Here is a fun kit that lets you build a metal water rocket launcher. The StratoLauncher IV Water Rocket Launcher has aluminum, plastic, and brass parts. You can use a 2gal tank for up to 30 launches. The StratoLauncher launches bottles over 350ft in the sky.
You can upgrade the kit to do more with it. Its 125 PSI safety valve prevents exploding water bottles.
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