Design of a Lunar Rover Utilizing Hydrogen-Oxygen Fuel Cell
Technologies
A Thesis
Presented in Partial Fulfillment of the Requirements for the Degree of Master of Science
for Aeronautical and Astronautical Engineering in the Graduate School of The Ohio State
University
By
Michael Phillip Snyder
Graduate Program in Aeronautical and Astronautical Engineering
The Ohio State University
2011
Master's Examination Committee:
Dr. Meyer J. Benzakein, Advisor
Dr. Gerald M. Gregorek
Copyright by
Michael Phillip Snyder
2011
Abstract
Future exploration of the solar system will depend on new designs and technologies that
are efficient and versatile. Roving systems have explored the Moon and Mars but current
means of supplying power are fragile and inefficient, or are considered hazardous to
launch. NASA’s Glenn Research Center developed criteria necessary for the design of a
robotic lunar rover with an extended exploration time. In order to satisfy these
requirements a versatile rover equipped with a hydrogen-oxygen fuel cell with a 1
kilowatt nominal power output was designed to operate in the lunar environment for
longer than 5 years continuously. Scaled testing of the rover was performed to predict the
performance of the lunar rover. Testing was performed at the Ohio State University’s
Aeronautical and Astronautical Research Laboratory in order to determined drawbar pull
and sinkage of the rover. Regolith mitigation strategies were investigated in order to
prolong the life of the rover by limiting and eliminating contamination caused by the
lunar dust.
ii
Dedicated to
To Diane, Walter, Andrea and everyone else who has helped me along to reach the
starting point of my journey.
iii
Acknowledgments
I would like to thank my advisor, Dr. Meyer Benzakein for all of his help and guidance. I
would also like to thank Dr. Paul Penko, Eric Joyce, and Joel Longo for their help with
this project.
iv
Vita
April 16, 1986………………………...
Born, Sandusky Ohio, USA
June 2004……………………………..
Diploma, Bellevue Senior High School
June 14, 2009…………………………
B.S., Aeronautical and Astronautical
Engineering, The Ohio State University
October- December 2009……………….
Graduate Research Assistant,
Aeronautical and Astronautical
Engineering, The Ohio State University
January- March 2010…………………
Graduate Teaching Assistant,
Aeronautical and Astronautical
Engineering, The Ohio State University
September- December 2010…………….
Graduate Teaching Assistant,
Mechanical Engineering, The Ohio State
University
January- March 2011…………………
Instructor, Aeronautical and
Astronautical Engineering, The Ohio
State University
April- June 2011……………
Graduate Teaching Assistant,
Mechanical Engineering, The Ohio State
University
v
Publications
Snyder, M.P. and Joyce, E.R., Lunar Extra-Vehicular Activities and Colonization
Strategies, AIAA SPACE 2008 Conference and Exposition, September 9-11,
2008, San Diego, California.
Snyder, M.P. and Joyce, E.R., Optimization of Fuselage Design for a Sounding Rocket
Using Composite Materials, 45th AIAA Joint Propulsion Conference and Exhibit,
August 2-5, 2009, Denver, Colorado.
Joyce, E.R. and Snyder, M.P., Solid Rocket Motor Design for a High Altitude Composite
Rocket, 45th AIAA Joint Propulsion Conference and Exhibit, August 2-5, 2009,
Denver, Colorado.
Snyder, M.P. and Joyce, E.R., Robotic Lunar Rover Design Utilizing Fuel Cell
Technologies and Regolith Mitigation Strategies, AIAA SPACE 2009 Conference
and Exposition, September 14-17, 2009, Pasadena, California.
Joyce, E.R. and Snyder, M.P., Lunar Legislation: Strategies for Developing and
Protecting the Lunar Frontier, SPACE 2009 Conference and Exposition,
September 14-17, 2009, Pasadena, California.
Snyder, Michael and Eric Joyce. "Development of Active Rocket Guidance at The Ohio
State University: a guidance system being constructed by undergraduate
students." Ohio State Engineer Spring 2009: 06.
Snyder, Michael and Eric Joyce. "Luna Plaustrum: building a test prototype lunar rover."
Ohio State Engineer Spring 2009: 08-10.
Snyder, M.P., Joyce, E.R. and Osborne, J.C., Component Propulsion System: A New
Philosophy for Exploration, Space Propulsion 2010, May 3-6, 2010, San
Sebastian, Spain.
Snyder, M.P., et al, A Mars Utility Vehicle Design with Incorporated Regenerative Fuel
Cell Technology and In-Situ Resource Utilization, 46th AIAA Joint Propulsion
Conference and Exhibit, July 25-28, 2010, Nashville, Tennessee.
Joyce, E.R., et al, Design of a Versatile Regenerative Fuel Cell System for Multi-Kilowatt
Applications, SPACE 2010 Conference and Exposition, August 30- September 2,
2010, Anaheim, California.
Snyder, M.P., et al, Mobile Instrument for Lunar Exploration Endeavors: The Design of a
Fuel Cell Powered Lunar Exploration Vehicle, SPACE 2010 Conference and
Exposition, August 30- September 2, 2010, Anaheim, California.
Jedrey, R., et al, Preliminary Mars Ascent Rendezvous Study, SPACE 2010 Conference
and Exposition, August 30- September 2, 2010, Anaheim, California.
