Ion Drives – The Engines of the Future
By Andrew Baker
Section: 1
Introduction
In this this review, I will be examining the idea that for the foreseeable
future, ion drives are a practical and efficient option for the exploration
and potential colonization of the solar system. This review will focus on the
issues of thrust, longevity, and the energy requirements of the ion drive.
For decades, it has been apparent that in order to facilitate travel to
distant objects both in our solar system and beyond, a form of propulsion
far superior to that of conventional rockets is needed. To that end, a
variety of alternative propulsion methods have been devised utilizing
many different methods to generate thrust from the impacts of photons to
the detonation of nuclear warheads, however this review will focus
exclusively on the ion drive which generates it’s thrust by expelling
electrically accelerated charged particles at high velocities out of the
engine.
While the first recorded mention of an electric propulsion source was by R.
H. Goddard in 1906, it wasn’t until 1958 that the first ion drive was built [1].
This was most likely due at least in part to the fact that during this time
frame, two world wars had occurred and the Cold War was well under
way, and with it, the development of chemically propelled ICBM’s. Ion
drives, on the other hand, don’t have much use as a propulsion source
from a military perspective and as a result were not a priority for research
and development, however as we progress into an era of space
exploration, the ion drive is beginning to look more appealing. While
there are several different types of ion drives, each using a slightly
different method to accelerate the ions, this review will focus primarily on
the gridded electrostatic ion drive.
Thrust, Velocity, and Fuel
Unlike conventional chemical rockets, ion drives produce very little thrust
(on the order of tens to a few hundred mN). For example NASA’s NSTAR
ion drive produces a max thrust of 92 mN while it’s experimental NEXT ion
drive produces a thrust of 236 mN [2]. While these thrusts may seem small,
Stuhlinger says “the exhaust velocity of a rocket should be as high as
possible” [1]. This is important because a spacecraft’s maximum velocity
is related not to the amount of thrust it can produce, but to its exhaust
velocity [1] and the exhaust velocities of ion drives far exceed those of
conventional chemical rockets. Because of this, ion drives are
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theoretically able to achieve velocities far greater (on the order of at least
5 times greater) than that of conventional rockets, even though they
generally have accelerations in the order of 10e-4 g’s [1]. This is extremely
beneficial for the exploration of our solar system as not only does the
higher spacecraft velocity allow for significantly reduced mission times
(which lead to huge cost savings), but as ion drives weigh less than
conventional rockets, the mass difference at launch can be replaced
with scientific equipment, (making each mission not only shorter, but more
productive) [4], or eliminated by the use of a smaller takeoff rocket which
is less expensive to launch [3]. Brophy [3] describes a sample return
mission from a comet where an ion drive propelled spacecraft could
bring samples back in just over seven years while a conventional
spacecraft would take over nine years just to get there with no option to
bring samples back. Noca [4] agrees with using ion drives for this and
goes so far as to say that SEP (Solar Electric Propulsion) is an enabling
technology for this type of mission. This mission has all of the abovementioned advantages over a conventional chemical rocket.
While ion drives produce significantly less thrust than a conventional
rocket, the same also holds true for their fuel consumption. They
essentially “sip” fuel (generally between 0.24 and 0.36 mg/s according to
Brophy and Noca). This means that they can carry significantly less fuel
then would conventionally be needed, which allows more space for
scientific equipment, or conversely a significantly increased time in which
the engine is capable of providing thrust. This combination of low thrust
and low fuel consumption means that ion drives have a high specific
impulse.
Impulse and Longevity
Specific Impulse is a term used to define the efficiency of a rocket engine.
It represents the force of the engine with respect to the fuel used per unit
time and for the purpose of this review, has units of seconds. Generally
speaking, the higher the specific impulse of an ion drive, the more
powerful it is. NASA’s current state of the art NSTAR class of ion drive has a
specific impulse of 3100 s [3], while its next generation of ion drives (NEXT)
has a specific impulse of 4100 s [2]. This is a huge improvement in only a
few years. Just imagine what kinds of impulses will be possible a few
engine generations down the road. If ion drives are appealing now, they
will be mission enabling in the future, and could become the preferred
propulsion method of both manned and unmanned spacecraft.
