This is an older project, but it might be interesting to post here. Just to be clear, it is NOT a pressure water rocket. It is a hot water rocket. The difference is that the vessel is almost completely filled up with water and heated up beyond the boiling point at room temperature. In my experiments it was heated up to 200°C most the time, which is equal to a pressure of 1.6MPa (16bar)(Fig. 5). Once the valve is opened the hot water flows trough the Laval nozzle like a shacken soda can. In the Laval nozzle (Fig. 1) a part of the water evaporates through the expansion and accelerates further.
As a pressure vessel was an aluminium CO2 vessel with a pressure rating of 25MPa chosen (Fig. 2). Aluminium looses its strength quickly with the rising temperature, but through the high pressure rating there is still a sufficient high safety factor.
In Fig. 3 the relation between the evaporation pressure p and the corresponding temperature T is shown. Above that line water is in its gas phase and below the line the water is in its liquid phase. There are obviously more phases and lines between those, but here just the relevant one is shown.
Since the the vessel is nearly completely filled with water, just a tiny bit of the water is in the gas phase when its heated up too 200°C. The theoretical usable energy for propulsion is h = 0.4MJ/kg (Fig. 4). It is the energy which gets released through the expansion form 1600kPa in the ideal case to the atmospheric pressure of 100kPa.
If the Laval nozzle is perfectly designed a major part of the theoretical usable energy can be converted into thrust. To examine the difference between theory and reality I did build a test stand for the hot water rocket. This test stand (Fig. 5, Fig. 6) includes a load cell to measure the generated thrust. Pressure and temperature sensors are used to determine the status through the heating up process.
All sensor data are transferred via a long serial cable to a computer. This allows them to be observed in real-time as well as the valve and the heating device can also be remotely controlled. There is no human interaction necessary during the charging and discharging phase, so a safe distance can be kept.
For most test a concrete wall enclosed space was used to avoid any risk in case the pressure vessel overheats or would fail for any other reason. The following video shows one of
the conducted tests. Fig. 7 and Fig. 8 shows the vehicle with the final propulsion system in the fuselage. The pressure vessel is insulated with fibreglass and aluminium foil with just the nozzle is sticking out.
This version has no valve built in. The nozzle itself gets blocked at its most narrow opening by a boom which is clamped to the nozzle and can be easily released under tension.