How Much Air is in my water rocket ?

Its a good question … How much air is inside that water rocket ??? The easiest way to think about it is .. what volume of standard pressure air is being squeezed into the water rocket when its pressured to a particular pressure.

The attached calculator is an excel spreadsheet which will work this out for you. The volume of (Standard pressure) air in the bottle when its pressurized is related to

– The volume of the bottle you are pressurizing
– The amount of water in the bottle
– The temperature of the air under pressure (The more you squeeze in the hotter it gets)
– The air temperature of the day you are doing the launch

The equations we used to derive the formula are included in the bottom of the spreadsheet and are from the Combined Gas Law. As we are not super cooling or super heating the air, these equations will approximate pretty closely the correct amounts

The equations use the following units – for volume (Litres), pressure (psi) and temperature (Kelvins). There are converters there if you use other units of measurement. Just fill in the Green squares in the spreadsheet and you can go wrong.

Download the excel file using the link below.

Volume of Air in Rocket Calculator

The default increase in temperature we have used is 22 degrees Celsius. This is based on the Day 64 Thermal Test results that George over at Air Command has on his website. This information and videos are here. Thanks George :)

Its interesting to experiment with changing Air Temperature in the bottle. You can see that but cooling the air you can get more in at the same pressure. More Air equals more thrust ….


This entry was posted on Thursday, August 27th, 2009 at 10:15 pm and is filed under Calculators. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.


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