Dropdown Nozzles for Water Rockets

An attempt to solve a niche water rocketry problem. Plan is to revise and expand this during June-September 2026.

What is it?

Motivation

In water rocketry, sometimes you want to narrow the opening where water comes out to lengthen the thrust phase. Ideally, you would still have a full-bore opening to allow for the piston-effect boost of launch tubes.

Description

This is a removable dropdown nozzle (aka T-nozzle) that fits with full-bore (21.74mm) soda/seltzer bottles in common worldwide production. It stays up inside the bottle and drops down once the rocket leaves the launcher. It is supposed to stay in the bottle. See the caveats below.

How to build it?

Additional Materials needed

• 15 x 22 x 3.5mm nitrile O-ring (same that is used for the Cores on the Marimo Labs Water Rocket Launcher. One for each nozzle in use, but you can re-use the O-ring when you remove it.

• Optional: silicone grease lubricant

Assembly (Video TBD and better pictures when design is verified)

• Choose a DROPDOWN-NOZZLE.

• They are marked externally with their internal diameter.

• Slide on one O-ring. 

• Optional: apply grease to O-ring.

• Insert DROPDOWN-NOZZLE and O-ring into the neck of the bottle.

• Slide on the LOCK-SLEEVE to push the O-ring in place.

• Use the GRIP and WRENCH to twist the LOCK-SLEEVE. This compresses the O-ring, widening it, and locking the nozzle in place.

Loading a Rocket

This can be finicky. Add water to the rocket. Then use the tip of the straw or launch tube spear the back of the nozzle. Note that the 10 and 12mm openings can fit around the straw, but smaller ones will not. The DROPDOWN-NOZZLE should fit inside the Launch Tube of Core C. It will be finicky to get on, but in theory it should work fine even if the DROPDOWN-NOZZLE is next to (not on top of) the Launch Tube.

Disassembly

• Use the GRIP and WRENCH to loosen the LOCK-SLEEVE.

• Remove the LOCK-SLEEVE

• Wiggle the DROPDOWN-NOZZLE side-to-side as you pull the nozzle out

Caveats

• This design is in very early alpha, which means it only recently stopped ejecting from the back of the bottle. It may not work. If you try it, whether it works or not, PLEASE DO post your experience with the version number, opening size, and pressure levels of success and failure. (Maybe don't rate it if it doesn't work ;-) ). This process of crowdsourced iterative testing is what made the Launcher design reliable.

• You DO need to print the DROPDOWN-NOZZLE and LOCK-SLEEVE with 95% infill so that it SINKS in water. The nozzle won't work if it floats.

Experiment

• Try different nozzle sizes with your rocket design.

• Try with a Launch Tube Core (Core C).

Open Research Questions

• What pressure can this nozzle design tolerate?

• What additional geometry tweaks will help retention?

• What is the optimal water path geometry?

• How can the process of putting the Core be made easier?

• Is the nozzle alignment on-axis?

Print Settings

DROPDOWN-NOZZLE and SLEEVE

• Layer height 0.2mm

• 4 walls

• Infill 95% rectilinear (at high density, fancy patterns just add complexity)

• Tested with Arachne slicing; Classic should be fine too

GRIP and WRENCH

Less sensitive.

• Layer height 0.2mm

• 3 walls

• 15% infill, any