Anima Mundi

TRF,

As we’ve been discussing weight I’d appreciate an opinion on end-grain balsa and fiberglass sandwich panel. I want to use that material, branded DecoLite, as ½-inch thick fin stock. An 8x4x1/2 sheet of DecoLite is $250. It’s half the weight of plywood and just as strong. Most important, the shear modulus of the balsa core material is 23,000 PSI regardless of shear direction. Shear modulus of 1/8-inch cell Nomex paper honeycomb is directional and as low as 5,000 PSI in the width direction.

I think the balsa panel is a better choice than paper honeycomb panel. It’s “stiffer” and less likely to flutter. Any opinions?

Material comparison and a picture are below.

Feckless

Feckless Counsel said:

TRF,

As we’ve been discussing weight I’d appreciate an opinion on end-grain balsa and fiberglass sandwich panel. I want to use that material, branded DecoLite, as ½-inch thick fin stock. An 8x4x1/2 sheet of DecoLite is $250. It’s half the weight of plywood and just as strong. Most important, the shear modulus of the balsa core material is 23,000 PSI regardless of shear direction. Shear modulus of 1/8-inch cell Nomex paper honeycomb is directional and as low as 5,000 PSI in the width direction.

I think the balsa panel is a better choice than paper honeycomb panel. It’s “stiffer” and less likely to flutter. Any opinions?

Material comparison and a picture are below.

Feckless

Click to expand...

Fleckless,

End grain balsa is typically a good core material, but then again so is a honeycomb aramid core, but in fact your skin material is much more important than anything you select for the core, i.e., if you put a layer of carbon fiber on either, you will do much more to affect flutter than worrying about the core material.

To be honest though, it is really hard to give you any feedback as you provided only one parameter (thickness, well maybe two with flexure) of nearly a dozen parameters that affect flutter. You need flight speed, fin shape, laminate stack up, (and modulus, flexure, shear, etc. of each material), body attachment, adhesive parameters, fillet size/shape, etc. My advice... Don't ask for feedback on flutter on this or any other rocket forum as most people will just provide hearsay and most, even experienced rocketeers, will give incorrect information. If you are going to be building big, expensive and/or high performance rockets I would spend a little money as insurance and invest some time determining your real (or a very close approximation of your) flutter performance. Purchase The Laminator for $29 and AeroFinSim for $50. Enter your material parameters, thicknesses and stack up into The Laminator and get the aggregate lamina parameters and then plug that into AeroFinSim along with the fin profile/shape, body diameter, adhesive parameters, etc. and get a "right" answer versus an opinion.

-Tim

Tim,

Thanks for your knowledgeable reply.

Does your direct experience support that industrially manufactured 1/2-inch thick Nomex honeycomb sandwich panel could be up to this project? I would like to use that given my weight target.

I agree fin flutter, especially the destructive sort, is a subject poorly understood by we amateurs. I have AeroFinSim. Problem is that only accepts trapezoidal fin shapes. So I can't exactly simulate my fin. But I did make an approximation to my design and materials. AeroFinSim's flutter velocity is below RockSim's maximum velocity prediction.

I also agree that skins play an important and complex role in ultimate panel properties. But please consider that, as a fin bends and twists, the shear properties of the core come to dominate "stiffness." A fin that's not stiff has lower natural frequencies that may become excited at lower velocities. That is why I've focused, perhaps fecklessly, on shear modulus.

Feckless Counsel

Feckless Counsel said:

Does your direct experience support that industrially manufactured 1/2-inch thick Nomex honeycomb sandwich panel could be up to this project?

Click to expand...

If you had to press me, I'd say no. It looks like the material has only a single layer of fiberglass and from what I have seen simulating laminates that won't be sufficient for fins of the span you are trying to achieve. You likely need a couple layers of medium weave carbon fiber applied at 45 degree angle, or potentially better three layers of unidirectional fiber applied a 0, 45 and 90. But, in the end the three sentences above are "crap" as they are stabs in the dark not knowing all your parameters as will be the comments of most others, as pointed out earlier. Why? Because, few, if any, have built a rocket of the size you are considering matched with the fin material you have in mind. Closest match for me is a rocket I'm working on now that uses 1/4" ACP Composites Nomex honeycomb-carbon fiber; but the rocket is only 6" diameter and the fin profile is considerably different. But a 1/4" fin is likely not appropriate for a rocket of the size you are considering.

