We continue with the Water Rocket analysis with a summary of the four equations developed so far on Page 1. Note that all of this analysis is relevant to the upwards thrust phase of the rocket flight, while the water is being expelled through the nozzle by the compressed air.
The basic rocket Force equation:
The thrust force in terms of the water expelled through the nozzle:
Bernoulli's equation, relating the pressure applied by the compressd air to the velocity of the exhausting water:
Note that the resulting thrust force is twice the pressure difference times the nozzle area:
Unfortunately the compressed air pressure P is not a constant during the thrust phase, but varies in a nonlinear manner with the expanding volume of the compressed air. This is the main reason for the extremely complex relations resulting from this analysis.
As the water escapes, the air volume increases, causing a decrease in pressure and a corresponding decrease in thrust. We consider this process to be adiabatic (no transfer of heat during the split-second expansion process), which allows us to relate the time variation of the pressure to that of the volume.
The adiabatic expansion process is derived from the energy equation applied to an ideal gas, and is developed in the section on Adiabatic Expansion Analysis leading to the following equation:
where:
P0 is the initial absolute pressure at liftoff [Pa]
V0 is the initial volume of the compressed air [m3]
k is the ratio of specific heat capacities [k = 1.4 for air]
P, V are the respective time varying pressure and volume of the compressed air during the thrust phase.
The volume variation of the compressed air due to the water escaping through the nozzle is given by:
Substituting equations 3 and 5 into equation 6 and simplifying, we obtain:
Equation 7 is the differential equation for the volume variation of the compressed air as a function of time t. It cannot be solved explicity since the volume V is deeply embedded in a nonlinear manner in the equation, hence we resort to a numerical solution. It is of extreme importance to solve it however, since only through the solution of equation 7 can we know when all the water has been expelled denoting the end of the thrust phase, leading to the determination of the Duration of Thrust.
The numerical solution of ordinary differential equations (ODEs) is an important generic problem in engineering, and you will no doubt learn various methods of solving them (such as the Runge-Kutta methods). The main reason for introducing this analysis here was to indicate the importance of the adiabatic process analysis in the study of water rockets.
For those of you who wish to continue with this analysis through to the development of the acceleration, velocity and height differential equations we continue with the final page of the water rocket analysis.
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to the Water Rockets Introduction page
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Engineering Thermodynamics by Israel Urieli is licensed under a
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