GREEN LIFE FOR THE RED PLANET

6 December 2002

Submitted by:

Lubos BRIEDA Vinh DAO

Dave LAVENDER Joe POLIDAN

Melvin PRESSOUYRE Nicolas SPITZ

Simon STEWART

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TABLE OF CONTENTS List of Tables ..................................................................................................................... iv List of Figures..................................................................................................................... v List of Abbreviations ......................................................................................................... vi

Chapter 1: Introduction and Problem Definition ................................................................ 1 1.1 Required Disciplines..................................................................................................... 2 1.2 Scope............................................................................................................................. 3 1.3 Societal Sectors............................................................................................................. 4 1.4 Needs, Alterables and Constraints ................................................................................ 4 1.5 Relevant Elements ........................................................................................................ 6 1.6 Interactions among Relevant Elements......................................................................... 7 1.7 Summary....................................................................................................................... 8

Chapter 2: Value System Design ........................................................................................ 9 2.1 Performance Objectives.............................................................................................. 10 2.2 Cost Objectives ........................................................................................................... 12 2.3 Analytical Hierarchy Process...................................................................................... 13 2.4 Conclusions................................................................................................................. 14

Chapter 3: System Synthesis............................................................................................. 15 3.1 Subsystem Elements ................................................................................................... 15 3.1.1 Operation ................................................................................................................. 15 3.1.2 Structure................................................................................................................... 15 3.1.3 Thermal Control....................................................................................................... 17 3.1.4 Automation .............................................................................................................. 19 3.1.5 Communication........................................................................................................ 21 3.1.6 Power ....................................................................................................................... 23 3.1.7 Climate Control........................................................................................................ 27 3.1.8 Lighting.................................................................................................................... 29 3.1.9 Water and Nutrients Delivery .................................................................................. 30 3.2 System Alternatives .................................................................................................... 34 3.3 Summary.................................................................................................................. 35

Chapter 4: System Analysis.............................................................................................. 36 4.1 Atmospheric Control Subsystem ................................................................................ 36 4.2 Communication Subsystem ........................................................................................ 39 4.3 Control and Sensory Subsystem ................................................................................. 40 4.4 Lighting Subsystem .................................................................................................... 42 4.5 Plant Growth Module.................................................................................................. 45 4.6 Power Subsystem........................................................................................................ 51 4.7 Structure Subsystem ................................................................................................... 55 4.8 Thermal Control.......................................................................................................... 59

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CHAPTER 5: Conclusions and Future Plans ................................................................... 66 5.1 Summary of Design Process ....................................................................................... 66 5.2 Future Planning........................................................................................................... 67

References......................................................................................................................... 69

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List of Tables

Table 1.1: IPGM Needs, Alterables, and Constraints.................................................. 5 Table 2.1: Weights for the Performance and Cost Objectives................................... 14 Table 3.2: Necessary Elements to Grow Plants ......................................................... 31 Table 3.3: Wheat Harvest Data Summary ± SE (per tray) ........................................ 33 Table 3.4: Subsystem Chinese Menu Approach ........................................................ 34 Table 4.1: Scores of Design Concepts ....................................................................... 39 Table 4.2: System Analysis for Atmospheric Control Subsystem............................. 40 Table 4.3: System Analysis for Control and Sensory Subsystem.............................. 42 Table 4.4: System Analysis for Lighting Subsystem................................................. 45 Table 4.5: System Analysis for Plant Growth Module .............................................. 51 Table 4.6: Critical Characteristics of Solar Cells and Fuel Cells .............................. 52 Table 4.7: System Analysis for Power Subsystem .................................................... 55 Table 4.8: System Analysis for Structure Subsystem................................................ 59 Table 4.9: Estimated Temperature Ranges ................................................................ 60 Table 4.10: System Analysis for the Thermal Subsystem ........................................... 65 Table 4.11: Final System Analysis .............................................................................. 65

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List of Figures

Figure 2.1: Objective Hierarchy ............................................................................. 10 Figure 3.1: Close-Up of the Imager Mars Patfhinder Camera Head4 ..................... 20 Figure 3.2: Mars Pathfinder Lander High Gain Antenna6 ...................................... 22 Figure 3.3: AIR-X Wind Power Generator (Southwest Windpower, Inc.)8 ........... 25 Figure 3.4: A Fuel Cell Used by the Space Shuttle Orbiter (UTC Fuel Cells)....... 26 Figure 3.5: Schematic of a System Using Zeoponics20........................................... 33 Figure 4.1: Concentrated Light Mode used by the Flower Structural Design ........ 44 Figure 4.2: Top View of Plant Growth Module Layout ......................................... 47 Figure 4.3: Modular Tray Concept ......................................................................... 48 Figure 4.4: Robotic Arm Concept........................................................................... 50 Figure 4.6: Tent Structural Concept........................................................................ 58 Figure 4.7: Multiple Dome Structural Concept ...................................................... 58 Figure 4.8: 2-D Representation of Heat Transfer ................................................... 61 Figure 4.9: Basic Heat Pipe .................................................................................... 63 Figure 5.1: Gantt Chart for the First Iteration.......................................................... 67 Figure 5.2: Gantt Chart for the Second Iteration ..................................................... 68

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List of Abbreviations AHP Analytical Hierarchy Process DSN Deep Space Network IPGM Inflatable Plant Growth Module LED Light-Emitting Diodes MOE Measure of Effectiveness NASA National Aeronautics and Space Administration NAC Needs Alterables and Constraints VSD Value System Design

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Chapter 1: Introduction and Problem Definition

Scientific interest in the exploration of our solar system has increased over the last

century. The next major step since landing on the moon will be the exploration of our

neighboring planet, Mars. Mars is one of the only other worlds in our solar system on

which there is a strong possibility of fostering life. A system of ancient channels has

been discovered on Mars. There is supporting evidence that these channels were once

riverbeds. These riverbeds imply that liquid water once existed on the surface. Water is a

necessary ingredient to support life. For many reasons, despite being an inhospitable

planet now, Mars is one of the only other viable locations to support life in the solar

system. Biological experimentation is the next step to further human understanding of

Mars.

The objective of this project is to design an autonomous inflatable plant growth

module (IPGM) for deployment on Mars that will grow several species of plants for a

period of five to seven years. This system would be a predecessor of future plant growth

systems for use in NASA human missions to Mars. To achieve this objective the design

must address many issues.

One issue is the transportation of the module to Mars. The module’s design will

include a minimal mass and minimal volume approach to the transportation phase of the

mission. Also, transporting any kind of experiment to Mars will take a long time. The

Hohmann transfer, which is the cheapest transfer orbit, takes approximately 8.5 months

(0.70 years).

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Another issue is the atmosphere on Mars. The Martian atmosphere has an

average pressure that is only 0.7% of Earth’s atmosphere at sea level. The composition of

the atmosphere is not the only obstacle. The average temperature is -55°C, which is not

favorable for growing plants.

Several other issues must be considered such as greenhouse structure, light

collection, water and nutrient delivery, thermal and power management1. Many scientific

disciplines must be studied to ensure that these issues are addressed. Also, the scope,

societal sectors, needs, alternatives, and constraints, and relevant elements to the problem

will be assessed and discussed.

1.1 Required Disciplines

Plants growing on Mars will be subjected to conditions unparalleled on Earth.

Although microorganisms have been found to thrive in harsh environments in the

extreme depths of Earth’s oceans, the objective of this project is to grow common plants,

such as spinach, tomatoes, carrots, soybeans, wheat and potatoes. Therefore, a system

must be designed to provide the basic environmental conditions required to sustain plant

growth. Designing such a system requires a collaboration of several disciplines.

Relevant disciplines include:

• Aerospace and Mechanical Engineering – Aerospace and Mechanical engineers

identify stresses associated with the initial launch, delivery vehicle separation and

final landing on the surface of Mars. They also design the structure of the plant

growth module as well as develop the devices needed for the thermal control of

the module.

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• Electrical and Computer Engineering – Electrical and Computer engineers

collaborate on implementing mechanisms for internal communication and

automation. They also design devices for relaying information to ground stations

on Earth, and plan for implementing emergency recovery procedures. Electrical

engineers are also responsible for selecting a power plant.

