Water rockets are great fun and if care is taken can provide lots of fun for all ages. There is a good introduction to them on Wikipedia.

The most difficult part of designing a water rocket is the launch mechanism. You must make a seal to the rocket while you are charging it with air and then break that seal in a controlled way so that the rocket flies into the sky. The mechanisms to achieve this are often complex and expensive.

My design is very simple. I spotted that a length of semi-rigid nylon pneumatic hose will fit nicely into the rubber hole at the bottom of a Schrader Tubeless Tire valve. There are two elements

• a cap with a length of nylon hose sticking out through which we will pump air
• a charging hose with a Schrader valve both ends. One end it is actually used as valve, you connect the pump to this one. The other end is simply used to seal to the tube from the cap.

You simply stick the nylon hose into the rubber hole of Schrader valve, and pump air into the bottle until the pressure is so great that it jumps out of the valve. I hung the rocket on a vertical stake to ensure that it went up.

 

Parts

• 1 x 2 litre fizzy drink bottle, with cap
• 2 x Schrader Tubeless Tire valves
• 2 or 3 m of semi-rigid nylon pneumatic tubing 8 mm diameter with 1 mm wall, DIN73378

Tools

• Craft knife
• Hot glue gun

To make the cap

1. 

   Note the simple X cut in the cap

   Take 5 cm of nylon tube and the cap.

2. Using the craft knife cut a cross in the cap and push through 2 cm of nylon tubing from the outside,
3. Bond tubing to the outside of the cap with lots of hot glue

Prepare one of the valves

One of the valves is not used a valve. It is only used as seal to the rocket. Take the core out of this valve by unscrewing it with a jeweller’s screwdriver.

Fit the prepared valve to one end of the hose

Fit the other valve to the other end of the hose

I welcome your comments on this design and any suggestions for improvements.