Hydrodynamic Study of a Water-Propelled Rocket: An Undergraduate Experiment in Fluid Mechanics

An undergraduate experiment has been developed to measure the performance of a pressurized water rocket and compare test data to an analytical model developed from fluid momentum. A rocket and test stand were developed to measure the net thrust of the rocket, as well the air pressure and temperature inside the rocket, as a function of time. The model compares well to test data from four conditions, combinations of two initial water heights and two initial air pressures. The air in the rocket was assumed to under go polytropic expansion, and a value of 1.1 for the polytropic exponent was found to best fit the model to all experimental conditions. Experimental observations also 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 is also developed for the experiment and is described indetail.