From Vat19:

Launch a 2L bottle over 100 feet into the air with AquaPod!

Combining a readily-available launch vehicle (a 2L bottle) with water and air pressure, AquaPod delivers amazing results easily and safely.

Simply partially fill a 2L bottle and slip it over the launch tube, add air pressure with any standard bike pump, and then pull the launch string to “ignite” your water rocket. Launch the same “rocket” over and over simply by adding more water and air.

AquaPod prevents over-pressurization with a built-in relief valve. Intended for ages 14+.

Edit: The product is currently sold out on Vat19, but you can get it from Amazon.com right here.

[AquaPod Water Bottle Rocket – Vat19 | Amazon.com]