Ion Thrusters

Let’s Talk Science Fiction

Star Wars A long time ago in a galaxy far, far away… Star Wars is a movie produced by Lucas Films Ltd. If you are wondering why I have a slide dedicated to Star wars, I am a huge nerd and love it. That and it is relevant to the subject of Ion Thrusters. The next slide will explain. Hopefully. If you believed Star Wars was scientifically accurate in most regards, you may want to rethink the beliefs you hold about it. There is nothing else important being written here. Props if you kept reading it though. More props if you actually managed to read fast enough to make it to this point in this rather unimportant part of text. Honestly just adding filler at this point so the animation plays and has some actually words to display. Shame I can’t tilt this like the actual Star Wars intros but I digress. Onwards to science! Star Wars A long time ago in a galaxy far, far away…

This should look familiar

TIE Fighters The name of this iconic Starfighter is actually an acronym for “Twin Ion Engine.” It is a reference to the real life technology of ion thrusters. However, if you had not guessed, it does not share any real aspects of ion thrusters, aside from the name of course.

How would TIE Fighters be in real life? If TIE fighters had ion thrusters that functioned like the ones we had now, they would be going considerably slower. It would make a snail proud how slow it would be going. Space fights would have taken much longer to begin and that would have been boring to watch, hence why it only has the name in common.

Science time!

Some History The concept of Electric Propulsion (or EP) was first conceived in 1906 by Robert Goddard. In 1916, Goddard built an Ion engine and showed it produced thrust. In 1964, NASA scientist Harold R. Kaufmen built and successfully tested an Ion engine that used Mercury as reaction mass in the suborbital flight of SERT 1. The physics of EP was first described expansively in a book by Robert Jahn, in 1968.

More History The technology of ion propulsion that used cesium and mercury, along with mission design and trajectory analysis, was published by George Brewer in 1970. 1998, NASA launched Deep Space 1, which used an Ion engine. Deep Space 1 flew by the asteroid 9969 Braille and comet Borrelly. The success of this mission paved the way for future missions powered by Ion engines.

Ion Thrusters: What are they? An ion thruster is a type of electric propulsion used to for spacecraft propulsion. They are well known for little propellant, comparatively to other forms of propulsion, and thus can travel great distances. Ion thrusters, and other electric propulsions, are also easier to launch. The reason is because they have less launch mass due to reduced propellant. They are cheaper to launch and often used on small spacecraft for long range missions.

How do they work? As the name suggests, “An ion thruster ionizes  propellant by adding or removing electrons to produce ions” (Dunbar). Most ion thrusters use electron bombardment. This is when a high energy electron (negative charge) is collided with a propellant atom (neutral charge). This releases electrons from the atom and creates a positively charged atom.

How they work Part 2 The gas produced is made up of positively charged ions and negatively charged electrons, which results in no overall electric charge. If you hadn’t guessed, this is plasma and plasma can be affected by magnetic and electrical fields.

How they work part 3 “In most ion thrusters, electrons are generated with the discharge hollow cathode by a process called thermionic emission” (Dunbar). These electrons are attracted to the discharge chamber walls, which are charged to a high positive potential by voltage applied by the thruster’s discharge power supply. The propellant atom is then injected into the discharge chamber for Electron bombardment.

How they work part 4 High strength magnets prevent electrons from reaching the discharge walls too easily, increasing the amount of time they are in the chamber and increasing to probability of ionization. “The positively charged ions migrate toward grids that contain thousands of very precisely aligned holes (apertures) at the aft end of the ion thruster. The first grid is the positively charged electrode (screen grid)” (Dunbar)

Almost done. “A very high positive voltage is applied to the screen grid, but it is configured to force the discharge plasma to reside at a high voltage. As ions pass between the grids, they are accelerated toward a negatively charged electrode (the accelerator grid) to very high speeds (up to 90,000 mph)” (Dunbar)

Final explanation slide The final step is for the positive ions to be accelerated out of the thruster as an ion beam. Thrust is achieved! Electrons are also released from the neutralizer, another hollow cathode, to make the beam neutral. An important step as it prevents ions from being drawn back to the spacecraft. Ions being drawn back would reduce thrust and erode the spacecraft, which is bad.

But wait, there’s more! The most commonly used propellant in ion propulsion is xenon. This is for a few reasons: High Atomic mass and is easily ionized, thus creating a desirable amount of thrust. Inert and has high storage density, which makes it great for storing on spacecraft. “Ion thrusters have an input power need of 1–7 kW, exhaust velocity 20–50 km/s, thrust 25–250millinewtons and efficiency 65–80” (Wikipedia contributors).

Inside an ion thruster: Hall thruster version

What makes it different from other EP? Ion thrusters are not the most promising of the different kinds of electrical propulsion but it is the most successful in practice. Ion engines have been shown to be very reliable and have flown several successful mission. Research into other EP types are still ongoing and ion thrusters may be replaced in future with a more efficient form of EP.

Another picture

What are they used for? As mentioned in an earlier slide, they are used for a wide variety of missions. Here are a few: SERT-I and II, July 20, 1964 and Feb. 3, 1970. Deep Space 1, 1998 Hayabusa, 2003. SMART-1, 2003 Dawn, September 27, 2007 They are also used to for altitude adjustments of spacecraft, usually to keep them in orbit.

The problem with Ion Thrusters Going back to the beginning of this presentation, they do not produce a great deal of thrust very quickly. “An ion drive would require two days to accelerate a car to highway speed” (Wikipedia Contributors). They are only usable in the vacuum of space, as they do not work when ions are present outside the engine. They always have to rely on conventional chemical rockets to be launched into space.

Here is a video

Recap So while they are great for missions that require little initial thrust or travel a great distance and can keep spacecraft in orbit, they are not good for anything else and are useless in an atmosphere. So while the TIE fighters would probably get great mileage from an ion thruster, they would be useless as star fighters. Good thing George Lucas took some liberties with science in his movie.

Work Cited Dunbar, Brian. "Ion Propulsion." NASA. NASA, 11 Jan. 2016. Web. 08 Dec. 2016. Goebel, Dan M., and Ira Katz. Fundamentals of Electric Propulsion: Ion and Hall Thrusters. Hoboken, NJ: Wiley, 2008. Print. Rogers, Charles W., B.A., M.S., Ph.D. "Ion Propulsion." Salem Press Encyclopedia Of Science (2016): Research Starters. Web. 8 Dec. 2016 Wikipedia contributors. "Hall effect." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 1 Dec. 2016. Web. 1 Dec. 2016. Wikipedia contributors. "Ion thruster." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 24 Nov. 2016. Web. 24 Nov. 2016.