Abstract : The system specific impulse (Iss) was determined to be the equivalent of specific impulse (Isp) of an idealized system having the same net effectiveness as the actual system but with zero mass of all structural equipment except the payload. THE Iss for chemical and nuclear propelled launch vehicles is compared on the basis of specific impulse, structural factors, equivalent velocities, and number of stages. With Iss, it is possible to separate the system net effectiveness of a multistage vehicle from the downgrading of payload-carrying capabilities introduced by the staging and to recognize that the system effectiveness for each stage is proportionate, or that it is a simple summation if the staging is non-proportionate.