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.
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.
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.
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.
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.
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.
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.
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.
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 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).
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.
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.
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!