What's more, strictly speaking it's a form of turbine, but it's not really close to a typical functional turbine where you're typically redirecting your axial flow via fixed nozzles and stators to impact your turbine blades face on.The governing equations for something like this would be somewhat related to turbomachines, i.e. the fins are acting like turbine fan blades that generate power or torque, in this case to rotate the model rocket. Turbine design theory could be found in a college or graduate level engineering textbook. However, most of the typical design equations for rotating turbomachines usually assume a constant operating condition, i.e. RPM and inlet/exit velocity, or slowly changing inlet conditions, a model rocket goes through a large velocity range very quickly and the RPM would not be constant.