I was arguing with my NASA friend the other day about what would make a more efficent propellent for an ion engine. Is it a lighter ion or a heavier ion. THis was what I derived. Assume a charge q of mass m is accelerated through a potential V. The mass of the ship is M. Working through the problem with the tools an intro physics student could comprehend, I arrived at an equation that gives you the change in velocity for the ship after consuming ONE available ion. v = Sqrt[2qVm/(M^2)] I'm pretty confident in this. The units work out and all variables are in the correct place intuitively. i'm confused on one point. I already have velocity as a function of mass of one particle. Do I need to divide by mass of one particle to get to velocity change per unit mass? I think i'm confusing particle mass with bulk mass somehow.