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8/18/2019 Masters Special Problem 9800
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Viability of Dual Propellant Mars Transportation
Vehicle
Marius D Popescu1 Georgia Institute of Technology, Atlanta, GA, 30318
Electric Propulsion methods have been at the forefront of vehicle concepts that
travel to Mars.
Nomenclature
g =gravitational constant, 9.81m/s2
µ =gravitational parameter
I. Introduction
A significant cost
II. Literature Review
It is known that varying specificimpulse and thrust to weight on a vehicle can
be advantageous. Historically this has almost
exclusively been done by staging, but is
advantageous even if only one stage is used.
In chemical propulsion, multiple fuels are
often used to vary thrust to weight and
impulse between stages, but it has also been
shown to be theoretically practical in a single
stage simply due to the tradeoff between
specific impulse and density impulse.Furthermore there is slight more benefit in
having a single engine than separate engines
burning in parallel [1] which led to a few
conceptual designs in dual fuel dual expander
rocket engines, which burned both
hydrocarbon and hydrogen fuel within
concentric combustion chambers and
expanded through a shared nozzle. In
addition to reducing propellant mass fraction,
using multiple propellants may increase
thrust efficiency, benefit the propellant tank
system design, and reduce cost impulse (that
is that one fuel may be cheaper to provide the
same ΔV). In general the same advantageous
could be applied to electric propulsion, and
be even more effective. An electric propulsion
device that can vary its impulse and thrust
over a large range means it could be in a more
optimal specific impulse regime for both
planetary escape and interplanetary travel
parts of a mission. However, even a small
range can increase performance; it also
benefits from better flexibility and
management of power levels throughout a
mission. For instance, as mass decreases
during a mission, the optimal Isp increases,
and changes in power level or large gravity or
drag losses may optimize Isp lower. Being able
to vary modes also may make it possible to
burn continuously which further reduces trip
times. It has been shown that varying specificimpulse in any propulsion device and electric
propulsion in particular can provide
significant reduction in propellant cost and
mass and/or trip time for interplanetary
missions as shown in the figure below. [2] It
is therefore of interest to develop an electric
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propulsion device that can vary its impulse
aka Variable Specific Impulse Electric
Propulsion (VSI EP). This is precisely the
reasoning behind the development of
VASIMR.
The above table shows that despite
additional gravitational losses and not being
able to utilize the Oberth effect, low thrust
NEP can reduce the amount of propellant
needed, ignoring time constraints. As shown
in Figure 1, adding variable impulse to the
device can further improve mass fraction.
Furthermore the same paper by Acta
Astronautica states:
“ It is quite interesting to notice how the variable-Isp
thruster permits a 10% mean reduction of the
propellant consumption. This value is slightly lower
(about 9%) if the unavoidable upper and lower
limits for the Isp are imposed at reasonable levels,
but it can double in special cases (like for long
missions with gravity assist and low total
propellant mass). It has to be recognized that up to80% of the achieved propellant mass savings could
be obtained using dualmode thrusters, considerably
simpler to develop and qualify.” [2]
There has been some research done
exactly as to what range of specific impulse is
necessary and optimal for a given mission or
vehicle. Unfortunately, the referenced paper
does not explicitly state the bounds, but one
can infer the range to be about 2800-3500s
for the mission to Mars, furthermore the
paper is not clear on what the maximum and
minimum specific impulse they found when
the bounds were not imposed.[2] A similarstudy conducted by the company that created
VASIMR, Ad Astra, found that on a mission to
Mars allowing the specific impulse to vary
between 4000s and 30000s vs a constant
specific impulse of 5000s given a power input
and initial mass allowed them to save about
15% of propellant. [6] It is worth noting that
the paper did not state whether or not the
constant specific impulse value was
optimized, and that this range of specificimpulse is not particularly close to the
currently realized range of 2000-10000. Both
papers nevertheless demonstrate significant
predicted propellant savings.
Considering the above results, there is
currently some amount of work being done
on variable specific impulse or multi-mode EP
devices. Currently, specific impulse is
principally varied by modulating electrical or
heating parameters in devices. Hall effectthrusters can vary their specific impulse by
increasing or decreasing the discharge
voltage and beam current. The range between
minimum and maximum specific impulse for
currently available hall thrusters is
approximately 500 to 1000s with the typical
maximum specific impulse around 3000s.