Dunn, J., et al, 3D Metal Printing in Space: Enabling New Markets and Accelerating the
Growth of Orbital Infrastructure, Space Studies Institute Space Manufacturing
2010 Conference, October 29-31, Mountain View, California.
vi
Fields of Study
Major Field: Aeronautical and Astronautical Engineering
vii
Table of Contents
Abstract…………………………………………………………………………………... ii
Dedication……………………………………………………………………………….. iii
Acknowledgements……………………………………………………………………… iv
Vita………………………………………………………………………………………. v
List of Tables…………………………………………………………………………….. x
List of Figures…………………………………………………………………………… xi
Chapter 1: Background
1.1 Interplanetary Rover Exploration………………………………………... 1
1.1.1 Lunokhod…………………………………………………………... 1
1.1.2 Lunar Roving Vehicle……………………………………………… 2
1.1.3 Martian Rovers……………………………………………………... 3
1.1.4 Current Designs……………………………………………………. 4
1.2 Fuel Cell Technologies......………………………………………………. 5
Chapter 2: Requirements
2.1 Rover Design Requirements……………………………………………. 10
2.2 Mission Requirements………………………………………………….. 10
2.3 Regolith Mitigation……………………………………………………... 11
Chapter 3: Design and Testing
3.1 Design…………………………………………………………………... 13
viii
3.1.1 Base Station……………………………………………………... 13
3.1.2 Rover……………………………………………………………..16
3.2 Testing………………………………………………………………… 27
Chapter 4: Results……………………………………………………………………... 31
Chapter 5: Conclusions…………………………………………………………………37
References……………………………………………………………………………. 39
ix
List of Tables
Table 1: Mass Break Down……………………………………………………………... 17
Table 2: Range, Payload, and Velocity…………………………………………………. 19
Table 3: Parameters for Rover and Scaled Rover………………………………………. 29
Table 4: Theoretical Results……………………………………………………………. 32
Table 5: Experimental Results………………………………………………………….. 33
x
List of Figures
Figure 1: General Fuel Cell Operation………………………………………………….. 6
Figure 2: Flow-Through Fuel Cell and Water Separation Schemes……………………...7
Figure 3: Non-flow-through PEM Fuel Cell……………………………………………...8
Figure 4: Base Station…………………………………………………………………... 14
Figure 5: Male Port Adapter with Nozzles……………………..………………………. 16
Figure 6: Power Allotment……………………………………………………………... 18
Figure 7: Rover Frame………………………………………………………………….. 20
Figure 8: Stress Analysis of Frame…………………………………………………...… 21
Figure 9: Track System…………………………………………………………………. 22
Figure 10: Motor Performance………………………………………………………….. 23
Figure 11: Tread Design………………………………………………………………... 24
Figure 12: Rover with Navigation System……………………………………………… 26
Figure 13: Assembled Rover Front View (Dimensions in meters)……………………... 27
Figure 14: Assembled Rover Side View (Dimensions in meters)……………………… 27
Figure 15: Scaled Tread Pod……………………………………………………………. 28
Figure 16: Regolith Test Bin……………………………………………………………. 29
Figure 17: Sinkage Test………………………………………………………………… 31
Figure 18: Pressure Contours for Helium………………………………………………. 34
Figure 19: Velocity Contours for Helium………………………………………………. 35
Figure 20: Pressure Contours for Nitrogen……………………………………………... 35
xi
Figure 21: Velocity Contours for Nitrogen……………………………………………... 36
xii
Nomenclature
A
Area
b
Track Width
ESA
European Space Agency
c
Cohesion Factor
F
Force
k
Parameter
GRC
Glenn Research Center
l
Track Length
H
Hydrogen
m
Mass
I
Current
n
Valence Electrons
JPL
Jet Propulsion Laboratory
p
Normal Pressure
LRV
Lunar Roving Vehicle
u
Exponent of Deformation
M
Molar Mass
x
Length
MCFC
Molten Carbonate Fuel Cell
v
Velocity
MSL
Mars Science Laboratory
z
Sinkage
NASA
National Aeronautics and
Space Administration
φ
Internal Shearing
Resistance Angle
O
Oxygen
PAFC
Phosphoric Acid Fuel Cell
PEM
Proton Exchange Membrane
R
External Resistance
RTG
Radioisotope
Thermoelectric Generator
SOFC
Subscripts
C
Pressure-Sinkage
c
Cell
d
Density
n
Normal
Solid Oxide Fuel Cell
t
Track
Sol
Martian Day
φ
Pressure-Sinkage
W
Weight
xiii
Chapter 1: Background
The ability to explore and examine unknown terrain is critical to the future exploration of
our solar system. Future mission plans require the ability to stay, for an extended period
of time, on the surface of the planetary body that is being explored as well as gather as
much information as possible for subsequent missions (1). In order to extend the overall
time of exploration while simultaneously reducing the cost of a mission, robotic
exploration is a vital method to examine the surface of a planetary body. Specifically, the
exploration of the lunar surface cannot be fully completed without, at minimum, the aid
of robotic rovers due to the extreme and hazardous environment. Currently, only one type
of vehicle has remotely explored the Moon and several others have travelled to Mars. A
new type of rover, capable of serving multiple types of missions, is needed. The National
Aeronautics and Astronautic’s Glenn Research Center (GRC) determined a list of
requirements necessary to attain this goal. It is the intention of this thesis to satisfy all of
the criteria developed in order to create the next generation of robotic lunar rover. Future
manned and unmanned exploration programs will greatly benefit from information
gathered by robotic operations and from the complimentary functions they can serve.
1.1 Interplanetary Rover Exploration
1.1.1 Lunokhod
In the early years of the race to land man on the Moon, the Soviet Union designed the
first rover that would be used for remote exploration. This rover was designed initially in
order to survey areas of the lunar surface destined for future Cosmonaut landings (2).
1
However due to several set-backs, including the failure of the launch of first Lunokhod
mission, Lunokhod would land and would serve the purpose of being the only robotic,
mobile explorer to land on the Moon (2). There were two Lunokhod rovers that landed
and traversed the lunar surface. The first rover, Lunokhod 1, landed and operated for 11
lunar days (nearly three times the design life) and explored over 10 km of terrain (3).
Lunokhod 2 operated for four lunar days and explored 37 km of the surface (4).
