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Short Review on Electric Propulsion System: Ion Thruster

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Ion thrusters have proven to be a suitable and ef-ficient alternative to conventional propulsion systems. With very low demand on fuel due to very high specific impulse generation, ion thrusters can easily compete with chemi-cal propulsion systems, even if the produced thrust is much 5 lower. The system can be used for various mission demands like orbit station keeping for geostationary satellites, orbit and attitude controlling and multi-goal missions. Whereas chemical propulsion is highly unsuitable for deep space mis-sions, ion thrusters are also making it possible to reach out 10 further into deep space. This paper represents a short re-view on electric propulsion systems, specifically on the Ion Thruster with its design and functions.
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Date: 24 August 2014
Short Review on Electric Propulsion System: Ion Thruster
Tanzim Kawnine1and Maria Kawnine1
1Lule˚
a University of Technology
Correspondence to: Tanzim Kawnine
tankaw-0@student.ltu.se
Abstract. Ion thrusters have proven to be a suitable and ef-
ficient alternative to conventional propulsion systems. With
very low demand on fuel due to very high specific impulse
generation, ion thrusters can easily compete with chemi-
cal propulsion systems, even if the produced thrust is much5
lower. The system can be used for various mission demands
like orbit station keeping for geostationary satellites, orbit
and attitude controlling and multi-goal missions. Whereas
chemical propulsion is highly unsuitable for deep space mis-
sions, ion thrusters are also making it possible to reach out10
further into deep space. This paper represents a short re-
view on electric propulsion systems, specifically on the Ion
Thruster with its design and functions.
1 Introduction15
The propulsion of choice for science fiction writers has be-
come the propulsion of choice for scientists and engineers
in space engineering. The efficient use of fuel and electri-
cal power enable today’s spacecraft to travel farther, faster
as well as cheaper than any other propulsion technology cur-20
rently available.
In Figure 1, a general classification scheme of propul-
sion systems is depicted. The concept of Ion Thruster can be
found as a branch of electrostatic rocket propulsion systems.
Electric propulsion systems branch out to electro thermal,25
electromagnetic and electrostatic rocket propulsion, which
indicates that the concept of electric propulsion systems is
indeed a very well known and well studied concept, that
might offer an alternative to conventional propulsion sys-
tems. These conventional systems can be seen in the other30
two branches of rocket propulsion: thermal and nuclear sys-
tems. Especially the thermal-chemical system with its vari-
ations of propellant is the most commonly used propulsion
system in space engineering.
Fig. 1. Propulsion System Classification
Generally, there are two types of propulsion systems. Sys-35
tems that are performing functions such as orbit transfer are
referred to as primary propulsion, whereas systems which are
associated with the orbital attitude and control are referred to
secondary propulsion (Fortescue et al., 2011).
For apogee injection, the launcher carries the satellite into40
an elliptical transfer orbit. To further reach geostationary or-
bit, a thrust level of 400 – 600 N is applied to move the satel-
lite into the required circular orbit.
The task for a secondary propulsion system is dependent
on the specific type of propulsion as well as the space mis-45
sion. In general, the tasks can be summarized as follows: The
propulsion system is used for orbital control, to reach the de-
sired position after initial injection from the launcher. It fur-
ther maintains the satellite in the required position during its
operational time. Eventually, at the end of life (EOL) of a50
satellite, the propulsion system is used for the injection into
a ”graveyard”. Besides orbit controlling, the propulsion sys-
2 T. and M. Kawnine: Ion Thruster
tem is also vital for the attitude controlling of the satellite.
