System-specific Impulse, Issp is a measure that describes performance of jet propulsion systems.
The Issp allows a more accurate determination of the propulsive performance of jet propulsion systems than the commonly used Specific Impulse, Isp, which only takes into account the propellant and the thrust engine performance characteristics.
Therefore, the Issp permits an objective and comparative performance evaluation of systems of different designs and with different propellants.
The Issp can be derived directly from actual jet propulsion systems by determining the total impulse delivered by the mass of contained propellant, divided by the known total (wet) mass of the propulsion system.
A more detailed presentation of derived mathematical formulas for Issp and their applications for spacecraft propulsion are given in the cited references.