System-specific impulse

System-specific Impulse, I_ssp is a measure that describes performance of jet propulsion systems. A reference number is introduced, which defines the total impulse, I_tot, delivered by the system, divided by the system mass, m_PS:

I_ssp=I_tot/m_PS

Because of the resulting dimension, - delivered impulse per kilogram of system mass m_PS, this number is called ‘System-specific Impulse’. 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.

The I_ssp 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. This allows a quantitative comparison of for example, built systems.

In addition, the I_ssp can be derived analytically, for example for spacecraft propulsion systems, in order to facilitate a preliminary selection of systems (chemical, electrical) for spacecraft missions of given impulse and velocity-increment requirements. A more detailed presentation of derived mathematical formulas for I_ssp and their applications for spacecraft propulsion are given in the cited references. In 2019 Koppel and others used I_SSP as a criterion in selection of electric thrusters.

See also

• Specific Impulse