The level of thrust to perform attitude controlling depends
on the size of the satellites and lies between 1 and 22 N (Ley55
et al., 2008). For interplanetary missions, the propulsion sys-
tems have additional task to perform such as a precise course
correction during the flight for a number of years as well as
deceleration of satellite to swing into an orbit of a planet or a
moon.60
In the 1950’s, the Glenn Research Center began their work
on ion propulsion which is related to the electric propulsion
system. The first operational ion propulsion system in space
was the Science Electric Rocket Test 1 (SERT 1) which flew
in July 1964 and total 31 minutes of operational time before65
it returned to Earth. Today, electric propulsion systems are
commonly used for station keeping of geostationary commu-
nication satellites. Currently, the NASA Evolutionary Xenon
Thrusters (NEXT) is under development which will reduce
mission cost and trip time. NEXT is capable of performing70
wide variety of missions to targets of interest such as Mars
and Saturn.
In the following it will be shown that ion propulsion sys-
tem is suitable for the tasks that are specified for secondary
propulsion system and also long-term interplanetary mis-75
sions.
2 Space electric propulsion
As mentioned earlier, the electric propulsion has been an ac-
tive area of development since the dawn of space flight. But it
is only in recent years that electric propulsion has been used80
in commercial, scientific and military missions. Thrusters for
electric propulsion systems use ionisable gases for propel-
lant. For interplanetary missions, it is used as a main propul-
sion to achieve a high v, e.g. Deep space 1, SMART-1,
Hyabusa, BepiColombo. Most of the electric propulsion sys-85
tems have low thrust levels compared to chemical thrusters,
in the order of some mN up to 1 N, but the overall perfor-
mance level is greater than that of chemical propulsion sys-
tems by a factor of 10 – 20.
Chemical propulsion relies on the stored internal energy90
and is limited with respect to its specific energy in the
molecular bonds of its propellant, whereas energy in electric
propulsion is obtained from an external power source such
as solar panels. The propellant to electric propulsion systems
can be accelerated to very high velocities, hence achieve a95
very high specific impulse. The internal energy in chemi-
cal propulsion limits the maximum specific impulse to about
450 s, in contrast to electric propulsion specific impulses of
over 17,000 s have been obtained in the laboratory (Curran et
al., 1991; Ley et al., 2008). This is certainly a great advantage100
of electric over chemical propulsion.
Thruster types and functions
There are three different main groups in electric propulsion
technologies:
1. Electro thermal propulsion105
2. Electrostatic propulsion
3. Electromagnetic propulsion
Electro thermal propulsion, as a combination of electric and
chemical propulsion system, uses electromagnetic fields to
generate plasma to increase the temperature of the bulk pro-110
pellant. The thermal energy, which is transferred to the pro-
pellant gas, is then converted to kinetic energy. Hydrogen,
helium, ammonia are used for this type of propulsion sys-
tem (Fortescue et al., 2011). Electro thermal system perfor-
mance in terms of specific impulse, Isp, is between 500 –115
1000 s, but still greater than chemical propulsion which can
be referred to ”mono-propellant”, ”bi-propellant” and ”cold
gas thrusters”.
Electrostatic propulsion uses electric fields to acceler-
ate ionized propellant gas. The ionization is performed by120
means of DC discharge, radio frequency or electron syn-
chrotron (Ley et al., 2008). The positively charged particles
are then neutralized by adding electrons from a neutraliser
outside of the acceleration zone. The physical principle of
the ion thruster as a part of the electrostatic propulsion will125
be described in a greater detail in Section 3.
By applying both electric and magnetic fields, an ion-
ized propellant gas is accelerated which is known as electro-
magnetic propulsion system (Jahn, 1968). Using this tech-
nique, both ions and electrons are exhausted out of the ve-130
hicle without the need of neutralization (Choueiri, 2009).
Examples include Hall thrusters, pulsed plasma thrusters
(PPT), pulsed inductive thrusters (PIT), and magnetoplasma-
dynamic thrusters (MPDT).
3 Physical principle of the Ion Thruster135
The ion propulsion system (IPS) consists of five units:
the power processing unit (PPU)
the power source
propellant management system (PMS)
the control computer and140
the ion thruster.
