Why am I making centering rings?

The Rocketry Forum

Help Support The Rocketry Forum:

Duncan.Byers

Well-Known Member
Joined
Mar 18, 2022
Messages
74
Reaction score
29
After cutting gobs of custom centering rings for re-sized tubes, I stopped and went 🤔. Why circles and not straight lines to do this? (It's sloppy work, I know....)
20220521_171721.jpg

Any one of the thousands of you who are far smarter than me have a good reason to keep cutting rings??? This is far far easier.....
 

mh9162013

Well-Known Member
Joined
Apr 25, 2022
Messages
185
Reaction score
132
Location
Western, KY
Might have to do with the fact that it could be difficult to get all 4 peices of wood (or cardboard) to be cut to the right width so they touch the MMT and the inside of the main body tube, with little to no gaps. Then there's still the risk of the MMT being off-center. This risk exists when using 2 centering rings, but those are easier to use/install without worry about wobble or gaps between the MMT, centering rings and inside of the main body tube.

Centering rings might also have less weight in many situations.
 

Duncan.Byers

Well-Known Member
Joined
Mar 18, 2022
Messages
74
Reaction score
29
Might have to do with the fact that it could be difficult to get all 4 peices of wood (or cardboard) to be cut to the right width so they touch the MMT and the inside of the main body tube, with little to no gaps. Then there's still the risk of the MMT being off-center. This risk exists when using 2 centering rings, but those are easier to use/install without worry about wobble or gaps between the MMT, centering rings and inside of the main body tube.

Centering rings might also have less weight in many situations.
You might be right and I thought of that but if you're having to cut custom rings you've got the measurements anyway. Distance from the MM to the inside of the tube. If they're all the same width shouldn't be an issue I wouldn't think. And they don't have to be long at all so weight might not be an issue. But I'm not sure which is why I asked the question.
 

Paul Howard

Well-Known Member
Joined
Jan 8, 2022
Messages
86
Reaction score
51
Location
Corvallis, Oregon
Hmmm.,,,well there is still the need to stop the ejection charge pressure/gases from escaping out of that arrangement instead of pushing your parachute/streamer and nose/paylaod-section off - unless this is a booster and you're making good use of gap-staging.

That said, I have done EXACTLY what you are doing in conjunction with centering rings-Especially at the forward end of the motor mount and and either put vent-holes in the rear centering ring for hot engine pressure to escape but not the actual ejection charge, or, omitted the rear centering ring altogether. I did both so that I have a more solid base immediately under the fins if I glue the fins to the outside of the tube for more strength, but now I tend to use "through the body" fin mount to the motor tube with centering rings.
 

Duncan.Byers

Well-Known Member
Joined
Mar 18, 2022
Messages
74
Reaction score
29
Hmmm.,,,well there is still the need to stop the ejection charge pressure/gases from escaping out of that arrangement instead of pushing your parachute/streamer and nose/paylaod-section off - unless this is a booster and you're making good use of gap-staging.

That said, I have done EXACTLY what you are doing in conjunction with centering rings-Especially at the forward end of the motor mount and and either put vent-holes in the rear centering ring for hot engine pressure to escape but not the actual ejection charge, or, omitted the rear centering ring altogether. I did both so that I have a more solid base immediately under the fins if I glue the fins to the outside of the tube for more strength, but now I tend to use "through the body" fin mount to the motor tube with centering rings.
I'm not sure how it would escape....oh. 🤦‍♂️ Never mind. So I cut just one ring.... Thanks! That, sir, is the information that I was looking for. I would sure as hell have smashed a rocket on the ground.
 

smstachwick

LPR/MPR sport flier with an eye to HPR and scale
Joined
Jul 29, 2021
Messages
1,242
Reaction score
1,080
Location
Poway, CA
I'm not sure how it would escape....oh. 🤦‍♂️ Never mind. So I cut just one ring.... Thanks! That, sir, is the information that I was looking for. I would sure as hell have smashed a rocket on the ground.
It’d be a good way to make a gap-staged booster though
 

roytyson

Well-Known Member
Joined
Oct 31, 2017
Messages
367
Reaction score
224
More importantly, why do we put in motor tubes? Why not just glue in rings with ID the same as the motor?
 

Paul Howard

Well-Known Member
Joined
Jan 8, 2022
Messages
86
Reaction score
51
Location
Corvallis, Oregon
More importantly, why do we put in motor tubes? Why not just glue in rings with ID the same as the motor?
You probably "could" do that, but there is the matter of retaining the motor in place so (A) it doesn't go shoot up into and through the rocket body, but that is solvable with secondary "stop ring" at the top of the motor similar to what is done in conventional motor mount tubes, and (B) so the motor stays in place with some type of retainer clip or threaded ring system and the ejection charge pushes the parachute/stream & nose-cone/payload-section instead of ejecting the motor, which also there is probably a way to make such a device. I personally am now such a big fan of through-the-body fin mounting having fins glued to the motor mount, centering rings and the body tube that I personally would still prefer a motor mount. But, if one were competitive enough about LPR altitude, streamer, helicopter or parachute duration competition, you're idea might be worth the small weight reduction combined with other weight reduction methods.
 

