 In SI units, impulse is measured in newton-seconds (N·s) and "I"ssp in N·s/kg.The "I"ssp allows a more accurate determination of the propulsive performance of jet propulsion systems than the commonly used Specific Impulse, "I"sp, which only takes into account the propellant and the thrust engine performance characteristics.Therefore, the "I"ssp permits an objective and comparative performance evaluation of systems of different designs and with different propellants.