We finished building the water rocket thrust measuring rig today, and did four firings to test it out. The first two tests did not use any dampening on the lever arm and as a result it was virtually impossible to see the scale needle as it flew around so much.
We have plotted the initial thrust curves of an 8L rocket with a 7mm nozzle using only water and then again using foam and the difference in curve shape was quite substantial. We will not publish these initial results just yet, as we would like to confirm the data by repeating the experiment a number of times and then averaging the results. Full data will be published once we obtain it. Hopefully in the next few days.