bjphoenix

Well-Known Member
Joined
May 3, 2011
Messages
1,075
Reaction score
243
After cutting gobs of custom centering rings for re-sized tubes, I stopped and went 🤔. Why circles and not straight lines to do this? (It's sloppy work, I know....)
View attachment 519573

Any one of the thousands of you who are far smarter than me have a good reason to keep cutting rings??? This is far far easier.....
I built a motor mount like that over 50 years ago, and even then I had the idea that it would be stronger than using cardboard rings.
I had a scratch built BT55 rocket, probably my largest with one motor. Back then Estes did not make C motors, only B and below. Then we somehow stumbled on FSI and their 21mm C motors so I ordered some. I pulled the motor mount out of that rocket using pliers and a screwdriver, rolled a new tube out of paper to fit the 21mm motors, put strips of balsa on it and glued it in similar to your photo. I don't recall how I sealed it, maybe I did make one crude ring. Somewhere back in that era I also built a BT60 rocket with 3 motor cluster. I sealed the gaps between those motor tubes by stuffing in some tissue and soaking it in glue. Fast forward to now and I still have that BT55 rocket but no more 21mm motors. I made a new 18mm mount for it so I could launch it again but after a few launches I decided to retire it. For MPR I do a similar thing by extending fin roots to the motor tube because I think it makes the strongest motor mount.
 

Duncan.Byers

Well-Known Member
Joined
Mar 18, 2022
Messages
74
Reaction score
29
I built a motor mount like that over 50 years ago, and even then I had the idea that it would be stronger than using cardboard rings.
I had a scratch built BT55 rocket, probably my largest with one motor. Back then Estes did not make C motors, only B and below. Then we somehow stumbled on FSI and their 21mm C motors so I ordered some. I pulled the motor mount out of that rocket using pliers and a screwdriver, rolled a new tube out of paper to fit the 21mm motors, put strips of balsa on it and glued it in similar to your photo. I don't recall how I sealed it, maybe I did make one crude ring. Somewhere back in that era I also built a BT60 rocket with 3 motor cluster. I sealed the gaps between those motor tubes by stuffing in some tissue and soaking it in glue. Fast forward to now and I still have that BT55 rocket but no more 21mm motors. I made a new 18mm mount for it so I could launch it again but after a few launches I decided to retire it. For MPR I do a similar thing by extending fin roots to the motor tube because I think it makes the strongest motor mount.
I would think you could use the same thing with fin roots without having to extend them all the way to the MM. Fin roots extend into the body, then the MM with the risers - apply glue to the risers on both the top and one side of each riser, insert and rotate so that the glued face of each riser engages a fin root. Or cut slots for the fin roots, glue in the MM with risers directly next to the slots and put the fins in that way. Might be easier than fiddling with tighter tolerances to get the fin roots exactly right? I'm always looking for easier ways to make up for my habit of measuring once and cutting twice.
 

Dane Ronnow

Well-Known Member
Joined
Dec 15, 2020
Messages
469
Reaction score
272
Location
Las Vegas, NV
Off the top of my head, I'd say to support the body tube circumference. I wouldn't like a body tube that would squeeze at the MM. Besides, wouldn't putting the four pieces in with any pressure at all on them tend to 'square' the BT?
 

Duncan.Byers

Well-Known Member
Joined
Mar 18, 2022
Messages
74
Reaction score
29
Off the top of my head, I'd say to support the body tube circumference. I wouldn't like a body tube that would squeeze at the MM. Besides, wouldn't putting the four pieces in with any pressure at all on them tend to 'square' the BT?
If you get the measurements wrong possibly. But unless there's some magic method I'm missing I can be far more accurate cutting a straight line than an arc or circle and in my experience it's entirely possible to mis-cut a ring. And if the risers are putting too much pressure on the tube hit 'em with some sandpaper the same way you would with a ring.
 

Duncan.Byers

Well-Known Member
Joined
Mar 18, 2022
Messages
74
Reaction score
29

lakeroadster

👀 Oddroc Scum 👀 CAD Monkey Swingkinder
Joined
Mar 3, 2018
Messages
4,639
Reaction score
4,120
Location
Central Colorado
After cutting gobs of custom centering rings for re-sized tubes, I stopped and went 🤔. Why circles and not straight lines to do this? (It's sloppy work, I know....)
View attachment 519573

Any one of the thousands of you who are far smarter than me have a good reason to keep cutting rings??? This is far far easier.....
Centering rings might also have less weight in many situations.
Might? :dontknow:

Yours is a "manufacturing methods" question. Far, far easier for you may or may not be so for others. Buy a fly cutter, you could make 6 centering rings in the time it would take to make 4 spokes.

Plus you have to ask yourself "What' my end goal here"?

The method you propose is far from ideal from an engineering perspective if minimizing weight is also a prime factor. And in rocketry, minimizing weight is almost aways a prime factor.

