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MARS OR BUST, LLC
1.0
Mission Summary
2.0
Systems Engineering and Integration
3.0
Mars Environment and In-Situ Resource Utilization
4.0
Structures Subsystem
5.0
5.1
Electrical Power Distribution and Allocation Subsystem
Overview
The power subsystem will manage, distribute and store power throughout the Mars
habitat. Both mobile and stationary sources of power will be present within the habitat to
provide for all the functions of the habitat and mission.
5.1.1
Level 2 Requirements
The following requirements were derived from the DRM and level 1
requirements:
Table 5.1: Level 2 Requirements
Requirement
Number
5.1
5.2
5.3
5.4
5.5
Requirement Description
Supply sufficient power with 3-level redundancy
Supply power while reactors are being put online
Transfer power from reactor to habitat
Distribute power on a multi-bus system
Provide storage and interfaces for rovers/EVA suits
5.6
5.7
5.8
5.9
5.10
5.11
5.12
5.13
Interface with transit vehicle power sources
Regulate voltage to a usable level
Include a fault protection system
Provide an emergency power cutoff
Follow maintenance and safety procedures
Have power dissipation capability
Be flexible in order to allow for future expansion or addition
Mass must not exceed 3249 kg (including in-transit power)
Source
DRM 1.3.3.7
DRM 3.6.4.6
DRM 3.6.3.7
Derived
DRM
3.6.4.6.2
Derived
Derived
Derived
DRM 3.6.3.7
Derived
Derived
DRM 2.4.4
DRM 3.6.3.4
5.1.2 Power Profile
The power profile (table 5.2) was created by collecting peak power usage from
each subsystem during various mission modes. These modes include both non-crewed
and crewed times as well as a survival added with battery and non-battery, in case the
nuclear reactor is not operational during survival mode. The crewed active day and night
modes push the limits of the allocated 25 kW to the habitat. However, these numbers
could be reduced with further revisions from each subsystem. Also, the survival modes
have relatively high power usage to allow the capability of an EVA. If the habitat were
to go into a survival battery mode, ECLSS would not have the water filtration system on
and the habitat would run off the storage tanks. This is why the battery and non-battery
power survival values differ.
Table 5.2: Power Profile
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Noncrew
Crew
Power Consumed by Habitat During Specific Mode
Mission Mode
Time in Mode
Total (kW)
Landing
6.9
Set-up Time dependent process
11.4
Survival (battery) => energy balance required
6.1
Survival (Nonbattery) 12 hours
6.1
Day:
Active
24.9
Non-active
9.1
Time dependent process
Survival (battery) => energy balance required
Survival (Nonbattery) 12 hours
Night:
Active
Non-active
10.4
12.4
26.9
11.1
Time dependent process
Survival (battery) => energy balance required
Survival (Nonbattery) 12 hours
10.4
12.4
5.1.3 Mass/Power/Volume Breakdown
The mass of electrical cabling inside for the habitat can be estimated for different
gauges of wire used. Aluminum wire would most likely be used due to its low mass and
high conductivity. Below is a table showing several different gauges of wire as well as
their mass for 5000 meters.
Table 5.3: Electrical Cabling Mass and Thermal Breakdown
Gauge
1
2
4
6
8
10
12
14
16
18
20
Diameter
(cm)
0.73
0.65
0.52
0.41
0.33
0.26
0.21
0.16
0.13
0.10
0.08
Area (cm2)
0.42
0.34
0.21
0.13
0.08
0.05
0.03
0.02
0.01
0.01
0.01
Mass
(kg/m)
0.11
0.09
0.06
0.04
0.02
0.01
0.01
0.01
0.00
0.00
0.00
Mass for
5000m of
cabling
572.30
453.76
285.41
179.46
112.91
71.00
44.64
28.10
17.65
11.11
7.00
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Resistance
(ohms/km
@ 20C)
0.68
0.86
1.36
2.16
3.32
5.29
8.40
13.35
21.26
33.79
53.48
% Loss
for 25
kW
2.35
2.97
4.73
7.51
11.54
18.35
29.16
46.36
73.82
117.34
185.69
MARS OR BUST, LLC
The masses were calculated using simple volume calculations with 2.699 g/cm3 as
the density of aluminum. Table 5.2 also shows the resistance and resistance losses using
a total of 25 kW. Using this table, we can only estimate the total mass for electrical
cabling in the habitat. We chose to use 5000 meters of cabling for an estimate, but the
table also has a mass per unit length for future calculations, as the total cabling becomes
more apparent. Using the table, we estimated the mass to be around 150 kg and heat
losses from resistance to be around 10%. Any higher losses would be too inefficient and
should result in choosing a larger gauge. We also realize that an actual habitat would use
a combination of smaller and larger gauges depending on the load.
