Centering Ring Thickness

[deadalus52]

Hello everyone,

I'm finishing up my preliminary plan for my level 3 attempt. I've already purchased a Loc Precision Bruiser EXP-3 kit, and although I've seen a few people saying it'll take mid range M's in bone stock configuration... I'm not seeing it. Yes, the TTW fin mounting gives extra strength, and yes, glassing the exterior will help a little as well, but given the stock centering rings are 1/4" and only the aft ring is only slightly thicker. Is there really enough meat for the aeropoxy to grab on to? Which brings me to a more general question: Is there a general rule of thumb for centering ring thickness when using engineered plywood? I can't seem to find any data out there for this in terms of high power, and while I've built many a rocket, I've never really had to think about strength due to them being kits or "Oh 1/4" seems reasonable for an L1 build."

 

[rharshberger]

The 1/4" will be fine, even if it was built properly with Titebond wood glue. If worried get one of Charlies Thrust plates https://www.scpconcepts.com/rocket-accesories.html

The thrust plate will transfer much of the thrust directly to the airframe instead of the CR's taking some of it along with the fin roots.

 

[Steve Shannon]

Hello everyone,

I'm finishing up my preliminary plan for my level 3 attempt. I've already purchased a Loc Precision Bruiser EXP-3 kit, and although I've seen a few people saying it'll take mid range M's in bone stock configuration... I'm not seeing it. Yes, the TTW fin mounting gives extra strength, and yes, glassing the exterior will help a little as well, but given the stock centering rings are 1/4" and only the aft ring is only slightly thicker. Is there really enough meat for the aeropoxy to grab on to? Which brings me to a more general question: Is there a general rule of thumb for centering ring thickness when using engineered plywood? I can't seem to find any data out there for this in terms of high power, and while I've built many a rocket, I've never really had to think about strength due to them being kits or "Oh 1/4" seems reasonable for an L1 build."

Do the math. Draw a diagram for yourself showing a cross-section of the rocket with all the glue joints and then look at how the force of the rocket motor is transferred to the rocket. Look at the area between the centering rings and the MMT and where the retainer sits and between the centering rings and the body tube. I think you’re probably okay, but you can prove it to yourself if you do the calculations.

 

[deadalus52]

Do the math. Draw a diagram for yourself showing a cross-section of the rocket with all the glue joints and then look at how the force of the rocket motor is transferred to the rocket. Look at the area between the centering rings and the MMT and where the retainer sits and between the centering rings and the body tube. I think you’re probably okay, but you can prove it to yourself if you do the calculations.

What exact calculations? Plywood breaking strength? Epoxy holding strength?

 

[ksaves2]

What exact calculations? Plywood breaking strength? Epoxy holding strength?

Exactly, not everybody can do the math. Some people with degrees have problems doing structural analysis. Kurt

 

[deadalus52]

Exactly, not everybody can do the math. Some people with degrees have problems doing structural analysis. Kurt

Especially when I'm a recently graduated electrical engineer who normally deals with angry pixies and bits rather than chips and tree carcasses

 

[terryg]

You are not dealing with a 1/4 inch centering ring alone. You are working with the structural strength of two centering rings and thru the wall fins bonded together as a structural unit, which is much stronger then a centering ring by itself.

 

[Dad Man Walking]

Especially when I'm a recently graduated electrical engineer who normally deals with angry pixies and bits rather than chips and tree carcasses

OK, so step back from the slide rule and let's all just think about it for a minute...

1. The epoxy on the edge of the CR is not the only thing holding the CR to the motor tube and the airframe. There should be fillets at the CR/motor tube connection, and fillets between the CR and the airframe. Those lock the CRs pretty securely to both tubes.

2. The CRs are not the only component transferring the load from the motor tube to the airframe of a TTW rocket. The fins are bonded to the motor tube at the root (with good fillets, of course) and to the airframe, also with fillets. Net result is that you have more than doubled the strength of the connections between the motor tube and the airframe.

So...it is unlikely that the motor tube will find a way to tear free from the CRs. And even less likely that the motor tube can break free if it's held in place with a proper TTW construction.

So can we just get on to some of the fun stuff -- like what electronics are you going to use, what color will you paint it, will you give it a name, when will you finish it, where and when will you launch it and give us a launch report? With pics, of course. This forum has rules, you know.

