System-specific impulse

13:36, 26 September 2006 (UTC)PEER

System-specific Impulse, I_ssp

To describe the 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:

                      <Issp=Itot/mPS>            			

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 (Ns) and I_ssp in Ns/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 e.g. built systems. In addition, the I_ssp can be derived analytically for e.g. 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 is given in reference (1).

(1) Peter Erichsen; Performance Evaluation of Spacecraft Propulsion Systems in Relation to Mission Impulse Requirements; Proceedings of the Second European Spacecraft Propulsion Conference, 27-29 May, 1997 (ESA SP-398, Aug. 1997).