HHWRSA

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 ….

Here is the video .. more to come

I think the video sums it up pretty well. We had about 1 hour to do a few tests of the MkVII water rocket car so we went down to our launch track and had 2 runs.

It certainly takes a lot more effort to get the 6.75L bottles up to 120psi. We calculated there is about 39.8 L of (standard pressure) air squeezed inside the 6.75L bottles.

First Test – 1500ml water and 120psi

This wasnt a bad start. The veer to the right was most likely a wheel alignment issue. It was a lot faster than previous launches and even after 70m managed to hit the gutter with enough velocity to cause it to roll and land on its head. We did a few unpowered pushes to get the wheels aligned a bit better.

Second Test – 1500ml water and 120psi + foam

This test was a bit of a disaster. We must have over-compensated on the wheel alignment as it just veered constantly off to the left and hit the gutter at full thrust. The impact sheered off the front wheel and bent the rear metal L bracket holding the rear wheel. It still had enough speed after the crash to make it all the way across the road. The front wheel was a good 25m away from the main car.

Damage

The damage wasnt too bad. The front wheel needs replacing and the rear L bracket holding the rear wheel needs straightening. The bottles themselves are fine. The fins are bent a bit but just needs straightening. We will be able to have it fixed for a few more tests this coming weekend.