• Planetary Science – Planetary scientists provide information on the Martian

atmospheric conditions and the mineral composition of Mars’ soil.

• Biological Systems Engineering and Horticulture – Members of these disciplines

are responsible for identifying environmental and nutritional conditions required

for plant growth and integrating individual systems to sustain these conditions. In

addition, systems must be developed to recycle decomposed organic materials

into usable nutrients.

The list of the required disciplines shows that scientists and engineers will be the

main contributors to this project. The scientists and engineers working together to

produce this design will need to limit their design to fit the scope of the project.

1.2 Scope

Transfer of the plant growth module to the surface of Mars is not included in the

mission requirements. Therefore, the scope of this mission does not include the

determination of the optimal transfer orbit, nor does it include the selection of the launch

vehicle. Items included in the scope are:

• Study of atmospheric and surface properties of Mars

• Definition of requirements for sustaining plant growth

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• Design of a platform providing the necessary environment

• Design of a structure that can survive the impact induced by landing and the harsh

environment on Mars

• Handling of emergency situations

• Design optimization

The major factor in the determination of the scope was to assume that the module has

already landed on Mar’s surface. Therefore, the items included in the scope all focus on

growing plants in the module. While scientists and engineers will be the main

contributors to the project, other societal sectors will be affected.

1.3 Societal Sectors

The design, production and use of the IPGM incorporate several different actors.

The National Aeronautics and Space Administration is involved in many aspects of the

project such as the IPGM transport and material requirement. This project will be the

first experiment involving the growth of a life form on another celestial body. Thus, this

project may interest biologists and medical professionals. Media and public relations

officials are needed to provide information to the public concerning the growth and

development of the plants on Mars.

1.4 Needs, Alterables and Constraints

The different needs, alterables and constraints (NAC) associated with the Martian

IPGM are displayed in Table 1.1. The sole need for the project is to grow plants on Mars

for a period of five to seven years. Table 1.1 lists the plants that could be selected for this

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mission. Plants use photosynthesis so they need ample supplies of carbon dioxide, water

and sunlight. The atmospheric control system can provide these supplies in different

ways. The design of the subsystem includes some other alterables such as the module

design, the material selection and the power management. Finally, autonomy of the

system and biological information processing add to the alterables.

Table 1.1: IPGM Needs, Alterables, and Constraints Categories Elements Needs: Grow plants on Mars for a period of 5 to7 years Alterables: Autonomy of the system Power management Material selection Plant species (Spinach, Tomatoes, Carrots, Soy beans, Wheat, Potatoes) Module design All subsystem level designs Exact gas composition in the module /Atmospheric control Biological information processing Constraints: Must fit into the payload envelope of the Delta II 7925 3.0 composite

fairing launch vehicle (maximum diameter: 2.74 m, overall length: 5.028m, maximum mass: 905 kg) while allowing room for orbit transfer and landing crafts1

Surface location of the IPGM is restricted to the Gusev Crater located at 14.5°S/186°W1

Design life shall be 5 to 7 years IPGM should be able to grow the following items or a combination of the

items (Spinach, Tomatoes, Carrots, Soy Beans, Wheat and Potatoes) External structure must be deployed from an unmanned vehicle and must

have an internal frame structure such that it is stowed in a tight payload compartment, while also having the capability to be automatically deployed.

Adequate water must be maintained for plant development Must provide an adequate lighting system of 400-700 nm to support

Photosynthesis Must provide an environment to satisfy the following parameters

Temperature: 10 to 30°C Relative humidity: 40 to 90% CO2 Partial Pressure: 0.1 to ~3 kPa O2 partial pressure: >5 kPa Inert gas composition: optional Ethylene gas: < 50 ppb equivalent at 100 kPa total pressure1

Must remove or scrub excess ethylene gas

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1.5 Relevant Elements

The plant growth module operates in a harsh environment. There are frequent

dust storms with winds in excess of 100 mph that could severely damage the module. In

addition, the temperature on the surface of Mars fluctuates from -130°C to 30°C2. This

wide temperature range may cause unpredictable material degradation. As a result, there

is a need for thermal protection and resistance against physical phenomena that occur on

Mars. The module will require a rigid external structure that is effective while not

excessive in mass. The module structure will require advanced polymers, metals, and

ceramics that provide both mechanical and thermal protection.

Most of the module subsystems will require power to operate. Examples include

lights to facilitate photosynthesis, pumps to extract rarified Martian gases,

communication systems, and water and nutrient delivery systems. Initially, the energy

can be stored in battery units but this energy will be exhausted and other power sources

will need to be used. Hence, the natural resources on Mars, such as sunlight, must be

utilized to produce energy for all the electronic systems aboard the module.

Another important issue is the threat that this device may pose to the environment

of Mars. The module needs to be free of any contaminates at the end of the system’s

lifetime to prevent damage or extinction of possible microorganisms that may already

exist on the Martian surface. Environmental concerns need to be resolved in order to

avoid criticism from organizations that oppose the introduction of biological materials to

remote locations in our solar system.

Water and nutrient delivery is also a main concern. The plants need a continuous

supply of CO2, N2, and O2 to sustain a successful growing cycle. Nitrogen and potassium

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are crucial elements for plant growth and exist in scarce amounts on the Martian surface.

Perhaps these components can be formed in a fertilizer reactor aboard the module. Plant

waste will have to be recycled and possibly discarded to maintain a healthy ecosystem.

The plant waste can be recycled into useful organic material to grow new plants.

However since the space under the inflatable biosphere is limited, some of the waste may

have to be discarded to prevent accumulation that may cause choking within the module.

Autonomy is an important aspect of the project. The system needs to operate

effectively after deployment for a life cycle of 5 to7 years. Advanced control systems will

have to be implemented to regulate all plant processes and will require contingency plans

in case of control system failure or damage. Communication with onboard computer

systems will be required to upload modifications to onboard computer systems on the

plant module system.

All these elements are required for a successful system. In order to achieve this

goal, there will be interactions between these elements.

1.6 Interactions among Relevant Elements

The plants that will grow in the IPGM will require the support of an artificial

environment in order to thrive. The module structure will need to provide this

environment. The structure will need to shield the plants from Mars' harsh temperature

changes and atmospheric pressure, as well as from the Sun's radiation. The structure will

allow adequate sunlight to enter. The structure will also maintain the proper gas

composition and a bed of soil in which the plants will grow.

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The automation system will need to recycle the atmosphere and soil in the module

to maintain the desired plant growth conditions. The automation system will also need to

remove any plant-threatening toxins that build up in the module. The transmission of

biological data could be automated as well.

The automated features of the module will require a power source. The power

source must also provide sufficient power to the communications equipment to transmit

biological data to Earth and receive commands from Earth. The module structure must

physically support the power, communications and automated plant support equipment.

With the subsystems interacting in these ways, the module will successfully

complete its mission.

1.7 Summary Chapter 1 gives an introduction and mission objective to the Mars missions.

Information and disciplines needed for the problem were also introduced. The problem

definition identifies the needs, alternatives, and constraints, scope, and relevant elements

of the problem. The following chapters outline the process of producing a preliminary

design. The value system chapter is established to evaluate possible solutions to reach

one optimal design for the mission.

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Chapter 2: Value System Design

The purpose of the Value System Design (VSD) phase is to specify each mission

objective in a design concept. The VSD will select the best alternative design based on

specific attributes of each mission objective. The objectives are compared to each other

and assigned a relative weight via the use of the Analytical Hierarchy Process (AHP).

Also, the VSD applies the AHP at all levels of the hierarchy, which leads to a weighted

expression for the measures of effectiveness (MOE). Next, the VSD is comprised of two

broad categories: efficiency and effectiveness. Efficiency is a measure of how well an

objective performs its task, whereas effectiveness determines the extent of which the task

fulfills the requirements.

The key to designing the most efficient and effective IPGM will depend on

maximizing the performance and minimizing the cost. The objectives hierarchy, shown in

figure 2.1, displays all of the mission objectives that characterize all design concepts. The

hierarchy is divided into two main parts: the performance and cost objectives. The legend

on figure 2.1 indicates if the measurement should be maximized or minimized. Some

measurements are assigned factors of effectiveness or efficiency since there is not a direct

method of measurement. The factors will be quantified by using a 1-10 scale, with 10

being the best score.

