This blog covers the day to day progress of water rocket development by the Air Command Water Rockets team. It is also a facility for people to provide feedback and ask questions.

Friday, April 13, 2007

Convergent Divergent Nozzles

In order to see if a water rocket can benefit from CD nozzles (de Laval nozzle) while using foam we have built a couple to hopefully test this weekend.

One is a 9mm nozzle (aluminium above) and the other a 7mm (black plastic). The divergent part is conical in shape and has a slope of 4 degrees from the centerline. This value was obtained by reviewing the video and seeing at what angle the spray emerges. The actual angle of the spray varies over time from quite wide to quite narrow near the end as the pressure drops.

The average angle was about 4.7 degrees during the middle of the burn. We chose a value of 4 degrees because our thinking was that if it was more than 4.7, there would be nothing touching the sides during the middle of the burn and hence ineffective. That is just a guess and not based on any scientific or mathematical calculations.

We just want to test to see if there is a noticeable effect when a CD nozzle is used with foam compared to a straight through nozzle. If there is a difference, further tests will be performed to see how the shape can be optimised.



Anonymous said...

Nice Work George. I made a 7mm Nozzle today but taking a hollow tube with the ID being 7mm and made it to fit unside the normal Gardena nozzle. Now i am going to epoxy it.

George Katz said...


Be careful with 7 mm nozzles as the amount of thrust they produce on take off is quite small particularly at lower pressures and the rocket can be much more unpredictable. But they are fun to watch.

We only use the 7mm nozzles for tests to make the burn longer and smaller in magnitude to make it easier to measure.