Original Research

The calculation of the thrust of a rocket motor

J. H. Knoetze
Suid-Afrikaanse Tydskrif vir Natuurwetenskap en Tegnologie | Vol 12, No 3 | a565 | DOI: https://doi.org/10.4102/satnt.v12i3.565 | © 1993 J. H. Knoetze | This work is licensed under CC Attribution 4.0
Submitted: 09 July 1993 | Published: 09 July 1993

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J. H. Knoetze,, South Africa

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Abstract

Traditionally the thrust of a rocket motor is calculated by first calculating the thrust coefficient and then multiplying it by the product of the throat area and pressure. The thrust coefficient is calculated using a standard gas dynamics equation. This equation assumes that the combustion products are a single component, non-reacting ideal gas and that the flow through the nozzle is isentropic. The thrust coefficient is a function of the ratio of specific heats, y, the area ratio of the nozzle and the motor and ambient pressures. Standard methods exist for calculating the tosses due to deviations from the assumed flow. The combustion products of modern composite propellants contain a significant portion of condensed species (primarily A1₂O₃), while the composition of the combustion products changes continuously as the products move throught the nozzle. Some uncertainty therefore exists with regard to which value of y to use and how to handle the condensed species. The assumption o f an ideat, non-reacting gas can be el iminated hy as.mming the process to he isentropic and to calculate the thrust hy using the thermodynamic state and composition of the combustion products in the motor and nozzle exit. This can be achieved by using any of the standard thermochemistry programs available in the rocket industry. It is thus possible to use the results of a standard thermochemistry program directly in an alternative method for calculating thrust. Using this method only the mass flow rate (which is a function of pressure, throat area and effective caracteristic velocity) and the results from the thermochemistry program are needed to calculate the thrust. The advantages of the alternative method are illustrated by comparing the results of the two methods with a measured thrust curve.

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