Feasibility of the Development of a
Hybrid Ion Drive/ Solar Sail Propulsion
System
Prepared for:
Administrator Charles Bolden
Administrator, NASA
Prepared by:
Andrew Baker
11/16/2014
This report addresses the feasibility of implementing a hybrid Ion Drive/ Solar Sail
propulsion system on manned and unmanned spacecraft for the exploration of
our solar system and beyond.
Table of Contents
LIST OF VISUALS
III
EXECUTIVE SUMMARY
IV
INTRODUCTION
1
PURPOSE, SCOPE, AND METHODOLOGY
1
SOLUTION OVERVIEW
1
ION DRIVES
SOLAR SAILS
2
2
SOLUTION CRITERIA
3
TECHNOLOGICAL AVAILABILITY
RELIABILITY AND LONGEVITY
CONSISTENCY WITH ORGANIZATIONAL GOALS
3
3
3
EVALUATION OF CRITERIA
3
TECHNOLOGICAL AVAILABILITY
ION DRIVES
SOLAR SAILS
CONCLUSION
RELIABILITY AND LONGEVITY
ION DRIVES
SOLAR SAILS
CONCLUSION
CONSISTENCY WITH ORGANIZATIONAL GOALS
ION DRIVES
SOLAR SAILS
CONCLUSION
3
3
4
4
5
5
6
6
6
7
7
8
CONCLUSION
8
RECOMMENDATION
8
REFERENCES
9
List of Visuals iii
List of Visuals
Figure 1 - Solar Sail Forces and Motion
2
B. N. Cassenti. “Optimization of Interstellar Solar Sail Velocities,” Journal of The British
Interplanetary Society, vol. 50, pp. 475-478, 1997.
Table 1 - Performance Characteristics of NEXT vs. NSTAR SOA.
4
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.
Executive Summary iv
Executive Summary
For decades, it has been apparent that in order to facilitate travel to distant objects both in our
solar system and beyond, a form (or forms) of propulsion far superior to that of conventional
rockets is needed. This report will focus exclusively on the ion drive and the Solar Sail both of
which are classified as Low-Thrust Propulsion. This report will examine the feasibility of a hybrid
Ion Drive/ Solar Sail propulsion system for the purpose of the exploration of our solar system and
potential travel to others.
Ion drives have been successfully used in space since the mid 1960’s. The most recent advances
(whose specifications have been summarized in Table 1) have been the development of NASA’s
current state of the art NSTAR drive, with a maximum thrust of 92 mN, and the next generation
NEXT drive with a maximum thrust of 236 mN [2].
Ikaros, the largest solar sail to date (14 meters) produces 6.35 μN of thrust over its area of 14 m^2
[9]. This thrust from the solar sail takes place at a distance of 1 A.U. from the sun and can be
calculated by the equation p = E/C where p is momentum, E is energy, and c is the speed of
light. At 1 A.U. the energy from the sun is 1631 W/m^2 so the equation becomes p = 1631/(3e8)
 4.53 μN for a completely absorbent solar sail and twice this for a completely reflective solar
sail.
While both ion drives and solar sails are reliable over the long term, they both suffer from
limitations such as the fuel and energy requirements of the ion drive and the next to negligible
thrust produced by solar sails deep in interstellar space. Using both in conjunction could
mitigate most of the limitations of each individual propulsion source as solar sails could provide
power for the ion drives and ion drives could supplement the thrust when the sunlight becomes
to diminished to have a noticeable effect on the solar sail.
While we currently have the technology to build ion drives we still do not have the ability to
produce solar sails of sufficient size. As early tests of solar sails have been successful, if they could
be produced at the required size, they would be able to realistically propel a spacecraft.
While ion drives and solar sails are both good options individually for spacecraft propulsion in the
foreseeable future, their combination in a hybrid propulsion system has the benefits of both with
reduced drawbacks. Ion drives require a large amount of energy, which the necessarily large
solar sails can easily provide allowing for a greater acceleration for a longer duration. It is
concluded that the recommendation of a hybrid ion drive/solar sail propulsion system be
implemented only after we have developed the technology to produce sufficiently large solar
sails.
It is recommended to use a hybrid Ion Drive/ Solar Sail propulsion system for exploration of the
solar system and beyond when in the near future we have solved the problems inherent with the
production of large solar sails.
