last updated: 16th August 2017 - Day 187 & Day 188 Skittles in Microgravity

Safety First

Search

Site Index

Tutorials

Articles

Rocket Gallery

Labs

Where To Buy

10 Challenges

Links

Blog

Glossary

Contact Us

About


Construction - Basic

Body

Ring Fins

Flat Fins

Nozzle

Nosecone

Construction - Advanced

Robinson Coupling

Splicing Bottles #1

Splicing Bottles AS#5

Reinforcing Bottles

Side Deploy #1

Side Deploy #2

Mk3 Staging Mechanism

Multi-stage Parachutes

Fairings

Construction - Launchers

Gardena Launcher

Clark Cable-tie

Medium Launcher

Cluster Launcher

Launch Abort Valve

Quick Launcher

How It Works

Drop Away Boosters

Katz Stager Mk2.

Katz Stager Mk3.

DetMech

Dark Shadow Deployment

Articles

Recovery Guide

Parachutes

How Much Water?

Flying Higher

Flying Straight

Building a Launcher

Using Scuba Tanks

Nozzles

Video Taping Tips

MD-80 clone

Making Panoramas

Procedures

Burst Testing

Filling

Launching

Recovery

Flight Computer

Servo Timer II

V1.6

V1.5

V1.4

V1.3, V1.3.1, V1.3.2

V1.2

Deploy Timer 1.1

Project Builds

The Shadow

Shadow II

Inverter

Polaron G2

Dark Shadow

L1ght Shadow

Flight Log Updates

#188 - Skittles Part #2

#187 - Skittles Part #1

#186 - Level 1 HPR

#185 - Liquids in Zero-G

#184 - More Axion G6

#183 - Axion G6

#182 - Casual Flights

#181 - Acoustic Apogee 2

#180 - Light Shadow

#179 - Stratologger

#178 - Acoustic Apogee 1

#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

#160 - Chasing Rockets

#159 - Measurement

#158 - Dark Shadow

#157 - Polaron G2

#156 - Foam Flights

#155 - Down The Barrel

#154 - Revisits

#153 - ClearCam

#152 - Mullaley, Axion G2

#151 - Competition Day

#1 to #150 (Updates)

 

FLIGHT LOG

Each flight log entry usually represents a launch or test day, and describes the events that took place.
Click on an image to view a larger image, and click the browser's BACK button to return back to the page.

Day 64 - Thermal Testing
2.1L bottle under test. With pressure gauge, temperature gauge and a watch.
Air is supplied from a SCUBA tank through a pressure regulator.

Date:  27th July 2008
Location:
Workshop
Conditions:
Pleasant
Team Members at Event:
PK and GK

Over the last few months there have been some great conversations on the Yahoo Water Rocket forum relating to the maximum pressure a bottle can hold and how temperature could affect the bottle's strength. Mike, Trevor, Christian, David, Gary, Cliff, Henning, Danny, Ralph, Pat and others all had interesting comments on the subject so we decided to run a number of experiments to see how the temperature changes inside a rocket.

As more experiments are done over the next few months, they will be added to the list below to keep them all together.

Theory

Most water rockets are made from PET bottles. The strength of the PET material is affected by temperature. Most rocketeers know that PET can be softened and shaped with hot air or hot water. The lowest temperature where this effect begins is known as the glass transition temperature. Wikipedia lists this as both 75 degrees C and 69 degrees C (see table) however, the actual value depends on a number of factors such as additives and the microstructure of PET.

The main question is whether under normal circumstances a rocket made out of PET bottles can reach such a high temperature where the rocket can fail at a lower pressure compared to the same bottle tested hydrostatically. Due to the nature of hydrostatic testing the bottle is cooled by the water and hence does not experience the same sort of heating as a rocket on a launch pad.

A number of factors can affect the temperature inside the rocket and hence the walls of the bottle.

Sources of heat include:

  • Sun - With the greenhouse effect the sun can raise the temperature inside the bottle above the outside ambient temperature. It is the same effect as you get in a closed car on a hot day. The colour of the bottle or any dark paint or tape on it can have a significant effect on the internal temperature.
  • Pressurisation - By far the largest contribution to the increase in temperature inside the rocket is due to pressurisation. As air is compressed the temperature increases.

  • Air source - The air supply type also has a contributing factor to the temperature. When filling from a compressor, the air is heated by the compressor, and warm air comes out of the air hose. When filling from a tank, the air comes out cooler due to expansion. The higher the rate of flow the cooler it is. The length of hose also has an effect on heating or cooling the air from the air source. A long black hose can also heat the air when exposed to the sun.