According to Harvey, the eight wheeled rovers had a 756 kg and 840 kg mass,
respectively, standing 4.42 m in length, 2.15 m in width, and 1.92 m in height. The rover
utilized solid wheels and their wheel base was 2.22 m by 1.6 m. During the mission, the
Lunokhod rover was remotely controlled from Earth and could climb 20o slopes and
could traverse side slopes of up to 45o. The anticipated top speed was only 2 km/hr and a
radioisotope generated heated the rover during the lunar night, when the systems were in
a hibernation mode (2). Lunokhod’s batteries were charged by photovoltaic cells that
were covered by a dome in order to mitigate the lunar regolith contamination. The
photovoltaic array was uncovered when the rover was stopped so that it could charge
while the tires were not throwing the regolith into the sky and covering up on the
photovoltaics (4).
Even though the speed of the rovers was comparative to a slow walk, the rovers carried
scientific instruments, performed analysis, and most importantly demonstrated the ability
to survive the lunar night. This would be the only robotic roving attempts to ever take
place on the lunar surface. The Lunokhod missions prove that, while difficult, continual
exploration is possible in an environment as harsh as the Moon. The Lunokhod roverwas
limited by power since it was only meant to carry a specific payload and operate on a
1
specific mission platform. The use of solar power limited its’ exploration to lunar days,
leaving a large portion of mission time in a hibernation mode in which the rover used its
radioisotope generator for keeping the systems warm enough to survive.
1.1.2 Lunar Roving Vehicle (LRV)
During the later Apollo missions, Apollo 15, 16, and 17, the National Aeronautics and
Space Administration (NASA) wanted to expand the area that the astronauts were
capable of conquering. One method, that was inevitably used, was to mobilize the
explorers so that the rock and dust samples were obtained from a wider variety of
regions. The LRV was implemented to provide the carrying capabilities required to hold
two astronauts and scientific samples. Each of the four wheels was driven by independent
motors and was comprised of a woven metal mesh with attached titanium chevrons which
would flex and pull regolith for added traction (5). The vehicle’s power was supplied by
two batteries containing 8.3 kW-hr of energy (6). The LRV was targeted to have a mass
of only 181.8 kg and it was collapsible in order to fit in one of exterior quads of the lunar
module that delivered it to the surface (5). The maximum payload the LRV could
support was 440 kg which includes the mass of the astronauts that were necessary to
operate the vehicle (6). The rover was manually driven and was incapable of remote
operations. The LRV was designed as an exploration aid to ferry astronauts and lunar
samples from the lunar module to an exploration site, and then back the lunar module. It
was incapable of independent exploration of any type with the exception of a remote
television camera that could be operated from the Earth (5). The LRV was equipped with
a communications system that could transfer and receive information to and from Earth
(6). An environmental control unit enabled the rover’s subsystems to reject excess heat
2
generated while operating by radiating the heat to space when the LRV was stopped (6).
The LRV is a great example of how a large (1.83 m x 3.1 m x 1.12 m (6)) vehicle is
capable of traversing the lunar environment with relative ease of operation.
1.1.3 Martian Rovers
There have been three rovers that have successfully landed and explored the Martian
surface. Mars Pathfinder rover, or Sojourner, which was launched in 1996 and the Mars
Exploration Rovers (MERs), named Spirit and Opportunity that were launched separately
in 2003.
Pathfinder
The 6-wheeled Mars Pathfinder rover, Sojourner, had a mass 10.6 kg and measured 68
cm long by 48 cm wide by 28 cm tall (7). A vehicle of this size was considered to be a
micro-rover (7). According to Stone, the rover’s solar panel and battery system was
capable of producing 1 W-hr of energy and the primary mission was only planned to have
a duration of seven days. Mars Pathfinder proved that a low-cost, scientifically
significant, exploratory rover could be developed and launched within three years. The
rover could only reach a speed of .04 km/hr and could only traverse .5 km away from the
lander (7). The rover survived for a period of approximately twelve weeks on the Martian
surface (8). Pathfinder created the foundation for future programs that utilized rapid
development methods by proving that the concept can work for space missions.
MERs
The six wheeled MERs require 900 W-hr per Sol or roughly 880 W-hr per Earth day to
operate even though the time of operation was only a small fraction of a Sol (9). Each
rover is powered by a battery recharged by photovoltaic cells (8). The rover Spirit was in
3
operation for 2010 Sols and traveled 7.73 km while Opportunity is currently on Sol 2750
and has traveled 33.93 km to date (10). The primary missions for the MERs were
designed for 90 Sols and the vehicles were exposed to temperatures ranging from -100o to
0o C (8). Each MER stands at 1.4 long by 1.2 m wide by 1.5 m tall and are 176.5 kg (11).
The top speed of the rovers is 0.2 km/hr (8). The MERs proved that extended exploration
is possible with no direct maintenance and prolonged exposure to harsh environments.
Methods and practices created for Mars can be used as a stepping stone for the
development of a lunar rover.
1.1.4 Current Designs
Presently, there are no rovers designed specifically for use on the Moon. Several vehicles
are being developed in an effort to win the Google Lunar X-Prize but unfortunately the
majority of the details regarding these projects remain proprietary. However, based on the
criteria of the contest, the rovers are most likely small in stature, incapable of traversing
adverse terrain, and will not be on functioning on the surface for an extended period of
time. This can be assumed because the main objective of the Google Lunar X-Prize is to
have a robot travel 500 m across the surface while the funding has to be 90% private (12).
The funding limitation and the short range requirement indicate that a small rover with a
limited amount of power will be utilized.
Current NASA and European Space Agency (ESA) efforts are focused on Martian
exploration. NASA has launched the Mars Science Laboratory (MSL) and NASA and
ESA are working together to eventually launch the vehicle containing the ExoMars rover.
There are no known plans for a rover utilizing a fuel cell power source.
