Carbon reinforced fins question

For balsa fins reinforced with carbon, is one layer of 3K fabric enough for high-power, or would you recommend more layers? Here are the specifications for the fabric:

50" 3K Carbon Plain Weave

Hexcel AS4 (12.6x12.6 weaves/in)

Thickness Weight Width Tracers Fiber

0.009" 0.23mm 5.9oz/200gsm 50in, 127cm None AS4 640Ksi 33.5Msi

ManOntheMoon said:

For balsa fins reinforced with carbon, is one layer of 3K fabric enough for high-power, or would you recommend more layers? Here are the specifications for the fabric:

50" 3K Carbon Plain Weave

Hexcel AS4 (12.6x12.6 weaves/in)

Thickness Weight Width Tracers Fiber

0.009" 0.23mm 5.9oz/200gsm 50in, 127cm None AS4 640Ksi 33.5Msi

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It depends on the flight profile, fin span, fin thickness, end grain or straight grain balsa, lots of variables. Not enough information.

Exactly- more info is needed.

I can say I did a rocket last year that did this. It was a 54mm minimum diameter, phenolic airframe, designed specifically for the Aerotech i65.

The fins were 1/16” balsa core with a single layer of 5.9 ounce carbon fiber. The fins held up fine.


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I am building a 4" diameter rocket with the following max data vaules from RockSim simulation details on a K660 motor:

Max data values:
Maximum acceleration:Vertical (y): 563.110 Ft./s/sHorizontal (x): 15.838 Ft./s/sMagnitude: 563.110 Ft./s/s
Maximum velocity:Vertical (y): 1038.4318 ft/s, Horizontal (x): 14.8001 ft/s, Magnitude: 1042.9823 ft/s
Maximum range from launch site: 868.33990 Ft.
Maximum altitude: 7841.53544 Ft.

The fin span is 4" and the thickness is 1/4", B-grain balsa.

End grain.

I will wait for the real pro's to weigh in on this, but my gut tells me the mode of failure will be peeling of the carbon away from the b-grain balsa, end grain balsa would be much better (balsa has little peel resistance but is strong in tension).

My Big Daddy's are a single layer of carbon over the stock balsa fins. The 54mm version was somewhat heavier carbon as I recall - maybe something like 9 oz. It has flown many times on J motors to a simulated 9K (never flew an altimeter). Obviously, the fins aren't very large.

Jim

I'm afraid the balsa wood option introduces too many risks for me. I'm rethinking my build strategy and switching to a carbon reinforced birch plywood option.

Have you considered basswood?

What Mach number? What fin span, root length, tip length, fin thickness, fin material and sweep length? And altitude of highest Mach? And tube outer diameter? These are min questions here for a Aero Fin Sim by Dr.Cipolla . It'll tell you what Mach the fins flutter to death at harmonic vibration wise as long as you got the material properties and fin data right. Fin span, material, profile ,and thickness affect fin flutter the most. Every fin has a Mach limit. Naca4197TN is a manual calc you can do with larger error.

If you got a Rocsim file or Open Rocket file that might help other questions. AeroFinSim has limits on solid materials flat plate. I never got a straight answer on some airfoiled USRC SEDS fins last year. Had to flutter tips and root on taper but it assumes flat plate single material. Your looking into more mechanics material theory crap on multi materials and advanced mechanics on composites (that parts beyond me)... If you got some flutter sims going you get a ballpark of thickness and material to run.