NASA Fires Up Powerful Lithium-Fed Thruster for Trips to Mars

[eppur_se_muova]

5. Hydrogen is being used there as ejectable mass, nothing more. It's handled by mechanical pumps and pressure.

In contrast, ionized gases/plasmas interact with both electric and magnetic fields to produce that equal-and-opposite thrust. The electrostatic ion engine seems to be favored by high mass/charge ratio -- high m, low v means a lower delta-mv for the same delta-m(v-squared), I guess. Thrust is controlled by the electric field, whereas the classic rocket equation relies only on thermal energy (from combustion, or nuclear reactor) to give gas molecules velocity. I'm assuming MPD (a variation on MHD, presumably) creates large magnetic fields through discharges in the conducting plasma, but I'm really guessing there -- I'd really like to see a basic-math treatment of both.