SpudFiles

Very easy to make. You need a 500ml coke bottle, this particular drinking cap and Schrader valve attached to the bottom.

Fill the half of it with water and pressurize it to 5-6 bars. While you are pressurizing it, use your other hand to grip the cap since it cannot hold the pressure itself.

To launch it, just throw it in the air or release it like I did. Be careful not to get wet like me

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Neat! I never though of using one of those caps like that before.

Needs flammable gases though.

Hahaha nice work! The flammable gases suggestion is a good one.

Coincidentally, I've been experimenting with Coca Cola bottles too. I took a regular bottle cap, drilled a 5.5 mm hole in there, and threaded a presta valve with some teflon in it. Screwed the cap on a 1.5L bottle, pumped it up to 11(!) bar, and shot it with my air gun from some distance.

It so loud that my neighbours thought I'd blown up myself. Really recommend it. (not really)
At least it's fun to shoot at!

Regards

I've used a quick disconnect fitting in a similar way..
The narrow outlet seemed to benefit the rocket effect.