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Timmi's Water Rocket Car - 2.11.2003

This autumn we had two of our nephews from Ludwigsburg as our guests. Naturally, some pressure-driven experiments were on the schedule.

Original toy car

This is the original toy truck with the destiny to serve as a carrier for water/air jet propulsion.

Tim with our WaterRocket Car

Here is 8 year old Tim, proudly presenting our rocket vehicle. It is the chassis of  the truck above, enhanced with a 1 liter coke bottle, cable-tied to the back. We asked the Pink Panther to be test pilot.

Gardena nozzle with reduction

The car was operated with a Gardena type launcher. A regular bottle cap was glued into the grey Gardena bit. The plain Gardena nozzle (8 mm, right) was reduced to 3.5 mm with a wooden reduction (left), made on my Unimat lathe (description here).

Preparing the start ...

The technical crew, building up pressure. We used only low pressure, being so close to the vehicle. The launch was triggered with a string, off-axis to the exhaust jet.

The biggest problem was to keep the car driving on a straight line. The wheels are made from some hard plastic. The car was always skidding all over the pavement, mostly turning over several times, driven by the vast water jet force.

The first remedy was taping stones into the car front (see picture). Did not help much, except making the car heavy and slow.

WR car: wheeles with friction rubbers

Our second remedy was a rubber "coating" for the wheels: an old bicycle tube was cut into 2 cm wide strips. They covered the wheel surface well (partly removed for the photo). This increased the surface friction by much.

WR car: guide string

The rubber coated wheels were not sufficient either. So we drilled holes into the bumper bars. Then we threaded a string through the two holes (back hole shown here) and tightened it close to the ground.
The visible filings around the hole and on the trailer pin developed after some 5 starts.

Enlarged launch crew

The complete launch crew:
niece Clara (on  unicycle), nephew Tim (holding WR car), wife Susanne (holding guide string), husband Ulrich (holding wife).

Animated GIF of WR car

This animated gif shows one of the launches. Tim enjoyed pulling the start rope. Clearly visible are the wild direction changes right after start, in spite of the tightened guide string.

Future correctives, which remain as homework until the next session:

  1. Fins will not help much for initial stability, since the speed of the car is still low during this phase.
  2. Better is a launch tube or a similar type of initial guidance.
  3. Two slats, one on each side of the car during say the first 2-3 meters.
  4. Mount the bottle slightly upward on the chassis to increase surface friction during the period of highest acceleration.

We had a very instructive and amusing afternoon on the autumnal schoolyard, nearby our house.



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Stand / Last Revision:  22.11.03

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