 

[OverTheTop]

Do the math. Draw a diagram for yourself showing a cross-section of the rocket with all the glue joints and then look at how the force of the rocket motor is transferred to the rocket.

What he said. You can make some approximations (seriously coarse estimations) to really simplify the shear forces and where they are acting. The TTW tabs effectively bond the outer airframe to the MMT, which will transfer the thrust through to the airframe. You will be surprised how strong it is.

You have done a degree so they have taught you how to think about a problem. It's not important what you actually learned along the way. You don't have to be trained in that field to work it through. I assume you passed, so you have the skill.

I think you will find you don't need a fancy thrust plate, but if you want it nice and pretty, go for it :wink:

 

[Steve Shannon]

What exact calculations? Plywood breaking strength? Epoxy holding strength?

Start with shear strength of the epoxy, but all have to be considered. They’re not typically difficult.
Force / Area = Stress. Rearrange as needed.
Let say that you have three centering rings. The area of each centering ring where is touches the motor mount tube is 1/4 inch (the thickness) times the circumference of the hole the tube passes through, right (2 x pi x radius). Just to make life easier let’s say that’s a 2 inch radius (4 inch motor mount) and we’ll cheat and use 3 instead of pi. That’s less than pi so we’re actually being conservative. So, the area of the glue joint for a single centering ring is 1/4 x 2 x 3 x 2 = 3 in^2. As someone pointed out, this doesn’t include fillets. I think it really emphasizes the importance of fillets in fact.
Looking at the material properties of Aeropoxy 6209:
https://www.ptm-w.com/technical-lib...oducts Bulletins/AEROPOXY ES6209 Bulletin.pdf
we see that the aluminum to aluminum tensile lap shear strength at 77°F is 2900 psi. That’s the only lap shear strength I found in my 15 seconds of googling. Notice how it goes down to 700 psi at 140°F.
Again, we’re going to err significantly on the side of caution. We’ll assume you painted the rocket black and you’re flying it at Airfest on one of the hot days, so it’s entirely possible the inner spaces, sitting in the sun, are hot. We use 700 psi. Psi is actually lbs./in^2.
700 psi x 3 in^2 = 2100 lbs.
In other words, the glue joint at the interface between the centering ring and motor mount tube, for a single centering ring, holds a ton+ and you have multiple centering rings, plus fin tabs, etc.
So epoxy isn’t the weak link. Using the same areas look up material properties for your different components. Here’s a document discussing the shear strength of birch plywood.
https://ac.els-cdn.com/S18777058173...t=1519295597_5f755dd82c4ed9313804ab72b0ba03ea
It lists the shear strength of plywood as 7.11 mPa, which is 1031 psi.
The force of the motor is simply the peak thrust. I would apply a margin of two or more just to be conservative.
As long as you prepare your joints right, I see no problem.


Steve Shannon

 

[AeroAggie]

...
It lists the shear strength of plywood as 7.11 mPa, which is 1031 psi.

Steve Shannon

Steve, I don't think this property is what you would use to analyze the plywood here. The glue joint is a shear strength analysis, mostly. The centering ring itself is subject to bending, not shear. (well, there is shear, but it's mostly transverse not in-plane which I think is what the shear strength you quoted is based on). If the centering ring is thin enough to deflect more than about half its thickness, then the glue joint analysis really starts to turn into peel strength, not pure shear.

I'll see if I can scare up some good examples of how this analysis is done for non-structural engineers. If a person wants to do their own research, Roarks Formulas for Stress and Strain has all the equations you need for circular plates.

The biggest hurdle for doing any analysis of this type is the plywood material properties. Not all woods are created equal. You can use published values for adhesives and expect close enough results, but unless you buy wood that conforms to a particular specification (like MIL-P-6070) the properties are a guess.

That being said, multiple 1/4" plywood rings and a good epoxy or wood glue should be way more than adequate. Add TTW fins and you're more better. Add foam-filled on top of that and you're more betterer again, albeit with a potentially significant weight penalty.

 

[T-Rex]

Thanks Steve and Aggie! I have never seen an analysis like that before, nor really considered it. Obviously it had to be done at some point, but all us mere mortals just accept that it works. (or in the case of the OP, question if it works)
The majority of this forum is filled with "you are/have overbuilt" when someone asks about thicknesses or glue.

