NORTH POLE EXPRESS — REVISION B
Internal, removable fin mounts / PETG and 0.6 mm nozzle prototype

START WITH THE SMALL FIT TEST
FIT_TESTS/01_SHORT_CLEARANCE_B.3mf
Three female clearances: 0.15, 0.25 and 0.35 mm PER MATING FACE, plus one male.
About5 g of model plastic for the whole set. Use one PETG color. Follow the
object names/left-to-right order and mark the coupons before removing them.
Pick a freely sliding fit with minimal visible rocking. Do not force a jam.

NEXT TESTS
FIT_TESTS/02_FULL_LENGTH_TRACK_FIN_B.3mf checks full-length sliding, bowing,
and the real closed front cap. The supplied female uses0.25 mm per face.
FIT_TESTS/03_TAIL_KEEPER_FUNCTION_B.3mf checks the short tail, keeper, thumb
screws, insert fit, retention and tip movement with economical dummy levers.
The long coupon is not locked; the short keeper coupon is not full length.
Also assess the real full-length fin locked in the actual full-length body.
If a different clearance wins, regenerate that interface consistently; do
not scale the fin or tube. This prototype is not a zero-play or flight claim.

FULL FILES
North_Pole_Express_24_ALL_PARTS_B.3mf —14 separate physical parts on one plate.
REFERENCE_ONLY/North_Pole_Express_24_ASSEMBLED_B.3mf —same parts in assembly.
DISPLAY_ONLY/North_Pole_Express_DISPLAY_FILLERS_B.3mf —four optional narrow
fillers for covering the slots when displaying the rocket without its fins.
Display fillers are not flight fins and are excluded from the flying mass.

WHAT CHANGED
- The old outside strips are gone. The visible fin root begins at22.55 mm
  radius, close to the nominal22.5 mm tube skin. The T-head is inside.
- Four permanent, blind tracks have continuous inner and side walls plus
  closed forward ends. Fin clearance is outside the intended gas boundary.
- Both centering rings are notched around those fixed tracks. Their permanent
  adhesive interfaces still need sealing/fit verification.
- A nonrotating aft keeper blocks the four shoes from sliding backward.
  Remove its two finger screws and the keeper, then slide fins straight aft.
- The forward recovery-leader hole moved to225 degrees between tracks.
- A local, full-length backing rib increases the coupler key's thin web to at
  least1.2 mm for the0.6 mm nozzle. Check recovery packing past that local rib.

HOW THE LOCK WORKS
1. Keep the fin shoes and their tracks free of adhesive.
2. Slide each fin FORWARD from the tail into its internal track.
3. Fit the keeper straight onto the aft end; it does not twist.
4. Secure the two M3 thumb screws. The keeper provides positive axial retention;
   the captured T-heads provide the radial retention and bearing surfaces.
5. To remove fins, undo both screws, withdraw the keeper, and slide fins aft.
   The permanently bonded motor mount remains in place.

The present geometry has0.5 mm nominal axial running clearance. Thumb screws
retain the keeper; they do not prove that radial/lateral play is eliminated.
Evaluate the supplied fit coupons, actual locked tip motion and repeated use
before treating this interface as accepted or copying it into another rocket.

HARDWARE FOR THE FIN KEEPER
- Two M3x6 mm under-head thumb screws, with no washers in the nominal stack.
- Two M3 heat-set inserts, assumed4.6 mm OD x4 mm long.
- Printed pilots are4.2 mm diameter and5 mm deep, with a solid roof above.
- The2.4 mm keeper leaves3.6 mm screw projection; M3x8 can bottom out.
- Head envelope: <=8 mm diameter, <=4 mm height. Larger heads need a check.
Verify actual insert dimensions, installation instructions, screw engagement,
pullout resistance and clearance before use. Avoid distorting the thin tube
or adjacent tracks while installing inserts. Do not send the proxy QA G-code
to a printer; select your own machine/material settings in Bambu Studio.

OTHER PURCHASED PARTS
Commercial24 mm motor (E12 is provisional, delay TBD);120 mm paper motor tube
assumed24.5 mm ID /26 mm OD; commercial E-length motor hook and thrust ring;
parachute, cord, heat protection, adhesives and any secured balance hardware.
Centering rings have26.4 mm holes; match them to the actual purchased tube.
Motor tube: assumed z=-8..112; rings z=8..11 and105..108.
Hook is at45 degrees. Guides and forward leader are at225 degrees. Keeper
screws are at135/315 degrees. These angles are viewed in assembly coordinates.

BODY / NOSE ASSEMBLY
Body sections are joined permanently with the keyed40 mm internal coupler.
Install and inspect motor mount, leader and insert hardware before closing
the body. The nose's25 mm shoulder remains removable for recovery. Its separate
anchor has two tether holes and a central M4 ballast-bolt provision. Select and
mechanically secure any balance hardware during dry assembly BEFORE bonding
the anchor; access after bonding is not guaranteed.

DISPLAY FILLERS
DISPLAY-01/02/03/04 occupy tracks at0/90/180/270 degrees respectively. The
nameplate front is270 degrees. Their narrow curved faces continue the red/ivory
wrap. Use the same keeper to retain them. They let the fins be stored separately;
the rocket is only a display model in this configuration.

PRINTING / VALIDATION
Owner process: PETG,0.6 mm nozzle. Exact filament brand and profile remain open.
Supplied orientations: body/tracks upright; fins on their internal shoe roots;
keeper/rings/anchor flat. Inspect thin features, closed-track cap bridges, fit,
brims and supports. Keep supports out of the sliding cavities. Import colors
are presentation metadata, not configured AMS slots; the two greens can map
to one green filament.

Main B passed digital closed-solid, nominal-interface and continuous insertion
checks. A local H2S0.6 / stock PETG Basic /0.18 mm monochrome diagnostic slice
also completed without slicer warnings. This does not qualify the owner's
actual multi-color process, physical fit, load capacity, sealing or flight.

Printed solid-volume estimate:210.8 g at assumed1.25 g/cm3, about18 g above A.
Purchased parts and balance hardware are extra. The new aft mass changes the
balance assessment. Review B's mass/CG and motor analysis; A's20–30 g ballast
illustration is obsolete for B. No fixed ballast mass is prescribed. E12,
delay, actual loaded CG, launch guidance, strength/heat and recovery still
require validation. Current geometry is a reusable-interface PROTOTYPE.

The printed assembly is612.4 mm tall including keeper (about24.1 inches),
before screw heads and motor projections. Fin span remains145 mm.
Original A files are preserved. Source and independent reviews are in the
project's design/, qa/ and simulation/ folders. See also
REMOVABLE_FIN_INTERFACE.txt for what can be shared across future rockets.