Mars Science Laboratory
4
The MSL mission features a large rover named Curiosity. According to NASA’s Jet
Propulsion Laboratory (JPL) the rover measures 3.05 m long, 2.74 m wide, and 2.13 m
tall. The rover weighs 900 kg and contains a mobile geology lab, several cameras, and a
laser capable of vaporizing rock particles for analysis. The primary mission is designed to
last one Martian year and will explore the surface for signs of past microbial life and if
conditions presently exist below the surface for life to be present today (13). The rover is
planned to traverse 200 m per Martian day and be powered by a Radioisotope
Thermoelectric Generator (RTG) which was supplied by the United States’ Department
of Energy (14). This generator produces a continuous 110 W to power the rover’s
systems (14). If this mission is successful, it will mark the first time that a large,
unmanned rover will have successfully explored a non-terrestrial body.
ExoMars
The ExoMars rover is a joint mission between NASA and ESA. The rover is currently in
development and will be six wheeled, solar powered, and able to autonomously cover
100 m per Sol (15). Although the details of the mission are incomplete, it can be assumed
that the rover will be of MER stature.
1.2 Fuel Cell Technologies
Fuel cells are devices that generate direct current electrical energy from chemical
reactants (16). The convention is for two reactants to combine within a chamber that
contains an anode and cathode which enables a flow of electrons, creating power (16).
Figure 1 shows the general operation.
5
Figure 1 (16): General Fuel Cell Operation
There are four major types of fuel cells that are currently being popularly used. They are:
Proton exchange membrane (PEM), Solid oxide fuel cell (SOFC), Molten carbonate fuel
cell (MCFC), and Phosphoric acid fuel cell (PAFC) (16). Joyce et al affirm that PEM fuel
cells have several advantages compared to other types of fuel cells. PEM fuel cells are
simple in design and construction. Once a power demand is realized, the cells can easily
be stacked on top of one another or placed in series to supply the required power. PEMs
also have short start up times in comparison to the PAFC and the SOFC but longer start
up times compared to the MCFC (16). The operating temperature of PEM fuel cells are
lower than the other mentioned types (17), making integration into an overall system
easier. Less mass and fewer design considerations would need to be devoted to thermal
control and management when using PEM fuel cells. There are two design possibilities
6
with PEM fuel cells, flow through and non-flow through which are depicted in Figure 2
and Figure 3 respectively. Also depicted in Figure 2 are the two types of water separation
for flow-through fuel cells, active and passive.
Figure 2 (16): Flow-Through Fuel Cell and Water Separation Schemes
The active and passive water separation methods both circulate a reactant, which is
excess in the power producing reaction, along with the product to a separator. The
remaining reactant is then put back through the cell. In the case depicted, hydrogen and
oxygen are the two reactants. The excess reactant in a hydrogen-oxygen fuel cell is
oxygen and the circulating oxygen removes water from the cathode surface (16).
7
Figure 3 (16): Non-flow-through PEM Fuel Cell
The water produced in a non-flow through PEM fuel cell is transferred by a hydrophilic
membrane and stored with no circulating of the reactant (16). A non-flow through fuel
cell is lighter and less complex due to the elimination of the water and reactant separator.
Non-flow through fuel cells tend to be more expensive, partially because most active
research in fuel cells is in the automotive industry which use oxygen from the atmosphere
are flow-through in design. Fuel cell technologies have been utilized in many space
programs including project Gemini, project Apollo, and as recently as the Space Shuttle
(18). A regenerative fuel cell system has the capability of supplying electrical power to a
sustained, long duration mission due to the ability to recycle products back into their
8
original reactants in order to reduce the amount of mass needed to be carried by the
powered system (16).
9
Chapter 2: Requirements
2.1 Rover Design Requirements
The requirements set for this rover evolved from a study performed at NASA’s GRC
detailing the challenges along with existing conceptual designs (19) and funded through a
grant from the Ohio Aerospace Institute. The guidelines were developed during meetings
with scientists at GRC and are as follows: 1 kilowatt nominal power supply without using
batteries or RTGs, greater than 500 kilogram carrying capacity, minimum of 250
kilometer range, five year mission design life, and continuous operation. A five year
design life requires that the rover survive for approximately 62 lunar days. Power will
need to be available to supply a scientific payload that will be selected after the rover is
designed and developed. The rover, therefore, will need to be a lunar mule, capable of
carrying a variety of payloads to explore a wide range of possible landing sites. Another
important constraint placed on the development is that the rover be designed utilizing
current technologies. There can be no advanced research and development on any
component involved with the construction. No current designs, already in operation or in
development, utilize a hydrogen-oxygen fuel cell, making the off-the-shelf requirement
significantly more difficult.
2.2 Mission Requirements
The primary mission requirement is that the rover be capable of traversing the lunar
surface, both mare, or darker areas, and highland regions. The mare regions are flat,
relative to the mountains and valleys of the highland areas however varying sized craters
10
are present in both (20). In order to navigate these terrains it is recommended all lunar
roving vehicles be able to climb and descend a slope of 25o since craters commonly have
only 5-10o slopes and this leaves a significant margin of mission flexibility (21). Any
successful mission exploring that lunar surface will have to navigate and investigate these
different regions effectively and without constant reroute to avoid obstacles that
commonly occur. Another important aspect in the mission is to continually survive the
lunar night and the temperature flux that occurs during the transition between night and
day. During lunar night, which lasts for approximately 14 Earth days, the temperature
can reach as slow as -150o C and can reach 100o C during the day (22). In addition to the
electrical power dedicated to lighting the rover’s way, a significant amount of heat needs
to be generated to keep vital components operating during night.