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Figure 2.1: Objective Hierarchy

2.1 Performance Objectives The performance of the IPGM is divided into 16 relevant sub-objectives that are

to be maximized or minimized. First, power and thermal efficiency are important to

maximize to maintain an IPGM that does not waste stored energy on the inside of the

dome area or on the surroundings. An efficient power and thermal system must be

Earth

Cost

Design

Performance

Maximize

Minimize

Measurement

Legend rem/year)

Power efficiency

Power output

η

Volume of growth area

Effectiveness factorAutomation effectiveness

Power (Watts)

Gas partial pressure (P)

Volume (m^3)

Atmospheric control

Radiation exposure Radiation (

Communication ability Transfer speed (kbps)

Growth conditions

Thermal efficiency

Structural stability

based modification

Dome deployment reliability

Material effectiveness

Structural strength

Use of Martian atmosphere

Science Experiments

Mass of structure

Production costs Cost ($)

Cost ($)Operation cost

Mass (kg)

Reliability factor

Number of inputs

ηT

Effectiveness factor

Displacement (cm)

Strength,MPa

Growth factor

Composition of dome atm.,%

Data storage capability

Reliability factor

Number of inputs

ηT

Effectiveness factor

Displacement (cm)

Strength,MPa

Growth factor

Composition of atm.,%

Data storage capability

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applied in conjunction with an adequate insulation scheme to maintain suitable growth

conditions since nighttime temperatures on Mars can reach –100°C. Next, optimizing the

volume of the growth area will allow room to grow more plants which will provide a

higher product yield. Another important issue is to regulate the temperature, pressure and

gas composition of the IPGM’s internal atmosphere. The pressure and temperature should

be maintained near the standard atmosphere conditions on Earth (1 atm and 25°C).

However, changing the partial pressure of the gases inside of the dome may be favorable.

For example, some plant species may require a different partial pressure of carbon

dioxide (CO2) to maximize its produce. In addition, radiation shielding will be required to

prevent damaging solar radiation from degrading the biological systems aboard the

IPGM. The MOE for radiation absorbance will be in rem/year, which could be measured

via a Geiger counter or other radiation sensing devices. Furthermore, the IPGM will be

at an advantage to incorporate some of the gases in the Martian atmosphere into the

growth area. Utilizing the external environment will allow the IPGM an increased level

of autonomy while reducing the weight of stored supplies needed for the mission from

Earth. Next, communication, modification capability, and automation are important

issues for maintaining a responsive system that will operate ideally over the lifespan of

the IPGM. Communication will require data to be periodically downloaded to obtain

information on the status and conditions of all the subsystems. Earth-based modification

will be important during an unexpected event on Mars (i.e. natural climatic change on

Mars, solar flares, dust storms, etc.). The automation systems aboard the IPGM will

execute many tasks during the mission. Examples include lighting schemes, deliver

nutrients to plants, regulate internal atmosphere, and recycle plants and organic wastes.

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Also, the IPGM will need to perform many scientific missions while resting on Mars.

The ability to store data is the MOE for maximizing the effectiveness of the scientific

missions. The last category of concern is the structure of the IPGM. All structure

concepts will consist of the solid frame and the inflatable dome. The strength of the

frame must be maximized to withstand the vibratory forces at launch and the impact from

the landing. Optimizing the strength of the frame is measured by the mechanical

properties of the applied materials (e.g. Youngs modulus, shear modulus, and bulk

modulus). Finally, the dome deployment system must be reliable. Snags or tears during

the deployment of the inflatable dome could lead to mission failure.

2.2 Cost Objectives The other key to optimizing the design of the IPGM is to minimize the cost of the

entire mission. The mission cost is comprised of three objectives: launch weight,

production cost, and operations cost. First, launch weight is proportional to cost (

$10,000/kg). Therefore reducing the weight of the payload can have a large effect on the

cost of the mission. However, reducing the mass of the IPGM below the maximum

capability of the Delta II rocket will not result in large savings since the launch vehicle

for the mission has been arranged. Next, production cost must be considered in the

mission architecture. Production costs can be large due to the level of technology that

will be built into the subsystems of the IPGM. For example, the thermal and power

systems will be built from components that are made from expensive next-generation

polymers and high-quality metals like titanium, gold, and platinum. Last, operation costs

will be a factor that contributes to cost. Operations will include surface deployment,

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post-landing survival testing, start-up, daily operations and maintenance for continuous

plant/harvest, and shutdown.1

2.3 Analytical Hierarchy Process The VSD objectives were compared to each other via the analytical hierarchy

process (AHP). The AHP was applied at both “maximize performance” and “minimize

cost” levels. The AHP provides means to determine the quality, or value, of each

alternative design, which is formulated in system synthesis. The following formula

(equation 2.1) is used to compute the overall value of an alternative design:

V = w1(w1.1M1.1+w1.2M1.2+…+w1.nM1.n) + w2(w2.1M2.1+ w2.2M2.2 +w2.3M2.3) (2.1)

where w1 represents the weight of the performance objectives and w2 represents the

weight of the cost objectives. The weight for the performance is considered to be twice as

important as the cost, therefore, w1=2/3 and w2=1/3. The performance objectives were

analyzed using a matrix technique whereas the few cost objectives were compared by

intuitively assigning fractional relative weights (Table 2.1). M represents a MOE and is

assigned a value from 0 to 1, with 1 being the best value for a design concept. There are

16 components of the performance value since there are 16 objectives. Each design

concept will be analyzed by applying (2.1) to obtain an overall value of the design.

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Table 2.1: Weights for the Performance and Cost Objectives

Performance Objectives Weight, %Power efficiency 4.88 Power output 2.04 Atmospheric control 12.6 Volume of growth area 2.04 Automation effectiveness 10.8 Radiation exposure 11.7 Communication ability 6.61 Growth conditions 5.49 Thermal efficiency 11.6 Material effectiveness 2.84 Static stability 3.56 Dome deployment reliability 1.52 Earth-based modification ability 5.90 Structural strength 8.64 Use of Martian atmosphere 4.27 Measurement capability 5.49 Cost Objectives Mass of structure 30.0 Production costs 50.0 Operations cost 20.0

2.4 Conclusions The VSD is a design phase used to specify each element of a mission concept to

an overall design. The objectives of this design project are compared to each other by

applying the AHP at both levels of performance and cost. The performance weights are

determined by executing an analytical matrix whereas the cost weights are intuitively

determined on a fractional contribution basis. The alternatives that are produced in the

design synthesis stage are compared to one another by applying equation (2-1), which

was formulated by the AHP.

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Chapter 3: System Synthesis

The purpose of system synthesis is to review alternative elements and find

conceptual design solutions for the IPGM. A list of alternative subsystem elements is

created using a “Chinese menu” approach. Each subsystem element is described briefly

followed by the advantages and disadvantages of the different proposed alternative. This

chapter defines the subsystem elements, the conceptual design solution from the

subsystem elements and the summary.

3.1 Subsystem Elements

3.1.1 Operation The IPGM is required to grow plants on Mars for a period of 5 to 7 years. The

IPGM must also execute surface deployment, post-landing testing, start-up, daily

operation and maintenance, and shutdown. Also, all operations must function in

automated fashion. The ideal solution for the design is to create a biosphere on Mars that

replicates the conditions existing on Earth. The attainment of each of these goals is

satisfied by the proceeding subsystems.

3.1.2 Structure The IPGM structure may consist of a single module or multiple modules. The

single module concept provides a single environment for the plant growth and a bigger

growth volume. The single module provides a simpler configuration for integration with

the subsystems. The multiple modules concept provides a uniform environment

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throughout the modules. Consequently, the multiple module design has a smaller growth

volume. Each module will have the same geometric shape and design. The multiple

modules provide a fail-safe design, where if one module fails, the other modules can still

operate successfully. Also, both concepts are required to be deployable.

The IPGM external structure must be able to withstand the harsh Martian

atmosphere, but must be light, durable, and deployable into a full module to provide

adequate growth volume. Hence, the structure subsystem is divided into three

classifications: inflatable, rigid and inflatable, or rigid.

