Prediction of Water Rocket Performance

Launching a water rocket is an effective means to demonstrate the application of the equations of a projectile motion. Water rockets are relatively safe to handle; the use of water as the propellant mitigates the fire hazards associated with combustible fuels. When modeled accurately, in addition to the equations of motion, the analysis of the performance of the water rockets also entails the applications of the energy or Bernoulli's equation, equations of state, and the numerical solutions to the partial differential equations. The nozzle flow theory is also involved. In this work, the water rocket performance parameters such as maximum altitude, maximum speed and the average specific impulse are calculated while accounting for the time variation of the air pressure inside the rocket, the mass of the rocket, and the flow velocity at the nozzle exit. The results from the analysis are compared with the measured values. The most important finding is that the choked air flow at the exit, which occurs after the rocket runs out of water continues to accelerate the rocket considerably. The analysis presented here is expected to serve as a reliable reference for the prediction of water rocket performance.