Gorr, 6:00
L11
THE ION ENGINE: SPACE PROPULSION SYSTEM OF THE FUTURE
Jake Stambaugh (jvs7@pitt.edu)
INTRODUCTION: THE ADVANTAGES OF
THE ION ENGINE
chemical reaction that combines volatile solid or liquid
components to release a large volume of gas, which propels
the rocket in the opposite direction [3].
A major engineering challenge of present-day space
travel is the management of fuel and maximization of its
efficiency. Replacing current chemical-based rockets with
ion engines would allow for variable specific impulse: the
ratio of the thrust produced by the engine to the weight flow
of the propellants [1]. A variable specific impulse, as is
generated by an ion engine, allows a rocket to apply
different amounts of thrust at different stages of its journey,
while a chemical rocket has to generate a large amount of
thrust at the beginning to carry its large fuel payload out of
Earth’s gravity well [2][3]. A rocket operating with an ionic
propulsion system achieves its maximum velocity in space
and reaches its destination sooner than a chemical rocket is
capable of. For example, a ship propelled by an ion engine
could reach Mars from Earth in 39 days, while the Curiosity
rover, as a reference, took 10 months to reach its destination
[2][4]. The ability for a shuttle to travel this quickly through
space extends the range of manned missions deeper into the
reaches of our solar system. Missions into space like these
have always attracted me to the astronautically related fields
of engineering. This has fostered a personal interest in the
physical and chemical sciences that make these marvels of
space travel possible. The process of researching the ion
engine has further expanded my understanding of the
physics behind rocketry, and allowed me to apply the
physics knowledge I have acquired in high school and
college classes to a real engineering topic.
How the Ion Thruster Works
HOW THE ION ENGINE DIFFERS FROM
CHEMICAL ROCKETS
According to Newton’s third law of motion, every force
has an equal and opposite reaction force. Rockets accelerate
using this principle by creating contained explosions in
chambers with one open end. The gases released during
these explosions move at very high speeds; as they bounce
off the inside of the chamber, they exert forces on the rocket.
If the gas particles bounce off of every side of the chamber
they would exert a net force of zero on the system, causing
no acceleration. However, since one end of the chamber is
open, the particles can escape without applying any forces to
the open side. This creates a net force on the system in the
direction opposite to the open end of the chamber, so the
rocket accelerates in the desired direction. The magnitude of
this acceleration is referred to as the rocket’s thrust. In a
chemical rocket, the thrust is generated by an exothermic
University of Pittsburgh, Swanson School of Engineering 1
Oct. 30, 2012
Ion propulsion engines use ionized plasmas as
propellants to generate thrust. Neutral plasma atoms are
injected into the engine chamber where they are bombarded
with high energy electrons that are generated by a discharge
cathode. When the atoms are bombarded with electrons,
each atom loses one of its own electrons to collide with new
atoms. This turns more neutral atoms into ions, which
release more electrons creating a chain reaction. The engine
is set up with a positively charged electrode towards the
front of the thruster (upstream) and another electrode with a
negative charge at the downstream end. The electromagnetic
forces of these electrodes direct the ions downstream [5]. In
modern variants of the ion engine such as the Ad Astra
Rocket Company’s VASIMR, radio waves tuned to the
rotational frequency of the ions superheat the plasma to
temperatures reaching one million kelvins. Since the
frequency of the radio waves can be manually adjusted, so
too can the heat of the plasma, giving the engine a variable
specific impulse [6]. Finally, the plasma ions accelerate
down the length of the thruster until they exit as ion jets. The
ion jets are collectively called the “ion beam,” which creates
the thrust of the engine [5].
HOW THE ION ENGINE CAN IMPROVE
CURRENT MEANS OF SPACE TRAVEL
In a traditional chemical rocket launch, the fuel is ignited
inside the rocket, and the entire capacity is expended within
roughly two minutes. The rate at which the fuel is burnt can
be controlled slightly by the shape of the nozzle, and once
ignited the rocket’s burn cannot stop and restart [4]. The
goal of a space launch is normally to escape Earth’s
gravitational well before the fuel runs out. Then the rocket or
attached spacecraft can drift through space. Consequently,
the rocket is out of fuel when it exits Earth’s atmosphere,
and its only option is to drift passively to its destination.
