Balsa for centerging rings

[jqavins]

I don't (yet) build medium or high power rockets so I am not sure when one has to start giving up on paper, cardboard, and balsa, and start using stronger materials for a rocket.

You probably need to go to stronger, heavier material a little bit later (higher power) than you actually will. As a group we have a tendency to overbuild. So when you start to think "Maybe I should use quarter inch plywood", you're probably still good with one eighth. When you start thinking "Maybe I should glass this heavy wall cardboard tube", you probably don't need to. It's not always a bad thing, because at a little bit bigger and higher lower you eventually really will need to; better to have it and not need it than to need it and not have it.

The problem comes when each generation adds a little bit more cautious overbuilding on top of the cautious overbuilding that the prior generation taught - and didn't know they were doing and teaching - and before you know it you've got five pound, fiberglass and plywood mid power birds. But I guess this is all off topic, isn't it?

 

[Ez2cDave]

I don't (yet) build medium or high power rockets so I am not sure when one has to start giving up on paper, cardboard, and balsa, and start using stronger materials for a rocket.

You will know when you blast a Motor Mount Tube completely through a rocket . . .

Dave F.

 

[GrimmReaper]

“As a group we have a tendency to overbuild.”

If that’s true, then this is most certainly “my” group of rocket buds. I’m just getting back into rocketry after a long hiatus. At the end of my previous era, I was overbuilding some of the larger LPR kits to handle 29mm motors and the cardboard centering rings and white glue didn’t seem appropriate. (Estes Honest John and the smaller 1/64 scale V2) I ended up using plywood rings that were pretty thick, which required more nose weight, which required a different engine, ad nauseam.
What I’m wondering is if I glue two balsa rings together (90 degree offset), seal the outsides with Sig sanding sealer, and use epoxy for the fillets, will this work to strengthen the CRs enough to handle mid-power engine loads, say Aerotech F and some G engines? I’ve always used that Sig Sanding sealer to strengthen my balsa fins — they’re a bear to sand down afterwards, but I enjoy sitting in the garage workshop listening to some good blues guitar while I sand away in peace.

 

[jqavins]

What I’m wondering is if I glue two balsa rings together (90 degree offset), seal the outsides with Sig sanding sealer, and use epoxy for the fillets, will this work to strengthen the CRs enough to handle mid-power engine loads, say Aerotech F and some G engines?

The short answer is yes.

The not so short answer is:

• Plain basswood may be stronger than laminated balsa for the same thickness, or even for the same weight. Laminated basswood surely would, and is probably overkill, but doesn't weigh much as overkill goes.
• Sealing the edges is pointless. Your're going to glue them into a tube and glue another tube into them, so all that glue and the tubes give all the sealing the edges need and more.
• Epoxy is unnecessary for strength (though it has other advantages).

 

[GrimmReaper]

The short answer is yes.

The not so short answer is:

• Sealing the edges is pointless. Your're going to glue them into a tube and glue another tube into them, so all that glue and the tubes give all the sealing the edges need and more.

I don’t think I was quite right in saying, “the outsides.” What I meant was sealing the flat parts of the CRs. It gives my fins a stiffer feel, which I (perhaps incorrectly) equate to strength. Maybe that would end up making a CR too brittle.

 

[jqavins]

OK, it's probably my bad, assuming that when you wrote of sealing something, it must mean sealing a cut edge. I shouldn't assume.

OK, sealing the faces. I'd say it's unnecessary. For fins, the stiffness you get from that may be of some value, but I doubt it. I also doubt very much that it's going to cause any trouble with brittleness. Sealers like that are meant for finishing, and for fins is probably helping you get a nice finish, but structurally I'll bet heavily that it's doing nothing to speak of.

But you were asking about centering rings. For a centering ring, all the same goes as for fins, except that no one cares about the finish. And that the strength and stiffness requirements for centering rings are surprisingly mild. The ring structure, the way that both the inner and outer edges are glued to tubes, is a really strong stricture. Card stock is fine at least up to D engines, so you really don't need anything special for F and G. Even the tendency of weak woods like balsa to split is mitigated by that structure. What I wrote above about plain basswood being as strong or stronger than laminated balsa is true for fins and even truer for centering rings since splitting is less of an issue.

I'm not immune to the overbuilding urge, and I don't buy my own line of "Just use plain balsa, it'll be fine", even though it probably will. But I do buy my own line that there's no need for anything fancy. Use plain basswood or birch ply and yeah, it'll be fine.

 

[GrimmReaper]

Thanks, jqavins. You folks have more experience at this rocket science. I was assuming that the thrust of F or G engines would overpower plain card stock CRs, so I was fishing for something still lightweight. Basswood is probably the way to go.