For a 11.5" V2 I am also in the process of building, I will be using two 1/4" lite ply panels with cut outs to provide a frame which will have 1/4" styrofoam inserted; these panels will then be sandwiched with one layer of carbon fiber in the middle and two layers on the outside of each giving me a finished thickness of ~0.575. I have not completed simulation of that stackup and the rocket flight profiles, but I believe I will be in the ballpark. But again, I will verify this for my rocket, its dimensions and profiles only. The application of the same stackup on a different rocket and its specific flight profile will be completely different.

Feckless Counsel said:

I have AeroFinSim. Problem is that only accepts trapezoidal fin shapes. So I can't exactly simulate my fin.

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Yes, but your fins are not too far off from a trapezoid, just make sure your span is close, shape is approximate and the overall area is represented.

Feckless Counsel said:

But I did make an approximation to my design and materials. AeroFinSim's flutter velocity is below RockSim's maximum velocity prediction.

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Well if that is the case, then you should be worried and take the simulation one step deeper. There was a recent experience of a large rocket where, from all appearances, the preflight homework was not done. Anyone that is considering launching a rocket of any significant size owes it to the rocket community to do the preflight homework to assure stability and structural integrity of their design/build (but that is just my opinion).

Feckless Counsel said:

I also agree that skins play an important and complex role in ultimate panel properties. But please consider that, as a fin bends and twists, the shear properties of the core come to dominate "stiffness." A fin that's not stiff has lower natural frequencies that may become excited at lower velocities. That is why I've focused, perhaps fecklessly, on shear modulus.

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From what I have seen in my simulations flutter is most impacted by fin thickness, then fin shape, then skin material characteristics then the impact from other things (adhesive, fillet shape, core material) fall off considerably. I am not an expert in this area, but for a limited view you can take a look of an excerpt from my L3 documentation that provides a commentary on the flutter analysis I did for that design.

Feckless Counsel said:

Jim,

Excellent reply and exactly the sort of experience I’m seeking. Thanks.

Feckless

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As a follow up, I dug through some pics I took of one of the best set ups I've seen on a large diameter rocket such as you are attempting.

These are serious strong rod and receiver's. Note how little overlap in full size airframe there is.
The rods have grooves near the end that were locked into place when the airframe was assembled by a rotating disk that had Keyholes in it. It "captured" all rods at once. At apogee an airbag [car type bought at junk yard for 20.00] Rotated the disk, then de-ployed semi-gently pushing the 2 sections apart. You can see the quicklinks for recovery attachment.

Most folks don't realize it But: Airbags are 2 step devices. First a cable pulls the seat belt tight ,restraining you from movement when the bag actually deploys a instant later. This achieved by a pyrotechnic device .[piston in a cylinder with cable attached on one end]
Charge fires, moving piston and cable, rotating disc to "unlock" position. Then bag inflates. If A doesn't happen, then B cannot. So it has built in passive safety feature. It can easily be hooked up with the wires that come on it , to any triggering device, with enough current to operate it. Here an altimeter. I don't recall if he had to boost the juice. This was Steve Eve's [Saturn project] elegant simple solution to a difficult problem. We talked at great length about the design and problems with shear, and this was his answer to it.
Yes it worked!



FYI also a very beefy rail guide set up, screwed through into CR's with blocks laminated on the sides. They are hanging door roller slides. [barn door] The rail was just the tube that's attached to the barn. [Unistrut]

Just some food for thought.

patelldp said:

My guess is closer to 175-200# loaded.

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Feckless...

First of all, let me say this is a great thread and I am learning a lot following the engineering discussions. I like ambitious projects, and you have obviously put a lot of thought into this and have done a substantial amount of planning into what you are undertaking. Your tube rolling jig alone is very impressive. Keep it up.

