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.