In order for this design to be feasible, the rover must be able to fit on a conceivable lunar
lander. The recently cancelled Ares V rocket was designed to be compatible with a
lander capable of supporting any future lunar landing attempt with the rover. The
complete regenerative fuel cell system could be integrated on the rover, but in order to
optimize performance, several components can be separated into a base station and
operate in parallel with the rover systems. Solar activity will require protection to prevent
damaging effects during the rover’s operation. Radiation, coronal mass ejections, and
magnetic anomalies will have to be considered in order for the rover to survive its five
year primary mission.
2.3 Lunar Regolith Mitigation
The top most layer, or regolith, of the lunar landscape is covered with a powder that can
range in depth from several meters to a dusting. This layer is composed of pulverized
11
rock that has accumulated over billions of years from the constant impacts of projectiles
on the lunar surface. The highland areas have smaller sized regolith particles when
compared to the mare areas (23). The smaller sized particles make the risk of sinking into
the surface and, consequently, allowing a vehicle’s wheels or tracks to become stuck
more probable. Areas classified as “soft soil” create hazards for vehicles as demonstrated
during the Apollo 15 mission where the LRV got stuck and, due to the low weight in the
lunar environment, the astronauts simply moved the rover to fix the problem (24). This
type of fix cannot be performed in a robotic mission so other mitigation factors must be
used to counter this possibility.
12
Chapter 3: Design and Testing
3.1 Design
Robotic aids must be developed to conduct a wide variety of mission-related tasks if an
initiative is taken to explore the lunar surface in the future. The mission’s objectives set
requires a versatile vehicle with a regenerative fuel cell system. The regenerative system
will be split up into two main components in order to maximize the rover’s performance.
The rover will contain the fuel cell system consisting of the following components: an
oxygen storage tank, a hydrogen storage tank, and a water storage tank. The rover will be
equipped with the primary communication system and all of the exploration and driving
equipment with the exception of base station instrumentation and cameras.
3.1.1 Base Station
The split architecture of the regenerative fuel cell system requires that a base station
contain the following components: electrolyzer, storage tanks for the generated hydrogen
and oxygen, solar panels for powering the electrolyzer, and secondary communications
systems. There will be a transfer dock on both the rover and base station that will enable
the transfer of water generated from the rover’s fuel cell to the base station and the
hydrogen and oxygen from the base station to the fuel cell storage tanks.
Power System
The base station, depicted in Figure 4, is 4.5 m long, 6m wide and 4m tall and will be
powered by four strings of solar panels containing two sets of cells each. They are
13
extended from the core of the base station upon landing and the radiators, which are used
to dissipate excess system heat, are located beneath the mounting positions of the strings.
BP SX3200 200 W photovoltaic cells were used for analysis. The photovoltaic cells are
multi-crystalline silicon and are 0.16 x 0.16 m each. The cells produce 0.49 V at 8.16 A
at their maximum power output capability. The cells have an 11.9 % efficiency and the
solar arrays need to have a combined total of 32 m2 area in order to power the electrolyzer
and environmental control systems (25). The accepted average solar flux at the lunar
surface, which is approximately 1 astronomical unit away from the sun is 1366 W/m2 and
varies by 3.3% due to the eccentricity of Earth’s orbit around the sun (26). This allows
for the solar array area to produce up to 1801 kW-hr of energy, which is 29.5% greater
than the 1270 kW-hr required for continuous surface operation. These power
consumption values are based on the performance of the commercially available Giner
PEM electrolyzer (27). This allows the electrolyzer to be operated during the lunar night,
at a range of solar flux conditions, and will power the systems after the solar cells’
performance degrades. Degradation occurs with prolonged operation, non-optimum sun
incidence angles, and by contamination cause by dust particles settling on the surface of
the panels.
Figure 4: Base Station
14
The solar panels will charge a lithium-ion polymer battery that has a capacity of 11 kWhr. This will enable the battery to be capable of storing all of the energy required to run
the vital systems during a lunar night. The battery has a wide range of operational
temperatures (213.15 K to 253.15 K), weighs approximately 81.5 kg, and occupies a
volume of 0.032 m3 inside the base station (28).
Electrolyzer
Electrolyzers are devices that reduces a compound into component elements (16). An
electrolyzer is needed to form a regenerative power system using a hydrogen-oxygen fuel
cell. According to Joyce, during electrolysis, a solution is subjected to direct current and
is decomposed. The direct current is supplied by the batteries that have been charged by
the solar panels on the base station. The hydrogen and oxygen combine in the fuel cell to
create energy and the byproduct of this reaction is water (16). In the electrolyzer, the
water is decomposed such that at the negative electrode, an external electrical supply
provides the electrons and the protons are removed from the source such that:
4H+ + 4e- 2H2
and at the positive electrode:
2H2O O2 + 4H+ + 4ewhich completes the reaction and the oxygen (O) and the hydrogen (H) are collected and
pressurized into storage tanks (29). Due to the high operating pressure of the electrolyzer,
once the gases are decomposed, they can be slowly pumped up to the storage tanks’
pressure in a reservoir prior to being fed into the tanks. The operating pressure of the
electrolyzer is 60% of the pressure of the rover and base storage tanks (27).
15
Docking Station
The base station must contain a docking station that supports the refueling of the rover
and discharge of the rover’s water tank. This docking station must contain three ports,
one each for the hydrogen, oxygen, and water, and these ports must be protected from
contamination due to the regolith. In order to mitigate this contamination, a series of
nozzles will be placed along the male docking mechanism as depicted in Figure 5. An
inert gas stored in the rover, will be blown through the nozzles in an effort to clear any
dust that may be present on both the male and female port adapters before insertion and
transfer begins.
Figure 5: Male Port Adapter with Nozzles
3.1.2 Rover
The rover is designed to operate with a 1 kW hydrogen-oxygen fuel cell and have a mass
of 750 kg. Due to the mass of the vehicle, tracks are used for propulsion and this enables
a range of 550 km. The rover must operate continuously for five years and have the
capability of performing scientific experiments and conducting extensive exploration of
16
the lunar surface. The 750 kg mass will be broken down as shown in Table 1. All masses
were taken from CAD and commercially available components of the rover.