The mass of the power subsystem was calculated to include all of the equipment
that would be used in a regulated system. This includes the charge control, batteries,
regulation, conditioners, circuit breakers, and wire needed to carry out the full mission of
the subsystem. Because this is the initial design phase and all of the components are not
known as of yet, the mass of the power system is an estimation dependant upon the
power output. The masses of the equipment were estimated using the ratio of 11.4
kg/kW for all equipment less the wiring and batteries [Larson and Pranke, 2000]. Having
a 25 kW system the mass comes to 285 kg. The batteries were sized to provide 10 kW of
power to keep the habitat running until power could be restored or provided from the
rover. The habitat will have 24 hours to run on batteries. Using Li-ion batteries with a
specific power of 170 W*h/kg and the specified power needed and time, a mass of 1411
kg of batteries is needed. There is also mass that needs to be allocated for spares. This
was decided to be set at 100 kg for spare breakers, or other equipment that may need
replacing. A table showing all of the mass calculations and the total mass required for the
system is shown below. The estimated volumes are shown as well. The volume for the
batteries was estimated by using the energy density of 160 W*h/L [Larson and Pranke,
2000]. Battery volume plus the estimates for the rest of the system gives a total of 3.6
cubic meters.
Table 5.4: Power System Masses and Volumes
Power System Masses and Volumes
Battery Mass
Li-Ion
Regulated system mass
W*h/kg
W
170
10000
11.4
25
Time (h)
Spares (breakers, etc.)
Total mass w/o wires
24
Volume (m3)
kg
1411.765
1.5
285
2
100
0.10
1796.765
Total Volume
3.60
The total mass and volume breakdown is shown in table 5.4.
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Table 5.5: Mass/Volume Breakdown
Power Subsystem Technologies
Wires/Cabling
Component
#
Wires/Cabling
Add.
Weight
(kg)
Weight
(kg)
Total
Weight
(kg)
150.00
Power
(kW)
Total
Power
(kW)
2.5*
Totals
150
Total
Volume Volume Crew Time
(m3)
(m3)
(hrs/day)
0.10
2.5
*Amount of heat generated
0.1
Batteries
Component
#
Li-ion
Add.
Weight
(kg)
Weight
(kg)
Total
Weight
(kg)
1411.77
Power
(kW)
Total
Power
(kW)
10*
Totals
1411.77
Total
Volume Volume Crew Time
(m3)
(m3)
(hrs/day)
3.00
10
3
*Amount of power produced, not needed
Regulated System
Component
Regulated System
Spares (breakers, etc.)
#
Add.
Weight
Weight
(kg)
(kg)
285.00
200.00
Total
Weight
(kg)
Total
Power
Power
(kW)
(kW)
25.00
Total
Volume Volume Crew Time
(m3)
(m3)
(hrs/day)
1.00
Totals
485
25
1
Grand Totals
2046.77
37.50
4.10
5.1.4 Input Output
The input/output diagram (figure 5.1) for the power subsystem is fairly simple.
Power is taken in from the nuclear reactor and distributed throughout the habitat.
Approximately 2.5 kW of heat is output to the thermal subsystem and command is input
from the C3 subsystem.
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Figure 5.1: Input/Output Diagram
5.2
Design and Assumptions
5.2.1 Assumptions
There were several assumptions made in the design of the power subsystem.
Even while in transit, the habitat requires a minimum amount of power. It was assumed
that this power would be supplied to the habitat before it arrives on Mars. Also, the
nuclear reactor would already be on the surface of Mars to provide the habitat with power
along with the cabling used to transfer power from the reactor to the habitat. An assumed
25 kW is allocated to the habitat with a maximum of 160 kW to the whole system.