 

[BABAR]

Old thread originally (2005!)

can’t speak for high power.

for low power, certainly you CAN make centering rings from balsa, if you don’t make a “ply” with different preferably right angle grain directions (for 2 ply) it’s gonna be weak.

also practically, it’s a bit more challenging to cut a ring in a ring in balsa (again certainly do-able) especially with “width” between inner and outer circumferences (BT-5 to BT-20 or BT-20 to BT-50.). Again do-able.

but card stock (or for larger diameters, like BT-20 to Pringles Can, foam board) is easier to make, cheaper (use a cereal box), and works just as well. Negligible if any weight difference.

just because you CAN do something doesn’t mean you should.

 

[Ronz Rocketz]

I tried epoxy and CA with balsa because I'm too lazy to order the correct size of plywood CR. Pulled the shock cord out of the CR. I like the idea of sandwiching with manila cardstock.

 

[bjphoenix]

I think we overestimate the amount of force applied to a centering ring. A black powder motor doesn't put all that much force into a motor mount. I had a BT60 rocket land in the water a couple of years ago and I didn't pull the 18mm motor out immediately, by the time I tried to pull the motor out it was really stuck and I had to pull hard, then harder, then eventually it pulled the entire mount out by tearing up the cardstock centering rings. I may have applied 5 times the force that a C6 could put on the mount. Beyond 18mm black powder motors, maybe beyond 24mm black powder motors, it is likely a different story and would require stronger materials.

 

[jqavins]

Except in "off-nominal flight termination" the centering rings will never see more force than the peak thrust delivered by the motor. According to ThrustCurve.org, the greatest peak thrust of any BP motor on the market comes from the E12, and is 33.3N. That's just under 7.5 lbf. So, in your head or in you hands, turn your rocket nose down and place an eight pound weight in or on the motor mount. If it doesn't break, the CRs are strong enough.

 

[prfesser]

I have flown a mid-power rocket that has centering rings made from two layers of 3/32" balsa glued cross-grain. For insurance I added three rectangular gussets of 3/32" balsa (single layer) between the centering rings. Handles an F motor just fine, and I'd not hesitate to fly it on a full G.

It's but one example, but I'd do it again for any midpower or maybe even a smaller high-power rocket.

 

[cwbullet]

You can make centering rings out of anything, but you need multiple layers for the thinner or softer stuff.

 

[DaHabes]

The short answer is yes.

The not so short answer is:

• Plain basswood may be stronger than laminated balsa for the same thickness, or even for the same weight. Laminated basswood surely would, and is probably overkill, but doesn't weigh much as overkill goes.
• Sealing the edges is pointless. Your're going to glue them into a tube and glue another tube into them, so all that glue and the tubes give all the sealing the edges need and more.
• Epoxy is unnecessary for strength (though it has other advantages).

For my scratch LPR builds, I've switched 100% to basswood. I'll also use it on some MPR builds, and in some cases I'll ditch a kits balsa fins and cut my own basswood fins, especially if I am going up a MMT size and want TTW fins. Balsa is really hard to find now, expensive, and given the difference in strength, you can generally go down one size in basswood (e.g., 3/32" basswood vs. 1/8" balsa) for a nominal weight increase and much greater strength. I've grown tired of balsa fins breaking on hard landings.

 

[jqavins]

Indeed, price and availability are factors not considered in my answer above. For fins, papering balsa goes a long way but sure, basswood makes sense.

That "nominal weight increase" is at the wrong end of the rocket for stability, so in a few cases it may be amplified by the need for additional nose weight. Just something to keep in the back of one's mind. Especially when upsizing a motor mount, when the added motor weight is likely to push you to nose weight where none was required originally.

 

[DaHabes]

My view on papering is that it's tricky. You absolutely cannot paper the root edge of the fin where you glue it to the rocket. I've made that mistake and had the paper rip off the fin as it got jarred loose on landing. If you don't get the edge of the paper at the edge of the fillet radius, it's going to need spot putty, which is more weight on an LPR rocket. Then there is the leading-edge issue - if you round the leading edge then you have this fuzzy paper crap to deal with, usually with CA.

All-in-all, basswood eliminates all of these issues. It's just easier to work with and stronger.

 

[lakeroadster]

The core of most wind turbine blades is balsa.

I've used cardstock, raw balsa, papered balsa, home made balsa plywood, basswood, papered basswood, homemade basswood plywood, and even strips of masking tape as centering rings... never have had a failure yet.

It's not so much the materials that are being used, as it is the order and manner of the finish designed structure.

 

[DaHabes]

The core of most wind turbine blades is balsa.

Didn't know that. No wonder balsa is hard to find and expensive. When you look at prices for common sheets at National Balsa, basswood is cheaper (3/32' basswood is stronger than 1/8" balsa, with a very modest weight penalty).

