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070224-001 Air and water mixing static fire test
Date 24th February 2007      3:00pm - 5:00pm
Tested by GK & PK
Location Sydney, Australia
Test Procedure Static Fire Test Procedure
Related 070224_002 - Thermal tests of skin temperature and internal temperature


  • This experiment is carried out in order to understand what happens inside the lowest bottle of a Robinson coupled rocket. In particular what happens when some amount of water is left in the upper bottles.

Experiment Setup

The 8L Polaron III rocket was set up on the launcher and tied down for this experiment. A number of cameras and lighting were set up around the rocket.

Setting up cameras for the experiment.
... and a couple more.
It is a good idea to wear ear protection, as it can be pretty loud.
Firing the rocket by hand.


Static Fire Test #1

We filled the rocket with 2.5 L of water. There was about 600ml was in the top bottle, and about 1.9L in the bottom bottle. With an air gap in the top of the lower bottle.  This water configuration is quite easy to achieve, just pour the water in and let it partially fill the next bottle. As the top bottle is closed off the small Robinson coupling hole mostly keeps the water coming through to the bottom bottle and hence leaving an air gap. The nozzle was a 9mm Gardena nozzle. The rocket was pressurised to 110psi before being fired.

Following are frames from the slow motion video.
Just prior to launching test #1.
Initial bubble blow-through, extends all the way to the nozzle.
Jet of water from upper bottle creates a lot of bubbles in the lower bottle.
Lots of bubbles causes the spray to widen.
When bubbles reduce then flow from the nozzle improves.
During the air pulse, some remaining water is forced up the sides of the bottle.


Water Rocket Static Fire Test #1

We pulled the hose off the rocket and watched what happens to the water inside the bottle and the corresponding spray from the nozzle. The video clearly shows the jet of water from the top bottle dragging air with it penetrating all the way through the water in the lower bottle. This starts a process of generating a great many bubbles in the water against the nozzle and progresses up the bottle until the entire bottle is filled with bubbles so that it is milky in appearance. The nozzle spray also correspondingly widens the same way as we saw on the last launch day. The air bubbles in the water must be expanding as they exit the nozzle, and hence the wide spray.

After a while when the water runs out in the top bottle and water is only being pushed out of the bottom bottle, the nozzle spray narrows again and the water becomes a little more clear, although the surface is very rough and there is a substantial number of bubbles in it.

As the water is about to run out, the and the air pulse happens, the airspeed inside the rocket increases significantly.  The air flow is such that the remaining water actually gets pushed up the sides of the bottle and is kept there until the air runs out and the water then just drains out of the nozzle. It wasn't a lot of water but it was an unexpected result.

Static Fire Test #2

The second test involved only filling the lower bottle with 1.9L of water so that there was no water in the upper bottle. We used exactly the same pressure (110psi) as the previous test.


On launch the blow-through also happened, but the blow-through bubble quickly retracted again to the surface and the water stayed relatively bubble free for the duration of the burn. The nozzle spray was also relatively clean.

Water Rocket Static Fire Test #2

Conclusions / Analysis

  • Our experiment confirmed that the wide water spray observed last week was due to expanding bubbles rather than a faulty nozzle/seal as previously believed.
  • Designing a better coupling might mean that the rocket does not produce this "blow through" and allow the water to come out more evenly and hence more efficiently.
  • Water flows up the sides of the lowest bottle during the air pulse. Minimising this should also allow better flight efficiency.
  • To get a more efficient burn there should not be any water in the upper bottle above the coupling. Water often ends up there if you fill the rocket too quickly with air.

Things to consider

If the bubbles are produced in an efficient manner it may be possible to use that to an advantage with an expanding nozzle. Antigravity Research used detergent to create foam inside the rocket with an expanding nozzle to make the rocket more efficient. When perfected a technique based on the static fire test #1 may be used to generate the same effect without the viscosity penalty of using detergent.

If the temperature of the air inside the rocket drops below freezing at higher launch pressures, it may be likely that the during the air pulse tiny water droplets exiting could be freezing and exiting as ice or snow?


  • These were static tests that do not take into account the acceleration of the rocket, so the water may behave differently, but from last weeks tests we see that even accelerated rockets behaved in a very similar manner.
  • We purposefully recorded the rocket video sideways so that we could see more of the action.

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