Aerospace and Electronic Systems - May 2019 - 49

Vanegas et al.
from the vent loop, the same as the LiOH system. After
the canister is spent, it can be placed in a MetOx regenerator. This regenerator is a special oven housed at a central
location with a mass of 47.63 kg [12]. This requires crew
time and significant power (over 1 kW for up to 14 h), and
the enthalpy of reaction is À81 491 J/mol CO2 for MetOx.
The metal oxide canister consists of metal sheet
assemblies, a charcoal bag containing active charcoal, and
a cover panel. The charcoal and bag construction are the
same as those used in the LiOH CCC, meaning that the
MetOx canisters designed for the Extravehicular Mobility
Unit (EMU) are interchangeable with the EMU LiOH
CCC's. However, the MetOx CCC is significantly heavier
than the LiOH CCC at 14.52 kg per container. The scrubbing capacity is nearly identical at 0.54 kg/CCC before
recharging [12]. The mission timeline was analyzed to
determine the total amount of CO2 that needed to be
scrubbed. Using the size, mass, and enthalpy change for
the MetOx CCC's listed above, a total requirement of four
CCC's was calculated. This total includes the two CCC's
in the suit PLSS, which results in only two CCC's being
located on the rover (and thus inside the system boundary). This corresponds to a mass of 62.6 kg for the four
MetOx CCC's.

RAPID CYCLE AMINE (RCA)-SYSTEM SPECIFICATIONS AND
DRAWBACKS
RCA is currently being developed by NASA as a new
technology for carbon dioxide scrubbing and humidity
control. The RCA design consists of alternating absorbing
and desorbing beds that contain amine coated pellets
mounted in an aluminum foam substrate. This alternating
design allows a single RCA CCC to continuously scrub
CO2 without need for replacement. The amine used in the
swing bed also removes water vapor from the suit ventilation loop, eliminating the need for a condensing heat
exchanger, slurper, and rotary separator. The weight of
one RCA CCC is 7.26 kg and does not require replacement during the mission.
Due to the continuous operation capability, each astronaut and the rover only require one RCA CCC, for a total
of three canisters. Each CCC has a CO2 scrubbing rate of
0.15 kg CO2 /h. The dimensions of one RCA canister are
the same as MetOx and LiOH. Thus, the interchangeability of all three systems still remains an option for future
missions.

SYSTEMS COMPARISON AND INTEGRATION
Overall, the RCA and LiOH systems have significantly
lower weight than the regenerative MetOx system. At
14.52 kg as opposed to 2.9 kg per CCC and a life-span no
longer than LiOH, MetOx is the definitive third choice for
this mission. However, the ability for regeneration during
MAY 2019

the mission makes MetOx a superior system to LiOH in
long duration missions such as on the ISS, where the mass
of the regeneration oven is not a significant drawback. For
shorter missions, there are no significant benefits to having
the MetOx system over LiOH. However, since the LiOH
and MetOx canisters are interchangeable in the same
ECLSS system, this decision could be easily changed later
if mission duration increases. Alternatively, although
LiOH is a proven technology to ensure carbon dioxide is
removed from the environment, the canisters are not
rechargeable, so empty canisters (0.75 kg/canister) are
cumbersome to return to Earth if they are to be replenished. Finally, one of the drawbacks of RCA is that the
presence of a faint ammonia smell in the ventilation loop
occasionally causes discomfort to the astronaut.
There are benefits to using LiOH over MetOx for this
type of mission and RCA over both LiOH and MetOx, but
it is important to remain flexible. In the future, should there
be some change in mission requirements or operational
capabilities, this flexibility gives decision makers and mission planners more options. RCA weighs more than LiOH,
but the advantage of not needing to change cartridges during EVA, eliminating the need for a time-consuming activity that is potentially dangerous and difficult, is more
important than the additional mass required (4.36 kg additional/CCC). RCA is the better choice for the ECLSS system, but as a next-generation technology that is still under
development and has not yet been flight-tested, this paper
uses LiOH for the chosen system in all mass estimates and
the concept of operations. The interchangeability of the
systems means that it will be easy to switch to RCA later if/
when the technology becomes mature.

SUBLIMATOR DESIGN
While cooling is typically the main concern for lunar
exploration mission, during certain times of the lunar day
a situation may arise that requires heat transfer to keep the
astronauts warm. To allow the water running through the
LCG to be heated, rather than cooled, a bypass valve in
the LCG transport loop can be actuated by the astronauts.
A diagram of this setup (Thermal Control Unit) is shown
inside the Rover ECLSS in Figure 3. This setup is similar
to the design used in the Apollo PLSS backpack which
allowed different bypass levels depending on the amount
of cooling needed by the astronauts. The PLSS bypass
line simply allowed a lower flow of the LCG water over
the sublimator. In our design, the bypass valve will allow
water to bypass the sublimator hot plate and instead flow
to a heat exchanger which is exposed to direct sunlight.
The internal design of the sublimator housing and its
components, shown in Figure 5, is very similar to the
Apollo PLSS design [13] and was taken to be the base
model for this analysis. While future missions may use an

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Aerospace and Electronic Systems - May 2019

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