The term System-specific Impulse, ''I''
ssp is a measure that describes performance of
jet propulsion
Jet propulsion is the propulsion of an object in one direction, produced by ejecting a jet of fluid in the opposite direction. By Newton's third law, the moving body is propelled in the opposite direction to the jet. Reaction engines operating o ...
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
propellant
A propellant (or propellent) is a mass that is expelled or expanded in such a way as to create a thrust or other motive force in accordance with Newton's third law of motion, and "propel" a vehicle, projectile, or fluid payload. In vehicles, the e ...
s.
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
Specific impulse (usually abbreviated ) is a measure of how efficiently a reaction mass engine (a rocket using propellant or a jet engine using fuel) creates thrust. For engines whose reaction mass is only the fuel they carry, specific impulse i ...
References
{{Reflist
Spacecraft propulsion
Physical quantities
Classical mechanics