the REAR motor mount centering ring: needed?

[techrat]

OK, I get it that the upper centering ring needs to be there to seal the body tube so that the ejection charge goes up and out, popping the nosecone so the chute deploys.
That said, I've started building some rockets now with TTW (through the wall) Fins... and they connect to the motor mount tube, and after epoxying inside and out, gets covered by the rear motor mount centering ring.

Is this ring really necessary? I mean, it likely stops a ton of drag, but it also prevents easy access to the "fin can", and I can never again get in there once its sealed up without destroying the rocket. So, here's the question: Since the Fins themselves are supplying most of the thrust-bearing into the airframe, is that rear centering ring really *that* critical? Can I fly without it, or does that change the base drag somehow so badly that a stable rocket becomes unstable? Or is this a holdover from existing designs being scaled up, and everyone just assumes this piece is necessary because "tradition"?

 

[Steve Shannon]

OK, I get it that the upper centering ring needs to be there to seal the body tube so that the ejection charge goes up and out, popping the nosecone so the chute deploys.
That said, I've started building some rockets now with TTW (through the wall) Fins... and they connect to the motor mount tube, and after epoxying inside and out, gets covered by the rear motor mount centering ring.

Is this ring really necessary? I mean, it likely stops a ton of drag, but it also prevents easy access to the "fin can", and I can never again get in there once its sealed up without destroying the rocket. So, here's the question: Since the Fins themselves are supplying most of the thrust-bearing into the airframe, is that rear centering ring really *that* critical? Can I fly without it, or does that change the base drag somehow so badly that a stable rocket becomes unstable? Or is this a holdover from existing designs being scaled up, and everyone just assumes this piece is necessary because "tradition"?

You probably could get by without it, but your rocket won’t be as strong. The aft centering ring provides support for the body tube and the fin tabs. It also distributes the thrust more evenly to the fin tabs. I seat my motor retainer tight to the aft centering ring so the retainer, the aft centering ring, the fin tabs, and the forward centering ring all form a thrust bearing structure. The motor mount tube carries almost no thrust.

 

[Donnager]

One thing it does, is provide a center locator for the motor immediately next to the business end of the motor. While this may not be a big deal, I could envision some off-axis thrust/performance if the rear of the rocket deforms between fins, because it isn't being held round along its entire circumference, and you are only relying on fin fillets to hold the MMT centered.

I suspect we (rocket hobby people) overbuild in many places that are unnecessary. I tend to make my fin cans to where they are very solid, and I'm sure they are overbuilt.
For 75 and 98mm stuff, I would be nervous about no rear centering ring or thrust plate, (especially if the motor retainer screws to it). Not sure I'd want a motor with several hundred pounds of thrust just pushing on a MMT that had any significant unsupported length.

For cardboard airframes, that back ring also reduces rear damage on parachute recovery.

I probably won't test this.

 

[Hobie1dog]

I consider it and integral part of the rear of the rocket and I cannot imagine not using it.

 

[afadeev]

OK, I get it that the upper centering ring needs to be there to seal the body tube so that the ejection charge goes up and out, popping the nosecone so the chute deploys.
That said, I've started building some rockets now with TTW (through the wall) Fins... and they connect to the motor mount tube, and after epoxying inside and out, gets covered by the rear motor mount centering ring.

Yes, that's typical.

Is this ring really necessary? I mean, it likely stops a ton of drag, but it also prevents easy access to the "fin can", and I can never again get in there once its sealed up without destroying the rocket.

Necessary?
Strictly speaking not, when you have properly glued TTW aft fins.
But very VERY handy. Because without it:

• Aligning the motor mount tube (MMT) would very VERY tricky, if not impossible. Misaligned MMT is a recipe for highly non-vertical flight paths.
• High thrust motors would end up transmitting the thrust only through the front MMT centering ring (CR) and the fin tabs. Both of those are more likely to fail.
• The aft end of the rocket gets flimsier and more likely to crumple. Rockets usually impact one of the aft fins upon landing and especially during transportation (when most damage is incurred). Aft centering ring reinforces the "business" end of the airframe and the MMT.
• Open end "fin can" will become a magnet for dirt and moisture, further weakening the airframe and the MMT

On larger rockets where access to "fin can" is desired for internal fin fillets, folks temporarily install the aft centering ring, glue fins and MMT in place, then pull it out to work on the inside of the "fin can". Then seal it back up for good.

So, here's the question: Since the Fins themselves are supplying most of the thrust-bearing into the airframe, is that rear centering ring really *that* critical?

