STRUCTURAL ANALYSIS

Fin Attachment Strength

Mounting types, adhesive selection, and fillet sizing for secure fin attachment.

Failure Modes

Fin joint failure is distinct from flutter:

Failure Type	Cause	Depends On
Flutter	Aeroelastic instability	Fin geometry, material stiffness
Joint failure	Bond shear from aero load	Mounting type, adhesive, fillet

A fin can flutter off (material fails) OR be ripped off (joint fails) — whichever limit is lower determines the maximum safe velocity.

Mounting Types

Type	Strength	Best For
Surface Mount	1× (baseline)	Low-power, slow rockets
Through-the-Wall (TTW)	2-4×	Mid-power and up
TTW to Motor Mount	Maximum	High-power, supersonic

Surface Mount:      TTW to Motor Mount:
    Fin                    Fin
     |                      |
═════╪═════            ═══╪═╪═══  Body
                          │ │
                       ───┼─┼───  Motor mount

Adhesive Selection

Adhesive	Shear Strength	Notes
Wood glue (Titebond)	3-5 MPa	Porous materials only
CA (cyanoacrylate)	10-15 MPa	Brittle, poor gap filling
5-minute epoxy	8-12 MPa	Convenient, moderate strength
30-minute epoxy	15-25 MPa	Better strength, gap filling
West System 105/205	20-30 MPa	Excellent, standard for HPR
Rocketpoxy	25-35 MPa	Optimized for rocketry

Fillet Sizing

Fillets increase bond area and reduce stress concentration:

Recommended Radius

r_{fillet} = 2-3 \times t_{fin}

Tip-to-Tip Reinforcement

Laminating fins with fiberglass or carbon fiber tip-to-tip increases both joint strength and fin stiffness (raises flutter velocity).

SOURCES

References

NASA & Technical Reports

NACA TN-4197: Summary of Flutter Experiences — (1958)
Includes fin attachment load considerations

Community Resources

Fin Flutter Analysis Revisited — Apogee Components
Peak of Flight #615 - Updated flutter and joint analysis
Internal Fillets on TTW Fins
Rocketry Forum discussion on optimal fillet techniques
Tip-to-tip carbon fibre reinforcement — Cambridge University Spaceflight
Advanced fin reinforcement techniques

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