The inflatable structure deployment concept will decrease the structural mass and

storage volume. Some materials that already have been used for inflatable space

programs are polyamides, fluorocarbons, polyarylene-ether phosphine-oxide polymers

and polyesters.3 The polymeric dome can be inflated by using the necessary gases to

grow plants. The pressure difference alone stiffens the inflatable structure without the

use of structural members by providing a higher internal operating pressure than the

surrounding environment. In addition to reducing mass, the lack of stiff structural

members also increases packaging efficiency, which can decrease the launch payload

volume of the growth chamber.

The rigid structure deployment concept utilizes advanced materials to deploy the

module. Advanced materials such as “smart” materials have been developed and used in

many aerospace-related programs. Smart materials can be stowed in compact

compartments and, when released by a mechanism, restore their original shape. Smart

materials are light and can be used as shielding from Martian dust storms. Smart

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materials would allow the module to be deployable while providing the necessary space

for plant growth.

The rigid and inflatable structure deployment concepts utilize both smart

materials and inflatable materials. Combination of the two concepts may provide

advantages but may not be the best concept for the structural deployment.

The concepts described concern the deployment of the module structure to

provide adequate plant growth volume. The whole module structure must be rigid and

able to anchor to the Martian surface. The module structure geometry is restricted by the

Delta II payload geometry, which has a maximum diameter of 2.74 m and an overall

stowed length of 5.028 m.1 Therefore, the basic shape of the stowed module will be a

cylinder. The cylinder shaped module must be able to be automatically deployed into a

biosphere once landed on Mars.

3.1.3 Thermal Control

There are several alternatives to designing an effective thermal subsystem for the

IPGM. The function of the IPGM thermal control system is to regulate the temperature

of individual spacecraft components as well as the ambient temperature of the

environment on the inside of the module.4 The thermal control system must provide

thermal stability for both the transfer stage and the actual 5-7 year life cycle on Mars.

Radiation heat transfer dominates the course of the transfer orbit from Earth to Mars.

Convection heat transfer into the cool Martian atmosphere is the most important concern

while on the surface of Mars. However, some solar radiation penetrates the thin

atmosphere of Mars. There are three main alternatives for an effective thermal system

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aboard the IPGM. The following systems may be chosen to operate alone or in

conjunction with each other.

A thermal blanket can be encompassed around the spherical growth module since

the material is likely to be made from a thin un-insulated polymeric material. The

thermal blanket reduces the heat lost by convection into the cold Martian environment.

The thermal blanket would have to be deployed at nighttime or during periods of non-

photosynthetic plant activity since the useful solar radiation (of the sun) would be unable

to penetrate the blanketed dome. The device could be deployed with a mechanism

similar to a device used to deploy the canopy on a convertible automobile. The blanket

itself may consist of as many as 24 layers of different fabrics, including aluminized

kapton, mylar, dacron and other special materials. The blankets also provide protection

against micrometeoroids, which consist of the dust grains of rocky debris that litter the

Martian environment. Some advantages to this system are low maintenance, low cost,

and the simplicity of the system. Adding more moving parts and sunlight blockage are

drawbacks of this system.

Solar arrays could be used to provide thermal support while also providing power

to the IPGM. Solar arrays could be used to heat a fluid similar to antifreeze, which

would be circulated around the IPGM. The heat from the fluid would be convected to the

IPGM by means of small fans blowing on the coils of tubing. Advantages of this system

include unlimited solar energy, extensive lifetime and cheap operation cost.

Disadvantages are the relative size or bulkiness of the panels, the complications of

deploying the system, and the problems associated with long periods of reduced solar

activity.

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The electric components and dome atmosphere could be kept in adequate

conditions by using an electric heater. Also, heat can be obtained from the internal

resistance of the electronic components. The heater could get its power from batteries or

solar arrays. Heaters would provide a reliable source of heat while costing very little to

produce. However, heaters would draw away from the available power supplied to the

IPGM.

3.1.4 Automation

The module relies completely on the automation system to manage all of its daily

functions. Due to the vast distances between Earth and Mars, the communications lag is

too great to allow efficient control of the module from the ground. The module needs to

act autonomously to function efficiently. The automation requires sophisticated sensors

to monitor plant life signs, atmospheric conditions, materials recycling, energy

production and all other systems in the module. The automation subsystem can be

divided into several sub-subsystems. These are:

• Sensing

• Materials Transport

• Waste Management

• Communications

• Computer

The sensing system involves monitoring all functions of the module and

determining if there are any anomalies occurring. The sensing system has to monitor the

plant health, atmospheric conditions, radiation levels and power levels. Many types of

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sensors are thus required. Some are simple such as thermometers and barometers.

Others are more specialized such as CO2, O2, and ethylene sensors. A Geiger counter

may also be needed to detect radiation levels. Still or video cameras similar to the one

used for Pathfinder (see figure 3.1) may be used to get visual data on module and plant

conditions. Attitude sensors may be used during deployment to achieve the correct

orientation. Air speed and wind direction indicators may be utilized to monitor the

exterior atmospheric conditions.

Figure 3.1: Close-Up of the Imager Mars Patfhinder Camera Head4

Depending on how the plants are grown, the materials transport system has to

initiate new seed growth, move biological waste to the waste management system and

supply the plants with water and nutrients. This process may involve the use of robotic

arms to move the plant materials to the necessary locations. Another alternative for the

movement of materials may be conveyer belts. More detail for the transport system is

provided in chapter 4.

The waste management system needs to either dispose or recycle the waste.

Waste will be generated in several forms. One form is gaseous waste. Gaseous waste

includes ethylene and any other undesirable gases. The waste management system could

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remove these gases from the atmosphere using a combination of filters and pumps. Some

gases may be able to be recycled and used for other purposes. Another form of waste is

organic waste. It may be possible to recycle the plant waste by composting this organic

material. Bacteria might be required and the possibility to bring such organisms to Mars

is still unclear. The plant waste could also be burnt. The decomposition may be

accomplished by using the flammable waste gases produced by the plants, such as O2 and

ethylene. The gases and ashes would then be stored or expelled from the module.

The communications system needs to automatically transmit the data collected by

the sensors back to Earth. The communications system also receives commands from

Earth and relays them to the computer system. The computer system controls and

coordinates the functions of the other sub-subsystems. The system may include a

microprocessor, or it may be a set of micro controllers. It might have the capability to

detect and react autonomously to anomalies or it may transmit data related to anomalous

occurrence back to Earth and await instructions.

3.1.5 Communication

There are several methods to send the collected data from the module to Earth:

communicate directly from Mars to Earth as in the Mars Pathfinder mission or use one of

the two satellites (Mars Global Surveyor, Mars Odyssey) orbiting Mars to relay

information back to Earth.

The Mars pathfinder mission showed that communications with Earth from Mars

may be accomplished without the use of a satellite. The best way to do that is to use the

NASA Deep Space Network (DSN), like the Pathfinder Lander. The DSN is an

22

international network of antennas that supports interplanetary spacecraft missions for the

exploration of the solar system and the universe. The network consists of three deep-

space communications facilities placed approximately 120 degrees apart around the

world (USA, Spain and Australia) thus permitting constant observation of spacecraft as

the Earth rotates. They operate in two microwave frequency bands, S-band (2.3 GHz) and

X-Band (7.2 GHz TX uplink, 8.4 GHz RX downlink) 5. The Pathfinder Lander had two

different paths to communicate with the DSN: via its high gain antenna (shown in figure

3.2) and via its low gain antenna. The maximum data rate of the high gain antenna, based

upon the performance of the Pathfinder radio frequency subsystem is 11.06 Kbps.

Alternatively, a low gain antenna with a maximum data rate of 600 bps can be used to

communicate with Earth. Both antennas are capable of receiving the 7.2 GHz uplink

signal and transmitting the 8.4 GHz downlink signal.

