Electric Propulsion Types

In my experience, sci-fi often tends to lump the entire category of low-thrust, high-efficiency, electrically-powered engines into the label "ion thruster."

Although I won't say it's "wrong" -- this naming convention is just as valid -- the naming convention used in engineering is slightly different, with "ion thrusters" being just a subset of the larger category of electric propulsion (a name sometimes shortened to "eprop").

There are three main categories of electric propulsion -- electrothermal, electrostatic, and electromagnetic. We'll go over each of them here in a little more detail. Do note that this is focusing on concepts that are in the realm of "have been tested;" I will be ignoring photonic drives and whatnot.

Electrothermal Propulsion

Electrothermal propulsion uses electricity to heat propellant. Simple as that. Many are basically slightly-more-efficient monopropellant thrusters.

Although not totally accurate, perhaps the easiest way to explain them is thus: Isp is proportional to the propellant's velocity as it leaves the engine. Making the propellant hotter increases the velocity.

Here's some examples:

• Resistojet: A resistojet passes the propellant through a chamber with a filament in it. Electricity is passed through the filament; it heats up, and as the propellant passes over it, the propellant is heated in turn.
• Arcjet: In an arcjet, a high voltage between two electrodes creates a plasma arc through the propellant stream. This heats up the propellant. These are a bit more efficient than resistojets, but the plasma arc tends to be really good at eroding the engine walls, so they can be a bit lifetime-limited.

Electrostatic Propulsion

Electrostatic thrusters, in most cases, work by ionizing the propellant (thereby making it charged) and then placing it in proximity of a charged electrode, which it accelerates towards.

This category either is, or contains, the mythical category of "ion thrusters," depending what you count.

Generally, these tend to be the highest efficiency, but lowest thrust -- even moreso than electrothermal and electromagnetic engines, which are already efficient but low-thrust compared to typical chemical thrusters.

Some examples:

• Gridded Ion Thruster: the prototypical "ion thruster." A metal grid (a plate with a bunch of holes in it) is used as the accelerating electrode; positively charged ions are created in the chamber and then accelerate towards the grid, leaving the engine at high velocities.

Electromagnetic Propulsion

Electromagnetic thrusters, in most cases, use the Lorentz force to accelerate propellant. Consequently, it's really the magnetic field doing the work, with the electric field typically perpendicular to the intended direction of acceleration.

These usually require higher energy inputs than anything we've yet put in space -- consequently, they've only really been tested on the ground.

Some examples:

• Magnetoplasmadynamic thruster (MPDT): Kind of the prototypical electromagnetic thruster -- an MPDT injects ionized propellant into a chamber with a magnetic field shaped to accelerate it outwards uzing the lorentz force.