#post603717

Click to expand...

Makes so much more sense now; thanks for the great explanation.