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070309-002 - Jet Foaming tests
Date 9th March 2007      1:00pm - 3:00pm
Tested by GK and PK
Location Sydney, Australia
Test Procedure Static Fire Test
References N/A
Related 070309-001 (Performed same day)
070309-003 (Performed same day)


  • To investigate what happens when a foaming agent is added to the water.
  • To investigate how foam could be generated in flight.


Foam inside the rocket is ejected to provide a different thrust profile compared to just plain water and air. The foam provides a different density medium that acts as the reactive mass.

We developed the following technique to generate high density foam inside the rocket during launch and throughout the thrust phase. We call this technique Jet Foaming

A foaming agent is added to the water, and the water is arranged in the bottles such that the lowest bottle has a majority of the water but also has an air-gap at the top. (see diagram below) The upper bottle contains some water also. This water is held above the air gap due to the closed cavity in the rest of the rocket. Upon launch water is forced from the upper bottle into the lower water bottle at great pressure and this generates foam very quickly in the lower bottle. The foam generation continues even after the water runs out in the upper bottle as air then is continuously blown into the already generated foam.

This technique has the advantage that although it takes a little time to convert all the water in the lower bottle into foam, the blow-through effect ensures the first foam generated is delivered to the nozzle directly and instantly while the rest of the water is turned to foam.

While the use of foam inside of water rockets is not new (see Antigravity Research), at the time of writing we were not aware of others using this technique for a rocket to continuously generate its own foam during the boost part of a flight.

Experiment Setup

The 8L Polaron III rocket was set up on the launcher and tied down for this experiment. The standard static test fire procedure was used. A video camera was set up to film the lowest bottle to see the water's behaviour.

Setting up for static fire tests.
The rocket is attached to the launcher with the 2 meter extension.
Rocket configuration for static fire tests #7 and #8. This diagram shows the distribution of water within the rocket.

About 400ml of water was placed in the upper bottle, and about 1800ml in the lower bottle. We added about 80ml of kids bubble bath solution to the water.

Static Fire Tests #7 and #8 - Foam

Launch Pressure (PSI) Notes
3 110 9mm Nozzle, jet foaming configuration
4 110 7mm Nozzle, jet foaming configuration. Thrust phase lasted 7.28seconds

Water Rocket generates foam - Slow Motion


Don't try this indoors!
Another day at the office. "You did want the flowers washed dear didn't you?"
The after math of a foam test.
Frame from the close up video showing Jet Foaming in action.
The water level rises as foam is generated at the bottom.
With a 7mm nozzle, the thrust ended after 7.28 seconds.

During the filling process quite a bit of foam was generated and pushed up into the upper bottles, but as you see on the video, by the time we fired the rocket, the water was free of foam, with only low density foam sitting on top.

The jet foaming technique creates a lot of foam on the ground around the test area.

Conclusions / Analysis:

  • The Jet foaming technique is very effective at creating foam for the duration of the boost phase.
  • Quite a bit of low density foam is left in the bottle after the boost phase. We will need to measure the weight of the remaining foam, and perhaps try to develop a way of getting rid of this during the boost.
  • The thrust profile is changed significantly from normal air/water mix.
  • With the 7mm nozzle the rocket provided thrust for 7.28 seconds, which means that take offs are likely to be slow, but should be nicely sustained. Therefore this technique may be particularly useful in the sustainer of a two stage rocket.

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