Hall thrusters are currently the most
commonly studied potential bimodal electric
propulsion device and most already have
some specific impulse flexibility over a small
range. Although Ion thrusters would in theory
derive the most benefit from dual propellants,
they suffer from inefficiencies at lower
specific impulse operation and scalability
making them generally impractical for
manned interplanetary missions. There are
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attempts modifying ion engines to be partially
bimodal, the GIE NEXT attempts to improve
upon throttle-ability and specific impulse, but
over a large range of power and suffers
technical issues. Although no other flown
models have been specifically designed forvariable specific impulse, there are some
models being tested and designed, including
the T-220HT-HET currently being developed
and tested in Georgia Tech’s HPEPL. In
addition to more conventional Hall thrusters,
there are other short and medium term
technologies being pursued. The same paper
from Acta-Astronautica mentions some of
these: Hybrid electrostatic systems like
HET/GIE (ex. QinetiQ) which combines thetwo propulsion systems either in parallel or
as an integrated system (much the same
principle as the chemical rockets), and double
stage hall-effect thrusters (ex. SPT-MAG and
LABEN-ALTA DSHET), which separates the
ionization and acceleration regions of hall
effect propulsion. Shorter term Nested Hall
Thrusters (NHT) have a big potential in
covering a broad range of specific impulses
and thrust levels. Of course, there are also
longer term and more novel technologies
such as the VASIMR, PIT and HIIPER concepts
to approach the goal of VSI EP.
Currently the most prominent design
considering variable specific impulse for
electrically powered mars transit vehicles is
VASIMR. VASIMR stands for VAriable Specific
Impulse Magnetoplasma Rocket and is being
developed by the Ad Astra Company founded
and led by former astronaut Chang Diaz.
Simply stated, VASIMR essentially develops
thrust by heating a gas and converting its
perpendicular motion into parallel motion,
fundamentally similar to chemical rockets. It
consists of three stages which comprise three
main subsystems: the plasma injection stage
utilizing a helicon antenna, the heating stage
utilizing an Ion Cyclotron Resonance Heating
(ICRH) antenna, and the expanding stage
utilizing a magnetic nozzle. The design is
electrode less, meaning that the high
temperature plasma does not erode the
device and can handle high power densities.This is achieved by magnetic confinement
which ties all three stages together and using
RF power to produce and heat the plasma.
Thrust and specific impulse is varied
primarily by selectively partitioning the RF
power to the helicon or ICRH systems, along
with adjusting the propellant mass flow.
VASIMR experiences a few
technological challenges at the moment.
Contributing many of issues are the large andstrong magnetic fields it requires. This
presents 4 main issues: charged particles
remaining attached to field lines causing
greater beam divergence, losses, and charging
of the spacecraft, the powerful
superconducting magnets required are both
complex and heavy, the shielding that would
be required for both communication and
health considerations, and induced torques or
movement of charges due to electromagneticinteractions. In addition to this, VASIMR
suffers from significant thermal management
considerations, requiring rather large and
potentially heavy radiators. The latter issue is
only more profound when the power systems
are taken into account, which represents the
most important consideration with any
electric propulsion device. VASIMR gets
significant benefits from increasing power
levels, and since its proposed roles are
primarily interplanetary and/or large
payload coupled with fast transit, it is
generally advocated that the VASIMR
operates in MWs of power. Due to this Dr.
Chang Diaz has suggested nuclear power,
which additionally needs significant thermal
management, and is likely heavier than solar
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power. VASIMR has also yet to demonstrate
long term firing over the ambitioned specific
impulse range. As of today the VX-200 can
only be fired for less than a 60 seconds with
1.2 seconds being the average firing length
and needs to be cooled down over anextensive period of time, and the specific
impulse has only been optimized and
controlled between 780s – 4900s, far less
than the ambition range of 3000 – 30000s
[6][7].
So far, the demonstrated VX-200 has
been able to achieve maximum 51mN/kW at
1660s and 35 mN/kW at the maximum thrust
level, and thrust efficiencies varied from
around 10% to 72% at the highest thrustlevel with around 30% thrust efficiency
around the maximum thrust to input power,
for short periods of time. [7] On the other
hand Hall effect thrusters have demonstrated
levels of 90mN/kW at similar efficiencies or
better over a similar range. NASA’s 457Mv2
Hall thruster has demonstrated 76.4 mN/kW
at low power and 46.1mN/kW at max power,
with anode efficiencies between about 55 to
70%. [8] Furthermore the specific power ofVASIMR is projected to only come down to
about 1.6kg/kW for a 1MW case, 3kg/kW for
250kW case. [9] Whereas the hall effect
thrusters have demonstrated 1.3 kg/kW and
below at 6kW which improves with scaling.
[10] Nested hall effect thrusters are expected
to improve those values even further to
perhaps 0.5kg/kW for MW levels and be able
to vary impulse between 1000-5000s.[11] It
should be also considered that Hall effect
thrusters have been flown and tested
extensively, and can be fired for 1000’s of
hours. They have also been shown to be fuel
flexible with the 400M model operated on
both krypton and xenon. Benefits for dual
propellants can benefit any electric
propulsion device including VASIMR, which
can virtually run on any propellant and has
considered propellants such as deuterium
and krypton. However, until VASIMR has
demonstrated to be more competitive with
current and shorter term technologies, the
focus of the research will be more focused ondemonstrated propulsion devices such as hall
effect thrusters and electrostatic ion engines.
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