Seriously, thank you for showing us that it can be calculated.

 

[dr wogz]

reminds me of a table I saw someone build.

Wobbly legs.. One massive bolt securing them.. No wonder, although he didn't get it.. Yes, a massive nut, washer & bolt, tightened to wood crushing torque. No other structure or such to stop any of the twisting forces around said nut & bolt..


The TTW fin root / tab adds a lot of shear strength from the motor to the BT.. It's essentially an 'I' beam (although the loads are 90°..) The CRs at this point, could be 1/8", as they are just kinda holding the MMt concentric.. But don't!! they do add strength & cap off the structure, making another 'I' beam..

 

[Steve Shannon]

Steve, I don't think this property is what you would use to analyze the plywood here. The glue joint is a shear strength analysis, mostly. The centering ring itself is subject to bending, not shear. (well, there is shear, but it's mostly transverse not in-plane which I think is what the shear strength you quoted is based on). If the centering ring is thin enough to deflect more than about half its thickness, then the glue joint analysis really starts to turn into peel strength, not pure shear.

I'll see if I can scare up some good examples of how this analysis is done for non-structural engineers. If a person wants to do their own research, Roarks Formulas for Stress and Strain has all the equations you need for circular plates.

The biggest hurdle for doing any analysis of this type is the plywood material properties. Not all woods are created equal. You can use published values for adhesives and expect close enough results, but unless you buy wood that conforms to a particular specification (like MIL-P-6070) the properties are a guess.

That being said, multiple 1/4" plywood rings and a good epoxy or wood glue should be way more than adequate. Add TTW fins and you're more better. Add foam-filled on top of that and you're more betterer again, albeit with a potentially significant weight penalty.

I absolutely agree with you. When the centering ring bends you end up with adhesive trying to peel. My feeble attempts to research it in the middle of the morning had an illustration which indicated that exact thing. I assumed that was why the strength value for epoxy was so much less than the tensile strength (2900 psi vs. 7500 psi as I recall). What I didn’t consider was that the separate article for plywood didn’t indicate the same derate.
At that point I rolled over and went back to sleep. Mea culpa! [emoji851]


Steve Shannon

 

[Dad Man Walking]

...The glue joint is a shear strength analysis, mostly. The centering ring itself is subject to bending, not shear. (well, there is shear, but it's mostly transverse not in-plane which I think is what the shear strength you quoted is based on). If the centering ring is thin enough to deflect more than about half its thickness, then the glue joint analysis really starts to turn into peel strength, not pure shear...

...That being said, multiple 1/4" plywood rings and a good epoxy or wood glue should be way more than adequate. Add TTW fins and you're more better...

+1
AeroAggie is building on the point I was making. If the fins are locking the motor tube and the airframe together with the TTW construction, there is a lot of strength to resist the motor tube moving upward during thrust, and the CR's won't be subject to much force that would cause deflection.

The TTW construction needs to be analyzed as a system. The fins lock the motor tube and the airframe together, and share the load that the CRs would handle by themselves if the fins were surface mounted. If either part of the system (CRs and TTW fins) are strong enough by themselves to handle the expected loads, there's a lot of headroom in the system to handle many of the unexpected ones.

..Add foam-filled on top of that and you're more betterer again, albeit with a potentially significant weight penalty.

Not to get too far off topic, but this is something I've never quite understood. I have built a couple of small rockets (29mm high power capable) using Wildman's foam-and-fly setup. Was fun and fast to work with, and withstood "normal" flight loads just fine. But a hard landing on one of the rockets (flat spin onto the playa after a deployment failure) dislodged the fin that hit the ground first. I don't know much about the properties of the foam, but I can say it does not have anywhere near the bonding strength (resisting shear and tensile loads) of a good adhesive (epoxy on fiberglass, wood glue on cardboard and wood). It was ridiculously easy to remove all of the fins just by wiggling them until the foam disintegrated enough to free the fins. Based on my personal experience, I believe that foam is not a substitute for proper adhesives and construction techniques in a high power rocket if the goal is to add strength to the TTW setup. Foam has enough tack by itself to hold the fins in during nominal flight loads (on the small rockets I used it on). But if the TTW construction is being stressed to the failure point, the foam isn't going to do much more. My two cents...

