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Flight Log Updates

#230 - Tajfun 2 L2

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#228 - Tajfun 2 Elec.

#227 - Zip Line

#226 - DIY Barometer

#225 - Air Pressure Exp.

#224 - Tajfun 2

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#193 - Coming Soon

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#187 - Skittles Part #1

#186 - Level 1 HPR

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#183 - Axion G6

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#177 - Reefing Chutes

#176 - 10 Years

#175 - NSWRA Events

#174 - Mullaley Launch

#173 - Oobleck Rocket

#172 - Coming Soon

#171 - Measuring Altitude

#170 - How Much Water?

#169 - Windy

#168 - Casual Flights 2

#167 - Casual Flights

#166 - Dark Shadow II

#165 - Liquid Density 2

#164 - Liquid Density 1

#163 - Channel 7 News

#162 - Axion and Polaron

#161 - Fog and Boom

#1 to #160 (Updates)



Each flight log entry usually represents a launch or test day, and describes the events that took place.
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Day 27 - Foam Tests, Baffle Tests, and Burst Tests
Setting up for four new static fire tests.
We attach the rocket to the launcher with the 2 meter extension.
The baffle that screws onto the Robinson coupling, You can see the air holes on the side.
We have to manually fire it. A great way to cool down on a hot day.
Frame from the video showing just prior to launch.
Just after launch, there is no blow through.
Rocket configuration for static fire tests #7 and #8
Don't try this indoors!
Another day at the office. "You did want the flowers washed dear didn't you?"
I won't tell if you won't. 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.
Re-enforcing a bottle with another bottle.
They loosely fit together, and then the outer jacket is shrunk over the inner one.
On the test stand.
The top jacket separated. This will need a bit more work.
Stress fractures from 220 psi near the top of the throat. The bottle did not break.
Date: 9th March 2007      1:00pm - 3:00pm
Back Yard (site #6)
Mostly sunny with a little cloud cover.
(click the name for rocket details)
Name Capacity Notes
Polaron III 8 L A newer rocket rebuilt from the last launch attempt that failed on the launch pad. Fixed to launcher.

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

This week we performed a number of follow on static tests based on the previous tests. Last week the tests led us down two very different paths in controlling the water inside the rocket. The first led us to developing a baffle that fits on the Robinson coupling of the lowest bottle. This deflects the air preventing the "blow-through" effect.

The second path of investigation was to use the blow-through effect to our advantage to create foam inside the rocket.

Flight Day Events

Static Fire Tests #5 and #6 - Baffle

This test shows how water behaves inside the lowest bottle with the baffle in place. It seems to solve the blow through problem as well as the water being pushed up against the sides during the air pulse as was seen in the previous tests #1 and #2.

The baffle (see picture at left) replaces the coupling nut and is essentially a plugged tube with four 6mm holes radiating out from the center. Because the baffle is so close to the base of the bottle, the bottle lobes help direct the airflow downward to the surface of the water.

Water rocket with baffle in coupling

George | MySpace Video

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

Lessons learned

  • The baffle looks like it solves the blow-through problem, and doesn't allow air to pass directly into the nozzle from the lowest coupling.
  • The baffle also eliminated the water held in the lowest bottle during an air pulse.
  • At this point we don't know if and how much it reduces overall thrust from the rocket especially during the air pulse. - More tests needed.

We now have to test fly it to see what difference it makes during a real flight.

Static Fire Tests #7 and #8 - Foam


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.

Water Rocket generates foam - Slow Motion

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

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 at left) 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. [If you have additional references on foam work in water rockets please contact us. We would like to add them to the credits list.]


In this test the baffle was removed and the standard coupling nut put back. 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. (See diagram at left)

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 bit of a mess, so make sure you do it over a dirty floor that needs cleaning.

Lessons learned

  • 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. We will need to buy or build a logging load sensor to get some realistic measurements.
  • 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.

We appreciate all the support and helpful suggestions from the many people in the online water rocket community.

Burst Tests

We also tried a bottle re-enforcement technique based on Richard Wayman's technique described here:

We used a 1.25L bottle with one half of the jacket made from a 1.5L bottle, and the second half (the neck) from another 1.25L bottle. Because we don't have a heat gun we used hot water instead to shrink the outer bottle. It gave a very satisfactory result.

We had three attempts at blowing it up but on the first attempt the hose released the nozzle at 160 psi. On the second attempt a connector broke at a thread on our pressure regulator at 200psi most of the water was drained from the bottle through the pressure bleed valve...oops. We quickly replaced the connector and tried again.

On the last attempt at 220psi the bottle again separated from the hose, and although there was virtually no air in the bottle, the bottle took off and skidded along the grass for about 15 meters. We will attach the hose properly next time through a different connector.

Looking at the bottle after the tests it was obvious that this is likely to be a very good technique for building rockets that fly at 200+ psi. The top part of the jacket separated, but that is likely to be improved with larger overlaps, and perhaps glue.

Flight Record

Launch Rocket Pressure (PSI) Notes
1 Polaron III 110 9mm Nozzle, baffle on lowest coupling
2 Polaron III 110 7mm Nozzle, baffle on lowest coupling
3 Polaron III 110 9mm Nozzle, jet foaming configuration
4 Polaron III 110 7mm Nozzle, jet foaming configuration. Thrust phase lasted 7.28seconds

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