Applications of the Ion Engine
An ion engine can equip a rocket with a form of
propulsion after it leaves the atmosphere, making it possible
to reach distant locations in shorter amounts of time. A
potential use of an ion engine is as a second-stage thruster to
be used after primary chemical rockets propel the spacecraft
Jake Stambaugh
out of the atmosphere. This is more a plausible solution than
using a second-stage chemical rocket for deep space because
an ion engine and its fuel weigh far less than a chemical
rocket and its fuel. Therefore, it takes less energy to move
the spacecraft from Earth into space. An ion thruster is also
able to change its momentum by using very small amounts
of fuel due to its variable specific impulse. This allows it to
conserve fuel and continue firing the thrusters for a much
longer portion of the trip. Because of this, the spacecraft
reaches its maximum velocity in deep space, and greatly
reduces the time necessary to reach distant locations [6].
Systems like this are already being employed; for instance,
one is currently in place in NASA’s Dawn spacecraft, which
launched in 2006 [4].
The second useful application for an ion thruster is as the
only rocket on a spacecraft. This requires an engine that
propels itself into space from Earth’s surface. Currently, the
engines in development require five to ten megawatts of
electricity to generate enough thrust for this application.
However, this much power would require a source with a
comparable output to a nuclear power plant [6]. Presently,
potential solutions being tested include nuclear fission
reactors or large solar cells mounted to the rockets.
Unfortunately, these solutions are still in testing phases, and
are not yet ready for deep space [3].
Although the ion engine itself is not an ethically
ambiguous issue as much as it is a technical advancement,
the engineers responsible for developing and building it are
still expected to perform their duties in an ethical, honorable
and responsible fashion to honor the profession of
engineering [7].
THE VALUE OF RESEARCHING AN
ENGINEERING TOPIC FOR
ENGINEERING STUDENTS
This is a useful assignment because it gives students an
opportunity to pursue one of their own interests, but it also
shows them how to view a concept through the lens of an
engineer. For this assignment, students research an issue or
topic related to an engineering field that interests them. In
addition to learning about the topic, the student could have
to apply math, physics, chemistry and biology to understand
the context of the issue. This exemplifies the process of
applying scientific concepts to a real-world issue.
I encountered a problem when beginning to research the
ion engine for this paper. I found that many of the resources
and articles published on ion engines and electric engine
propulsion were addressing topics like specific impulse and
the methods through which ions are turned into plasma.
These were topics that I had never seen and didn’t
understand, so before researching ion engines I had to find
resources to explain these topics related to how the ion
engine worked. By researching and applying these smaller
conceptual topics to a larger practical one (the ion engine), I
feel that I understand these new topics better than if I had
been introduced to them during a lecture, and it seems that I
am not the only person who this holds true for.
ETHICAL CONSIDERATIONS FOR
ENGINEERS IMPLEMENTING THE ION
ENGINE
Engineers involved in the research of these ion engine
technologies must always abide by the National Society for
Professional Engineers’ code of ethics. The primary
propulsion mechanism of the ion engine is dangerously hot
plasma, so the engineers must prioritize safety when
designing any part of the engine [7]. Without the proper
attention to detail, a failure of the ion engine could
compromise the lives of astronauts depending on it. Any
engineer failing to solve a safety-related problem would
breach the first fundamental canon of the NSPE code of
ethics [7].
Beyond the scope of the safety of the engine and the
power source, the engineer’s relation with the employer and
the public must be a primary focus. Both the AIAA and
NSPE require their members to maintain an impartial
opinion and an honest relationship when dealing with the
public [7][8]. Because of the cost of developing the ion
engine, there are many parties financially invested in the
success of this engine. Any one of these parties could have
an interest in obscuring information about the ion engine for
political or financial reasons. Regardless, no engineer
involved with the ion engine can present the information as
anything but objective and truthful, and they must also alert
their employers to any conflict of interest in one of these
areas.
University of San Diego Engineering Program
An experiment conducted at the University of San Diego
evaluated the impact of a laboratory session in the freshmen
semester of an engineering program [9]. The experiment
observed the 2008 freshmen engineering class as they
attended two lectures per week about engineering design.
