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Team Members at Launch Event: PK, GK, AK, John K and Paul K.
Number of firings: 4

Today was a slightly different launch day. After reviewing last weeks photos and videos we noticed that both J4 II and Polaron III exhibited unusual nozzle spray behaviour and in one photo the water in Polaron III appeared cloudy just after takeoff. So we decided to perform some static fire tests to see what was going on.

Flight Day Events

During the week I managed to figure out (well actually read the manual) how to use our digital still camera to record video at 60 frames per second. Because of the faster frame rate it only records at a resolution of 320 x 240 but that was enough for our needs.

We set up Polaron III on the new launcher and tied it down so it wouldn't fly away. We knew that we didn't need to weigh down the launcher, because the rocket's thrust would be pushing against it and therefore providing an equal but opposite reaction force. But we did anyway, just in case Newton was asleep at the wheel on the day.

We set up a few cameras around and did the following experiments:

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.

Results

Water Rocket Static Fire Test #1


(If the video does not play, try the latest Flash player from Macromedia)

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 as the previous test.

Results

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


(If the video does not play, try the latest Flash player from Macromedia)

The next two tests were used to measure the temperature of the bottles inside and outside as they were filling and just after they emptied. No water was used in these experiments. The ambient temperature was 22 degrees C. All temperatures were measured with a tiny thermocouple connected to multimeter that can measure temperature. The thermocouple can measure rapid changes in temperature.

When measuring the skin temperature we just placed the sensor onto the bottle, but probably should have been shielded from the environment.

Static Fire Test #3

The rocket was filled to 50 psi and the outside skin temperature was measured during filling to be 25 degrees C. +3 above ambient.

When the air was let out, the thermocouple was placed inside the rocket and the air temperature was measured at 16 degrees C. -6 below ambient.

Water Rocket Static Fire Test #3

(If the video does not play, try the latest Flash player from Macromedia)

Static Fire Test #4

The rocket was filled to 100 psi and the outside skin temperature was measured during filling to be 27 degrees C. +5 above ambient.

When the air was let out, the thermocouple was placed inside and the air temperature was measured at 8 degrees C. -14 below ambient.

Lessons learned

• 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.
• The temperature measurements indicated that there is unlikely to be much effect on the PET bottle material with the pressures we regularly use.

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?

Notes

• 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.

What's Next?

• We will try to improve the coupling to prevent this "blow-through". We already have a simple idea that may fix this. More on that next time.
• We will need to do further tests with a different sized nozzles, and bottle capacities and shapes.
• We may try to get a consistent bubble forming scheme and use an expanding nozzle to see if it makes a measurable difference.

Flight Record

Notes to Self

• Wear ear and eye protection when performing tests like this.