My only input, FWIW, is be prepared for coming in over-weight. You will. Yes, most big projects are overbuilt (IMHO due to the square-cube law*), but it does beat the alternative. If you plan for 600 pounds of thrust and a 110 pound rocket, but your rocket ends up weighting 160 pounds (like mine did), you're gonna need bigger parachutes (more weight) and more thrust (more strength to support it = even more weight). It really adds up.

SMR said:

It is also interesting to see how far OFF I was on my original weight estimate. All the little things that get added as you go, i.e. all-thread, doubled-up rings, glass, etc., really add up in the end. The true craftsman among us are much better at building light.

Click to expand...

*Basically, when you upscale, you run into structural limitations. If something is 10 times as tall, it is also 10 times as wide and 10 times as thick. Therefore, a thousand times as heavy. Yet the cross sectional area of the structure is only 100 times as big, so the structure is only one tenth as strong.

Tim,

I completely agree. Every flyer, regardless of scale, has the sober duty to assure their model is safe and flightworthy.

May I congradulate you for the excellent paper on fin flutter. It’s really well done. I also wish to thank you for engaging me in this subject. Flutter has been a big concern and I’m glad to have colleagues with perspective. Certainly I have some reckoning ahead of me especially where fin design meets weight target. Maybe SMR and Dan’s experiences will be closer to what's acheivable?

Feckless Counsel

Fleckless,

I, like Sather (SMR), want to give you the utmost encouragement to keep this going and don't worry if you are a bit off on your initial estimates--its part of the process and challenge. Looking forward to this build!!!

-Tim

Feckless Counsel said:

Tim,

I completely agree. Every flyer, regardless of scale, has the sober duty to assure their model is safe and flightworthy.

May I congradulate you for the excellent paper on fin flutter. It’s really well done. I also wish to thank you for engaging me in this subject. Flutter has been a big concern and I’m glad to have colleagues with perspective. Certainly I have some reckoning ahead of me especially where fin design meets weight target. Maybe SMR and Dan’s experiences will be closer to what's acheivable?

Feckless Counsel

Click to expand...

TRF,

Received my quarter measure of gorilla snot today. Thanks for the recommendation daveyfire and jsdemar. I was hoping to roll the first tube this weekend. Unfortunately I have an emergency business trip that will keep me away until the 18th of November. Until then gents.

Feckless Counsel

TRF,

Back to the issue of fin flutter?

I agree that DecoLite's performance is marginal in this application. FinSim reports the "do not exceed" velocity as 480 miles per-hour. RockSim reports a maximum velocity of 400 miles per-hour.

I would, however, like to point out discrepancies in FinSim's output. Please refer to the attached data.

Figure 1 - actual fin geometry.

Figure 2 - FinSim input data. Note that actual fin geometry is unsupported and fin is modeled as a trapezoid. Material is birch plywood as a near equivalent to DecoLite.

Figure 3 - summary data on divergence and flutter velocities. Note the dark blue text. Divergence is velocity of torsional excitation. Flutter is velocity of flapping excitation.

Figure 4 - torsion flexure unsteady flutter data as noted in dark blue text of figure 3.

The discrepancies to note are in figures 3 and 4:

1. Figure 3 reports divergence speed below flutter speed. Figure 4 reports divergence speed above flutter.

2. Figure 3 reports divergence speed as 480 miles per-hour where figure 4 reports the speed as 755 miles per-hour.

First make sure you have the fin geometries defined correctly. It looks like your fin has a sweep of ~10" to 12" and you have it set at zero? And your rocket will fly higher than sea level as set now. Also the CG on your fin shape is farther back than the 0.45 position you have as a default. If you set the EA to 0.35 and the CG to 0.6 along with upping your altitude to 10K your divergence velocity moves above your flutter velocity to get closer to that calculated in the U-g method.

Tim,

Thanks again for the reply. Few would venture.

I did not realize EA and CG parameters are user inputs. Instead I thought those were calculated values based on input screen from figure 2. Is it my fecklessness or is there a documentation here? Bottom line is I'm skeptical of this application, its results and my ability to apply it properly.

May I suggest we open a sidebar of PMs and report our consensus back to this thread?

