Fin Construction¶

Fin Specifications¶

Property	Value
Quantity	3
Material	Birch plywood
Profile	Swept-back curved
Attachment	Through-the-wall (TTW)
Extends Below Tube	Yes

Unique Fin Design¶

The Peregrine's fins are distinctive:

Curved leading edge (reminiscent of 1950s sci-fi rockets)
Extend below the body tube base
Interlocking tab structure for alignment
Large surface area for stability

6-Point Attachment Method¶

The fins use Through-The-Wall (TTW) construction with 6-point epoxy attachment for maximum strength:

                    AIRFRAME (body tube)
                          │
    ┌─────────────────────┼─────────────────────┐
    │                     │                     │
    │  ④ External     ┌───┴───┐    ④ External  │
    │     Fillet      │       │       Fillet    │
    │                 │  FIN  │                 │
    │  ③ Internal     │       │    ③ Internal  │
    │     Fillet      └───┬───┘       Fillet    │
    │                     │                     │
    └─────────────────────┼─────────────────────┘
                          │
    ┌─────────────────────┼─────────────────────┐
    │                     │                     │
    │  ② Outside      ┌───┴───┐    ② Outside   │
    │     Fillet      │       │       Fillet    │
    │                 │  FIN  │                 │
    │  ① Inside       │ ROOT  │    ① Inside    │
    │     Fillet      └───┬───┘       Fillet    │
    │                     │                     │
    └─────────────────────┴─────────────────────┘
                    MOTOR MOUNT TUBE

The 6 Attachment Points¶

Point	Location	Purpose
①	Fin root to motor mount (inside)	Primary structural bond
②	Fin root to motor mount (outside)	Reinforces motor mount bond
③	Fin root to airframe (internal fillet)	Bonds fin to body tube interior
④	Fin root to airframe (external fillet)	Visible fillet, aerodynamic
⑤	Centering ring to airframe	Locks motor mount in place
⑥	Centering ring to motor tube	Completes the rigid structure

This creates an extremely strong integrated structure where the fins, motor mount, and airframe become a single rigid unit.

Construction Steps¶

1. Preparation¶

Sand fin edges lightly
Test fit in body tube slots
Ensure interlocking structure aligns properly

2. Tacking¶

Use thick CA to tack fins in position
Verify alignment with fin guide or laser level
Check all fins are parallel to centerline

3. Internal Fillets (Points ①, ③)¶

Apply epoxy inside motor mount area
Use the Kevlar pull-out technique for rear ring access
Ensure contact with motor tube and centering rings
Cure 24 hours before proceeding

4. External Fillets (Points ②, ④)¶

Mix epoxy (1-hour working time used)
Apply fillet material to external joints
Use popsicle stick to form smooth concave shape
Cure 24 hours

Build Notes¶

Epoxy Application¶

Liquid Epoxy Challenge

1-hour epoxy was quite liquid and leaked through small gaps in centering rings. Each side required separate 24-hour cure cycles:

Position rocket so epoxy pools at joint, not leaks away
Flip and repeat for opposite side
Total assembly time: ~4 days

Result¶

The 6-point TTW attachment creates a very rigid fin can assembly. Fins cannot flex or rotate relative to motor mount or airframe.

Fin Flutter Considerations¶

At expected velocities (under Mach 0.5), fin flutter is not a concern for:

Birch plywood construction
Proper fillet application
Normal H/I motor flights

Build Photos¶

Photos to be added - see Issue #8