Introduction 1
Introduction
For decades, it has been apparent that in order to facilitate travel to distant
objects both in our solar system and beyond, a form (or forms) of propulsion far
superior to that of conventional rockets is needed. They must be able to
provide thrust for long stretches of time and reach speeds impossible with
conventional technologies. 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 report 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 and the Solar Sail which utilizes the pressure of solar radiation to propel it.
Both of these are classified as Low-Thrust Propulsion.
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. Solar Sails
on the other hand, have a somewhat longer history. The first successful test of
the principles of a solar sail occurred in 1974 when Mariner 10 used the angle of
its solar panels for attitude control. While this was not technically a solar sail and
the mission was not designed to test the principles of a solar sail, it did effectively
prove the potential of the concept. While the practical applications of solar
sails are relatively recent (and for the time being mostly just prototypes), the
idea of using sunlight for propulsion is much older with Johannes Kepler
discussing the creation of “vessels and sails adjusted to the heavenly ether” in
the 17th century, and James Clerk Maxwell proving in 1873 that sunlight exerts a
small amount of pressure [8]. While solar sails are currently mostly limited to
prototypes, it is not a big leap to predict that we will be using solar sails on space
probes within the next 20 years.
Purpose, Scope, and Methodology
This report will examine the idea of whether or not a hybrid propulsion system
utilizing both Ion Drives and Solar Sails is not only feasible but a superior
approach to one based on either Ion Drives or Solar Sails alone for the purpose
of the exploration of our solar system and potential travel to others. Emphasis
will be placed on the issues of thrust, longevity, energy requirements, and the
limitations of each technology within the bounds of current technological
availability, reliability and longevity, and consistency with organizational goals.
Solution Overview
While both Ion Drives and Solar Sails have the potential to be the primary form of
propulsion for deep space exploration, they can be implemented together in
such a way that they complement each other and reduce their individual
weaknesses.
Solution Overview 2
Ion Drives
Ion Drives are a form of electric propulsion which utilize electric and magnetic
fields to accelerate charged particles to high velocities before expelling them
from the engine. Their primary drawback is that they require large amounts of
energy to operate.
Solar Sails
"Let us create vessels and sails adjusted to the heavenly ether, and there will be
plenty of people unafraid of the empty wastes." -- Johannes Kepler, 17th-century
astronomer
Solar Sails are exactly what they sound like, giant sails made of extremely thin
reflective materials that generate thrust from the transfer of momentum from
photons as they impact the sail (See Figure 1). Their primary drawbacks are that
they need to be extremely large and they produce very little thrust when they
are far away from the sun.
Figure 1: Solar Sail Forces and Motion [6]
There is a variation of solar sails that have embedded solar cells that generate
electricity from the photons along with thrust from them [9]. As solar sails have to
be large to produce a noticeable amount of thrust, they could also have the
dual function of generating energy for the ion drives which would enable them
to operate at maximum power at distances far greater than they would be able
to with a relatively small conventional solar array.
Solution Criteria 3
Solution Criteria
The below criteria will be used for the evaluation of this recommendation.
Technological Availability
Because our use of these propulsion technologies is dependent on our ability to
build them such that they are able to produce adequate thrust and are of
sufficient size, technological availability is defined as our ability to produce the
technology and the specifications of the most advanced forms of these
propulsion technologies developed to date.
Reliability and Longevity
Reliability and longevity is defined as the ability of solar sails and ion drives to
operate continuously and provide thrust for years without failing due to their
inherent limitations.
Consistency with Organizational Goals
Consistency with organizational goals is defined by whether or not the hybrid
use of these propulsion technologies allows for deep space exploration of our
solar system and beyond.
Evaluation of Criteria
Technological Availability
Technological availability is important for spacecraft propulsion. Without our
ability to actually construct these devices such that they are useful and
practical, they would just be theoretical ideas or interesting laboratory
experiments. Unlike conventional chemical rockets, ion drives and solar sails
produce very little thrust (on the order of tens to a few hundred mN for ion drives
and a few μN/m^2 for solar sails).
Ion Drives
Ion drives have been successfully used in space since the mid 1960’s. They have
been used on a variety of missions including orbit transfers, attitude adjustments,
drag compensation, and as the main propulsion system for some deep space
probes. The most recent advances have been the development of NASA’s
current state of the art NSTAR in the late 1990’s, which operates in the 0.5-2.3KW
region and can produce a maximum thrust of 92 mN, and the next generation
NEXT drive which operates in the 0.5-6.9 KW region and can produce a
maximum thrust of 236 mN [2] (See Table 1).