Figure 3.2: Mars Pathfinder Lander High Gain Antenna6

A second alternative for communicating with Earth is to use satellites orbiting

Mars. Of the two satellites currently orbiting Mars (Mars Global Surveyor and Mars

Odyssey), only Mars Odyssey has the necessary equipment to communicate with a

lander. Odyssey's telecommunications subsystem is composed of a radio system

23

operating in the X-band microwave frequency range and another radio system that

operates in the ultra high frequency range. The X-band system is used for

communications between Earth and the orbiter, whereas the ultra high frequency system

is used for communications between Odyssey and any landers present on the Martian

surface. A third satellite named Mars Express (project from the European Space Agency)

whose launch is scheduled for 2003 should also be equipped with an ultra high frequency

system allowing communications with any Martian lander.

The amount and the quality of data transmitted are then determined by the choice

of the communication’s subsystem, either direct communications or use of the satellites.

However, without communication the mission becomes useless. Thus, an ideal system

would be one which allows both communications through satellites and directly with

Earth. Therefore, an effective structure should include an X-band radio to communicate

directly with Earth, and an ultra high frequency system to communicate with satellites.

3.1.6 Power

Power to the module can be generated using solar cells, wind power generators,

fuel cells, or nuclear reactors. Solar cells harness the sunlight that penetrates Mars’ thin

atmosphere. The windmill concept utilizes the wind generated by dust storms on Mars as

a source of natural power. Fuel cells use chemical reactions and produce heat and water

as byproducts.

Solar power may provide a cheap and dependable source of energy for the IPGM.

The solar intensity on Mars is about half of the intensity on Earth. Therefore, larger

panels are required to compensate for the reduced amount of sunlight. For example, to

generate 300 W of electricity on Mars, the module would require two 300 W panels such

24

as ASE's 300DG/17 solar panels. This specific panel is 1.8 m long, 1.3 m wide, and has a

mass of 48 kg.

Observations of Mars’ climatic changes on Mars indicate that long lasting global

dust storms move enough dust particles to block the solar energy reaching the surface of

Mars completely. Such a condition could be hazardous for any module which relies

solely on solar energy as its power source. Wind power generators, or windmills, could

prove to be an effective back-up source of power during such storms. On Earth,

windmills require airspeeds of 10 m/s to produce a significant amount of electricity. On

Mars, the minimum required airspeeds are about 30 m/s due to the lower atmospheric

pressure8.

Sensors on the Mars Pathfinder and Viking modules recorded average wind

velocities of 5m/s. The peak velocity recorded by the Viking Lander was 25 m/s during a

local dust storm9. For mission safety reasons, both modules were landed at low and flat

regions. Much higher velocities usually exist in rugged terrains. Unfortunately, the

IPGM is to be located in the Gusev’s crater, where high wind velocities are unlikely to

exist. Thus, a combination of batteries and a wind generator could serve as a viable

secondary power source. However, the sufficient wind velocities on Mars are too

infrequent for the IPGM module to regard the wind energy as its primary power source.

Incorporating a wind generator to the module would only slightly increase the

total weight and volume. For instance, the AIR-X wind generator (see figure 3.3)

manufactured by Southwest Windpower, Inc. generates 250 Watts of power in a 10 m/s

terrestrial wind10 (or approximately a 30 m/s wind on Mars). The peak power output is

400 Watts at 13.5 m/s, but attaining such a high output on Mars is very unlikely. The

25

generator weighs only 6 kg and the diameter of the rotor is only 1.14 meters. The

generator does not require a tower, and could be easily incorporated into the support

structure of the module.

Figure 3.3: AIR-X Wind Power Generator (Southwest Windpower, Inc.)8

Fuel cells consist of two electrodes immersed in an electrolyte11. Hydrogen is

supplied to the cathode and oxygen is supplied to the anode. A catalyst in the cathode

breaks the hydrogen into an electron and a proton. While the proton travels to the anode

directly through the electrolyte, the electron is directed through a circuit, where it creates

electricity. The electron, proton and oxygen combine at the anode to form water.

A fuel reformer allows the fuel cell to utilize any hydrocarbon compound as a

fuel. Methanol is commonly used instead of hydrogen. Methanol is a safe chemical,

often found in window washing liquids. The reformer converts the hydrocarbon, water

and oxygen into hydrogen, carbon dioxide and carbon monoxide. Next, water is added

and carbon monoxide is converted to carbon dioxide and hydrogen. Hydrogen is directed

to the fuel cell cathode, and carbon dioxide is releases as exhaust12.

26

Fuel cells usually operate at temperatures between 150 to 200oC. Such a high

operating temperature usually indicates the need for a separate cooling unit; however, it

could be advantageous for the mission. The fuel cell could double as a heating unit for

the IPGM module. Several types of fuel cells exist. An alkaline fuel cell was used on the

Apollo mission. Since then, the fuel cell technology has improved: the three fuel cells

used on the space shuttle orbiter (Figure 3.4) provide 10 times the power for the same

unit size. Each 12 kW fuel cell weighs 118 kg and measures 36×38×114cm. The fuel

cells have collected over 87,000 flight hours and proved to be reliable. Their efficiency

rates are above 75%, which is a significant increase compared to combustion power

plants. Combustion power plants tend to be only 25% efficient12.

Recently, NASA's JPL developed a flat, portable 5W fuel cell (see figure 3.4).13

Another interesting development was the creation of regenerative fuel cells. Water

created by the fuel cell is split into hydrogen and oxygen using a solar powered

electrolysis. Hydrogen and oxygen are fed back to the fuel cell, which creates electricity,

heat, and water. Water is then re-circulated to the electrolyser, and the entire process

repeats. With the use of an external solar array, a re-circulating fuel cell could provide a

generous, long lasting power source for the module.

Figure 3.4: A Fuel Cell Used by the Space Shuttle Orbiter (UTC Fuel Cells)

27

There are a number of isotopes used for the nuclear power plants (strontium-90,

polonium, promethium, etc.). Nuclear power produces the most power per mass of fuel of

all the types of sources. However the structural mass of a nuclear power system is

substantial, and the power per kg of system mass is much smaller than for other power

sources. Also, radiation has adverse effects on the biomass. There should be enough

radiation shielding to protect the plants. However, this shielding would add weight to the

IPGM and thereby increase the cost.

Energy storage is provided by alternative types of batteries. Lithium-ion batteries,

nickel cadmium, lead acid, or zinc silver are examples of batteries that can be used to

store energy.

3.1.7 Climate Control

The atmosphere of Mars consists primarily of carbon dioxide, which makes up

95.3% of the atmosphere. The remaining fraction is divided among nitrogen (2.7%),

argon (1.6%), oxygen (0.15%) and water (0.03%)14. Carbon dioxide comprises only a

minute part of Earth’s atmosphere. The atmosphere on Earth is primarily made up of

nitrogen (77%) and oxygen (21%), with only small amounts of argon, carbon dioxide and

water.15 The average density of Martian atmosphere is less than 1% of Earth’s sea level

density. As such, the module needs to create and sustain an atmosphere favorable to

plant growth. The climate control interacts with the automation system to regulate the

atmospheric composition, pressure, humidity and temperature.

Due to the low ambient atmospheric density, the module needs to bring the

atmospheric gases from Earth, since extraction of sufficient amounts from Mars’

atmosphere would require power-demanding suction pumps. Depending on the size of

28

the module and calculated consumption rates, the system either releases waste gas

products into the Martian atmosphere, or recycles them. A common waste product of

plant growth is a carbohydrate ethylene, C2H4. All plants naturally produce ethylene to

regulate their life cycle. Perhaps, during the mission operation, sufficient amounts of

ethylene would accumulate to significantly shorten the plant life, and reduce the colony’s

reproduction rates. The Wisconsin Center for Space Automation and Robotics developed

an ethylene reduction module, which converts the ethylene gas into carbon dioxide and

water. The module uses titanium dioxide photo catalyst tubes to oxidize ethylene when

exposed to ultra-violet light16. The photo catalyst is not consumed during the operation,

which is a significant improvement over other methods of removing ethylene. Potassium

permanganate based filters need replacement every 30 days. Passive filters, which are

placed across the direction of air circulation, require no power to operate, but are

ineffective. These filters would need to be periodically replaced, significantly increasing

the amount of waste generated by the module.

The proper atmospheric composition is regulated using a system of pumps and

controllers. A CO2 monitor, such as the Green House CDM6000 model measures carbon

dioxide concentration using an infrared technology17. Measured values are relayed to the

regulator, which directs the pumping mechanism to release more carbon dioxide. A

suction pump or a release mechanism counteracts the build up of pressure in the module.