The power source is usually any source of electrical power
such as solar or nuclear power. A solar electric propul-
sion system (SEP) uses solar cells to generate power. A nu-
clear electric propulsion system (NEP) is using a nuclear145
heat source connected to an electric generator. The generated
T. and M. Kawnine: Ion Thruster 3
electric power by the power source is then converted by the
PPU, supplying the power required for each component of
the ion thruster. The generated electric power by the power
source is then converted by the PPU, supplying the power150
required for each component of the ion thruster, such as the
positive and negative grids, discharge chamber and the hol-
low cathodes. The PMS controls the propellant flow from the
propellant tank. The design of PMS is highly sophisticated
and it does not require moving parts.155
Figure 2 depicts the operation of an ion thruster. An ion
thruster moves ions by electrostatic repulsion. The neutral
Xenon propellant enters from the propellant tank. A hol-
low cathode emits electrons which impact the Xenon atoms,
pounding loose an electron and creating positive Xenon ions.160
The positive ions are then pushed by gas pressure through
holes in a positive grid. Then the electric field between the
positive and negative grid accelerates the ions such that the
ion beam is exhausted out through the nozzle. A hollow cath-
ode plasma bridge neutraliser is placed at the exit of the noz-165
zle which shoots out electrons to neutralize the ion beam.
Otherwise the ions would be attracted back to the negative
grid, cancelling out the thrust.
Fig. 2. Schematics of an Ion Thruster according to (NASA, 2007)
The performance of electrostatic thrusters can be obtained
by setting the potential energy in relation to the kinetic en-170
ergy of the ion (Fortescue et al., 2011). The potential energy
of the ion, as it leaves the ionization region, is determined by
the voltage potential applied between the ion source and the
exit plane. The ions achieves its kinetic energy as it is moving
through the electrostatic field.175
mi
2v2=qV, (1)
where miis the mass of the ion, qis the charge of the ion
and Vis the potential of the ion source and the exit plane.
Assuming that the power plant output (solar cells or nuclear
power source), W, is fully consumed by the ions into kinetic180
energy, then the thrust, F, to power ratio is simply:
F
W=r2mi
qV (2)
As the ion thruster produces small levels of thrust relative to
chemical thrusters, it generates higher specific impulse or the
higher exhaust velocities. It means that such a thruster can185
have fuel efficiency of 10 – 12 times greater than a chemical
thruster. As it has mentioned that the higher the rocket’s spe-
cific impulse or the fuel efficiency, the farther the spacecraft
can travel with a given amount of fuel. Because of the low
level of thrust production in ion thrusters relative to chemi-190
cal thrusters, it needs to operate in excess of 10,000 hours to
slowly accelerate the spacecraft to speeds necessary to reach
other planets in the solar system (NASA, 2013).
Ion thrusters are capable of propelling spacecraft up to 90
km/s. It can be compared to the conventional (chemical)195
propulsion system which was used in Space Shuttle, capa-
ble of a top speed of about 8 km/s. The trade-off for high
speed is the low thrust applied to the spacecraft. The draw-
back is that the ion thruster must be operated for a long time
for the spacecraft to reach its top speed.200
4 Design of Ion Thrusters
In the following there are two examples of ion thrusters
presented, NEXT and NSTAR. NSTAR stands for NASA
Solar electric propulsion Technology Application Readiness
and provides the Deep Space 1 (DS1) spacecraft with a205
xenon ion propulsion system. NEXT is NASAs Evolutionary
Xenon Thruster. The project is advancing the capability of
ion propulsion to meet NASA robotic science mission needs.
The system hardware has advanced to high state of maturity
and testing to date is very successful.210
4.1 NSTAR xenon-ion thruster
The Deep Space 1 spacecraft was the first spacecraft to suc-
cessfully implement the concept of the solar-electric propul-
sion system (Brophy et al., 2006). Designed to deliver to-
tal vof 4.5 km/s while only consuming 81 kg of fuel, the215
NSTAR xenon-ion thruster consists of four major building
blocks:
Xenon feed system (XFS),
Power processing unit (PPU),
Digital control interface unit (DCIU) and220
Thruster itself.
