Propulsion system assembly for an air vehicle, the propulsion system assembly comprising: a driveshaft rotatably mounted to a casing about a drive axis, the driveshaft comprising a first shaft end and an opposite facing second shaft end; a plurality of electric motor modules in axially stacked relationship with one another with respect to the drive axis and co-axially aligned with the drive axis to define an electric motor module stack, said plurality being an integer greater than unity, each electric motor module being configured for transmitting a torque to the driveshaft when coupled thereto independently of at least one other said electric motor module; each electric motor module comprising a controllable clutch arrangement for selectively coupling and decoupling the respective electric motor module with respect to the driveshaft to respectively enable and disable transmission of torque between the respective electric motor module and the driveshaft; wherein said propulsion system assembly comprises a housing member for enclosing therein at least said casing and said plurality of electric motor modules and configured for pivoting about a pivot axis different from the drive axis and thus is configured for selectively providing vertical thrust for vectored thrust flight and for selectively changing the thrust vector to provide horizontal thrust for aerodynamic flight; and wherein the propulsion system assembly is configured for operating at a design aggregate shaft power corresponding to said vertical thrust, and for operating at a modified design aggregate shaft power corresponding to said horizontal thrust, said modified design aggregate shaft power corresponding to said horizontal thrust being less than said design aggregate shaft power corresponding to said vertical thrust, and wherein when operating at said modified design aggregate shaft power, a reduced number of said electric motor modules are coupled to the driveshaft, said reduced number being at least one less than said integer, such that the sum of the module design shaft powers of the electric motor modules that remain coupled to the driveshaft matches or exceeds the modified design aggregate shaft requirement for aerodynamic flight. a driveshaft rotatably mounted to a casing about a drive axis, the driveshaft comprising a first shaft end and an opposite facing second shaft end; a plurality of electric motor modules in axially stacked relationship with one another with respect to the drive axis and co-axially aligned with the drive axis to define an electric motor module stack, said plurality being an integer greater than unity, each electric motor module being configured for transmitting a torque to the driveshaft when coupled thereto independently of at least one other said electric motor module; each electric motor module comprising a controllable clutch arrangement for selectively coupling and decoupling the respective electric motor module with respect to the driveshaft to respectively enable and disable transmission of torque between the respective electric motor module and the driveshaft; wherein said propulsion system assembly comprises a housing member for enclosing therein at least said casing and said plurality of electric motor modules and configured for pivoting about a pivot axis different from the drive axis and thus is configured for selectively providing vertical thrust for vectored thrust flight and for selectively changing the thrust vector to provide horizontal thrust for aerodynamic flight; and wherein the propulsion system assembly is configured for operating at a design aggregate shaft power corresponding to said vertical thrust, and for operating at a modified design aggregate shaft power corresponding to said horizontal thrust, said modified design aggregate shaft power corresponding to said horizontal thrust being less than said design aggregate shaft power corresponding to said vertical thrust, and wherein when operating at said modified design aggregate shaft power, a reduced number of said electric motor modules are coupled to the driveshaft, said reduced number being at least one less than said integer, such that the sum of the module design shaft powers of the electric motor modules that remain coupled to the driveshaft matches or exceeds the modified design aggregate shaft requirement for aerodynamic flight.