REMOVABLE FIN INTERFACE — DEVELOPMENT VERSION B

Reusable idea: a rigid captured T-shoe, two separated bearing regions, a blind
gas-side cassette, and an independently removable aft keeper. No fin glue,
printed threads or repeatedly flexed retention hooks are used in this version.

Defined shoe section:
  Neck3.0 mm thick; head7.2 mm wide x1.8 mm radial depth.
  Standard trial clearance0.25 mm per mating face; coupons0.15/0.25/0.35.
  Two25 mm bearing regions; slightly relieved intervening faces.
  Current length134.5 mm; forward stop and aft keeper faces are explicit.

For the45 mm body, head radial position is18.6..20.4 mm. Relative to the nominal
body skin these are3.9..2.1 mm inward. The host cassette, skin junction and ring
notches must adapt to the host body radius, wall, motor tube and available gap.
The same shoe profile does not make an arbitrary body or fin automatically
compatible. Root length, stop placement and retention face must match too.

Keep in the shared interface:
- Head/neck section and bearing-face conventions.
- A named per-face clearance, selected from prints in a known process.
- Entry direction, lead-in, shoe length, stop and retention datums.
- Dedicated permanent gas-side walls and serviceable mechanical retention.

Keep specific to each rocket:
- Curved skin connection, body diameter and local wall reinforcement.
- Motor-ring notches, hook/rod/cord paths and keeper screw-boss layout.
- Fin planform, mass, balance, aerodynamic/structural loads and recovery system.
- Material, nozzle, process, measured fit, repeated-use behavior and load tests.

This is the first fit prototype of the interface, not a frozen or universally
interchangeable standard. Validate short and long fits, locked tip movement,
repeated removal, keeper/insert retention and gas sealing before promoting it
to a common family part. Retain the successful process and physical measurements
with the interface revision, rather than relying only on nominal CAD clearance.

Editable implementation: design/removable_mount_b.py; host integration:
design/build_b.py and fins_b.py. ROCKET_SPEC.md is the controlling host spec.