Table 1: Mass Break Down
Item
Structure
Fuel Cell System
Storage Tanks (30)
Reactants
Drive System
Nav/Comm Systems
Environmental Protection
Total Dry Mass
Mass (kg)
208.5
6.9
135
128.24
201.5
50
20
750.14
Fuel Cell
The rover is designed to operate nominally at 1 kW and have a maximum range of 550
km. This distance allows for the rover to travel over 250 km from the base station, as set
in the design requirements, and gives a 10% performance margin. The fuel cell design
constraints were determined by using an ElectroChem 1 kW nominal, 6 kW peak,
hydrogen-oxygen fuel cell that is commercially available. The fuel cell will be non-flow
through in order to reduce the amount of complexity in the system. This fuel cell’s stack
contains 45 cells that have an area, Ac, of 0.023 m2 and a current density, Id, of 0.1 A per
cm2 (16). The total mass flow rate of the reactants, m , can be determined by Equation 1
which uses a modified version of Faraday’s Law (25).
(1
Where F is Faraday’s constant, M is the molar mass, and n is the reaction valence
electron number. Using a reaction efficiency of 50% (16) the mass flow rate for the
17
reactants was found to be 2.22 x 10-4 kg/s which yields an oxygen mass flow rate of 2.08
x 10-4 kg/s and a hydrogen mass flow rate of 1.35 x 10-5 kg/s. The power allotment for
the rover’s systems is shown in Figure 6.
Figure 6: Power Allotment
The total mass of reactants that will be used is determined by multiplying the mass flow
rate by the total time the rover will be in use. The maximum velocity of the rover must be
determined in order to find the total time the rover will take to traverse its maximum
range. Using the kinetic energy, combined with the energy required to overcome friction,
the velocity can be determined using Equation 2.
(2
18
Where m is the mass of the rover, v is the velocity, Fn is the normal force, and xt is the
length of contact of a track. In order to overcome surface roughness and soil compaction,
the rover is assumed to be going up a 1.5o instead of a flat terrain (24). The energy that
goes towards driving is limited to only 750 W and that yields a velocity of 0.95 m/s. The
amount of time it would take to cover the 550 km range is therefore 5.789 x 105 s. Table
2 compares maximum velocity and range of the rover for varying payloads up unto the
design payload of 500 kg, making the total rover mass 1250 kg.
Table 2: Range, Payload, and Velocity
Range (km)
550
515
486
463
440
423
Payload (kg) Velocity (m/s)
0
0.95
100
0.89
200
0.84
300
0.80
400
0.76
500
0.73
Totals of 120.42 kg of oxygen and 7.82 kg of hydrogen are required for the fuel cell to
operate over this time. Composite tanks can store gases at pressures exceeding 34.52
MPa making them an acceptable choice for the storage of the hydrogen and oxygen (25).
The operating pressure of the PEM is approximately 2.75 MPa allowing for the reactant
storage tanks to bleed into the fuel cell without having to increase the pressure (31).
Using the storage pressure of 34.52 MPa, the oxygen tank has a volume of 0.24 m3 and
the hydrogen tank has a volume of 0.25 m3. These volumes were determined using the
19
perfect gas law at an operating temperature of 270 K. Using the standard density of water
of 1 g/cm3, the water storage tank is 0.13 m3.
Structure
The primary structure of the vehicle is 6011T aluminum. Figure 7 is the basic frame used
for structural analysis. Using ANSYS finite element analysis software, loads were placed
Figure 7: Rover Frame
on the frame from all of the component’s mass at their points of attachment to the frame.
The maximum predicted payload of 500 kg was also added to the top of the rover frame
for the analysis. A distributed load for the internal components was placed on the bottom
section of the frame to account for the floor plate that will distribute the load on the
actual rover. The loads under lunar gravity were applied with a factor of safety of five.
This factor of safety will ensure that the rover can operate without failure under any
driving condition and possible drops from short distances. Figure 8 is the stress analysis
20
performed on the frame. The analysis shows that the structure does not fail under the
expected loading conditions. External shielding, in the form of aluminum and lead
Figure 8: Stress Analysis of Frame
plating, is added to the frame to limit the amount of radiation exposure to the internal
components of the rover. This plating also further strengthens and stiffens the structure
by their addition. An additional non-structural element, is multiple layers of nickel foil
with a KEL-F coating. KEL-F is a heat resistant material capable of absorbing
temperature of over 477 K, which is above any temperature to which the rover is
anticipated to be exposed (32). The multiple layers of aluminum, lead, and nickel foil
should also protect against space weather damage.
Drive System
The drive system for the rover consists of four “tread pods” that are mounted to each
corner of the frame. Each tread pod contains two separate track systems as shown in
Figure 9. The track system has one drive wheel in which the sprocket that will drive the
21
actual track is internal to the radius of the wheel. This is a regolith mitigation strategy
enabling the exterior covering of the tread pods to extend past the tracks in the upper
portion, limiting exposure. Tension rollers are on the top and bottom of the track
assembly and their role is to keep tension in the track to minimize slippage in the track
belt. Reducing this slippage will increase the overall efficiency of the vehicle as the
Figure 9: Track System
vehicle loses power in the transmission from track to soil when slip increases (33). The
road wheels are used to transfer the majority of the vehicles weight through the track belt
and onto the surface while the idler wheels keep the shape of the track and freely rotate.
