IEEE - Aerospace and Electronic Systems - September 2019 - 9

Doyle and Peck
centrifugally separated. This architecture has the added
benefit of providing passive spin-stabilization for the
spacecraft, maintaining its attitude much like a top or
gyroscope does, without the need for reaction wheels or
multiple attitude thrusters.
The two CisLunar Explorers spacecraft are major-axis
spinners with each spacecraft's principal axis aligned
closely to the axis of symmetry of the electrolysis propulsion thruster nozzle. The mass center shifts as propellant
is expended during the mission. Any such misalignment
causes an overturning torque that the spin stabilization
resists to some extent. To mitigate this shift as much as
possible, each spacecraft has been designed so that its
mass center begins slightly offset from the nozzle center,
on the near side of the nozzle relative to the propellant
tank. Then, as propellant is consumed and the mass center
moves away from the propellant tank, the mass center
tracks across the nozzle, eventually crossing its centerline
at approximately 50% fill fraction. Thus, throughout the
mission, the thrust axis is as close as possible to the center
of mass, minimizing the disturbance torque created by
electrolysis propulsion thruster pulses.
The moment of inertia of the spacecraft is reduced as
propellant is expended. However, the total angular
momentum remains approximately constant because
before the gaseous products of water electrolysis are combusted, they move from the propellant tank to the combustion chamber, and therefore closer to the spin axis. This
movement decreases the moment of inertia of the spacecraft as mass is transferred closer to the center of mass.
Yet, the spacecraft angular momentum remains constant
prior to each thruster pulse because no external impulse
has been applied yet. Therefore, the spin rate increases as
the moment of inertia decreases. The effect is called jetdamping and is similar to an ice skater drawing in their
arms as they spin. Eventually, the gas products are combusted and then expelled. The decrease in angular momentum at this point is very small because the combustion
products are expelled through a nozzle throat ideally centered on the spin axis. Therefore, over the course of the
mission, the spacecraft spin rate will increase, from an initial 6 rad/s after deployment and splitting, to a final value
of approximately 7.5 rad/s when propellant is depleted.
Spin-stabilization requires no active attitude control,
but reorientation is occasionally necessary. A single cold
gas thruster is all that is necessary to accomplish such
maneuvers. On each CisLunar Explorer, the cold gas
thruster is located in the arm of the spacecraft, with its
nozzle parallel to and as far away from the principal axis
as possible. This location maximizes the torque that the
attitude thruster exerts about the center of mass. This
reorientation torque does not significantly change as the
center of mass shifts because that shift is only a few percent of the total distance between the thrust axis and principal axis.
SEPTEMBER 2019

Liquid propellant in a spinning spacecraft sloshes
when the spin is disturbed, such as by a pulse from the
electrolysis propulsion or attitude control thrusters. Any
resulting nutation dissipates due to viscous effects in the
moving fluid [1]. There is an intentional symbiosis
between the attitude control and propulsion subsystems:
the same spin that is required to separate the water from
the electrolyzed gases is stabilized by the sloshing of the
water in the propellant tank.

PASSIVE THERMAL BALANCING
The water in the spacecraft propellant tank must remain liquid throughout the mission. This requirement derives from
the simple fact that the electrolyzers cannot electrolyze ice
but also because the sloshing of liquid water speeds up nutation damping to achieve a simple, stable spin about the thrust
axis. In principle, the expansion of water upon freezing can
damage the tank, but the tank here is designed with sufficient
ullage volume to accommodate this expansion. Because of
the importance of keeping the water in the propellant tank a
liquid, the propellant must be kept in an acceptable temperature range, which nominally depends on the pressure in the
tank. However, the freezing point of water is only negligibly
different from 1 to 10 atm. Water boils at temperatures far
above what the spacecraft electronics can survive (at maximum pressure, over 180 C). Thus, the propellant boiling is
not a concern.
Likewise, the spacecraft electronics, particularly the
lithium-ion batteries, also have acceptable temperature
ranges, although these are both narrower and colder than
that of the water. For example, the batteries can be safely
discharged between À20 to 60 C. In an eclipse, the spacecraft electronics can be kept warm enough to function by
their own waste heat; however, this waste heat can
become dangerous when the spacecraft is in sunlight and
oriented for maximum irradiance. Because the spacecraft
electronics would overheat well before the water boils,
and the water will freeze well before the electronics fail,
the desired temperature range throughout the spacecraft is
above the freezing point of water (0 C) and below the
battery maximum temperature (60 C). The spacecraft
achieve thermal survivability through a passive design,
heat sinking most of the spacecraft electronics to the propellant tank so that their waste heat is conducted away and
keeps the water from freezing. The smaller sides of the
spacecraft, parallel to the spin axis, are 20% covered in
GSFC Dark Mirror black paint to improve heat absorption
in the minimum-solar orientation.
The avionics dissipate approximately 1 W of power in
their quiescent mode. Several functions dissipate much
more heat, including the propulsion system's glow plug
(4 W), the attitude thruster's solenoid valve (7.3 W), and
the communications system when transmitting (7.5 W).

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IEEE - Aerospace and Electronic Systems - September 2019

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