How would one go about calculating ISp of a rocket with combined propulsion? E.g. the Space Shuttle during the phase of launch, with both SRBs and SSME active?
Of course there's the simple "experimental" method, taking wet and dry mass, and speed at the beginning and end of acceleration, but that is not very good if we don't have the complete, working rocket at hand. For the planning stage, we'd know the exhaust speed of different engines, their thrust and mass flow - how then would we go about finding the ISp of the whole?
Considering that various engine contribute a different amount of thrust, how to assign weights to their ISp to calculate the value for the whole craft?