As can be seen in this listing, the components are mostly
congruent with those listed in Section 3. The DCUI can be
seen as the control computer, which accepts high-level com-
mands from the spacecraft on-board computer and controls225
4 T. and M. Kawnine: Ion Thruster
both the XFS and the PPU. Aside from that, the DCUI is also
responsible for sending the propulsion telemetry data to the
spacecraft OBC. The XFS is similar to the propellant man-
agement system: It provides the propulsion system with the
xenon propellant and feeds electrons to the neutraliser cath-230
ode. The necessary power to run the thruster is converted by
the PPU. It converts the power from the solar arrays into nec-
essary driving currents and voltages for the engine.
After feeding the xenon propellant into the ionization
chamber, accelerated electrons are ionizing the xenon atoms.235
The ionization process is further improved by using perma-
nent magnets in order to attract and accelerate the electrons.
Two closely spaced, multi-aperture electrodes (grids) are
placed at the back end of the engine to accelerate the xenon
ions. The applied voltage is as high as 1.28 kV. Stripped off240
and ionizing electrons are collected and used for the neutral-
ization of the expelled ion stream at the back of the space-
craft. This is necessary to prevent the spacecraft from charg-
ing up negatively and re-attract the positive ion stream.
As the spacecraft is moving further away form the Sun,245
its solar arrays will produce less output power. These vari-
ations have to be taken into account for the design of the
thruster. NSTAR designed the engine in such a way, that it
is capable of operating within a power range of 0.5 kW up
to 2.3 kW (Brophy et al., 2006). Within this power range,250
the output thrust varies proportionally as well: Assuming a
singly charged xenon ion with mass mi= 2.1802×1025 kg
and charge q= 1.602177 ×1019 C, Equation 2 becomes:
F
W=r2×2.1802 ×1025 kg
1.602177 ×1019 C×1280V= 0.046 mN
W
Given the above power range, the thrust force ranges as fol-255
lows:
Fmin = 0.046 mN
W×500W= 23.055mN
Fmax = 0.046 mN
W×2300W= 106.055mN
F[23.055; 106.055] mN
260
In fact, the engine thrust measured in space for a PPU in-
put power of approximately 2 kW can reach up to 75.34 mN,
whereas it remains as small as 20.77 mN for a minimum in-
put power of 0.5 kW (Brophy et al., 2006). Variation of the
theoretical values from the measured values can be explained265
by inaccuracies of the input parameters as well as the fact,
that the engine input power does not equal the PPU input
power completely.
Another important factor that affects the measurement val-
ues is the engine efficiency to convert the given input power270
into kinetic energy of the ions. For the NSTAR ion thruster,
this efficiency is greater that 99.6% (Brophy et al., 2006),
which indicates a nearly perfect ion acceleration and can
therefore be ruled out as a factor of measurement inaccura-
cies.275
4.2 NEXT ion thruster
NASA’S Evolutionary Xenon Thruster, NEXT, retains criti-
cal heritage to NSTAR. It has similar concept and function
as its predecessor, only it is generating about three times
as much thrust as an NSTAR engine (NewScientist Space,280
2007).
Similar to the NSTAR system, NEXT implements a xenon
feed system (XFS), a power processing unit (PPU), a digi-
tal control interface unit (DCIU) as well as the thruster it-
self. Figure 3 depicts the system schematics for the NEXT285
propulsion system. As can be seen in this figure, the xenon
supply feeds the so-called HPA and LPA systems, before en-
tering the thruster, where HPA and LPA stand for high and
low pressure assembly, respectively. The LPA is part of a
single thruster string: a subsystem, which consists of an ion290
thruster, the LPA and a PPU. With the concept of thruster
strings, NASA enables missions to flexibly add strings if the
mission requires multiple, thus improving performance and
thrust. For redundancy purposes and failure tolerance, one
additional thruster string is compulsory for any mission de-295
sign (Schmidt et al., 2008).