For a small rocket that you want to build on Friday and fly on Saturday, foam is great. But if your goal is to reinforce a TTW project so that it will survive both planned and unforeseen events, I would always recommend that you pay attention to the details on the basic build (surface prep, good ahesives, good fillets), and add a tip-to-tip layup on the fin can for more strength.

 

[AeroAggie]

+1
AeroAggie is building on the point I was making. If the fins are locking the motor tube and the airframe together with the TTW construction, there is a lot of strength to resist the motor tube moving upward during thrust, and the CR's won't be subject to much force that would cause deflection.

The TTW construction needs to be analyzed as a system. The fins lock the motor tube and the airframe together, and share the load that the CRs would handle by themselves if the fins were surface mounted. If either part of the system (CRs and TTW fins) are strong enough by themselves to handle the expected loads, there's a lot of headroom in the system to handle many of the unexpected ones.



Not to get too far off topic, but this is something I've never quite understood. I have built a couple of small rockets (29mm high power capable) using Wildman's foam-and-fly setup. Was fun and fast to work with, and withstood "normal" flight loads just fine. But a hard landing on one of the rockets (flat spin onto the playa after a deployment failure) dislodged the fin that hit the ground first. I don't know much about the properties of the foam, but I can say it does not have anywhere near the bonding strength (resisting shear and tensile loads) of a good adhesive (epoxy on fiberglass, wood glue on cardboard and wood). It was ridiculously easy to remove all of the fins just by wiggling them until the foam disintegrated enough to free the fins. Based on my personal experience, I believe that foam is not a substitute for proper adhesives and construction techniques in a high power rocket if the goal is to add strength to the TTW setup. Foam has enough tack by itself to hold the fins in during nominal flight loads (on the small rockets I used it on). But if the TTW construction is being stressed to the failure point, the foam isn't going to do much more. My two cents...

For a small rocket that you want to build on Friday and fly on Saturday, foam is great. But if your goal is to reinforce a TTW project so that it will survive both planned and unforeseen events, I would always recommend that you pay attention to the details on the basic build (surface prep, good ahesives, good fillets), and add a tip-to-tip layup on the fin can for more strength.

Agreed! I’m not advocating foam in lieu of adhesive. The bond line of the foam is very brittle (its mostly air!). It does provide some strength in terms of shear transfer between motor mount and body tubes, but more importantly it provides stiffness to keep things from deflecting under load. The biggest benefit in my opinion is the stiffness and damping it provides the fins against flutter. You get something closer to a fully clamped cantilever boundary condition, rather than guided or simply supported.


TRA #16895 L2

 

[CORZERO]

Better go for that masters.

 

[AeroAggie]

Here's an example of what the freebody diagram and generalized equations for a centering ring would look like (Ref. Roark):

The annular line load (w) would be your thrust load. If you have two rings, I'd assume a 60/40 split for a conservative overlapping assumption. Say your thrust is 1000N, you would assume 600N of that is reacted at each ring if you were writing a margin of safety for structural analysis. If you have three rings, then I'd use a 40/30/30 assumption, and just size each one for 40%.

Do what Steve said for the glue joint shear stress. Use the equations above to determine the bending moment in the ring, and then use f=6M/bt^2 (flat plate bending stress) to check the ring for bending stress.

Please pardon the weird font sizing on the equations. I just did screen captures off a pdf. Page 465 in Roark if you're curious enough to google it.

 

[blackjack2564]

The TTW tabs effectively bond the outer airframe to the MMT, which will transfer the thrust through to the airframe. You will be surprised how strong it is.

I think you will find you don't need a fancy thrust plate, but if you want it nice and pretty, go for it :wink:

THIS ^ Steve said it first quite simply.
seems like everyone overlooked the obvious.

I've seen many big project with NO motor mount tube...just a couple CR's to keep motor centered between fins.
CR's act like thrust plate against to fins transferring load to airframe as stated above.

 

[SCP]

Hey thanks for the plug Rich! I'll get your check out in tomorrows mail!

