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USWaterRockets.com - Research & Development Report - March 3, 2013 - Water Rocket Bottle Expansion Thermal Pressure Testing.
   

Using Thermal Imaging to Analyze Water Rocket Pressure Tests

Does a water bottle rocket explode because the plastic bottle heats and softens when the air inside is expanding and stretching the plastic? We wanted to find out. The purpose of this experiment is to determine if bottle burst pressure is reduced because of the heat generated by the stretching bottle as it expands.

Before they fail, bottles stretch to unusual shapes.   Water Rocket Competitors need to know if they are reducing their rocket performance by filling them too fast and causing excessive heat.

It has been postulated for many years that the process of filling a water rocket with air pressure can cause the bottle to expand so rapidly that the plastic becomes warm and softens, reducing the ability of the bottle to hold maximum pressure. This experiment was conducted to prove or disprove the theory once and for all.

We wanted to find out the truth behind this theory, as it could teach us how to change our construction methods or pressurizing procedures so that we could increase the pressures we use to launch, resulting in higher flights.

This 2 liter Coke Bottle is stretched into a melon shape just prior to bursting. Is it heating up?   Water Rockets constructed using Spliced Bottles are more likely to burst when pressurized. The lessons learned in this test can help improve the capacity of spliced water bottle rockets.
Experiment Equipment:

To conduct this test, we needed to collect a number of different data points over time, so that the true temperature influence of bottle plastic stretching could be determined. The data points we need to record are:

1. The pressure inside the bottle.
For this we used the pressure logging system from our water rocket launcher.

The U.S. Water Rockets Portable Logger during testing.   Our portable logger was used to take log data of all of the sensors.

2. The temperature of the air inside the bottle.
Since air will raise in temperature as it is compressed, we need to know the internal air temperature so that heat generated by the compressor can be factored out. To determine the internal air temperature, we added a thermocouple to the air supply and passed it out to an auxiliary channel on the pressure logger. CA glue was used to seal the hole where the thermocouple wires leave the pressurized system.

A PVC end cap was used to mount the Thermocuple for recording the temperature inside the bottle.  A Type-J Thermocouple was inserted through the hole so that the temperature of the pressurized section could be read by the external logger.  The Type-J Thermocouple was fed into the pipe inside the bottle so we could log the temperature of the air in the bottle.

3. The temperature of the plastic at the point where the bottle is stretching.
The exact point where the bottle may stretch is difficult to know before the experiment, so placing a temperature sensor in exactly the right place would be much too difficult. Instead, we chose to monitor the temperature of the entire bottle using an infra red thermal imaging camera. This allows the temperature to be monitored over the entire visible surface of the bottle.

We used a FLIR Camera to record the temperature of the bottles under test to make sure we had a record of the temperature of the entire bottle.   The ambient temperature during the test can be found by looking at the background temperature in the FLIR Camera scene and matching the color to the scale on the right.

4. The stretching of the bottle.
This is recorded with a standard HD video camera. Additionally, we recorded the test at 300FPS using a high speed camera in case we needed to analyze the point of bottle failure by slowing down the rupture video.

The test was also recorded using High Definition and High Speed cameras as well as the Infra Red video.

5. Ambient Air Temperature.
The cold junction compensation sensor in the thermocouple input on our pressure logger provides a good source for logging the ambient air, and we also had a digital thermometer on hand, so we had good sources for the ambient temperature reading.

Digital thermometer used to record the ambient temperature.
Experiment Setup:

Our test setup consisted of 2 liter soda bottles which were partially filled with water to simulate a water rocket on a launcher. Each bottle was glued to a 22mm PVC pipe which was clamped in an upright position and connected to a 100 foot length of hose going to our compressor.

Test bottles were made by gluing a PVC launch tube and threaded 1/2" coupler to the neck of some prepared bottles.  A completed test bottle ready to be connected to the test rig.  We prepared several test bottles before running the test.

We made an effort to cancel out the heating effect of the compressed air by using the extra long hose as a heat sink. We even submerged a section of the hose in water to extend the heat sinking ability of the hose.

We attempted to use a long length of hose from the compressor to the test rig so that heat generated by the compressor would be dissipated. We even submerged part of the hose in water to improve the thermal transfer.