Aside from their high impulses, ion drives have another unique aspect;
they are able to run for prolonged periods of time. During one test
described by Brophy, the NSTAR was operated at full power for 8192 hours
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before being voluntarily shut down after having produced a total impulse
of 2.73e6 N s and using 88 kg of xenon fuel [3]. This is over 341 days of
continuous operation. Similarly, Patterson and Benson discuss a test of the
NEXT drive in which it was operated for over 9990 hours (more than 416
days) at full power [2]. While these may seem like a long time, the most
impressive part is that Brophy, Patterson, and Benson all agree that at the
ends of their tests, the engines showed no signs of being anywhere close
to failing. This shows that given adequate fuel and power, ion drives have
the potential to be operated continuously for over a year, perhaps more
at full power without fear of failing. This is a characteristic that is crucial for
any deep space mission.
Energy Requirements
The only potential drawback and real limitation of ion drives is their energy
consumption, which unlike their thrust and fuel consumption is anything
but small. NASA’s NSTAR drive requires between 0.5-2.3 kW while their
NEXT drive requires between 0.5-6.9 kW [2]. Remember that this power
requirement is for an individual drive and many systems in which these
drives will be used will contain upwards of three or four drives which will be
operated simultaneously (though not necessarily at full power). As a result
the power system must be able to generate enough energy to power all
the drives at full power. Currently, there are two practical methods that
can be used, nuclear and solar. Stuhlinger is a firm proponent of the use
of fission reactors while Brophy, Noca, Patterson, and Benson all believe in
using solar power. For the time being solar is the preferred method as
nuclear generation not only adds a significant amount of mass to the
craft, but also greatly increases its cost and complexity. The primary
downside to using solar energy to power the craft is that the energy that
can be generated from a solar array of a fixed size is directly proportional
to the distance the array is from the sun (If you double your distance from
the sun the power you can generate decreases by a factor of 4). This
however does provide an interesting side effect. For mission safety
reasons, NASA requires a safety margin on the operation of ion drives. This
means that while they can be operated at full power, the average power
at which they operate must be less than their maximum by a
predetermined amount. For the case of the NSTAR drive, even though the
maximum power is 2.3 kW, the average at which they are allowed to
operate is less than 2.1 kW [3]. Because of the solar array’s diminishing
capacity to produce power as the distance from the sun increases, craft
utilizing this power source automatically meet this requirement.
Conclusion
While it is obvious that there is still a lot of research and development to
be done with ion drives, their potential for use on both manned and
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unmanned spacecraft is just as obvious. As we enter an age in which our
curiosity cannot be satisfied by simply observing the planets and stars, we
are in need of technology that will enable us to actually go there. To that
end, ion drives are a practical and efficient solution for the foreseeable
future. Their low thrust is more than compensated for by their high
impulse, low fuel consumption, and ability to run for extended periods of
time, which give them the ability to attain velocities impossible with
conventional technologies. The only limitation we have come across is
the significant amount of energy they consume. As a result, research
should be directed into either finding a way to reduce this energy
consumption, or make our current methods of energy generation more
efficient. Ion drives are truly a propulsion source that we are only just
beginning to realize.
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References
1. E. Stuhlinger, Ion Propulsion For Space Flight. New York, NY:
McGRAW-Hill, 1964, pp. xviii – 10.
2. M. J. Patterson, S. W. Benson, “NEXT Ion Propulsion System
Development Status and Performance”, in AIAA/ASME/SAE/ASEE
Joint Propulsion Conference & Exhibit., Cincinatti., OH, 2007, pp. 1 –
17.
3. J. Brophy, “Advanced Ion Propulsion Systems for Affordable DeepSpace Missions”, Acta Astroautica, vol. 52, no. 2-6, pp. 309 – 316.
Mar, 2003.
4. J. R. Brophy, M. Noca, “Electric Propulsion for Solar System
Exploration”, Journal of Propulsion and Power, vol. 14, no. 5, pp. 700
– 707, Oct., 1998.