I can always give true objective feedback on this subject.....mainly because it grew out of a love for a hobby, and I don't need to (and never could haha) make my living from selling them. I made the first few for myself, after seeing one of the Red Glare regulars with a version of aluminum plate. It's in my nature to do something above and beyond for effort to eliminate human error, and use the best materials I have available. That is most of the fun for me. While technically it is true they can apply all the thrust load to the aft end of the airframe, without relying on any shear loading in the centering rings, and then you can even save weight due to no structural requirements on the rings. TTW fins obviously change that. There is some benefit to protection on the aft end, and also the convenience of having a machined accurate bolt pattern to directly accept the Aeropack retainers. The custom jobs get fun when we try to cluster motors with fancy retaining, or just fit odd size airframe or motors, etc. I did the first few because I have a machine shop, and started to offer them because it makes a very professionally done plate available to anyone, at what seems to be a fair price considering the work that goes into them. Also keep in mind I think it is Binder who has done some custom Thrust Plates that also look beautiful. We both do a lot of customs. I am happy to offer my opinion on his work also as top notch.

So to me, its interesting , there is always differing opinions on the use of them, I can tell you straight up (and reference the analysis by some really smart guys in this forum, I am an ME too and in general in alignment with them) - no, its not a "necessity" or every rocket out there would be losing the motor out through the nose, right? I have seen and heard of that happening a few times though, likely due to improper construction methods. I am not referring to motor CATO's but actually a motor thrusting up through the rocket. Still could happen with an aluminum thrust plate if there were some combination of other errors, but the odds are probably more in your favor.

I have some more fun ideas on more product lines, especially in the Av Bay area, bukheads, charge wells, etc. And similarly, my plan is for aluminum due to the workability (in a machine shop), material properties, availability, etc. If I was a cabinetmaker or had a top end wood shop my plans might be different.

The 1/4" will be fine, even if it was built properly with Titebond wood glue. If worried get one of Charlies Thrust plates https://www.scpconcepts.com/rocket-accesories.html

The thrust plate will transfer much of the thrust directly to the airframe instead of the CR's taking some of it along with the fin roots.

 

[jlabrasca]

Hello everyone... Is there a general rule of thumb for centering ring thickness when using engineered plywood? I can't seem to find any data out there for this in terms of high power, and while I've built many a rocket, I've never really had to think about strength due to them being kits or "Oh 1/4" seems reasonable for an L1 build."

While I am sure that Aeroaggie's response #18 cleared up all of your questions about this issue, Tim Van Milligan wrote some things about this topic that might be of interest

https://www.apogeerockets.com/education/downloads/Newsletter63.pdf

https://www.apogeerockets.com/education/downloads/Newsletter126.pdf

Centering rings can be a LOT thinner and lighter than you'd probably expect and the things that strengthen a rocket against damage in a hard landing are not all the same things that will strengthen it against the stresses it encounters while under thrust.

 

[ArchitectOfSeven]

Just my two cents one, the most efficient way to transfer load from the motor mount to the body is through the fins, not the centering rings. If it wasn't for the need to seal off the recovery bay, you can honestly survive without CRs just fine as long as your fins are bonded properly.

 

[ECayemberg]

Hello everyone,

I'm finishing up my preliminary plan for my level 3 attempt....I can't seem to find any data out there for this...

Real world data: the stock rings are plenty sufficient for an M. Here's a stock length Bruiser (30" shorter than the EXP) flying on an N2600. I think you've made a fine choice with a Bruiser EXP for a L3 shot!

[/IMG]

If you desire to reinforce something with fiberglass, there are two areas that could benefit: the fins and the top of the booster airframe. Not saying either NEED it, but if you must glass, make it worthwhile; here's the WHY:

Fins: Bruiser fins aren't a great design. I really like Bruisers and fly two fairly regularly, but the fins just aren't great for moving fast. The span is pretty large compared with the root, therefore they are a bit prone to flutter. Go low and slow, no worries; push it hard and you need to start thinking about flex and flutter. A layer or two of glass or carbon will help stiffen the fins.

Top of the booster airframe: I fly A LOT of paper airframes; I really enjoy how they're lightweight and easy to work with. Probably the most frequent "issue" I see with cardboard airframes is that they zipper (due to late or violent ejections, too short of recovery harness, etc.). A very simple proactive fix for this is to simply wrap the top of the airframe with a wrap or two of fiberglass. I use two wraps of 2" wide fiberglass tape and no longer worry about zippers. Don't need to glass the entire airframe, just where it counts.

Here's an example, which is part of a very similar Loc 7.5" Patriot build: https://www.rocketryforum.com/showthread.php?138759-Loc-MIM-104-Patriot-Build&p=1671261#post1671261