IEEE - Aerospace and Electronic Systems - September 2019 - 13

Doyle and Peck
The optical navigation system is able to solve the so-called
lost-in-space problem if the EKF diverges or following a
failure-for example, if the flight software fails or the avionics reboot unexpectedly due to, e.g., a single-upset
event caused by an energetic particle. The lost-in-space
solution initiates a particle filter that recovers the current
position in space and time. Thereafter, the spacecraft
returns to its EKF for trajectory determination [10].
The Raspberry Pi computer used for the optical navigation computations is also the flight computer for the
spacecraft. The Raspberry Pi series, despite being consumer-grade hardware, is radiation-tolerant up to 40 krad
(Si) [8]. Its specifications indicate that it is qualified for
temperatures ranging up to 70 C, and the CisLunar
Explorers program has verified the thermal survivability
in a thermal vacuum chamber up to 85 C.

POWER
The spacecraft uses a GomSpace P31u power module with
two 18650 lithium-ion batteries. Power is provided by SolAero ZTJM solar cells with monolithically integrated
bypass diodes, through the MPPT power conditioner on
the P31u. The instantaneous power available to the spacecraft varies depending on orientation but is typically an
average of about 8 W. During occasional, brief eclipses,
no power is produced; in these situations, the batteries provide the bus power.
Spacecraft power consumption in normal mode is
approximately 1 W, mostly for the Raspberry Pi and sensors.
The spacecraft requires 7.5 W to transmit and 7.3W to pulse
its attitude thruster and, therefore, dips into the batteries for
these purposes. However, it does not need to rely on the batteries to electrolyze (5 W) or fire the electrolysis propulsion
thruster by activating its glow plug (4 W).
During eclipse, the spacecraft not only produces no
power but also must heat itself to keep the onboard water
and electronics at safe temperatures. The use of onboard
heaters (7 W) and to the baseline power consumption
(1 W) means the spacecraft has an 8 W power deficit during eclipses, greater than at any other time. The battery
capacity and depth-of-discharge design are meant to
accommodate this situation.
Because of the spinning spacecraft architecture, the use
of bypass and blocking diodes is an essential part of the solar
panel design to help prevent damage from rapid switching of
power coming from different faces [23], [27]. Diodes have
been included for all cells but are especially important for
the panels on the side faces of the spacecraft.

WATER ISRU POTENTIAL
The CisLunar Explorers mission is designed as a pathfinder for water electrolysis propulsion technology and
SEPTEMBER 2019

to demonstrate the potential utility of ISRU resources
such as water when used in multiple subsystems of a
spacecraft. The previous sections described the subsystems of the CisLunar Explorers spacecraft and the
ways in which multiple subsystems leveraged the presence of water onboard the spacecraft. This section
identifies broader advantages of the use of water as a
multirole ISRU material and gives two examples of
existing mission concepts that could benefit from utilizing water electrolysis propulsion technology. The
next section expands on those examples and quantifies
the mass-saving benefits of water ISRU when used in
multiple aspects of a mission design.

UTILITY
Water is an excellent ISRU material, especially for future
crewed missions. Human crew will require water for
drinking, cleaning, and irrigation of crops. Astronauts on
the International Space Station use 3 L of water per day
for drinking, food, and hygiene, and an additional 1 L per
day for oxygen production [7]. Reclaiming and recycling
water and oxygen can reduce the net use per day. However, water supplies will still be depleted albeit at a lower
rate. Resupplying using water gathered in situ will reduce
the total launch mass required to supply any crew with
water for their needs.
Water electrolysis propulsion, described previously,
can provide commonality between crew supplies and
propellant reserves. This propulsion technology allows
for impulsive maneuvers, unlike electric thrusters, and
can achieve a specific impulse of up to 450 s, a significant improvement over the less than 200 s of steam propulsion, another impulsive water propulsion concept
[20]. An example use case for water electrolysis propulsion with refueling by ISRU would be the World Is Not
Enough (WINE) mission concept, developed as a collaboration between the University of Central Florida,
Embry-Riddle Aeronautical University, and Honeybee
Robotics [28]. WINE uses CubeSats, and possibly a
larger mothership, to explore small bodies such as asteroids, heating ISRU water into steam for propulsion to
leave the surface of each. The WINE team notes that
for bodies with significant gravity or where a separate
mothership is not present, alternative propulsion technologies with higher specific impulse than steam propulsion may be needed to perform the relatively high
DV transfers between bodies. They suggest electrolysis
propulsion as an option for this [28]. The use of water
electrolysis propulsion as an alternative or a supplement
to the WINE mission propulsion concept is discussed
further in following section.
Other uses are passive and will not consume water
supplies. Water can be used as a working fluid, as a heat

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IEEE - Aerospace and Electronic Systems - September 2019

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