Proper humidity is maintained using either a vaporizer, which evaporates water to create

steam, or a humidifier. The vaporizer could be integrated into the heating unit, which

could use steam as the source of heat. A mechanical humidifier uses a fan to blow water

collected by a wicking material. The second approach has the advantage of allowing the

29

automation system to regulate the humidity independently from the temperature, but has

the expense of an added overhead.

3.1.8 Lighting

The sulfur lamp is an efficient, powerful, bright, full spectrum light source that

has many different indoor and outdoor uses. Each bulb contains a small amount of sulfur

and inert argon gas. When the sulfur is bombarded by focused microwave energy it forms

plasma that glows brightly, producing light similar to sunlight. Because there are no

filaments or other metal components to break down, the bulb may never need

replacement. Only the magnetron needs changing.

The system is dimmable to 20% and provides a constant correlated color

temperature of 5,700 degrees Kelvin with a color rendering index of 79. Again, because

there are no filaments to degrade or alter its chemical composition, the light source does

not change color or intensity over time and continues to render objects close to their true

color.

LED. The main challenge for growing plants in space is supplying a sufficient amount of

light for photosynthesis. Potential electric light sources must have a high electrical

efficiency, small mass and volume, an excellent reliability and safety record, and an

optimal spectral output for photosynthesis and photomorphogenesis. Light-emitting

diodes (LED's), particularly in the red region of the spectrum (660 to 690 nanometers),

may meet this criteria. Maximal rates of oxygen evolution (a measure of photosynthesis)

are lower in wheat plants grown under red LED's as compared to control plants grown

30

under cool white fluorescent lamps, which holds true whether the plants are grown at a

high (500 µmol m-2 s-1) or low (50 µmol m-2 s-1) light intensity. The LED concept would

provide greater resistance to damage than the sulfur lamps.

3.1.9 Water and Nutrients Delivery The seeds will have to develop and grow after the module has landed on Mars.

Efficient water and nutrient delivery is therefore necessary. Indeed, most common

delivery systems on Earth cannot be applied in a low gravity environment. The major

possibilities that the delivery system can utilize on Mars include the following: Earth soil,

regolith, hydroponics and zeoponics. These alternative solutions have advantages and

disadvantages described in the subsequent chapters.

Biological experiments have shown that plants need some macroelements and

oligoelements to develop and survive. Arnon and Stout proposed that three criteria had to

be reached for the mineral nutrients of the soil to be considered necessary.

1. A given plant must be unable to complete its life cycle in the absence of the

mineral element.

2. The function of the element must not be replaceable by another mineral element.

3. The element must be directly involved in the plant metabolism. For example, as a

component of an essential plant constituent such as an enzyme, or it must be

required for a distinct metabolic step such as an enzyme reaction.

The necessary elements are listed in Table 3.2. In order to assimilate all these

elements, the plants must be provided a critical amount of elements per day. The

possibilities studied for the specific Mars conditions are described below.

31

Earth soil can provide all the elements needed by the plants as it does on Earth.

However, in a microgravity environment, the systems used on Earth are not efficient in

space. Using earth soil leads to difficulties in providing a uniform distribution and the

required amounts of water and nutrients in the plant root zone while preventing release of

free nutrients in the atmosphere17. Moreover, some living organisms are necessary to

aerate the soil and provide oxygen especially for a long period (5 to 7 years in this case).

Bringing bacteria or macroorganisms like earthworms could be difficult because of the

time required to go to Mars (8.5 months) and the exposure to high-level radiation.

The direct use of Mars soil could be another alternative to supply the nutrients for

plant. This solution has the advantage that it would not require any transportation of mass

from Earth.

Table 3.2: Necessary Elements to Grow Plants

Element Symbol Concentration in Dry Matter

(mmol/kg) Hydrogen H 60,000 Carbon C 40,000 Oxygen O 30,000 Nitrogen N 1,000 Potassium K 250 Calcium Ca 125 Magnesium Mg 80 Phosphorus P 60 Sulfur S 30 Chlorine Cl 3 Boron B 2 Iron Fe 2 Manganese Mn 1 Zinc Zn 0.3 Copper Cu 0.1 Nickel Ni 0.05 Molybdenum Mo 0.001

32

The loam found on Mars cannot be considered as a traditional according to soil

scientists. The soil is classified as Regolith, which is defined as soil devoid of living

organic matter. Moreover, the topsoil composition is uncertain and, even if known, the

landing site would have to be precise to allow an efficient extraction of the nutrients.

Extraction of the elements would require some moving modules, leading to an increase of

the overall costs and a minimization of the module volume used to grow plants.

Hydroponics is one of the most widely used systems to grow plants efficiently

both on Earth and in space. Hydroponics has shown some reliability in space application.

The different systems of hydroponics commonly utilized are:

1. Nutrients are supplied in liquid solutions

2. Plants are supported by porous materials, such as peat, sand, or gravel that

acts as a wick to relay nutrient solution to the roots.18

The advantages demonstrated by the use of such a system are numerous. For

example, extra oxygen can be provided to the roots thus permitting the plants to absorb

nutrients faster. Moreover, the plants do not have to probe the soil for nutrients (as in a

typical plant system on Earth) since they are directly delivered to the plant several times

per day. The benefits of hydroponics use can be measured in the growth rate, which is

generally 30 to 50 percent faster than for a soil plant grown under the same condition.19

Zeoponics is a recent system developed by NASA to grow plants in microgravity.

This system uses a composite called zeolite, a naturally occurring mineral group which

consists of over 50 different minerals. This substance is made of a porous crystalline

structure that remains rigid in the presence of water.20 The nutrients needed by the plants

33

are slowly dissolved following the demand of the plants by the release of ions. An

example of nutrient delivery system using zeolite is represented in Figure 3.5.

Figure 3.5: Schematic of a System Using Zeoponics20

Some experiments using zeoponics were performed at Kennedy Space Center

including the use of microporous tubes with and without various types of solid media,

and gravity-dependent systems.17 Results obtained are shown in Table 3.3 and show some

promising results for the use of zeoponics in the IPGM.

Table 3.3: Wheat Harvest Data Summary ± SE (per tray)

1 45 plants per tray which was equivalent to approximately 900 plants per m2 2 Aboveground dry matter (DM) = Spike DM + Straw DM 3 Chaff DM = Spike DM - Seed DM 4 Harvest index does not include roots

Porous Tubes

Porous Tubes +Zeoponics

Drip-Irrig. Zeoponics

Drip Irrig. P. Vermiculite

Aboveground DM (g)2 96.9 ± 3.5 90.9 ± 0.6 126.8 ± 5.0 148.9 ± 0.6 Straw DM (g) 33.7 ± 2.0 48.1 ± 0.1 78.8 ± 2.0 61.0 ± 0.3 Spike DM (g) 63.2 ± 1.7 44.7 ± 0.5 59.2 ± 3.2 98.9 ± 0.6 Chaff DM (g)3 23.0 ± 1.2 35.3 ± 1.1 46.2 ± 1.3 31.6 ± 0.3 Spike No 104 ± 6 218 ± 6 247 ± 6 155 ± 1 Harvest Index (%)4 41.5 9.4 10.3 45.1

34

From figure 3.5, depending on the association of elements to build the system,

results differ dramatically. The use of zeoponics show results in terms of above ground

dry matter produced. These results were obtained from the first generation of zeoponics

and before several technological advances.

Zeoponics can be another suitable support for the plants support on Mars. Also,

table 3.3 shows that the association of that system with different irrigation systems can

have different impacts. Consequently, the use of zeoponics seems to be a solution

depending on how it is used and for which plant it is used.

The system synthesis concerning the water and nutrients delivery showed that the

use of Earth or Mars soils will be a improbable choice. The disadvantages associated with

these substances are too important and more efficient solutions exist. The solutions are

the use of hydroponics and of zeoponics individually or in association.

3.2 System Alternatives

Alternative approaches to the design are achieved by applying a subsystem

Chinese menu approach. Table 3.4 displays various concepts for each subsystem.