5.2.2 Functional Diagram
The power system will provide power from the nuclear reactor. There will be two
reactors but only one will be used at a time, the other will serve as a backup. The power
will flow through a three bus, regulated system. The chart below shows the diagram of
the power flow. The system will allow for power to flow from the reactors to the
conditioner and regulator or to the batteries via a charge controller if the batteries are in
need of charging. The power can also flow from the batteries to the conditioner and
regulator if the reactors are offline. After regulating the power to a usable voltage the
power is transferred on either of three buses to the distribution hub. A three-bus system
is used for redundancy and three-level redundancy is needed because it is a life critical
system. From here the power is transferred to separate breakers that are specified for
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each subsystem with an extra breaker for life or mission critical systems that cannot be
turned off. Figure 5.3 shows, as an example, the critical components of the Command,
Control, and Communication under the life/mission critical breaker. Most ECLSS
functions are considered life or mission critical and therefore kept under the ECLSS
breaker. Under each subsystem there can also be branches to each component or a
breaker to a few components depending on the need of the component. Figure 5.4 and
5.5 show examples of the ECLSS and CCC subsystem in the way that they are setup.
The system uses circuit breakers to allow for equipment to be powered down while
connecting or disconnecting the equipment, reducing the risk of arching. The structure of
the system is designed to minimize the interference between subsystems while
connecting or disconnecting components within a certain system. The figures show a
basic structure for the layout of the power grid, however since all components are not
known as of yet, a complete system cannot be mapped out.
Figure 5.2: General Functional Diagram
Figure 5.3: Life/Mission Critical
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Figure 5.4: ECLSS
Figure 5.5: CCC
5.2.3 AC vs. DC/120V
The nuclear reactor will be outputting AC power, which is easier to transport at
high voltages over long distances. Based on requirements of today’s technology, it was
decided to use a voltage regulator and operate the habitat on 120V. Also, this is a safe
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operating voltage because there is little plasma interaction, which will reduce the risk of
arching. Since the habitat will be operating at 120 Volts and the reactor is 2 kilometers
away, AC current is ideal. However, the habitat will be using DC power so the input
current from the reactor will need to be converted. Coincidentally converting from AC
power to DC is easier and more efficient than DC to AC.
5.2.4 Contingency Power Supply
The power system, being life critical, needs two backups. There are two nuclear
reactors that can be used. One reactor at a time supplies the power to run the habitat and
the second reactor is on standby as the first backup. The second backup is the 10 kW
power supply in the pressurized rover. This supply can be connected to the habitat and
used to keep power supplied to critical systems. The batteries in the habitat will remain
charged and be ready to supply 10 kW of power for 24 continuous hours while the second
reactor is powered up or if needed, for the rover to return from a mission and be
connected to supply power. The batteries will insure the habitat is never powerless
during the times when the power load is switched from one source to another. While the
batteries will weigh the most, they will last the entire mission, as they will not be cycled
very often. Battery life depends on the amount of cycles. 10 kW is the initial full charge
and with an acceptable rate of degradation they will last well over 500 cycles. The 10
kW of backup power will be enough to power the habitat and keep the occupants alive
but may not be enough to sustain all or any current experiments or EVAs. Therefore
when running on the emergency 10 kW of power all unnecessary equipment needs to be
powered down and only life and mission critical systems will be powered.
5.2.5 Mission Operations Overview
There are some operations that will be required by the power subsystem during
the mission. In case of emergency, the power cutoff and restart was estimated to take 2
hours, occurring as little as possible. It will take 3 crewmembers with arching concerns
and potential damage to equipment. General power maintenance, such as replacing fuses
or resetting circuit breakers, would only take 20 minutes to perform with 2 crewmembers.
The maintenance will occur as needed, with only general electrical safety concerns.
Manual switching of lights, computers, etc. will only need one crewmember and occur as
needed. When the equipment is brought on-line or the power is cycled, the initial powerup and breaker switching will take 2 hours with 2 crewmembers. There is potential for
arcing and damage to equipment. This operation will occur at the initial habitat set up
and as often as the emergency power cutoff and restart occurs. The majority of the
equipment will have automated switching, which will occur as needed. There are only
concerns with error in the automation.