Meanwhile I'll continue to show "progress" on my feckless attempt at tube fabrication.

Feckless Counsel

Feckless Counsel said:

Thanks again for the reply. Few would venture.

Click to expand...

'cause most of us don't understand the math...

:blush:

TRF,

Rolled the first airframe tube and I'm here to tell you that fiberglass on this scale isn't that much fun. Rolling and saturating the cloth took 2 hours and 3 batches of epoxy at 20-ounces each. Because I'm feckless I managed to accumulate a fair number of persistent bubbles.

Next tube will be better. Need 3 good ones. Have material for 4.

Feckless Counsel

Hello,
First let me congratulate you on what is shaping up to be a very exciting project. You've had the input of some great builders with equally impressive track records. With that said I'd like to reccomend you contact the BOD at MDRA for a couple reasons. I'm not sure if you have a launch site chosen yet but MDRA has a long record of supporting large and unique projects. They would be happy, I'm sure, to support the launch as we have a fantastic launch support system, as many here can attest. In addition there are a more than a few large builders in the club and on the board who are a valuable source of information. I look forward to following this build and wish you luck.

Troy,

Thanks for your encouragement. I have presented these plans to Bob and Neil with favorable feedback. I wouldn't want to fly this anywhere but Higgs' Farm.

Feckless Counsel
MDRA 64

Feckless Counsel said:

Troy,

Thanks for your encouragement. I have presented these plans to Bob and Neil with favorable feedback. I wouldn't want to fly this anywhere but Higgs' Farm.

Feckless Counsel
MDRA 64

Click to expand...

Sorry, I didn't realize this was you.:blush: Is this the project we were talking about at APL?

TRF,

The first tube pulled free from mandrel with a little coaxing. I used the polyester (Mylar) sheet as handles and pushed the mandrel from tube using both feet. Evening photo with black Corolla attached.

As humor to those more competent than myself may I offer some initial learning:

1. Big jobs like this are akin to finishing drywall. Continuous experience matters.

2. I dressed the entire mandrel assembly with epoxy and then started to lay glass. That became a giant cylinder of flypaper. Next tube I'll lay a starter STRIP of epoxy and dress the cloth dry at least quarter turn.

3. Style 7628 as tight weave cloth promises high glass to resin ratios. Just note a tendency to trap air. If you're an amateur plan extra time to work bubbles.

4. I used "chip" brushes to lay epoxy. Maybe that's not the right tool. Perhaps I should have cut those brushes stiffer? Foam roller? Serrated roller? Squeegee? Would appreciate any feedback on working bubbles.

5. Laminating resin is necessarily thin. Use multiple, small batches of epoxy when rolling large tubes. Time required for this tube was about 2 hours lay-up.

6. Pulling the mandrel about 15 hours after lay-up is beneficial. The mass is still flexible but holds shape.

7. I wrestled mandrel from tube on a concrete garage floor. My zeal pushed the assembly from its padding and I have some galls to both tube and mandrel. Epoxy was still semi-soft and airframe received a greater portion of galling.

8. Satan's own spit wouldn't stick to PTFE mold release and it's super easy to apply. Thanks to David at TDK Propulsion for the recommendation.

9. Low odor epoxy in quantity reeks nonetheless.

Troy,

Yes. We discussed this project at APL and I am very grateful for your competent counsel. I hope you enjoy the photo albeit a bit dark. Three more sections and some fins should make that worthy of an MDRA lunch?

Feckless Counsel

Feckless Counsel said:

TRF,

4. I used "chip" brushes to lay epoxy. Maybe that's not the right tool. Perhaps I should have cut those brushes stiffer? Foam roller? Serrated roller? Squeegee? Would appreciate any feedback on working bubbles.

Click to expand...

I use chip brushes also... cheap and disposable. Just to get the resin from the mixing container to where it needs to be. Then roll the glass with a specialty fiberglass roller prior to applying peel ply, to push the cloth down and let the resin fully penetrate it. Especially with the tighter weave, which as you noted, is more likely to capture air bubbles. These rollers have little ridges to hold the glass down while letting the resin soak through.

