James Webb Telescope Issue - 22

Feature
Impacts of Low-Power Requirements on the LEMS HMS Design
Flight Center (GSFC) and has completed the TRL-6
qualification. This means that LEMS has completed the
design, and an engineering test unit (ETU) has been built
and tested in a simulated space environment. Thermal
vacuum testing has been performed, and the design works
as expected. A photo of the LEMS ETU is shown in Figure
2. As a TRL6 system, LEMS is ready to transition from a
research and development effort to a full-fledged flight
project payload. It is seeking flight project funding through a
proposal to NASA's Payloads and Research Investigations on
the Surface of the Moon (PRISM)1
.
power operations, during which all station subsystems are
" awake " and power-demanding tasks are conducted. The
balance between " hibernation " and " awake " allows LEMS to
remain power-positive and within operational temperature
limits. During " awake " periods, all science sensors are active
and collecting data. During " hibernation, " only SEISLEMS
and LMM collect data. The more power-hungry systems
- the Mass Spectrometer and LEIA instruments, the radio,
and even the processor card which handles the command
and data handling - were designed to only be active for a
roughly 10- to 20-minute window once every 24 hours. A
schedule over the full lunation showing estimated discharge
levels of the battery and the times when each subsystem
would be powered on is shown in Figure 3.
Figure 1 - LEMS Component Diagram - External (left) and Internal
(right) Layouts
LEMS is designed for a minimum mission duration of two
years. Designing a spacecraft that can survive and operate
on the moon for this duration required solving numerous
engineering challenges, including trade studies between
maintaining a survivable temperature and minimizing size,
weight, and power, often abbreviated as SWaP. The power
concerns had ramifications in each electrical subsystem -
the solar panels had
to be consistently
designed to charge
throughout the lunar
daytime, the battery
had to be large
enough to last the
whole lunar night, and
each subsystem had
to minimize power as
much as possible.
Figure 2 - LEMS Engineering Test Unit
during system-level integration
II. Reducing Power as Much as Possible
LEMS is power-sufficient and does not require external
power from its delivering asset to operate. Photovoltaic cells
generate power during the daytime and a high-efficiency
battery sustains operations at night. LEMS is also thermally
self-sustained and does not require radioisotopic heaters
nor active thermal dissipators to survive the hot lunar days
or cold nights. LEMS manages its thermal and power states
by alternating periods of low-power operations, during
which the station is in " hibernation, " with periods of highTHE
BRIDGE
Figure 3 - Operational cycle and subsystem status estimates over one lunation
III. The Hibernation Management System
If most or all the major payload systems need to shut down,
how would they turn back on? A photo of the components
of the LEMS Avionics Box is shown in Figure 4. The processor
card (E) in the image, manages the typical command and
data handling responsibilities, such as sending commands
and receiving telemetry from the other subsystems on LEMS,
as well as interfacing with the radio for communication with
Earth. However, as previously mentioned, even a subsystem
this simple requires more power than the battery can safely
provide for the long lunar night.
LEMS needed an auxiliary card that consumed very low
power while managing powering on and off the other
major subsystems. The HMS was conceptualized to handle
this challenge and act as a " timekeeper " and a " night
watcher " while the station is hibernating. The HMS will
follow a schedule stored in flash memory to control inhibit
signals, which act as an on-off switch to the electrical power
system card (D) in
Figure 4, and the
processor card. With
these inhibits set high,
LEMS enters a low
power state.
The HMS consists of
two Printed Circuit
Boards (PCBs)
Figure 4 - LEMS Avionics Box
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