Because the math behind waterrockets is quite complicated, we decided to build the pathfinder test-vehicle FAR (fixed airframe rocket).
As the name suggests, it is fixed to the ground, so not quite a "rocket" yet. But it is basically an early prototype of the planned Poseidon-Rocket, which is held down by load cells.
This enables us to measure the thrust curve and change in mass over time. With that data we should be able to build a mathematic model.
We conducted multiple thrust test, with FAR mounted on loadcells to measure the force.
Later tests used higher and higher pressures, up to 33bar. There were different variations of the tanks, valves and nozzle, for example the new carbon composite (opposed to previous PVC) tanks used in the latest tests.
FAR Block 3
PVC tanks, C-D nozzle, 30+ bar rated valve
FAR Block 4
Carbon composite propellant tank, PVC pressurant tank, C-D nozzle, 30+ bar rated valve