How to Calculate Rocket Nozzle Size

The performance of any rocket depends on the physical configuration of the nozzle, and properties of the fuel.

The performance of a chemically propelled rocket is largely determined by size and geometry of the engine and the conditions present in the combustion chamber. If one assumes, however, that the flow of propellant through the engine as a whole is isentropic (not taking entropy losses into account), then the computations involved in relating the flow characteristics to nozzle size become very simple.

Things You'll Need

• Scientific or graphing calculator

Instructions

1. Identify the specific heat ratio for the rocket's propellant. This is referred to in rocketry equations by the Greek letter gamma.

2. Perform the following calculation in a calculator or other computerized means of evaluation: Gamma*(2/(gamma + 1))^((gamma + 1)/(gamma -1))

3. Multiply the gas constant ("R") of the propellant by the temperature inside the combustion chamber of the rocket, then divide this product by the result of the previous step.

4. Take the square root of your result.

5. Multiply this number by the propellant mass flow rate, then divide by the pressure present in the combustion chamber. The result is the cross-sectional area of the smallest portion of the nozzle, or "throat".

References

• "Mechanics and Thermodynamics of Propulsion"; Phillip Hill and Carl Peterson; 1992

Resources

• "Gas Dynamics"; James John and Theo Keith; 2006