The entire drive train is enclosed to mitigate contamination. The drive wheels are each
driven by a motor comparable to the MB-EMU-75-2Q produced by Allied Motion. Using
Figure 10 and the diameter of the drive wheel, 0.254m, the gear ratios needed for the
22
transmission can be determined. Gearing the motor’s speed only affects the efficiency of
the drive system by a 2% reduction (33). Based on Figure 10, the gear ratios are
calculated to be ten, eight, seven, and six. These gear rations allow for the entire envelope
of operations. The transmission will be directly mounted onto the motor inside the
enclosed drive assembly.
Figure 10 (34): Motor Performance
Tracks were chosen for the drive system because they are more efficient at transferring
load to soil than a wheel and are less probable to get stuck in soil when compared to a
wheel based system (35). The increased complexity and, therefore, a higher risk of failure
has been countered in the design by robustness and redundancy. Each track system in the
tread pod is capable of detachment if it would bind or fail, allowing a decrease in friction
with the surface and not making the remaining functional track systems drag the disabled
system.
23
The rover will be steered by both skid steering, where one side’s tracks operate in one
direction while the opposite side moves in the reverse direction, and by linear actuators
moving the tread pods. An active suspension system is unnecessary due to the low speed
and unlikelihood of shocks from drops. In order for drops to occur that would justify the
need for an active suspension system, the navigation system would have to suffer a
primary and secondary failure, resulting in an immediate mission failure.
The track’s treads, Figure 11, were designed to be non-binding by using rounded
Figure 11: Tread Design (Dimensions in meters)
attachment joints. The tread pattern was iteratively developed through experimentation.
Various tread patterns were constructed and then tested using JSC-1a lunar regolith
simulant. The designs were ranked based upon the angle of tilt that the treads would need
to obtain before the regolith fell off and the ease with which the tread broke the surface of
the regolith. The initial design that ranked the highest was then modified until an
optimum design was determined.
24
Navigation and Communication
The rover will need to have an on-board navigation and communication system to allow
continuous exploration operations. Since the command and feedback time from the Earth
to the Moon is very small, approximately three seconds, a completely remote operation is
possible. However in order to utilize all of the potential exploration capabilities, the rover
must also be able to travel autonomously. The lack of both a substantial atmosphere and
objects that could be used as reference for distances on the Moon, a navigation system for
manual and autonomous operations must include a means to measure obstacles and
terrain. A LiDAR system that enables the mapping of terrain for a full 360o field of view
will be implemented to generate an interface that can be used to drive the rover (36).
Using a LiDAR system coupled with a Pathfinder type navigation system, the rover can
be navigated using only 95 Watts of power (7) (36). There will be two complete and
identical navigation systems for the sake of redundancy. This system contains three video
cameras, two located on the front of the rover and one on the back to enable driving in
both forward and reverse. The assembled rover complete with navigation systems
(LiDAR represented by white cubes) is shown in Figure 12.
25
Figure 12: Rover with Navigation System
Communications with Earth will be transmitted directly to and received directly from the
rover. This eliminates the need for a relay system and prevents the line of sight
restrictions that plagued the Pathfinder system (7). The communication system will
contain high and low gain antennas to maximize data transfer and efficiency. A system
with similar power as the MSL is more than sufficient for communicating with Earth
(13).
In order to navigate during lunar night or at low lighting conditions, high intensity lights
are needed. ARC HID lights will be utilized and placed on the back and front of the
rover. Four lights will be placed on the front and two on the back of the rover. Each light
uses 10 W and produces 550 lumens while only weighing 0.19 kg each (37). One light is
capable of providing sufficient illumination to support visual driving. Figure 13 and 14
are dimensioned CAD drawings of the completely assembled rover.
26
Figure 13: Assembled Rover Front View (Dimensions in meters)
Figure 14: Assembled Rover Side View (Dimensions in meters)
3.2 Testing
Manufacturing this rover on a full scale would be an expensive and difficult task.
Budgetary restraints alone prohibit this from being a reality during this project. In order
to test the fundamental drive concepts, a scale model must be used. Since the availability
27
of a lunar environment, approximately 1/6 Earth’s of gravity, is not feasible for the
testing required, all performance characteristics were calculated for Earth conditions as
well. The dimensions of the rover were reduced by 84% for the scale model. The scaled
mass was determined by taking the full scaled rover’s mass to tread area ratio (normal
pressure) and keeping it consistent. A single tread pod was constructed out of
Lynxmotion Tri-Track chassis and was used for testing. All values that were determined
were simply multiplied by four to obtain values for a full rover. Figure 15 is a picture of
the tread pod that was tested. The power used for the scale model was then determined by
maintaining a constant power to mass ratio between the full scale rover and the scale
model. Due to the structural loading limitations of the scaled tread pod, only the empty
condition could be tested.
Figure 15: Scaled Tread Pod
Table 3 is a list of parameters for the scaled and non-scaled, non-loaded rovers. The
normal pressure value has the contact factor parameter in the calculation. This factor is
the number of tracks systems multiplied by the area of the track and then multiplied by
the number of track links in contact with the ground. There are 10 links in contact with
the terrain for both the rover and scaled rover.
28
Table 3: Parameters for Rover and Scaled Rover
Parameter
Mass (kg)
Weight(kN)
Track Contact Length (m)
Track Contact Width (m)
Track Area (m^2)
Normal Pressure (kN/m^2)
Power (kW)
Power to Weight Ratio
Rover
750
7.36
1.52
0.31
3.77
0.41
0.75
0.10
Scaled Rover
48
0.41
0.25
0.05
0.13
0.41
0.041
0.10
A 1 m long x 1 m wide x 0.25 m deep sand pit was constructed at the Ohio State
University’s Aeronautical and Astronautical Research Laboratory in order to complete
testing. Sand is an inexpensive regolith simulant. It had many of the same soil-mechanics
characteristics and is ideal for testing the scaled rover (25). A bin of lunar regolith
simulant, Figure16, was used to investigate how regolith contamination affects
components and materials.