We chose a day to conduct the test where the ambient air temperature was as cold as possible. We wanted to insure that there would be as little external heating of the bottle and that changes in temperature would be easily detected by our Infra Red camera. The average temperature during testing was -9.6 degrees C (14.72 degrees F).

It was -10 degrees C on the day we tested, and was so cold the Ice Fisherman on Galway Lake were confined to their hut.

Note: we left the system to soak in ambient air for 30 minutes to allow the setup to equalize in temperature, but the water in the bottle was beginning to freeze even though we were agitating it. The water was not quite as cold as the surrounding air, but this did not seem to have any affect on the test.

With the compressor running and the dump valve open, we began the logging and video recording, and then closed the dump valve to begin compressing the air into the bottle. Once the bottle burst, we shut off the logging devices and saved the recorded data for analysis.

Data Analysis:

As you can see from our recorded data, the external temperature of the bottle did not change in any significant way over the course of the test due to stretching of the plastic. We observed the bottle temperature warming from -9 to +5 degrees C (15.8 to 41 degrees F) during the test, but this temperature increase tracks exactly with the temperature of the air inside the bottle, which is heating due to compression. Our attempts to mitigate the heat caused by compression did not work as well as we had hoped, but we can conclude from the test data that the most significant factor in heating the bottle is caused by the air being compressed into it, and not the self heating caused by stretching of the plastic.

Please click on the graphs below to open a viewer which provides text and a detailed analysis of each graph and an explanation of the analysis for that graph.

At the start of each test, the bottles would be close to ambient temperature inside and outside. The water was slightnly warmer because it would freeze if left to chill any further.  As pressure begins to be added, the air temperature inside the bottle rises, most likely due to compression inside the bottle. The air already inside the bottle gets no benefit from the radiator hose system we made.  The FLIR Camera shows the bottle getting slightly warmer, but the Thermocouple shows that the air temperature inside the bottle is causing the change the FLIR Camera observes.  The air temperature increase inside the bottle seems to taper off. This is likely due to the cooling effect of hose radiator system becoming more pronounced as the temperature difference between inside and outside gets larger.  At the exact moment of bottle failure, the air temperature inside the bottle recorded by the Thermocouple and the surface temperature of the bottle are exactly the same. It does not appear that the bottle has heated in any significant way due to stretching.  In the aftermath of the explosion of the bottle, the Thermocouple returns to ambient air temperature.

We conducted some additional experiments to see if stretching plastic caused a measurable heating effect. To test this we tried stretching and bending samples of plastic and recording the temperature change with the IR camera. These tests showed that the only way to generate significant heat was to repeatedly flex the plastic repeatedly in rapid succession, and simply bending or stretching the plastic one time did not generate any significant heat.

Samples of PET plastic were cut from a bottle to undergo a secondary test to confirm our findings.  We used a sample of PET plastic cut from a bottle to see if we could register any temerature change by manually stressing the plastic.  A single stretch of the plastic sample barely registered any change in tenperature. In this image, the sample was folded rapidly 20 times and it still only changed in temperature by about 10 degrees C.
Conclusions:

In conclusion we can make the following recommendations:
1. Compress the air into your rocket slowly over time to allow the heat to dissipate and radiate away. Rapid compression will cause a large heating effect of the air which could certainly contribute to bottle failure from softening.

2. Avoid other heat sources as much as possible, such as hot pavement or bonfires.

3. Avoid solar heating of the rocket. Do not paint the pressurized portions of the rocket, and if necessary paint them in light colors. The clear plastic will not heat the way a painted surface will when exposed to the sun.

4. Do not add colorants to the water in the bottle. Food coloring and dyes will absorb sunlight and heat the colored water, which will then heat the plastic.

5. You do not have to worry about the plastic stretching and self heating while pressurizing.

As always, practice all proper safety procedures when working with Water Rockets, and above all, have fun!

The pursuit of higher altitude flights has prompted a great deal of research into water rocket pressurization techniques. Water rockets made from plain bottles like this one are often launched with nearly 200PSI of pressure inside.   By helping determine best practices for pressurizing water rockets, we hope to improve safety for everyone enjoying the sport.
Research & Development Video:

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