S.1 Spacecraft Propulsion Systems Chapter 1: Introduction to ...

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For electric propulsion high impulse performance is not dictated by maximum exhaust
velocity, like for chemical propulsion, but rather by optimum values of thruster exhaust
velocity, ve opt, that can be elucidated schematically by the following figure:
With increasing exhaust velocity, ve, the combined mass of propellant and tank is decreasing
while the mass of the power supply is increasing. The point of intersection of the two curves
determines the minimum of the system mass by ve opt resulting in a maximum value of Issp.