Traditionally, thrust transfer is primarily handled by the CR, and especially the AFT centering ring when the motor retainer abuts the CR.
Applying perfect glue joints to the fin attachment points is harder, and more error prone. Especially on smaller diameter rockets. Not impossible, just much trickier to do reliably.
Applying a healthy bead of glue to the CR attachment points to MMT and airframe is much easier and more idiot proof.

Can I fly without it, or does that change the base drag somehow so badly that a stable rocket becomes unstable?

If you glued fins tabs well, you could fly without it.
I am not certain how much of an impact its absence would provide on drag, but you could model that, if curious.
You will also be much more likely to crunch the aft end of your MMT and airframe before and after the flight.

Or is this a holdover from existing designs being scaled up, and everyone just assumes this piece is necessary because "tradition"?

Many rockets do not have TTW fins, in which case CRs are irreplaceable.
With TTW fins, aft CR could be omitted, but doing so will offer no gains.
Basically, omitting CR is a solution to a problem that doesn't exist.

HTH,
a

 

[Mushtang]

Can I fly without it, or does that change the base drag somehow so badly that a stable rocket becomes unstable?

This would be a fun thing to spend a day finding out.

Build a rocket like you described but have the rear centering ring removable after the TTW fins dried (to make sure the motor is indeed centered in the back). Remove that rear centering ring and launch it about 10 times, then put it on with super glue and launch it 10 more times, all with the exact same motor.

If you can roughly measure height (or have it fly with an altimeter to be more accurate) then with all 20 launches you should have your answer. Come back and share it here.

 

[3stoogesrocketry]

You will get a lot of burning to the base of the model due to the Krushnic effect . The air in the fin can will be a dead spot and suck the heat up into the fin can.

 

[Donnager]

You will get a lot of burning to the base of the model due to the Krushnic effect . The air in the fin can will be a dead spot and suck the heat up into the fin can.

I'd be concerned about this as well as what I already mentioned.

 

[bjphoenix]

On larger rockets where access to "fin can" is desired for internal fin fillets, folks temporarily install the aft centering ring, glue fins and MMT in place, then pull it out to work on the inside of the "fin can". Then seal it back up for good.

I do this myself, on some mid power rockets with LOC category 2.6" tubing.
TTW fin tabs means multiple things- when I do it I extend as much of the fin root as I can through the tube. I've seen kits with TTW fins that only have a small part of the fin root extending through as a tab. If you have big tabs, then I don't think the rear centering ring is doing much. If you have small tabs then the centering ring will stiffen the rear of the outside tube. It stiffens the rear of the tube in either case, but there isn't as much force on the tube in between fins if the tube isn't trying to support the fins.

 

[afadeev]

I do this myself, on some mid power rockets with LOC category 2.6" tubing.

Same here. Occasionally.

If you have big tabs, then I don't think the rear centering ring is doing much. If you have small tabs then the centering ring will stiffen the rear of the outside tube. It stiffens the rear of the tube in either case, but there isn't as much force on the tube in between fins if the tube isn't trying to support the fins.

The following would be less of a concern with CF (carbon fiber) or FG (fiber glass) tubes.
With paper MMT tubes and reusable motors, I would worry about squashing the aft end of the MMT tube.

Without weatherproofing from aft CR, the tube will get soggy with environmental exposures in the field (unless you fly in a desert).
Without reinforcement from the aft CR, the motor thrust will be transmitted first to the motor retainer, then to the aft edge of the MMT paper tube. Slightly soggy paper tube will be challenged to maintain its integrity under those conditions. If the motor retainer abuts the aft CR, then the thrust is immediately transferred to the airframe.

Basically, no aft CR == no upside, plenty of downsides?

a

 

[bjphoenix]

Basically, no aft CR == no upside, plenty of downsides?

Thinking about it the other way around, there aren't many downsides to putting in the aft CR. The only problem I could see is if you have to replace a TTW fin that broke off in which case you could cut out part of the rear CR. Apogee has a recent video of repairing a fin from the outside, while watching that video I was thinking that I would have just cut out the rear CR and worked from there.

 

[Back_at_it]

I have asked myself the same question a few times. If you’re running TTW Fins then the rear ring doesn’t really do anything other than maybe help with aero. Honestly, if you’re running TTW fins then the forward rings only purpose is to seal the body tube. If I’m not running a baffle then there’s no reason the forward ring can’t be something like thin fiber. If I am running a baffle I’ll use plywood for the upper as there is a fair amount of pressure on top of that ring.

I have not left a rear ring completely out yet but I have made the rear ring out of the lightest possible materials I can find including poster board for rockets with thru the wall fins.

When doing this I’ll use a real centering ring on the rear to keep the motor tube aligned while gluing the forward ring in place as well as the first fin. After that I remove and replace it with something light weight. There are numerous builds where people with better equipment than I have made rear rings that were lightened with holes.