The effects of mass energy system, propellant density, electric power
and other parameters on the performance of propulsion systems with sep
arate source of energy are investigated. A simplified analysis that re
lates thrust and specific impulse in such systems is developed. The re
sults are presented in terms of variation of mass efficiency or payloa
d ratio with the external power supplied to the system. It is shown th
at the utilization of a separate source of energy is advantageous up t
o a determined value of the external power that depends on the satelli
te total mass and on its characteristic increment of velocity.