Modular Rail-Mounted Water Rocket Fins

Overview

This modular fin-mounting system for water rockets clips fins in place with a simple compliant locking mechanism, allowing broken fins to be replaced or fin configurations to be changed quickly and easily in the field without tools. Modularity is achieved with standardised rail geometry and the use of a separate retainer for securing the mount against a bottle. The system is compatible with the fantastic Marimo Labs water rocket launcher.

Designs for 3D-printed fins based on the relatively low-drag NACA0010 airfoil are included, as well as blank rails with empty slots to attach custom fins cut out of cardboard/corflute. STEP files for all parts are included to facilitate tolerancing and to encourage customisation and adaptation to different bottles or material widths.

Parts

Mount: Comes in 3-fin and 4-fin versions. Mounts rails at 30° from the vertical.

Retainer: Holds mount in place. Currently included retainer has only been tested to fit 1.25L Coke bottles.

Printed Fins: 40 mm and 60 mm chord lengths available.

Blank Rails: Compatible 30 mm-long rails with slots of various widths. Turn shapes cut from cardboard/corflute/etc into custom fins by gluing them into these slots.

Extended Rails: Same as blank rails but with leading edge extension for structural rigidity.

Recommended Print Settings

Tested in PLA+ with 3 walls, 4 top/bottom and 10% cubic infill. Fins printed with 2 walls, 3 top/bottom and 10% cubic infill. PETG+ or other filaments may be better for durability outdoors. No supports should be necessary. Mount should be printed with the wide end on build plate. Retainer has built in support that can be removed.

Bottle Compatibility

Currently compatible with 1.25L Coke bottles. Modification of the retainer may be all that is required for compatibility with similar sized bottles. Please get into contact for more help.

Changelog

20/06/2026 - Added extended rails

14/06/2026 - Initial upload