Bicycle valve adapter to garden hose for water rockets

This adapter takes all metal valve stems with an M8 thread (Dunlop and Schrader). Many bicycle tires have those - you might be able to just cut one out of an old tire.

The valve stem should be screwed in from the top (the side opposite the flange). If the valve stem has a nut I recommend keeping it on there, just on top of the remaining rubber from the tire - in the unlikely event of the plastic threads failing that'd prevent the valve stem from shooting out.

Screw the valve stem in until there's about 5mm of thread left between the rubber (or nut) and the adapter. Smear super glue all over the remaining thread, and then finish screwing it in. Optionally add some superglue on the top as well - it'll partially seep in. Let it stand upright for a few hours for the glue to settle and harden. The super glue is required to get the part air tight.

Once the glue has set insert it fully into a garden hose (something like Gardena Classic, 13mm or ½"), and fix it with a hose clamp.

Pressure limits are 6 bar with a Dunlop valve, or 10 bar with a Schrader valve.