  • Bottle stretching - As the bottle is pressurised and the walls stretch, that process in itself may generate some heat in the walls. (Thanks Christian and Trevor for pointing that out)

  • Ambient temperature - This is the outside air temperature. This can be significant if you are launching on a hot day.

Test Setup

The thermal tests were performed on a spliced pair of 1.25L bottles. The air was let in through the bottom and the thermocouple was fed in through the top lid and suspended about 15cm below the lid.

The thermocouple was connected to the multimeter and it was set on its thermometer setting. We also placed a watch next to it and a pressure gauge that read the line pressure fairly close to the bottle inlet. We set up a video camera to simultaneously record the temperature, time and pressure relationships.

We then filled the bottle at different rates to around 100psi. In some tests, we also filled the bottle with ~800ml of water which would typically be found in a rocket this size. The air bubbled through this column of water as would happen on a launch pad.

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

The Results

The ambient temperature was a cool 12C on the day of the tests.
 
Test Air Supply Test Start
Temp
(deg C)
Max
Temp
(deg C)
Pressure
(psi)
Time to reach max pressure
1 Tank Air only 12 35 110 36 seconds
2 Tank Air only 12 33 105 28 seconds
3 Tank Air only 12 33 100 16 seconds
4 Tank Air and Water 12 33 95 14 seconds **
5 Tank Air and Water 12 32 95 13 seconds **
6 Compressor Air only 12 21 30 * 52 seconds

*We only managed to get to 30psi with the little compressor before it decided to splutter and seize up. This was an old compressor already on its way out. We will have to perform this test again with a new compressor.

** Fills faster because of the reduced volume.

Other Tests

  • We measured the temperature of the air coming out of the hose before entering the bottle. During a slow fill rate the air temperature was close to ambient. On maximum flow rate the temperature reached 7 degrees C which was 5 degrees below ambient temperature.
  • Three months ago we ran some tests on small compressors to measure the air temperature coming out. The ambient temperature at the time was 17 degrees C. The temp started out okay when the compressor was first turned on at around 18 C. After about 2 minutes of unloaded operations (the compressor wasn't filling a rocket) the air temperature climbed to 29 C. At that point I stopped the test. That was a 12 C increase without any load.
  • On one occasion after air had cooled in the pressurised bottle, we let the pressure out rapidly and the air temperature dropped to -1 degree C.

Observations

  • From these few tests it is difficult to reach definitive conclusions about whether it is possible to reach 70 degrees in the bottle. We are waiting for warmer conditions in Summer when on a hot day temperatures in the shade can easily reach 35 degrees C. Filling to higher pressures of 130 - 140psi will also have an effect on further increasing the temperature. On a hot day filling to higher pressures would certainly be within the ball park figure of exceeding the glass transition temperature, but we won't be certain until further tests.

    We have observed an unusual stretch failure last year when we tried launching a rocket on a very hot day where temperature was around 40C and pressurised to 130psi.

    Pat had reported an incident where a bottle failed at 80psi when filled in around 5 seconds from a tank. It is uncertain how much heat may have contributed to this failure.
     
    Trevor reported that during one pressurisation test he measured 70 degrees C skin temperature with a remote laser aimed thermometer. He said that it was for a brief period of time and most likely attributed to the bottle stretching process.
  • We were quite surprised that the higher flow rate into the bottle did not increase the air temperature, but this could be partially explained by the fact that at the higher flow rates the air was 5 degrees cooler when it entered the bottle. Faster filling from a compressor could be different.

  • We were unable to test even faster flow rates mostly due to our air supply setup in that it has a number of quick release connectors with relatively small holes.

  • The other interesting result was that filling the rocket through the water column did virtually nothing to cool the air being compressed above the water.

  • Other consequences of filling rapidly and heating the air, especially on bottles sitting behind non-return valves is that the air pressure will drop once the air cools inside the rocket. Depending on various factors this could be as much as 10psi. This is particularly significant if you wait for a while before launching a rocket.

  • Because filling a rocket rapidly can raise temperature significantly it is advised to fill a rocket slowly to allow the air to cool.

<< Previous       Back to top      Next >>



Copyright © 2006-2017 Air Command Water Rockets

Total page hits since 1 Aug 2006:

George Katz - Google Plus