Tubes are looking fine. Rule #14 - everything costs twice as much and takes 3 times as long as you have budgeted for.

Second the rollers SMR pointed out. For wetting out you can't beat'm. One tip though, keep a gallon can of denatured alcohol handy as the roller will load up with excess epoxy quicker than you think. Swish it in the alcohol periodically to clear it and keep going. Clean it well when done or it will freeze up un you.

TRF,

Next tube I'll weigh the dry glass to get a better measure of ratios but:

Weight of the first tube is 14.8 pounds.
That's 8.8% heavier than my target weight of 13.6 pounds.
That's 17.3% less than Tim's thumb-rule estimate of 17.9 pounds.
Resin to glass ratio is estimated as 42:58.

Feckless Counsel

Feckless Counsel said:

TRF,

Next tube I'll weigh the dry glass to get a better measure of ratios but:

Weight of the first tube is 14.8 pounds.
That's 8.8% heavier than my target weight of 13.6 pounds.
That's 17.3% less than Tim's thumb-rule estimate of 17.9 pounds.
Resin to glass ratio is estimated as 42:58.

Feckless Counsel

Click to expand...

That is pretty good! How thick is the wall? Is it fairly consistent around the perimeter?

Tim,

Without trimming or sanding the wall thickness varies 0.070 to 0.080 inch at compass points about the circumference edge. I was expecting 0.055 to 0.065 inch. Better investigate that and thanks for another excellent question.

Feckless Counsel

TRF,

Second tube complete. Picture attached.

The second tube also has a few air bubbles and wrinkles. But it's much better than the first. Resin roller really helped. Two more tubes to roll and best 3 out of 4 fly.

Feckless Counsel

Man, That is one big nose cone, and rocket too!

I really like watching this thread.

SMR said:

I use chip brushes also... cheap and disposable. Just to get the resin from the mixing container to where it needs to be. Then roll the glass with a specialty fiberglass roller prior to applying peel ply, to push the cloth down and let the resin fully penetrate it. Especially with the tighter weave, which as you noted, is more likely to capture air bubbles. These rollers have little ridges to hold the glass down while letting the resin soak through.

Tubes are looking fine. Rule #14 - everything costs twice as much and takes 3 times as long as you have budgeted for.

Click to expand...

stantonjtroy said:

Second the rollers SMR pointed out. For wetting out you can't beat'm. One tip though, keep a gallon can of denatured alcohol handy as the roller will load up with excess epoxy quicker than you think. Swish it in the alcohol periodically to clear it and keep going. Clean it well when done or it will freeze up un you.

Click to expand...

Wow. Never knew about these things before.

Do they work with carbon fiber layups as well, or is it a "fiberglass only" thing? Most of my stuff is in the 3" diameter range using ~6 oz cloth, so how should they be sized (is a 4" wide x 1/2" diameter ok)?

Greg

TRF,

Seasons greetings and a happy holiday season to all. May I especially greet and thank Tim, Sather Dan, Jim, John, David, Mike, Greg, Troy and many other contributors to our project. Your wisdom, experience and precious time are humbly appreciated.

I'll have to pause construction for family and holiday travel. In that middling time I would appreciate any comment on:

> Light-weight fin design in the context of flutter.

> Coupling / separating concepts different than my proposal.

> Trimming airframe to perfect square.

Feckless indeed,

Geoffrey Howard
Columbia, Maryland
Traveling Man, MDRA, Tripoli, NAR

GregGleason said:

Wow. Never knew about these things before.

Do they work with carbon fiber layups as well, or is it a "fiberglass only" thing? Most of my stuff is in the 3" diameter range using ~6 oz cloth, so how should they be sized (is a 4" wide x 1/2" diameter ok)?

Greg

Click to expand...

They will work with Carbon as well though not as well as vacbagging. It mostly just works the bubbles out and excess resin. Beats the heck out of "stippeling".

stantonjtroy said:

They will work with Carbon as well though not as well as vacbagging. It mostly just works the bubbles out and excess resin. Beats the heck out of "stippeling".

Click to expand...

Thank you Troy for that info. I just bought one and hope to use it soon on a 3" project.

Greg