This Bottle Rocket base is designed to fit a PET bottle to use as a pressure vessel, with a cork and needle valve adaptor inserted in the end (see photos). Print 3 fins and slide into the centre. Some light sanding may be required to ensure the fit. Push the needle valve into a cork. Screw in bottle to centre, then 1/4 bottle with water. Insert cork attachment from underneath. Use a bicycle tire pump and pump until it launches. 

SAFETY WARNING : Please use the rocket under adult supervision. Do not have any part of your body standing over the rocket. Do not aim the rocket at anybody. 

This object was made in Tinkercad. https://www.tinkercad.com/things/kVSimfJ620z