<p>This brief report demonstrates a simple and straightforward approach to obtaining a time-free approximation for the optimal specific impulse of an electric propulsion system during preliminary mission design. We will show that accurate analytical estimates of the optimal specific impulse can be obtained directly from the mission characteristic velocity without the need to define a propulsion system firing time, as is typically required in prior works. In particular, all relevant mission and propulsion system parameters can be captured by a single non-dimensional system parameter, which enables direct prediction of the optimal specific impulse and corresponding minimum system mass fraction. The results of this work can be used in preliminary mission design studies in order to obtain an initial estimate for the desired propulsion system performance characteristics.</p>