Figure 16: Regolith Test Bin
The test pit verified aspects of the rover that had been predicted analytically. Drawbar
pull and sinkage both were determined experimentally in the pit. Several analytic results
29
were unable to be verified experimentally include: work performed compacting terrain,
towage work, and traction effort. A 222.4 N maximum load digital scale was used to
determine drawbar pull on the rover.
Computational work was performed with the structure (see previous section) and the
mitigation nozzles. The nozzles were analyzed using computational fluid dynamics to
determine if the effect was great enough to disturb dust that had settled on the surface of
the docking mechanisms.
30
Chapter 4: Results
The sinkage, z, was found by Equation 3 (33).
(
)
(3
Where p is the normal pressure, b is the width of track, u is the exponent of terrain
deformation, and both kc and kφ are pressure-sinkage parameters. The pressure-sinkage
parameters are working medium specific and dry sand was used in all cases. Kc and kφ are
given the constant values of .99 and 1528.43 respectively. The sinkage depth was tested
by placing the scaled tread pod on the surface, then removing it and measuring the depth
of the impression left in the sand. Figure17 depicts one of the tests of sinkage.
Figure 17: Sinkage Test
The work done to compact the terrain so that the tracks make a rut through the sand is
given by Equation 4 (33), where l is the length of the track.
(
)(
31
)
(4
The tractive effort of the rover was determined by Equation 5 (33).
(
)
(5
Where c is the cohesion factor, 1.04, and φ is the angle of internal shearing resistance
which is 28o. Once the tractive effort was determined, the drawbar pull was calculated
using Equation 6 (33).
(6
R is the external resistance of the rover which is negligible due to the low weight and
slow speed of the scaled rover. The scaled rover must move at the slowest possible speed
so that inertial influences are minimized (38). Finally the work done by towing was
determined using Equation 7 (33).
(
(
)( )
)(
(7
)
The results from the calculations are shown in Table 4.
Table 4: Theoretical Results
Theoretical Result
Drawbar Pull of Rover (N)
Sinkage (mm)
Work to Compact (J)
Towing Work (J)
Rover on Moon
308.53
3.03
533.54
810.98
32
Rover
1848.7
15.48
16320.4
24807.04
Scaled Rover
19
6.71
453.81
213.69
The experimental results from the testing of the scaled rover are found in Table 5. The
sinkage depth was estimated as no precision method for determination was available.
Table 5: Experimental Results
Run
1
2
3
4
5
Sinkage (mm)
5
3
5
5
5
Drawbar Pull (N) 16.68 15.79 16.37 14.46 15.97
Average
4.6
15.85
The error between the theoretical and the average experimental results for sinkage was
32%. This can be attributed to no source of precision measurement and no way of
determining if the depression distance on the sides, where it was measured, was
comparable to the rest of the depression. Shear forces in the sand on the edges would be
greater and would resist deformation if the sides of the pit are not far enough away from
the test space. The large space needed to verify this would require facilities and resources
outside of the possibility of this project.
The error between the theoretical and the average experimental results for drawbar pull
was 16.6%. The sources for error in this experiment can be attributed to a wide range of
sources. The power inputted to the rover and transmitted to the drive motor was likely
lower than anticipated. There was also assumed no track slip, which, even in minor
instances of approximately 1% could cause significant loss of available power. The other
probable source of error would be the assumptions made on the constants involving the
working medium. The calculations were based on dry sand, which, with any environment
with any humidity would be impossible to keep dry without a sophisticated
environmental control system.
33
The nozzles for the docking ports were analyzed using the CFD software Fluent.
Nitrogen and helium were used as a working fluid for this regolith mitigation strategy.
Both gases were analyzed at the same boundary and operating conditions. The reservoir
pressure was 34 MPa with the exit pressure being a vacuum. The initial mass flow rate
was set to .001 kg/s. The total pressures and the velocities at the exit were examined in
Figures 18- 21.
Figure 18: Pressure Contours for Helium
34
Figure 19: Velocity Contours for Helium
Figure 20: Pressure Contours for Nitrogen
35
Figure 21: Velocity Contours for Nitrogen
Based on the Fluent CFD simulations, the helium gas has a higher exit pressure and higher exit
velocity than the nitrogen gas. The higher exit pressure and higher exit velocity are desirable for
use in a regolith mitigation system. This will displace the maximum amount of regolith for a
given volume of inert gas. The rover will utilize helium as the inert gas. This should provide the
necessary force needed to clear the connections while not adding significant weight to the system.
36
Chapter 5: Conclusions
The rover designed can navigate the lunar surface environment for at least five years
using technologies that have already been developed. The 1 kW of power supplied by the
hydrogen-oxygen fuel cell can drive the rover, communicate with Earth, navigate, and
supply power for environmental controls and payloads while operating nominally. The
non-flow through fuel cell requires masses of hydrogen and oxygen that are capable of
being carried by the rover to allow it to carry out its 550 km required maximum range.
The rover can carry an additional 500 kg of payload on top of the 750 kg dry mass.
Regolith mitigation strategies were implemented in order to prolong the life of the rover
and allow more desirable operating conditions of the systems.
Results from the experiments indicate that the lunar rover scaling is possible but research
must be continued into finding accurate parameters to be used in the theoretical
calculations. A larger testing facility would have also benefitted the testing.
Theoretical results compared the lunar rover with a scaled prototype and the performance
required by the scaled rover can be accomplished using commercial off-the-shelf
components with minor modifications. The CFD works verifies that the inert gas nozzle
mitigation strategy is feasible and should be implemented in systems that care likely to be
contaminated by regolith.
Hydrogen and oxygen fuel cell technologies are a viable option for future rover designs.
Their performance is comparable to solar and RTG power systems that currently are
37
used. The regenerative fuel cell concept will enable long duration exploration and
influence the development of future manned and unmanned mission architecture.
38
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