Hydrodynamics and Thrust Characteristics of a Water-Propelled Rocket

An undergraduate experiment has been developed to measure the performance of a pressurized water rocket and compare test data with an analytical model developed from fluid momentum. A test stand allows the net thrust of the rocket to be measured, as well the air pressure and temperature inside the rocket, as functions of time. The model compares well with test data from four conditions (combinations of two initial water heights and two initial air pressures). The air in the rocket is assumed to undergo polytropic expansion, and a value of 1.1 for the polytropic exponent is found to best fit the model to experimental conditions. Experimental observations reveal that at higher air pressures the air mixes with the expelled water, resulting in a two-phase flow that reduces the net thrust of the rocket. A pedagogical approach to the experiment is described in detail.