Download Ion Thrusters

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.
A long time ago in a galaxy far, far
away…
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!
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.
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.
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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.
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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.
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.
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.
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.
“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.
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)
“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)
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.
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).
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.
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.
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.
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.
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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.