Evaluation of Criteria 4
Table 1: Performance Characteristics of NEXT vs. NSTAR SOA. [2]
The upper limit for the maximum velocity of ion drive technology is
approximately 354,000 Km/h for ion drives [7].
Solar Sails
The Ikaros solar sail on the other hand produced 6.35 μN of thrust over its area of
14 m^2 [9]. It is important to remember that this thrust from the solar sail takes
place at a distance of 1 A.U. from the sun (Earth Orbit) as can be calculated by
the equation p = E/C where p is momentum, E is energy, and c is the speed of
light. At 1 A.U. the energy from the sun is 1631 W/m^2 so the equation becomes
p = 1631/(3e8)  4.53 μN for a completely absorbent solar sail and twice this for
a completely reflective solar sail. This thrust could be significantly increased by
the use of either painted sails (which are solar sails coated in a material that
evaporates when a predetermined type of energy beam hits it, producing
increased thrust due to the evaporating molecules [12]) or lasers which greatly
increase the thrust produced by solar sails as they vastly increase the amount of
light impacting the solar sail [5]). Without these modifications however, the solar
energy of 1631 W/m^2 will decrease by a factor of four every time the distance
from the sun doubles. 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], or in the case of solar sails, the
velocity of the photons being absorbed and/or reflected. This leads to a
maximum velocity of approximately 322,000 Km/h for solar sails [7].
Conclusion
The solar panels used to power ion drives suffer the same effect as solar sails for
the energy generated as the distance from the sun increases. This leads to the
idea that as solar sails are inherently large (far larger than conventional solar
arrays), if they are the type that generates electricity by the absorption of
photons, this energy can be used to power ion drives at full or close to full power
at distances far beyond what they otherwise could even though the sails
Evaluation of Criteria 5
themselves only produce half the thrust. In fact, the Japanese solar sail
experiment Ikaros had solar cells embedded in the sail specifically to look into
the idea of using solar sails to power ion drives [9]. As solar sails would have to
be massive to match the thrust of ion drives, this is an acceptable tradeoff. This
also leads to the idea that because ion drives produce a much larger thrust
than can easily be produced by a solar sail, the size of the solar sail can be
drastically reduced. It is also important to note that while the velocities of a
hybrid propulsion system are not additive (though the system will approach the
larger maximum velocity of the two), their thrusts and accelerations are. Based
on the above criteria, while we have the ability to produce ion drives, as solar
sails are by necessity large and difficult to manufacture and have only been
used experimentally, I cannot recommend the use of a hybrid propulsion system
until we have developed the technology necessary to produce solar sails of the
required size.
Reliability and Longevity
Perhaps as important as our ability to build these technologies, their ability to
perform their functions for extended periods of time is crucial for the exploration
of our solar system and beyond. As the distances between celestial bodies are
immense and the thrust provided by these two types of propulsion systems is
small, their ability to reach their destinations in any reasonable amount of time is
directly dependent on their duration of operation.
Ion Drives
Ion Drives are the most efficient type of active propellant propulsion system yet
devised. While they 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, or conversely can carry more fuel for a significantly increased time in
which the engine is capable of providing thrust. The only potential drawback
and real limitation to the longevity 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]. This is the energy required per drive, and as any potential
spacecraft can be expected to contain at least three or four ion drives, this
energy requirement is substantially higher. Using the solar sails to generate the
energy for the ion drives could minimize this limitation (at least until the craft has
attained an acceptable cruising velocity and is in deep space)
Simply having enough fuel and energy however isn’t enough. To be useful, the
drive would need to operate continuously for years at a time without fail. This
however is not a problem. During one test described by Brophy, the NSTAR
(NASA’s current state of the art ion drive) was operated at full power for 8192
Evaluation of Criteria 6
hours before being voluntarily shut down [3]. This is over 341 days of continuous
operation. Similarly, Patterson and Benson discuss a test of the NEXT (NASA’s
next generation ion drive) drive in which it was operated for over 9990 hours
(more than 416 days) at full power [2]. Gizmag describes another recently
terminated test in which a NEXT drive was operated continuously for more than
48,000 hours (more than 5.5 years) before being voluntarily terminated [10].
