Question on "Hollow" Multilayer Fins

[CoyoteNumber2]

I don't know what the proper term is, but I have a question about fins that have a core layer with material cut out for weight reduction, like in this picture borrowed from Launch Lab:



I would like to make some fins like this with G10 skins, but I'm not sure how the hollow sections will affect fin strength and resistance to flutter. I'm aware of the Rule of Mixtures for composite / multilayer materials, but I don't know if it applies to this type of fin construction.

I'm wondering if the core layer can simply be ignored for estimating or calculating strength and rigidity. For example, if the above fin is 1/4" plywood and laminated with two 1/32" G10 skins, is the finished fin at least as strong as the two 1/32" skins by themselves? Or even a single-piece 1/16" G10 fin?

I have no doubt this can all be figured mathematically, but I'm wondering if there's a simple, practical approach to it.

C#2

 

[JohnCoker]

I doubt you will have issues if you skin with CF. After all, you're going to create a truss structure which will be a lot stiffer than normal slab fins. I've done this sort of thing a lot, including fins with just foam cores:
http://jcrocket.com/comanche-3.shtml#fins

 

[OverTheTop]

This is a really good way of getting the strength into the assembly and keeping the mass down by putting the solid skins further from the neutral axis of the part (where if you bend it it is neither in compression or tension). The further the skins are from the neutral axis the more stiff the assembly is. It is a cubic-law so doubling the thickness gets eight times the stiffness. It really gets the second moment of area working for you:
https://en.wikipedia.org/wiki/Second_moment_of_area
Depending on where the internal ribs are can affect the strength differently in different directions, creating an anisotropic composite part. Consider using an isogrid structure which presents relatively isotropic behaviour. Here is one I prepared earlier:
https://forum.ausrocketry.com/viewtopic.php?f=6&t=5019&start=42

Very happy with how it worked out. Could even do similar with a plywood core.
https://www.google.com/search?q=isogridhttps://en.wikipedia.org/wiki/Isogrid

I have no doubt this can all be figured mathematically, but I'm wondering if there's a simple, practical approach to it.

Sounds reasonable to ignore the core, calculating with the skins spaced apart, and probably not too far from the correct result.

 

[CoyoteNumber2]

@JohnCoker, @OverTheTop,

Thank you gentlemen for your replies. I like the isogrid approach; I may give that a try. And I will look into carbon fiber skins instead of G10 as well.

 

[deandome]

anyone remember Hexcel skis?
https://www.amazon.com/Aluminum-Honeycomb-Grid-Core-Cell/dp/B00WIUBELY
That's just one sample, you'll find other specs/thicknesses...and other mfgs https://pdf.nauticexpo.com/pdf/hexcel-composites/honeycomb/20367-5902.html

And tho they stopped making skis a long time ago, looks like Hexcel is still in business & make a lot of composite materials you might find interesting. They are/were an aerospace composite firm where the owner or someone figured their honeycomb might be good for skis. They were good, but didn't last super long. https://www.nauticexpo.com/prod/hexcel-composites/product-20367-467787.html

 

[OzHybrid]

@JohnCoker, @OverTheTop,

Thank you gentlemen for your replies. I like the isogrid approach; I may give that a try. And I will look into carbon fiber skins instead of G10 as well.

The main issue I have with CF is that it's a soft fibre. It's really strong when it's all bonded together but from what I've seen of Mach damage, it turns into a shaggy dog really quickly. Has anyone done comparative tests of CF and G10 fins and the damage each sustains when they get Mach burn. CF is thermally more conductive than GF and would seem to transfer the heat to the bonding resin quicker, destroying the bonding resin and destroying the strength at that point. Rinse and repeat more quickly after the first damage area. Protection of the leading edge would seem to be key for survival.
My 2c. YMMV

 

[OverTheTop]

The fins I discussed above had a nominal flight peaking at M2.14. The beveled leading edges were coated with a layer of JB Weld for thermal protection, since the edge of the lamination is exposed to the stagnation temperature along the length of the bevel. The flight was entirely nominal (from telemetry) and if I ever find the Apache sustainer I will let you know how the fins stood up during the flight.

Given the sweep angle of the leading edge it will be outside the Mach cone and suffering some heating in supersonic airflows normal to that edge. Not the full M2.14 (in a direction normal to the LE) due to geometry and resolving the airflow into two orthogonal components, normal and parallel to the edge.
https://www.grc.nasa.gov/www/k-12/airplane/machang.html
FYI if you can keep the sweep sufficient to stay within the Mach cone then the flow normal to the edge only sees subsonic air .

 

[OzHybrid]

The fins I discussed above had a nominal flight peaking at M2.14. The beveled leading edges were coated with a layer of JB Weld for thermal protection, since the edge of the lamination is exposed to the stagnation temperature along the length of the bevel. The flight was entirely nominal (from telemetry) and if I ever find the Apache sustainer I will let you know how the fins stood up during the flight.

Given the sweep angle of the leading edge it will be outside the Mach cone and suffering some heating in supersonic airflows normal to that edge. Not the full M2.14 (in a direction normal to the LE) due to geometry and resolving the airflow into two orthogonal components, normal and parallel to the edge.
https://www.grc.nasa.gov/www/k-12/airplane/machang.html
FYI if you can keep the sweep sufficient to stay within the Mach cone then the flow normal to the edge only sees subsonic air .

How many people could have benefited from that gem of info over the years...............