RADIOISOTOPE-DRIVEN DUAL-MODE PROPULSION SYSTEM FOR CUBESAT-SCALE PAYLOADS TO THE OUTER PLANETS Page: 3 of 3
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within a central core material. The ideal core material
must be capable of storing thermal energy, i.e., thermal
capacitor, and then dissipating that energy to a flowing
gas. Several materials have been identified elsewhere
as being capable of achieving this task relying on their
specific heat capacities, e.g., beryllium and boron tetra-
carbide . Instead, in this study the use of silicon as a
thermal capacitor material is being considered. Silicon
undergoes a latent heat of fusion (AH = 50.2 kJ/mol) at
1685 K . By taking advantage of silicon's latent heat
of fusion, when gas is flowed through the silicon core a
phase transformation from liquid to solid occurs. This
in turn, will dissipate energy from the core to the gas at
a constant core outlet temperature, yielding a constant
chamber temperature or turbine inlet temperature de-
pending on mode being used. For heat rejection, tur-
bine exhaust gases will be passed through flow chan-
nels in a solid lithium block. Having a high heat ca-
pacity, the lithium block absorbs the thermal energy
from the gas, which is then allowed to dissipate slowly
between pulses. This method has the potential to deliv-
er a low mass, compact heat rejection subsystem .
Mission Architecture: The trajectory analysis of a
mission architecture serves as a crucial step to deter-
mine the feasibility of both the mission and the primary
technology used for it. In this study the proposed pro-
pulsion system will propel itself from a geocentric orbit
using phasing maneuvers, i.e., perigee pumping. This is
accomplished by impulsing at the periapsis to induce
apogee raising until transition in to the correct helio-
centric orbit can be achieved for the interplanetary
phase. Figure 3 shows a possible trajectory using peri-
gee pumping. This technique of orbital escape aligns
well with the RTR concept, where propellant is in-
jected into the thermal capacitor and out of the nozzle
and is then allowed to "recharge" through each orbit. In
essence, the high thrust aspects of the thermal propul-
sion mode allows for a much quicker orbital escape
then what is achieved through electric propulsion
alone. Additionally, by employing a thermal propulsion
mode, launch mass is minimized by negating the need
for an upper stage motor. Once a heliocentric orbit is
achieved the electrical mode will be employed power-
ing either the communication or electric propulsion
subsystems. Utilizing the high efficiency of electric
propulsion through the interplanetary phase will aid in
decreasing overall transit times.
An instrumentation package for an Enceladus mis-
sion with focused objectives can be assembled to fit
with the limited constraints of a 6U package. Table 1
outlines possible instruments to be included. This study
assumes the CubeSat payload has a dedicated radioiso-
tope-based thermophotovoltaic (RTPV) battery with an
output of 5 - 10 W .
Figure 3: orbital escape process utilizing the dual-mode
In the exploration of the outer planets maintaining
communication becomes about available power. On an
Enceladus mission, a 10 W battery is not capable of
handling the data transfer needed. However, utilizing
the propulsion system's electric mode, the power needs
at location can be maintained. Preliminary results indi-
cate a 25 kg core can produce roughly 25 kW over a 6
min blowdown yielding manageable data transfer rates.
Table 1: instrument package for mission architecture.
Mass [kg] Volume [cm3] Power [W
X-Ray/Gamma 0.18 175 2.5
Spectrometer 0.23 180 -
High Res. 0.166 501 0.66
Mass 0.25 32 0.5
Conclusion: Preliminary numbers indicate a pro-
pulsion system can be designed to deliver a 6U Cube-
Sat payload to Enceladus orbit with less than a 1,000
kg launch mass. Additionally, such a propulsion system
allows for flexibility in both the payload size and mis-
sion destination with small changes in the launch mass.
Ultimately, the proposed propulsion system not only
extends the capabilities of CubeSat platforms but also
extends involvement of outer planetary exploration to
small research and university communities. This pro-
pulsion system provides the need of a low mass system
for exploration to the outer planets where solar-electric
and chemical-based propulsion systems are not feasi-
 Jerred N. D. et al. (2012) AIAA Space 2012,
Paper #5152.  Morgan S. et al. (2011) NETS 2011,
Paper #3303.  Rosaire G. C. et al. NETS 2013, Pa-
per #6736  O'Brien R. C. et al. (2009) J. Nucl. Ma-
ter., 393, 108-113.  Gaskell D. R. (2003) Intro. to
the Thermodynamics of Materials, 4th Ed., 587. 
Howe T. M. et al. (2012) NETS 2012, Paper #3059.
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Jerred, N. D.; Howe, T. M.; Howe, S. D. & Rajguru, A. RADIOISOTOPE-DRIVEN DUAL-MODE PROPULSION SYSTEM FOR CUBESAT-SCALE PAYLOADS TO THE OUTER PLANETS, article, February 1, 2014; [Idaho Falls, Idaho]. (digital.library.unt.edu/ark:/67531/metadc871763/m1/3/: accessed October 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.