19. It will be of interest to calculate the thrust of the engine by considering the engine as a whole, as the resultant thrust should be equal to the sum of the individual gas loads previously calculated.
20. Although the momentum change of the gas stream produces most of the thrust developed by the engine (momentum thrust = ), an additional thrust is produced when the engine operates with the propelling nozzle in a 'choked' condition (Part 6). This thrust results from the aerodynamic forces which are created by the gas stream and exert a pressure across the exit area of the propelling nozzle (pressure thrust). Algebraically, this force is expressed as (P-P 0 ) A. Therefore, assuming values of mass flow, pressure and area to be the same as in the previous calcula- tions i.e. by combining the gas loads on the individual engine locations.
21. On engines that operate with a non-choked nozzle, the (P-P 0 ) A function does not apply and the thrust results only from the gas stream momentum change.