0zo-9<L00 5 Fig. 2 Optimized Mass Fractions were v, is a"loss velocity", equal to the velocity to which one ion would be accelerated by the voltage drop A o. Notice how this simple expression already indicates the importance of a high atomic mass propellant A o is insensitive to propellant choice, and so v, can be reduced if m is large. Equation(9) also shows the rapid loss of efficiency when c is reduced below v Using(9), we can rewrite(5)as 16.522, Space Propulsion Prof. manuel martinez-Sanchez Page 5 of 1916.522, Space Propulsion Lecture 2 Prof. Manuel Martinez-Sanchez Page 5 of 19 were vL is a “loss velocity”, equal to the velocity to which one ion would be accelerated by the voltage drop ∆ φ. Notice how this simple expression already indicates the importance of a high atomic mass propellant; ∆ φ is insensitive to propellant choice, and so vL can be reduced if mi is large. Equation (9) also shows the rapid loss of efficiency when c is reduced below vL . Using (9), we can rewrite (5) as