This is our take on the classic "Tornado Tube", used for joining soda bottles together to create huge water rockets.
A short video about this design can be viewed here: https://youtu.be/j6wLRHK3ATE
The "tornado tube" name comes from a science toy used to connect bottles together to create a vortex when water moves from one bottle to another. Water rocket builders quickly seized upon the science toy and repurposed it as a water rocket coupler. However, the science toy version is notorious for leaking pressure and also known for choking the flow inside the rocket.
Our design uses an external o-ring seal on each end, which we discovered when designing our Gardena water rocket nozzle (you can find it among our other things). This creates an unrestricted flow between bottles for optimum performance.
To use this design, you will need a #213 O-ring for each end, or you can optionally use a flat hose washer between the bottles, but this does not seal as well.
Caution: You use this part at your own risk. We cannot guarantee the safety of this part due to the differences between printers and filaments. Always safety test parts like this and follow proper safety measures when dealing with Water Rockets. Use with adult supervision.
I've printed in ABS, PETG, and PLA. They all come out great. If you threads are tight and hard to get on the bottle, try scaling your print larger by a tiny amount. You could also be overextruding and need to calibrate the extruder.
The overhangs are all created to require no support, so you should only need minimal cleanup.