Table 3.4: Subsystem Chinese Menu Approach Subsystem Concept 1 Concept 2 Concept 3 Atmospheric Control Earth-supplied Mars-supplied N/A Communications Direct Satellite-Relay N/A Control and Sensory System Centralized Distributed N/A Lighting Direct Concentrated Bulbs/Diodes Plant Growth System Modular Trays Robotic Arm Jungle Mode Power Fuel Cell Solar Cell N/A Structure Flower Multiple Module Tent Thermal Thermal Blanket Solar Heat Pipe Electric Heater

35

The attainment of each alternative approach is measured by the relation to the

objectives that are inherent to the VSD. The alternatives for the subsystems are analyzed

in System Analysis to formulate 3 possible design concepts. The idea behind the

subsystem approach is to generate 3 possible designs that have unique approaches to

maximizing or minimizing the performance and cost objectives in the VSD. The first

design concept seeks to maximize performance while neglecting the cost while the

second concept enforces strict budget requirements. The third concept is derived by

blending the other two concepts into a compromised design.

3.3 Summary

The system synthesis chapter develops three possible design concepts for the

IPGM. The design concepts are developed using the subsystem Chinese menu approach.

The design concepts in this chapter are the preliminary conceptual designs that are

analyzed in the following chapter to determine the optimal design solution.

36

Chapter 4: System Analysis

The System Synthesis chapter introduces the available concepts for each

subsystem as well as narrowing down the options to the most feasible alternatives. The

alternatives listed in Table 5 are further explored and ranked in the System Analysis

chapter. Each subsystem concept is graded on feasibility, reliability, performance and

originality. Grades are assigned on a point basis, with available points being an odd

number from 1-9. A grade of 1 point indicates an inferior design, while 9 is reserved for

a concept that clearly outperforms the remaining alternatives. The feasibility category

ranks the system based on the difficulty of the physical implementation of such a

concept. Designs featuring fewer factors capable of failure receive a higher ranking for

reliability. The performance grade is based on how well the alternative satisfies the goals

identified in Figure 2.1. The originality category rewards alternatives featuring novel

ideas. Although cost is an important factor in space design, the project is not restrained

by an operational budget; thus cost is not included in system analysis. Results are

summarized in tables following each subsystem, and overall IPGM concept is presented

in Table 4.1.

4.1 Atmospheric Control Subsystem

The atmospheric control subsystem is responsible for maintaining the internal

pressure, gas composition and relative humidity favorable for plant growth.

Approximating the growth dome as a hemisphere with a 2 m radius, the total volume

enclosed by the outer shell is 4/6πr3, or 20 m3. Assuming that 20% of the indoor space is

occupied by the supporting hardware, the total volume of air inside the module is 15 m3.

37

At a pressure of 1 atm, the standard atmospheric density, ρ0, is 1.2 kg/m3. At this

condition, the mass of the internal atmosphere is 16.4 kg. This result represents the

minimum weight of gases that needs to be transported from Earth. However, plants

convert carbon dioxide, CO2, and water, H20, into glucose, C6H12O6, and oxygen, O2,

during the photosynthesis. Assuming that the plants convert the entire content of CO2

available daily, the total consumption of carbon dioxide over a seven year period is

approximately 14.7 kg, since air contains only 0.035% of CO2. This calculation does not

account for additional gas consumption generated during growth related processes such

as the production of ethylene.

The RFP requires that the module leakage be kept below 1%. Therefore it is not

possible to bring an additional 14.7 kg of carbon dioxide and periodically release the gas

from a storage tank. The Atmospheric Control Module will need to replenish the

consumed CO2 by recycling waste gases, or by harvesting the Martian atmosphere.

Concept 1 as identified in Table 4.1 relies completely on gas recycling. Concept 2

obtains necessary gases by compressing and separating required gases directly from the

atmosphere.

Concept 1 is simpler and has smaller power requirements. Since it operates

independently of outside conditions, it could be easily tested in a laboratory on Earth.

However, such a system may not be sufficiently effective to recycle gases for seven

years. Concept 2 is not feasible, since the Martian atmosphere is too sparse and does not

contain oxygen and water vapor at useful densities. Carbon dioxide, however, makes up

approximately 95% of the atmosphere, and only a light mechanical gas compressor

would be needed to extract an almost pure CO2. A combination system, which recycles

38

oxygen and water vapor, but supplements the process with the native carbon dioxide,

could be implemented. Such a system would be more feasible than Concept 1 since it is

not known if a recycling system can be developed and effectively perform for the

duration of the mission. A disadvantage of the combination system is that it requires a

gas compressor, which adds to the weight and power requirements. A mechanical

compressor capable of developing 1 atmosphere of pressure on Mars weighs

approximately 17 kg and requires 200 W of power.2 The air could be compressed instead

by utilizing the variation between the day and night temperatures on the planet. Such a

temperature-swing adsorption compressor requires only a minimal power and utilizes

very few moving parts.3 As such, it is reliable but lacks space environment heritage.

Concept 1 receives 5 points for feasibility, because a sufficient recycling system

may not be created. It receives 7 points for reliability since it contains fewer elements

than the combination system. It receives 7 points for performance and 5 for originality.

Concept 2 is not practical and thus receives the lowest rating for feasibility, reliability

and performance. It scores 7 points for originality, since it is a more novel idea than

concepts 1 and 3. Concept 3 receives 7 points for feasibility, since it incorporates a less

demanding recycling unit. Its reliability ranks at 5 points since the addition of the

compressor increases the likelihood of a failure. It receives 5 points for performance

because it performs at a greater power expense than concept 1. Finally the originality

gives 5 points.

39

Table 4.1: Scores of Design Concepts Alternative Feasibility Reliability Performance Originality Total

1) Earth-Supplied 5 7 7 5 24 2) Mars-Supplied 1 1 1 7 10 3) Combination 7 5 5 5 22

4.2 Communication Subsystem

The communication subsystem is responsible for data transfer between the IPGM

and a ground control station based on Earth. As such, it needs to co-operate with the

automation system to receive module status parameters to up-link, as well as to direct the

automation controller to adjust its procedures according to downloaded modifications.

Two alternatives for the subsystem were identified during System Synthesis. Alternative

1 uses a high power antenna to communicate directly with Earth. The second alternative

uses one of the satellites in orbit around Mars as a data relay. The module could also use

the satellite relay during normal operation. A backup antenna would be provided for an

emergency up-link with Earth. This combination would allow the module to report its

status to the ground control in the event of the relay-satellite failure.

All three concepts have been utilized by other space missions, and thus neither

concept is superior in the field of feasibility and originality. Concept 1 receives a grade

of 7 points for reliability, since the system is self-sufficient, and the performance of a

high-gain antenna was verified by the Mars Pathfinder mission. Performance of this

alternative is not satisfactory due to its higher power consumption and lower transfer

rates. Concept 2 is not reliable since it is dependant on secondary sources. The IPGM is

required to grow plants for a period of 5-7 years. A possibility exists that during this time

the relay satellites around Mars would be de-orbited, or would malfunction. Such an

event would terminate the useful life of the module, since it would be impossible to

40

transmit the colony health status information to the ground station. Performance of this

alternative is high, since the system requires only a low power antenna. Concept 3

received 7 points for reliability, since it incorporates a backup high-gain antenna that

would transmit status information to Earth in the event of the relay-satellite malfunction.

The high-gain antenna would be used only in an emergency situation; normal operation

would be performed using a low power antenna. Low power requirements earn this

alternative 7 points for performance.

Table 4.2: System Analysis for Atmospheric Control Subsystem

Alternative Feasibility Reliability Performance Originality Total 1) Direct 5 7 3 5 20 2) Satellite-Relay 5 3 7 5 20 3) Combination 5 7 7 5 24

4.3 Control and Sensory Subsystem

The Control and Sensory System is responsible for collecting data from various

sensors, interpreting obtained data and adjusting internal module conditions to maintain

plant growth. The system is required to operate without user intervention. The control

system can either consist of a central processing unit or can operate as a network of

microcontrollers individually responsible for their subsystems. The first of these

alternatives requires a computer capable of running simple programs consisting of

decision trees and loops. An 80386 processor running the MS-DOS operating system is

capable of performing such operations. A hard-drive unit is used to store data from

sensors. The stored data includes atmospheric conditions, as well as photographs of the

colony. Data is collected and stored until transmitted to Earth on a periodic basis. The

IPGM is controlled by a derivative of the commercially available IBM RAD6000

41

computer, which is radiation-hardened to survive the life cycle on Mars.28 Centralization

of the automation algorithm also simplifies real-time updates to the module. Only a

minimal amount of scientific data on plant growth in space exists; thus the threshold

values controlling the operation of specific subsystems will need to be estimated. After

observing the initial performance of the system, new operating constants may be

uploaded to the control unit. A centralized location of the module control software

allows easy updates by replacing the preset values stored on the computer hard-drive.