Fig. 3. NEXT system elements, simplified (Schmidt et al., 2008))
The improved ion thruster, NEXT, shows the following
characteristics:
0.54 – 6.9 kW input power,
>236 mN maximum thrust,300
4190 s maximum specific impulse,
which enables high power missions with fewer thruster
strings and is also beneficial for a reduced propellant mass,
enabling more payload and a lighter spacecraft.
Repeating the above calculation in the NSTAR subsec-305
tion (Subsection 4.1), the theoretical maximum thrust can be
achieved. Assuming the same input variables for Equation 2,
one can get:
F
W= 0.046 mN
W
Fmax = 0.046 mN
W×6900W= 318.17mN310
T. and M. Kawnine: Ion Thruster 5
As can easily be seen, the maximum output thrust exceeds
the stated characteristics given above. This is not only due
to inaccuracies of the input variables, mi,q,Vand W. For
the NEXT ion thruster, the previously neglected efficiency315
does play an important role in this calculation: Compared
to its predecessor, the thruster shows a reduced efficiency
of 70% (NASA, 2008), which reduces the maximum output
power to
Fmax = 0.046 mN
W×6900W×0.7 = 222.72mN,320
which is - nevertheless - an enormous improvement com-
pared to the NSTAR ion thruster.
Fig. 4. NEXT ion engine during testing (NewScientist Space, 2007)
5 Conclusions
Ion thrusters have proven to be a suitable and efficient al-
ternative to conventional propulsion systems. With very low325
demand on fuel due to very high specific impulse genera-
tion, ion thrusters can easily compete with chemical propul-
sion systems, even if the produced thrust is much lower. The
system can be used for various mission demands like orbit
station keeping for geostationary satellites, orbit and atti-330
tude controlling and multi-goal missions. Whereas chemical
propulsion is highly unsuitable for deep space missions, ion
thrusters are also making it possible to reach out further into
deep space.
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Article
Full-text available
A space shuttle is launched into space with the help of a rocket. A rocket is a machine that develops thrust by means of rapid expulsion of matter i.e. expansion of propellant. The major components of a chemical rocket assembly are a rocket engine, various types of propellants (the majority of which consist of fuel and an oxidizer i.e hypergolic liquid propellants), a frame to hold the components, control systems and a cargo such as a satellite. A rocket differs from other engines in that it carries its fuel and oxidizer internally, therefore it will burn in the vacuum of space as well as within the Earth's atmosphere. The cargo is commonly referred to as the payload. A rocket is called a launch vehicle when it is used to launch a satellite or other payload into space. Also it is a non-reusable launch vehicle. Once a rocket is launched, it will be irrecoverable. At present, rockets are the only means capable of achieving the altitude and velocity necessary to put a payload into orbit. Therefore the most important part of the rocket remains the propulsion system. It is the propulsion system that generates the thrust which enables the rocket to achieve the velocity that can counter the earth’s gravitational force and take the rocket into space and thus, it is pivotal to research about new ways to propel the rockets, increase efficiency and reduce operation costs, etc. Ion thrusters have proven to be a suitable and efficient alternative to conventional propulsion systems. With very low demand on fuel due to very high specific impulse generation, ion thrusters can easily rival the chemical propulsion systems, even if the produced thrust is much lower. The system can satisfy various mission demands like orbit station keeping for geostationary satellites, orbit and attitude controlling and multi-goal missions. Whereas chemical propulsion is highly unsuitable for deep space missions, ion thrusters are also making it possible to reach out further into deep space. This paper represents a basic functioning of electric propulsion systems, specifically on the Ion Thruster with its design and functions.
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