5.3
Verification of Requirements
5.3.1 Level 2 Requirements
The power group was as able to design the subsystem to meet all level 2
requirements. Table 5.6 shows the level 2 requirements and the methods used to meet
them. Mainly, the 3-level redundancy was met with a back up reactor and solar panels.
Batteries were also implemented for safety purposes. Also, the mass came out to be 2050
kg, which is well below the allocated 3250 kg recommended by the DRM.
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Table 5.6: Requirements Verification
Req.
Doc. Report
No. No.
Requirement Description
K1
5.1
Supply sufficient power with 3-level
redundancy
K2
5.2
Supply power while reactors are being put
online
K3
5.3
Transfer power from reactor to habitat
K4
5.4
Distribute power on a multi-bus system
K5
5.5
Provide storage and interfaces for rovers/EVA
suits
K6
5.6
Interface with transit vehicle power sources
K7
5.7
Regulate voltage to a usable level
K8
5.8
Include a fault protection system
K9
5.9
Provide an emergency power cutoff
K10 5.10
Follow maintenance and safety procedures
K11 5.11
Have power dissipation capability
K12 5.12
Be flexible in order to allow for future
expansion or addition
K13 5.13
Mass must not exceed 3249 kg (including intransit power)
Design
Back up reactor, solar panels, batteries, and
pressurized rover power supply
Report
Sec.
5.2.2
5.2.4
Batteries
Cabling
Multi-bus system
5.2.4
5.1.3
5.2.2
Batteries, connectors
Connectors
Voltage regulator
Circuit breakers
Emergency power cutoff
Mission operations
Regulation
5.2.4
5.1.3
5.2.3
5.2.2
5.2.2
5.2.5
5.2.2
Designed flexibility in layout
mass = 2050 kg
5.2.2
5.1.3
5.3.2 Failure Modes Effects Analysis
There are potentially two failures that can occur in the power subsystem. The first
one is a power outage, which can arise two different ways. One is power outage to the
whole habitat and the second possibility is power outage to specific subsystems.
Following the flow charge, one can observe that the power outage to specific subsystems
can occur due to many different causes, damaged power line, damaged transformer and
damaged connector. If one of the above causes occurs, then the crews must fix or replace
the specific technology that had been damaged. However, during this time, the backup
power must be used. Whether the backup power from the power subsystem or the battery
located at each subsystem will be used is determined by the technology that had been
damaged. For instance, if the power line is damaged, then the power subsystem cannot
provide the specific subsystem with backup power that the problem is being addressed at
and the battery located inside the specific subsystem will be used for all the life critical
components in the subsystem.
Power outage to the whole habitat can occur when the reactor or the transformer
is damaged. Fixing and replacing will have to be considered. However, the time in which
the problem can be fixed is indefinite so the battery will be used as backup power during
the night cycle and the high efficiency solar panels will be used during the day cycle as
back up power for life-critical components and charging the battery for night cycle usage
until the problem can be fixed. Power supply from the rovers is also an option here.
The second major problem that the power subsystem can encounter is the voltage
at outlets inside subsystems does not meet the requirement of 120 V. This can occur
when the transformer is damaged and doesn't distribute the right voltage to each
subsystem's outlet. The problem can affect the habitat two different ways. Either the
whole habitat’s outlet is affected or some specific outlet is affected. Knowing which
outlets have been affected will help the crew locate the problem. If this occurs, backup
power will be used to makeup the voltage so life-critical components of each subsystem
can operate properly while the problem is being repaired.
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Figure 5.6: Failure Tree
5.4
Future Considerations:
After this first iteration, some things need to be further analyzed before moving
on to the next step in the design. First, a more detailed power profile could be
constructed with more mission modes. Also, the hardware used to transfer and distribute
the power will need to be specified. Although the system mass came out to be below the
allocated budget, another iteration could be preformed to lower the mass even more. The
effects of electromagnetic interface and electrostatic discharge also need to be taken into
consideration.
6.0
Appendices
6.1
Appendix A: Acronyms
Please add any acronyms that you use in your section here.
DRM
KISS
MOB
Design Reference Manual
Keep It Simple and Stupid
Mars Or Bust
6.2
Appendix B: References
Please add any references that you use in your section here.
6.3
Appendix C: Acknowledgements
Please add any acknowledgements specific to your section here.
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