While the duration of these tests is significant, the most impressive part is that
Brophy, Patterson, Benson, and gizmag all agree that at the ends of their tests,
the engines showed no signs of being anywhere close to failing. This is even
more impressive when one realizes that the NSTAR was designed to have a
design life of 8000 hours at full power [4]. Not only do ion drives meet their
design lives, they surpass them. This shows that given adequate fuel and power,
ion drives have the potential to be operated continuously for years on end at full
power without fear of failing. This is a characteristic that is crucial for any deep
space mission.
Solar Sails
Solar Sails are extremely reliable and can easily operate for long periods of time
due to their nature. They have very few if any moving parts, which means they
are unlikely to break down or tear unless something physically happens to them
and as they do not require fuel or energy, they cannot run out. As long as light
shines on them they will provide thrust. One of their main drawbacks however
(this is also one of the reasons they are so large) is the fact that as their distance
from the sun doubles, the amount of thrust they produce is reduced by a factor
of four. This means that while they may still be physically capable of providing
thrust, at sufficient distances from the sun, the amount of thrust they generate
will become negligible. This is of course assuming that the sail is not impacted
and damaged by space debris which is a topic not covered in this report.
Conclusion
As described above, both ion drives and solar sails are incredibly reliable over
the long term, however they both suffer from potentially debilitating limitations
such as the fuel and energy requirements of the ion drive and the next to
negligible thrust produced by solar sails deep in interstellar space. Using both in
conjunction however would mitigate most of the limitations of each individual
propulsion source as solar sails could provide power for the ion drives in deep
space and ion drives could provide thrust when the sunlight becomes to
diminished to have a noticeable effect on the solar sail. Based on the above
criteria, I can recommend the use of a hybrid propulsion system as each
propulsion source can mitigate the limitations of the other.
Consistency with Organizational Goals
Consistency with organizational goals is important because if ion drives and solar
sails cannot meet the goals and objectives of any space agency (the
exploration and research of space via both manned and unmanned
Evaluation of Criteria 7
spacecraft) in terms of the exploration and potential colonization of our solar
system and beyond, they should not be per sued as a means of spacecraft
propulsion.
Ion Drives
Ion drives have a long history of success as a propulsion source for spacecraft.
They have been used on a wide variety of missions both in earth orbit and
throughout the solar system. The European Space Agency (ESA) has so far only
used ion propulsion to compensate for atmospheric drag on satelites [13] but
has plans to use ion drives as the primary source of propulsion on its future
BepiColombo mission to Mercury [14]. NASA on the other hand has been using
ion drives on spacecraft since the launch of Deep Space 1 in 1998 [3]. Research
is continuing to improve the performance of ion drives as is demonstrated by the
existence of the NSTAR and NEXT drives [2]. Both of these have the proven
ability to operate for years without failing and propel spacecraft to velocities
that conventional propulsion sources are unable to achieve. As a result, they
allow for faster, more efficient, and less expensive exploration of the solar system
than is conventionally possible, while also allowing more mission options such as
completing a comet sample return mission in 7 years while it would take a
conventional rocket more than 9 years just to reach the comet [3]. While the
thrust they produce is small, it more than doubled between ion drive
generations and if this trend keeps up, ion drives will become an even more
appealing form of propulsion whose only drawback is its energy consumption.
This will give space agencies even more of an incentive to use ion drives as one
of their primary forms of propulsion, and allow them to better complement solar
sails in a hybrid propulsion system.
Solar Sails
While solar sails have a long conceptual history, the actual production and use
of solar sails as a propulsion source is much newer and is still contained for the
most part within the experimental phase. NASA’s earliest actual use of the
concept was for attitude control of a probe by changing the angle of its solar
panels. Its first attempt to fly an actual solar sail, Nano Sail D (10 square meters),
failed when the delivery rocket was lost however its successor Nono Sail D2 was
successful [8]. Other tests such as JAXA’s (Japan Aerospace Exploration
Agency) Ikaros (14 meters wide) have also been successful [9]. These tests
clearly demonstrate that we can successfully produce small solar sails. What’s
more, Ikaros contained small solar cells to generate electricity as a test for
potential use as a power source for ion drives in the future. This demonstrates
that JAXA is at least considering an ion drive/ solar sail hybrid propulsion system
[15]. While this is a good first step, in order to be useful in a hybrid propulsion
system, the solar sail would have to be far larger. For an interstellar mission
powered solely by solar sails, the estimates make the size of the required sail
roughly comparable to the size of Texas [11]. It is a huge jump between building
Evaluation of Criteria 8
a 14 meter solar sail and building one the size of Texas. As a result, until we are
able to construct solar sails on anywhere close to this scale, solar sails are unable
to meet the organizational goals of any space agency.
