Why Are There Two Different Types Of Electric Space Engines, And How Do They Work?

Electric Propulsion is now a dominant force in space propulsion (pun intended) - in the last few decades more and more spacecraft have adopted Ion thrusters for orbit maintenance or deep space maneuvering, enabling performance that was previously beyond the capabilities of chemical thrusters.
There are Two main systems in common use, Gridded Ion thrusters are most used in deep space missions, while Hall Effect Thrusters are commonly found on satellites in Earth Orbit.

Follow me on Twitter for more updates:

I have a discord server where I regularly turn up:

If you really like what I do you can support me directly through Patreon

Contains some great animations from:
Thomas Marks
Plasmadynamics and Electric Propulsion Laboratory
University of Michigan

[scottmanley]

For everyone asking why it’s important to neutralize the exhaust, if you don’t then the positively charged Ions will get pulled back towards the spacecraft, and that will result in the thrust being cancelled out. So, it’s essential that the charge on the ions get removed in the process.

[ostlandr]

Wonderful to hear that "It doesn't match the models, but it works, so we'll just go with that." And the fact that it doesn't match the models means that the models need to be reworked, which means we'll learn cool new stuff.

[reeeeeeee]

A correction is that the electrons do hit the walls. At 11:53, it is shown that the electrons orbit along the magnetic field lines. And from the thruster illustration, the magnetic field lines do not go around the annulus; the field lines cross the gap radially outwards. This means the electrons are constantly orbiting the field lines and reflecting from each wall of the annulus, this causes erosion in the anode channel. The electrons slowly drift in a circle (but in fact they are bouncing back and forth between the inner and outer wall) due to the hall effect. The hall effect is a side effect, it does not play a critical role in confining the electrons. The mechanism responsible for confining the electrons is the gyration of the electrons about the magnetic field lines. Thus "Hall Effect thruster" is a bit of a misnomer, "Hall thruster" would be more accurate.

Regarding the higher mass flow rate of the Hall thruster, this is not really the major mystery. The reason why Hall thrusters can achieve a higher thrust density is because it effectively avoids the space-charge effect. In an ion thruster, the space between the grid only contains positively charged ions. This means there is a critical density where the amount of ions per unit volume is so high they start to repel each other. This prevents scaling the thrust up on a gridded ion thruster without scaling the size. In a Hall thruster, the ions and electrons exist in the same space, so the plasma is effectively quasi-neutral, and the electrons and ions shield each other (sort of) from the space-charge effect. This allows for higher thrust density.

However, the mystery lies in how to confine the electrons within the magnetic field effectively. The electrons diffuse out of the magnetic field at a higher rate than expected of classical models, but also slower than other proposed models, so we dont have a complete understanding of the mechanism that allows electrons to escape being trapped by the magnetic field lines.

[thecma]

An interesting consequence of the design of the hall effect thrusters is that they impart a swirl torque on the spacecraft along the axis of the thruster, so spacecraft using hall effect thrusters may need to desaturate inertial attitude control systems (reaction wheels/CMGs) periodically

[jonathanedelson]

Small correction at time 4:50. If you double the exhaust velocity you quadruple the energy needed per unit reaction mass, but you also double the thrust. So for the same thrust you only need half the reaction mass and double the power.

Jon

[Randelia]

Thank you so much for the ion/hall breakdown! Fascinating content. NASA continues to ignore my resume but thanks to you, KSP is ready to hire me!

The Hall effect thruster sounds something straight out of a science fiction book. It is unbelievable that we can build it and it works.

[ProjectPhysX]

Cool video, thanks! Small correction at 8:04: the gap width has no effect on ion velocity. Only the electric potential/voltage dictates ion velocity, not the distance between the charged grids.

[Chooie]

Some years ago I used to work in an inspection lab with a very nice computed tomography system, that is to say a machine built to take 3d x-rays of what was put in there simply speaking and outputting a volume of data that could be analyzed. The customer was aerojet rocketdyne and what I was inspecting was the rear outer rings of hall effect thrusters that were being tested for lifetime limits and erosion is definitely the best way to describe the appearance of the ceramic material. There were straight channels along the direction of thrust I forget how long they were running but for that to happen it was definitely a long time.

[randomm]

Thank you for that explanation of the hall effect. From watching AVE teardowns i knew that the hall effect had to do with magnets and electricity but didnt know what it was

[richb]

Thanks Scott for explaining how these thrusters work and applications.

[heiner]

Great explanation of Hall Effect Thrusters, I have heard a lot more about ion thrusters previously on ESA missions. Just to add FEEPs (Field Emission Electric Propulsion) may gain in popularity after decades of struggle. The slit FEEPs were actually removed from LISA Pathfinder spacecraft and replaced with a cold gas subsystem previously flight qualified for GAIA. But now various Cubesats are using the Enpulsion supplied needle FEEPs, which hopefully will meet lifetime requirements as there are proposed for missions like the AWS Protoflight model with a total spacecraft mass of about 135kg. So it could be that FEEPs are used for smallest spacecraft and the other electric propulsion technologies for larger satellites.

[Deltarious]

Ion thrusters have always been one of the very few real technologies that seem truly "SciFi" to me

[clevergirl]

A man of his word! Scott said he'll make a video on this subject in the last deep space update and he delivered!

[OolIi]

Scott I gotta say, your old OG explanation space vids thought me more science than school. Its amazing to see you still going keep it up✌🏼✌🏼

[arneschroder]

10:06 made my laugh! Electron „I wanna go there!“ „Eh eh“, answers the magnetic field, „you‘re gonna go sideways!“

[matterwave]

Yeah!! Ion source time :D
I'm working on electron impact ion source, your videos on ion thrusters are always a good discussion material with my colleagues, thank you for the good work Scott!

[JamesSimko]

I could listen to you speak all day... something about the way you can get information across is amazing

[carltheshivan]

Scott, I'd like to see a follow up about plasma thrusters. There are designs that do away with electrodes so the erosion problem is avoided. Stuff like helicon and mpd thrusters are cool bits of tech that we could use if space agencies ever considered doing more ambitious missions.

[brianmatthews]

Very interesting video. I've worked a lot with ion beam millers using these guns based on the same technology to produce mems.