The second concept uses multiple microcontrollers responsible for monitoring and

controlling individual subsystems. Such a system can work directly with the analog data

received from the sensors. A drawback of a distributed system is the increased

redundancy of the internal communication infrastructure. Combined length of wiring

would be increased since controllers for subsystems such as lighting and atmospheric

control need to share data from same sensors.

The first alternative, which uses a centralized computer, receives 7 points for

feasibility. The reliability is lower, since a malfunction of the centralized computer

would terminate the mission. This alternative receives 7 points for performance because

the centralized operation allows a better method for control algorithms to cross-

communicate. The centralized location also generates a more compact and more

compatible module. Concept 2 receives 3 points for feasibility, because it is not known if

a system consisting of microcontrollers could effectively collect, process, save and

communicate data. Reliability is higher than for concept 1 because a failure of one

control unit would not necessarily impair the mission. Performance of a distributed

system is kept low because such a system would require redundant wiring and data

42

storage units. Power requirements for a distributed system would be higher, and the great

number of individual components could complicate the module deployment. Both

concepts are given 5 points for originality, since neither demonstrates a novel engineering

idea.

Table 4.3: System Analysis for Control and Sensory Subsystem Alternative Feasibility Reliability Performance Originality Total

1) Centralized 7 5 7 5 24 2) Distributed 3 7 3 5 18 4.4 Lighting Subsystem

Plants require light intensity of at least 50 W/m2 for a 12-hour cycle to sustain

photosynthesis. Mid-day intensity levels around 125 W/m2 are needed for a healthy plant

growth. The lighting subsystem is responsible for maintaining the required light

intensities.

The first alternative for the subsystem works with a transparent dome to allow the

maximum sunlight to reach the colony. Light intensity on Mars is less than on Earth, due

to the greater distance from the Sun. The mean solar constant on Mars is 43.1% of the

1,368 W/m2 available on Earth, or 590 W/m2. This value represents the maximum

intensity available on Mars. The actual magnitude fluctuates during the day. The

lighting module needs to generate supplemental light during the dusk and dawn hours by

incorporating light bulbs, or light emitting diodes. This concept indiscriminately

illuminates the entire module. Such an operation may not be desired, if the growth

module occupies only a small fraction of the total dome volume.

The direct sunlight alternative requires a clear dome. As such, the dome is not

properly isolated. Analysis of heat transfer out of the module presented in the thermal

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subsystem section indicates that an un-insulated dome with outer Kapton shell looses

7.3 kW/m2 of heat. A hemispherical dome with a 2 m radius would lose 275 kW of heat

if left un-insulated, all of which will need to be regenerated by the thermal subsystem.

The power available on Mars is not sufficient to allow such massive heat loss. The

concentrated light alternative works with an opaque dome in which an outer shell is filled

with an insulating material. Figure 4.1 shows the drawing for this concept. The bottom

section of the module houses the support infrastructure in an isolated unit covered with

clear walls. A concentric ring of flat mirrors surrounds the growth section, in a

configuration that resembles a flower, and direct sunlight is reflected into the module

through the clear walls of the basement. The interior of the module is covered with a

highly reflective material such as aluminum foil. Thus, the dome acts as a concave

spherical mirror. The focal length of a spherical mirror is:

2rf = (4-1)

After passing through the focal point, the light begins to diverge. Desired light intensity

can be achieved by altering the distance between the focal point and the growth module.

Simple variation in the design of the growth section would accomplish this; the inflatable

section could form the top third of a sphere, instead of being a hemisphere. This Flower

Concept allows the top section to be fully insulated. Since hot air rises, the top section of

the module is the primary source of heat loss. The concentric ring of flat mirrors is

divided into several segments, each connected to an actuator arm. The backside of the

mirrors is covered with an insulating material. At night, the mirror segments rotate to

cover the clear walls of the module’s basement. This alternative requires a greater

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amount of internal lighting than the concept using a clear module. However, the total

energy expenditure is smaller due to the reduced amount of thermal leakage.

Figure 4.1: Concentrated Light Mode used by the Flower Structural Design

The third alternative for the Lighting System relies completely on light bulbs or

diodes. Such a system would work with a completely opaque dome. The advantage of

this system is its simplicity. A completely enclosed system is easy to implement and

does not require the additional weight associated with flat mirrors and their actuators.

However, it does not demonstrate any adaptability to the Martian environment. This

alternative is also prone to failure since no backup light source is available in the case of

bulb damage.

Concept 1, which uses direct sunlight with supplemental bulbs, receives 7 points

for feasibility due to the simplicity of its design. Reliability receives 7 points because, in

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the case of bulb malfunction, the system would be able to operate in a reduced light mode

using the available sunlight. Performance is inferior due to the large heat leakage

associated with a clear module. Direct sunlight is commonly used in terrestrial

greenhouses; therefore, this alternative received only 3 point for originality. The second

concept, which uses a system of flat mirrors in conjunction with an interior reflective

dome to deliver concentrated sunlight to the Plant Growth Module, requires a greater

amount of engineering, and thus receives 5 points for feasibility. This system is also

more susceptible to failure due to the greater number of components. Yet its

performance is high, as it allows for a great thermal insulation while benefiting from the

naturally available light source. Originality receives 9 points. Third concept, which

provides illumination by solely employing light bulbs and diodes is simple to engineer

and thus receives 7 points for feasibility. The reliability is low since damage to the light

bulbs during the launch/land stage would impair the mission. Performance receives 5

points since the concept allows for thermal insulation but does not benefit from the

available light sources. Lastly, originality receives 3 points since the concept does not

employ any novel ideas.

Table 4.4: System Analysis for Lighting Subsystem

Alternative Feasibility Reliability Performance Originality Total 1) Direct 7 7 1 3 18 2) Concentrated 5 5 7 9 26 3) Bulb/Diode 7 3 5 3 18

4.5 Plant Growth Module

The Plant Growth Module is responsible for germination of seeds, initiation of the

life cycle, delivery of necessary nutrients, removal of mature plants and a restoration of

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consumed nutrients. The possibilities for the modules are identified; of these, two

generate a controlled environment in which the plants are mechanically removed and

composted. The last possibility, named the Jungle Mode, merely initiates the growth.

Plants are then left to reproduce by natural means.

Temperature on Mars rarely reaches the melting point of ice. The low average

temperature results in a layer of permafrost up to 1 km deep. A module which would

utilize Martian soil for plant growth, would need to include a drilling unit as well as a

powerful defroster. Such components are beyond the capabilities of the IPGM.

Therefore, a soil-based horticulture would require soil to be delivered from Earth. Using

soil to grow plants would negatively increase the total payload weight, because the soil as

well as extra nutrients would be required since the nutrients tend to dissipate in the soil.

A hydroponics system eliminates these problems, as it delivers nutrients directly to the

roots, and uses reusable pebbles to provide the basic physical support to the plant body.

Plants are grown in trays filled with solid pebbles. A membrane forming the bottom side

of the tray allows the roots to pass to the nutrient solution flowing below the tray. A

controlled Plant Growth Module needs to remove mature plants from the growth trays as

well as to seed new plants. A general layout for such a module can be as shown in Figure

4.2. Plants are grown in trays, organized in columns. Each column is fed with nutrients

from an individual pipe flowing from a nutrient distribution system. The separation of

nutrient flow into columns allows vegetables with different requirements to be grown by

altering the nutrient concentration for each pipe. This concept also utilizes two nutrient

delivery tanks. Nutrients are passed from one tank to the other, and then the flo