Conclusion
While we currently have the technology to produce and build the ion drive
aspect of the hybrid propulsion system and have successfully used them on
probes within our solar system, we still do not have the ability to produce solar
sails of sufficient size. However, that being said, early tests of solar sails have
been successful (and even support the idea of using a solar sail to generate the
electricity for the ion drives) and if they could be produced at the required size,
they would be able to efficiently propel a spacecraft and fulfill their
requirements as part of a hybrid ion drive/ solar sail propulsion system. Based on
the above criteria, I recommend the use of a hybrid ion drive/ solar sail
propulsion system once we have solved the issues related to producing large
solar sails.
Conclusion
While ion drives and solar sails are both good options individually for spacecraft
propulsion in the foreseeable future, their combination in a hybrid propulsion
system has the benefits of both with reduced drawbacks. Ion drives require a
large amount of energy, which the solar sails can easily provide at distances far
exceeding that which is possible with traditional solar arrays. This would allow for
a greater acceleration for a longer duration. The only problem with this is that
we do not currently have the ability to produce solar sails of the size that would
be required. Based on the evaluation of the criteria above, it is concluded that
the recommendation of a hybrid ion drive/ solar sail propulsion system be
implemented only after we have developed the technology to produce
sufficiently large solar sails. If implemented before this, the solar sails would not
only be unable to produce the desired thrust, but would also have a diminished
capacity to mitigate the drawbacks of an ion propulsion system.
Recommendation
1. It is recommended to use a hybrid Ion Drive/ Solar Sail propulsion system for
exploration of the solar system and beyond only when we have solved the
problems inherent with the production of necessarily large solar sails.
References 9
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 Deep-Space
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.
5. P. Galea. (2010, September 1). Solar Sails for the Icarus Mission (1st ed.)
[Online]. Available: http://www.icarusinterstellar.org/solar-sails-icarusmission/
6. B. N. Cassenti. “Optimization of Interstellar Solar Sail Velocities,” Journal of
The British Interplanetary Society, vol. 50, pp. 475-478, 1997.
7. M. Wolverton. (2010, February 22). New Space Engines May Trade Fuel For
Photons (1st ed.) [Online]. Available:
http://www.popularmechanics.com/science/space/deep/4346578
8. D. Coulter. (2008, July 31). A Brief History of Solar Sails (1st ed.) [Online].
Available: http://science.nasa.gov/science-news/science-atnasa/2008/31jul_solarsails/
9. E. Howell. (2014, May 7). Ikaros: First Successful Solar Sail (1st ed.) [Online].
Available: http://www.space.com/25800-ikaros-solar-sail.html
10. D. Szondy. (2013, June 26). NASA’s NEXT ion thruster runs five and a half
years nonstop to set new record (1st ed.) [Online]. Available:
http://www.gizmag.com/next-ion-thruster-duration-record/28067/
References 10
11. M. Wall. (2013, March 12). First Interstellar Spacecraft May Use Texas-Size
Solar Sail (1st ed.) [Online]. Available: http://www.space.com/20169interstellar-spaceflight-solar-sail.html
12. B. Christensen. (2005, February 11). Earth To Mars in a Month With Painted
Solar Sail (1st ed.) [Online]. Available: http://www.space.com/790-earthmars-month-painted-solar-sail.html
13. ESA. (2009, April 6). GOCE’s Electric Ion Propulsion Engine Switched On (1st
ed.) [Online]. Available:
http://www.esa.int/Our_Activities/Observing_the_Earth/GOCE/GOCE_s_el
ectric_ion_propulsion_engine_switched_on
14. ESA. (2006, August 31). The Magic Of Ion Engines (1st ed.) [Online].
Available: http://www.esa.int/Our_Activities/Space_Science/SMART1/The_magic_of_ion_engines
15. Japan Aerospace Exploration Agency. (2008). Solar Power Sail
Demonstrator “IKAROS” (1sr ed.) [Online]. Available:
http://www.jspec.jaxa.jp/e/activity/ikaros.html