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We now have a water rocket that is aerodynamically sound. We know that we will be able to pump it up to a pressure of between 4 and 6 BarG (between 60 and 90 psig) and we can measure it. So, how do we know how much water to put in it?
We need to know its tare weight, capacity, diameter and nozzle dimensions to be able to work out how much water it will need for a flight with the greatest height.
We can measure its nozzle, body diameter and weight it empty to get its tare weight but we have changed its capacity so we don't know that any more - the volume of liquid it had when you bought it was not the same as its nominal capacity either and in addition, there has to be a certain amount of ullage (head space) so as to take into account the expansion of the liquid when it gets hot so that the bottle doesn't burst in the shop. All we can do is measure it and the best way to do that is as follows. . .
Weigh the rocket empty (you will need this for the computer model anyway). Fill it to the top with water and weigh it again. Take the former from the latter and you have your capacity (close enough) as, for the purposes of water rocketry, 1 gramme equals 1 cm³.
These figures were then fed into my computer model and the weights in the table below and graph on the right were calculated to be the optimum for the pressure range considering the diameters of the different rockets.
To put them into practice, put a piece of gaffer tape along the side of the rocket and weigh in the optimum amount of water. Mark on the gaffer tape where the water comes to, screw a top on, invert it and make another mark (in such a way that you will not be confused - possibly using an arrow pointing upwards). This will make life easier when in the field and you haven't got access to the scales.
If your rockets have tare weights or capacities that are different to these, you can use the above graph to work out roughly the right weight of water optimised for height - this assumes that the rocket capacity and diameter are roughly in proportion.