Finally had chance to remix a model https://www.printables.com/model/131862-desktop-whomp-rocket I and my son had a lot of fun with!

The idea was if the water could be used to launch the rocket, helping with the hard-to-achieve sealing in the original model. I also redesigned everything to print in vase mode to get rid of the seam that does not do much good in terms of sealing nor friction.

After cleaning the burr on the bottom of the rocket, just slamming the piston with the rocket into the water somewhat works, but not much…. this was the original idea. I can imagine with sanding the piston and some practice the rocket can be launched with success…. but I am lazy…. so I added the actual piston body. Fill it with water and push the main piston into it. Even thought the tolerance there is generous, the density and viscosity of the water will make the path of pushing water into the main piston and in turn the trapped air into the rocket much more appealing. Adding few drops between the rocket and piston will help with building the pressure even more.

CAUTION- HIGH SPEED PROJECTILE: Do not shoot yourself in the face, anyone else in the face, or anywhere else, for that matter.

NOTE THE DIFFERENT EXTRUSION WIDTH FOR THE ROCKET to get the clearances right.

Notes from practical testing (put the toy and child in the bathtub, add water, stir gently):

• It is actually more toys than I expected :D 
  • the splash rocket
  • funnel, optionally using rocket as plug from the inside
  • rinsing cup
• both just splashing the funnel into water and using the piston works, good enough for the rocket to stay in the bathtub.
• top of the rocket could use a slicing modifier to make it thicker - 0.8mm to be more sturdy vs 0.5mm the rest of the rocket for weight.