What you need
A 2 litre pop bottle
To be safe the pressure must not exceed 5bar (70psi) this is about the amount of pressure you can get from a cheap bike pump.
Below are instructions for our prototype, but it has some obvious limitations. Our car is not particularly aerodynamic and has wobbly wheels, how could you improve on this? Would a remote controlled starter be appropriate? How will you keep your car on course?
Lots of experimentation will help you achieve the right balance of air and water in your car. A large nozzle would maximise acceleration but be over too soon. Is this quicker than a smaller nozzle that gives you less power for longer?
What we did
1. On the bottom of a fizzy pop bottle there is a small round mark where the plastic is slightly thicker, we drilled a hole through this thicker plastic.
2. We then cut round the valve on an old bike tyre and using a knitting needle poked this through the bottle and screwed it into the hole in the pop bottle.
3. We made a hole in the cap of the bottle slightly smaller than our plastic tubing and threaded the plastic tubing through the cap.
4. We fitted the plastic connector to the plastic tubing on the bottle top, then taking another length of tubing and tied a really tight knot in it. We connected this to the other end of the plastic connector. (This knotted piece of tube acts like a cap over the plastic connector - removing this starts the rocket car).
5. Having finished the engine we made the vehicle using 4-inch plant pot bases for wheels, a knitting needle cut in half for the axles and the 2 plastic biro covers as axle covers. The main body of the car was made from a cardboard box.
The bottle goes off with a lot of force so make sure you are not in front of it!
If you use our bicycle valve design, make sure the valve is good and tight, and watch out for it popping out of the bottle. (We've never seen this happen but it would be nasty).
Don't over pressurise the bottle. If the base indentations pop out - you have gone too far. Stop pumping immediately!