The instructors also added two labs per week where the
students were broken into small groups to perform tasks with
LEGO MINDSTORMS robotic systems, following and
adapting the design processes from the textbook and lecture.
Additionally, the instructors administered written
assignments after every lab to track student progress. The
results showed that by applying the material taught in lecture
to a practical problem, many of the students showed an
increase in measured knowledge level and the majority of
the population showed a moderate, high or superb
understanding of the material based on test scores [9].
These results mirror my own experiences in applying
engineering concepts to a larger problem. Applying a
concept to a practical issue requires a firm grasp of the
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Jake Stambaugh
[6] S. Upson (2009). “Rockets for the Red Planet.” IEEE
Spectrum. (Online article). http://spectrum4-stagingdev.elasticbeanstalk.com/aerospace/space-flight/rockets-forthe-red-planet.
[7] “NSPE Code of Ethics for Engineers.” National Society
of
Professional
Engineers.
(Website).
http://www.nspe.org/Ethics/CodeofEthics/index.html
[8] “AIAA Code of Ethics” American Institute of
Aeronautics
and
Astronautics.
(Website).
https://www.aiaa.org/Secondary.aspx?id=4324
[9] T. F. Schubert, F.G. Jacobitz, E. M. Kim (2011).
“Student perceptions and learning of the engineering design
process: an assessment at the freshmen level.” Research in
Engineering
Design.
http://link.springer.com/article/10.1007%2Fs00163-0110121-x
theoretical elements of the topic. In the case of the
University of San Diego experiment, their robotics lab
helped reinforce the engineering design principles that were
taught. In my case, explaining the mechanics of an ion
thruster solidified my understanding of thrust, specific
impulse, and ionization; researching and applying the codes
of ethics related to my intended field of engineering helping
me to gain an intuitive knowledge of their content and
application.
CONCLUSION: THE ION ENGINE AND
THE FUTURE OF SPACE TRAVEL
Ion engines are currently one of the most promising
propulsion sources for future deep space exploration. Their
high fuel efficiency makes them lighter than chemical
rockets and allows them to travel quickly through space.
Current research efforts are focused on refining the design of
the engine and engineering power sources capable of
sustaining high levels of electricity, allowing for even
greater flight range. This broadens the possibilities for space
travel that would have previously been impossible. For
example, on a manned mission to Mars, resources like food
and breathable air are limited due to a rocket’s restricted
payload. By decreasing the time it would take to get to Mars,
an ion engine could reduce the amount of these resources
required by up 80% [6]. This, in turn, lightens the payload
and requires less fuel to launch as well as shortens the trip
for the astronauts.
This assignment helped me to learn about the
complicated challenges that engineers are attempting to
overcome and the strict ethical codes they follow. However,
in spite of these hurdles I believe that the ion engine is a key
component of our future in space. As NASA’s interest in
manned missions to the corners of our solar system seems to
fade, technologies like the ion engine are the instruments
vital to sending man into the cosmos once again.
ADDITIONAL SOURCES
[10] T. Benson (2012). “Rocket Thrust.” NASA Glenn
Research
Center.
(Website).
http://www.grc.nasa.gov/WWW/k-12/rocket/rktth1.html.
ACKNOWLEDGEMENTS
Courtney Elvin – For keeping me on task and her myriad of
edits
Kerbal Space Program – For their fun and helpful simulation
of rocket physics
Hurricane Sandy – For giving me a reason to stay indoors
and write
REFERENCES
[1] T. Benson (2008). “Specific Impulse.” NASA Glenn
Research
Center.
(Website).
http://www.grc.nasa.gov/WWW/k-12/airplane/specimp.html
[2] L. Grossman (2009). “Ion engine could one day power
39-day trips to Mars.” New Scientist. (Online article).
http://www.newscientist.com/article/dn17476-ion-enginecould-one-day-power-39day-trips-to-mars.html?full=true.
[3]
M.
Brain
(2010)
“How
Rockets
Work”
HowStuffWorks.com.
http://science.howstuffworks.com/rocket3.htm.
[4] M. Wall (2012). “NASA's Huge Mars Rover Curiosity:
11 Amazing Facts.” Space.com [Online blog].
[5] “NASA – Ion Thrusters.” (2010)
Glenn
Research
Center
http://www.nasa.gov/centers/glenn/about/fs21grc.html.
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