A lightweight centering ring distributes the loading, equally to the full circumference of the inner tube and to the full circumference to the outer tube.

Last question: Why use 4 spokes, when 3 would have worked?
 
Last edited:

lakeroadster

👀 Oddroc Scum 👀 CAD Monkey Swingkinder
Joined
Mar 3, 2018
Messages
4,639
Reaction score
4,120
Location
Central Colorado

Duncan.Byers

Well-Known Member
Joined
Mar 18, 2022
Messages
74
Reaction score
29
Might? :dontknow:

Yours is a "manufacturing methods" question. Far, far easier for you may or may not be so for others. Buy a fly cutter, you could make 6 centering rings in the time it would take to make 4 spokes.

Plus you have to ask yourself "What' my end goal here"?

The method you propose is far from ideal from an engineering perspective if minimizing weight is also a prime factor. And in rocketry, minimizing weight is almost aways a prime factor.

A lightweight centering ring distributes the loading, equally to the full circumference of the inner tube and to the full circumference to the outer tube.

Last question: Why use 4 spokes, when 3 would have worked?
I'm not so concerned over weight for most of my stuff. Not competing at least not now. I get all the advantages of rings and like them, I really do! Just asking the question since I don't have a fancy fly cutter but now that you mention it..... And yes, three would work. No particular reason for four versus three. And as for the weight issue I wasn't thinking ultimately of having the supports run the entire length of the MM.
 

mh9162013

Well-Known Member
Joined
Apr 25, 2022
Messages
185
Reaction score
132
Location
Western, KY

Duncan.Byers

Well-Known Member
Joined
Mar 18, 2022
Messages
74
Reaction score
29
I use a compass cutter, like this one: https://www.amazon.com/Circle-Cutter-Compass-Premium-Grade-Patented/dp/B003D8C784/ref=sr_1_8?crid=28YD30AGKHSFB&keywords=compass+cutter&qid=1653318119&sprefix=compass+cutter,aps,89&sr=8-8

I use it to make my own centering rings. It's not the best and takes some fiddling to get right. But it's a whole lot easier and more accurate/precise than using an X-acto knife blade by hand.
Yeah I've got a couple of those and they're awfully fiddly things. Someone else suggested using a fly cutter and I might just try that.
 

Duncan.Byers

Well-Known Member
Joined
Mar 18, 2022
Messages
74
Reaction score
29
Might? :dontknow:

Yours is a "manufacturing methods" question. Far, far easier for you may or may not be so for others. Buy a fly cutter, you could make 6 centering rings in the time it would take to make 4 spokes.

Plus you have to ask yourself "What' my end goal here"?

The method you propose is far from ideal from an engineering perspective if minimizing weight is also a prime factor. And in rocketry, minimizing weight is almost aways a prime factor.

A lightweight centering ring distributes the loading, equally to the full circumference of the inner tube and to the full circumference to the outer tube.

Last question: Why use 4 spokes, when 3 would have worked?
Have you used a fly cutter on paper rather than wood?
 

Duncan.Byers

Well-Known Member
Joined
Mar 18, 2022
Messages
74
Reaction score
29
Paper, no. Cardstock, yes. For paper I'd cut the o.d. with scissors and the i.d. with an X-Acto knife.

You bring up a good point though. Woodn't (;)) cardstock be a good material instead of thick wood for the spokes?
That was my plan - to use cardstock. The picture that I posted is actually a build with through-tube fins rather than exterior fins. I know, I know, wind cocking. Built it for giggles to launch on a windless day with no one around for miles.

So the bits work well on cardstock huh? Going to have to try it. Looking at the bits I would have thought it would end up making a beveled edge on the card stock.
 

lakeroadster

👀 Oddroc Scum 👀 CAD Monkey Swingkinder
Joined
Mar 3, 2018
Messages
4,639
Reaction score
4,120
Location
Central Colorado
That was my plan - to use cardstock. The picture that I posted is actually a build with through-tube fins rather than exterior fins. I know, I know, wind cocking. Built it for giggles to launch on a windless day with no one around for miles.

So the bits work well on cardstock huh? Going to have to try it. Looking at the bits I would have thought it would end up making a beveled edge on the card stock.

It does make a beveled edge. The glue said no worries, just more surface area. ;)
 

Duncan.Byers

Well-Known Member
Joined
Mar 18, 2022
Messages
74
Reaction score
29
I recant my observation of what is "easier" and what is not. I've tried multiple different circle cutters and found the closest to a winner I could find. It's simple but works pretty darned well. Got it off Amazon:

NT Cutter Compass Circle Cutter, 13/16 Inches ~ 5-7/8 Inches Diameter, 1 Cutter (C-400P)​

A little less than eight bucks.
20220525_192930.jpg 20220525_193010.jpg 20220525_193013.jpg 20220525_193037.jpg

Top screw adjusts centering point length and the bottom screw locks the point in. The blade is easy to adjust and cuts nicely. Outer circumference first then center.

20220525_213500.jpg
 

Attachments

  • 20220525_211159.jpg
    20220525_211159.jpg
    54.2 KB · Views: 0
Top