1/8x6x24 Balsa sheet = $5.86
3/32x6x24 Basswood sheet = $4.88

All the more reason to just switch to basswood and be done with it.

 

[lakeroadster]

All the more reason to just switch to basswood and be done with it.

Basswood 0.289 oz/cu-in
Balsa 0.098 oz/cu-in

I like basswood, but I wouldn't say 3 times the weight is a "modest" penalty.

If you're using Basswood, also consider Pine... 0.306 oz/cu-in

 

[jqavins]

Weight per square inch in more important than per cubic inch when it come to fins and rings, so cut the basswood density by a quarter for the reduction from 1/8" thickness to 3/32". (And I bet 1/16" would do fine for centering rings.) Then consider the small fraction of weight that the fins and rings contribute to the whole rocket, and I would call it very modest indeed. (Except in those rare edge cases where it's amplified by the need to add nose weight, and even then it's not big.)

 

[lakeroadster]

Weight per square inch in more important than per cubic inch when it come to fins and rings, so cut the basswood density by a quarter for the reduction from 1/8" thickness to 3/32". (And I bet 1/16" would do fine for centering rings.) Then consider the small fraction of weight that the fins and rings contribute to the whole rocket, and I would call it very modest indeed. (Except in those rare edge cases where it's amplified by the need to add nose weight, and even then it's not big.)

square inch vs cu-in? The weight is what is being considered

Joe, have you ever cut a centering ring from 1/16" balsa or basswood sheet? You'll end up with a bunch of pieces of a broken centering ring.

I typically use 1/8" thick wood and paper both sides. That helps to minimize the tendency of the wood from fracturing during the cutting process.


S

 

[BigMacDaddy]

foam-core board

Does foam core board work for centering rings? The foam does not melt?

I had not thought of that for centering rings but it would be nice and thick and two posterboard layers should make it pretty strong but light.

 

[jqavins]

square inch vs cu-in? The weight is what is being considered

I'm not sure what you're saying there, but yes, the weight of the finished part is what what's being considered.

Joe, have you ever cut a centering ring from 1/16" balsa or basswood sheet? You'll end up with a bunch of pieces of a broken centering ring.

I have, a few times. And I eff it up every time, no matter the thickness, the ID and OD, or the material. For me it's laser cut or purchase. Maybe a CNC router; I haven't done that yet, since the only place I have access to one has a laser cutter too. Once I unearth my drill press and get it going I'll want to buy a fly cutter.

But the weight reduction from going to 3/32" was @DeHabe's point to start, not mine.

 

[lakeroadster]

I'm not sure what you're saying there, but yes, the weight of the finished part is what what's being considered.

You stated:

Weight per square inch in more important than per cubic inch when it come to fins and rings..

More important? Weight is weight...

 

[jqavins]

Weight per cubic inch is the weight of a volume of material, obviously, so if you are comparing two parts with equal volume it's the figure you need. But there are few rocket parts where the volume is fixed when you compare this material to that; solid nose cones are the only ones I can think of.

Weight per square inch is for stock of given thickness, and is what you must use for things like centering rings (and fins).

You listed the densities of balsa and basswood, basswood's density being about 2.9 times greater, then commented that it's not a modest increase. I'm saying that, in this application, that's not the right apples-to-apples comparison because it's a finished part that matters, not the bulk material. A given centering ring in 3/32" basswood is not 2.9 times heavier than the same size centering ring in 1/8" balsa; it's only 2.2 times heavier. And for strength, I just bet that 1/16" basswood would be sufficient, so only about 1.5 times heavier than the balsa one.

 

[lakeroadster]

Weight per cubic inch is the weight of a volume of material, obviously, so if you are comparing two parts with equal volume it's the figure you need. But there are few rocket parts where the volume is fixed when you compare this material to that; solid nose cones are the only ones I can think of.

Weight per square inch is for stock of given thickness, and is what you must use for things like centering rings (and fins).

You listed the densities of balsa and basswood, basswood's density being about 2.9 times greater, then commented that it's not a modest increase. I'm saying that, in this application, that's not the right apples-to-apples comparison because it's a finished part that matters, not the bulk material. A given centering ring in 3/32" basswood is not 2.9 times heavier than the same size centering ring in 1/8" balsa; it's only 2.2 times heavier. And for strength, I just bet that 1/16" basswood would be sufficient, so only about 1.5 times heavier than the balsa one.

The reality is (for centering rings, the topic of this thread) balsa is probably 5 times stronger than what is needed. Unless the centering ring has a very large O.D. vs I.D.

But the biggest hurdle with this thread, and our conversation, is:

• What is the mission of the rocket?
• What loadings will the centering rings experience?
• What methods do you have to make its components?

Ya really have to define these, to determine what will work in each application.