System-specific impulse
Encyclopedia
The term System-specific Impulse, Issp is mostly used to describe the performance of jet propulsion
Jet propulsion
Jet propulsion is motion produced by passing a jet of fluid in the opposite direction to the direction of motion. By conservation of momentum, the moving body is propelled in the opposite direction to the jet....

 systems. A reference number is introduced, which defines the total impulse, Itot, delivered by the system, divided by the system mass, mPS:
Issp=Itot/mPS

Because of the resulting dimension, - delivered impulse per kilogram of system mass mPS, this number is called ‘System-specific Impulse’. In SI
Si
Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...

 units, impulse is measured in newton-seconds (N·s) and Issp in N·s/kg.

The Issp allows a more accurate determination of the propulsive performance of jet propulsion systems than the commonly used Specific Impulse, Isp
Specific impulse
Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...

, which only takes into account the propellant and the thrust engine performance characteristics. Therefore, the Issp permits an objective and comparative performance evaluation of systems of different designs and with different propellant
Propellant
A propellant is a material that produces pressurized gas that:* can be directed through a nozzle, thereby producing thrust ;...

s.

The Issp 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 Issp 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 Issp and their applications for spacecraft propulsion is given below.
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