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I am in high school, and am doing a major research project on Russia. Part of that is a section on the space race, and ion engines/hall effect engines have come up several times. Unfortunately, Google and other searches on this site have turned up a whole lot of things that make no sense or do not pertain to space propulsion. Could someone please explain how a Hall effect thruster works, and why they seem to be so much more powerful than anything else we've sent into space, in terms that a high school student could understand?

** Note: I state more powerful because of a graph posted in a PDF by the air force. Because I do not have 10 rep, i cannot post the picture, but going to this link will bring up the PDF, the graph is on page 3.

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Should be careful what you mean by "more powerful" - compared to chemical rockets ion engines are very (propellant) efficient, but the thrust is also very low and the electrical power required is significant. What in the wikipedia explanation is unsatisfactory? What concepts are you having trouble understanding? They use electric fields to accelerate positively charged Xenon ions to produce thrust. The problem is that to produce ions they need to strip electrons off the Xe atoms. The electrons aren't useful for thrust so they try to trap them with magnetic fields - hence name "Hall effect." –  Michael Brown Feb 12 '13 at 1:47
Thank you, could you provide another link? no matter what else is said, I have the type of teacher that will not accept Wikipedia. At all. So while Wikipedia has a understandable definition, I am hoping for links to other things that I can draw from. Also, I am looking for more comparisons to other engines/fuels, as this is over the space race as a whole. –  Harper Feb 12 '13 at 2:15
The Wikipedia article you've linked to seems to provide a considerable amount of information on the design and functioning of these devices. Can you be more specific about what you don't understand? –  dmckee Feb 12 '13 at 3:07
That was linked by someone else in an edit, but i do not understand why Hall thrusters are so much more powerful, in terms of power per specific impulse (I do not understand quasi-neutral plasma and the Child-Langmuir charge saturated current limitation in the article), how they compare to other rockets for specific jobs (in orbit transfers, earth orbit to mars flights), and how viable they really are for those jobs. Wikipedia says they have a "100% success rate" but a small thrust compared to chemical rockets, does that make them less viable for the job? –  Harper Feb 12 '13 at 3:28
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2 Answers

up vote 2 down vote accepted

If you want to avoid Wikipedia, look at the following NASA resources:

Overview of Hall Effect Thrusters

Ideal Rocket Equation

The NASA overview is short and easy to read, so there's no need to re-explain how the thruster works. To expand on the discussion in this question, there are (at least) two circumstances where a Hall thruster is a much better choice than a chemical rocket.

  1. When you need high precision stationkeeping: if you want to make precise adjustments to the orientation of a satellite, it helps to be able to control thrust in very small increments. This is a big challenge for chemical rockets, but Hall thrusters can easily adjust their thrust in milliNewtons by changing discharge voltage and/or propellant mass flow rate.

  2. When you need a high ratio of thrust produced to propellant used: Ion thrusters will usually outperform Hall thrusters here, but compared to chemical rockets the Hall thruster is still a much better choice. Read about Specific Impulse to lean more.

As you probably know, the weight of components is immensely important for space launch, so scientists and engineers make significant efforts to minimize weight wherever possible. For propulsion, if we can make efficient use of our propellant, then we don't need to carry as much of it, thereby reducing weight. The second link above shows the ideal rocket equation


where $\Delta{u}$ is the change in the spacecraft's velocity, $MR$ is the ratio of empty spacecraft mass (no propellant) to full mass (full of propellant), and $v_{eq}$ is related to the exhaust velocity of the propellant. So, suppose you have a propulsion system and you plan to use all of your propellant to get the biggest increase in velocity possible: you want to maximize $\Delta{u}$ for a fixed $MR$. As you can see from the equation, to maximize $\Delta{u}$ you must make the exit velocity $v_{eq}$ as large as possible.

This is where the differences between propulsion systems become very clear. You can perform additional research as needed, but its not hard to find sources for the following typical exhaust velocities:

Liquid fueled chemical rocket: under $5000$ $m/s$

Hall thruster: around $20000$ $m/s$

Ion thruster: around $50000$ $m/s$

When people tell you that Hall thrusters are "more powerful" than chemical rockets, they really mean that Hall thrusters give you more thrust for a given amount of propellant mass. In terms of thrust alone, chemical rockets win: There is no electric propulsion device in existence that can launch a rocket from earth's surface into orbit. Chemical rockets create high thrust by moving an enormous amount of propellant mass. Another good parameter for additional plasma thruster research is Thrust-to-power ratio ($T/P$).

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Thank you, I don't know how I didn't find that NASA page, but this answered all of my question. –  Harper Feb 12 '13 at 22:05
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Its not more powerful, its rather more efficient. Its basically a plasma device which produces plasma and ions are thrown out by mean of positive potential. Now the space craft will be charged positive over time, to compensate this a different component releases electrons hence maintaining charge.

Ions are used as it is more heavy and gives useful thrust. its used for satellite re orientation and maintaining track.

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But how does it compare to a chemical rocket? –  Harper Feb 12 '13 at 19:00
As i told more efficient, which means you need fewer kg of working material for same thrust, but of course you cannot lift off with ion engine. While in space its useful as you need little thrust (Power, work rate) for maneuvering, hence its useful in such situation. You see, you have to carry little fuel for same thrust the Chemical engine. –  Samir Chauhan Feb 13 '13 at 3:56
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