04ices-187 Systems Engineering Evaluation of a Mars Habitat Design Klaus, D., Lloyd, T., Howard, H., Fehring, J., Matthews, D., Ellis, T., Stephens, J., Jairala, J., Rowley, K., Sauers, C., Chluda, H., and Morris, K



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04ICES-187

Systems Engineering Evaluation of a Mars Habitat Design

Klaus, D., Lloyd, T., Howard, H., Fehring, J., Matthews, D., Ellis, T., Stephens, J., Jairala, J., Rowley, K., Sauers, C., Chluda, H., and Morris, K.

Aerospace Engineering Sciences Department, University of Colorado, Boulder, CO 80309 USA

Copyright © 2004 SAE International

ABSTRACT

The overall system architecture of a habitat intended for human occupancy on the surface of Mars was analyzed as part of a graduate aerospace engineering design class at the University of Colorado during the 2003 fall semester. The process was initiated by summarizing and deriving the governing requirements and constraints based on NASA’s “Reference Mission for the Human Exploration of Mars” (Hoffman and Kaplan, 1997; Drake, 1998). With emphasis placed on requirement identification and documentation, a baseline design was established that incorporated functional subsystem definition and analysis of integration factors such as structural layout, mass flows, power distribution, data transmission, etc. In addition, a ‘human-in-the-loop’ focus was stressed by designating a subsystem termed Crew Accommodations. To further support this function, a Mission Operations team was established to ensure that relevant crew health and well being factors were included as integral components of the habitat design and operational planning. Generic human spacecraft design requirements, detailed in the Man-Systems Integration Standards (MSIS, NASA STD-3000 Rev. B, 1995), were incorporated as applicable throughout the process. Results from the integration analysis were used in conjunction with detailed subsystem operational and volumetric requirements to assess compatibility of floor plan options proposed in various existing architectural habitat concepts. The resultant conceptual design, therefore, represents a unique merger of a traditional systems engineering approach with both architectural interests and human factor considerations.



background

project description



Design Reference Mission (DRM) - This design exercise used the basic mission outlined in versions 1.0 (Hoffman and Kaplan, 1997) and 3.0 (Drake, 1998) of the NASA Mars DRM. This document outlines a large-scale extended-stay human mission to Mars consisting of three separate crews over 10 years.

Key Assumptions - This design effort encompassed the surface habitat only, not transit or external equipment. However, external interfaces were included. The habitat was designed to fully function only on the surface of Mars, and will not be inhabited during transit. This design therefore focuses only on the surface operations of the habitat, although relevant aspects of launch, transit, and Mars landing were considered. Also, it was assumed that the mission architecture delineated by the DRM would be present upon arrival of the habitat, including a nuclear reactor providing 160 kW of power, with 25 kW allocated to the habitat. The cabling to transfer the power is included with the reactor. The in situ Resource Utilization (ISRU) plant, along with 2 large pressurized rovers capable of moving the habitat, will be located on the surface. Several small rovers will also be present. The planned launch vehicle will be capable of lifting 80 metric tones (mt) to LEO. Transit, launch, entry, descent, and landing modes were considered, but not included as key drivers for this design.

Other assumptions include that the crew will have the capability to perform EVAs. There could be no dependence on the Crew Transfer Vehicle (CTV). Communication satellites would be in orbit around Mars. Up to a 40-minute communication delay would exist with Earth. Physical profiles of all crewmembers would fall between the 5th percentile Japanese female and the 95th percentile American male human profiles. Environmental factors at the habitat site would be within the conditions found by Viking and Pathfinder (Tillman 2003).



Engineering Requirements - The top-level requirements taken from the DRM are to support a crew of 6 for 600 days without re-supply while maintaining the health and safety of the crew, as well as minimizing the dependency on Earth. Other key requirements include utilizing the 80 mt launch vehicle and deploying the habitat two years before the first crew with a 10-month standby capability between crews.

Design Philosophy - As with all space missions, it was essential to minimize mass, power and cost for this project. However, mass proved a bigger driver than power because the launch capability was well defined, while the power source has yet to be designed. The goal of this design was to focus on an overall systems engineering approach incorporating human factors and infrastructure interfaces. Hardware choices were limited to technologies with TRLs of 7 or better, and subsystems were designed to handle worst-case scenarios to establish a baseline design. The design met the full redundancy required by the DRM without analysis of reliability. Planetary environmental protection and mission justification factors were not considered, as they were deemed programmatic rather than engineering decisions. The design was based heavily on the key DRM requirements, as reevaluation of these top-level requirements was not within the scope of this project.

Baseline Design Description - The habitat has a total pressurized volume of 616 m3, an unused volume of 211 m3, an overall mass of ~68000 kg, and a maximum power consumption of 43 kWe. The overall geometry and structural layout is shown in Figure 1.

Figure 1. Habitat structural layout.



SYSTEMS ENGINEERING – The responsibilities of the Systems Engineering team were to ensure cohesiveness of the habitat design and fulfillment of all mission requirements. Specific tasks include identifying and deriving requirements from the DRM, delegating those requirements to the subsystem teams, reviewing and reconciling subsystem designs and coordinating subsystem interfaces. This team also worked closely with the Mission Operations (MO) group to ensure that consideration of human factors was addressed from the beginning of design. They teamed with project management to oversee, organize and direct the subsystem teams, develop report and presentation templates, establish comprehensive project schedules, conduct meetings, and provide expertise to individual subsystems. Special attention was given to integration of the habitat with the overall mission elements, including rovers, cargo landers, and nuclear power plants.
The design was split into 12 subsystems at the beginning of the project termed ‘Mars or Bust’ (MOB), including Program Management along with Systems Engineering and Integration. The remaining subsystems and their functional descriptions follow. The in situ Resource Utilization (ISRU) interface subsystem is responsible for the interface between the ISRU plant and the habitat’s consumables storage. The Structures subsystem provides a habitable volume and structural supports for the habitat and subsystem components. It is also responsible for the overall layout of the habitat, taking into account mass distribution, radiation protection and thermal considerations. The Electrical Power Management and Allocation (EPMA) subsystem stores and distributes power from the nuclear reactor. The Environmental Control and Life Support system (ECLSS) is responsible for supplying necessary consumables and maintaining a ‘shirtsleeve’ environment. The Thermal Control subsystem is responsible for all thermal control and heat dissipation except the cabin air heat exchanger, which is the responsibility of ECLSS. The Crew Accommodation (CA) subsystem is responsible for incorporating human factors into the design (along with MO) and providing the day-to-day equipment required by the crew for hygiene, maintenance and medical needs. The Command, Control and Communication (C3) subsystem supports and manages the habitat’s data flows by providing data processing and communications equipment. The Robotics and Automation subsystem is responsible for interfacing with mission robotics and designing major structural mechanisms such as the radiator deployment device. The Extravehicular Activity Subsystem (EVAS) is responsible for designing the airlock and the interface between the habitat and the EVA suit and pressurized rover. The Mission Operations team is responsible for scheduling operations, delineating automated and crew-operated tasks, and addressing safety and efficiency concerns.

KEY DESIGN DRIVERS AND CHALLENGES

MARS ENVIRONMENT - The Mars Environment team’s main objective was to collect all environmental parameters for the surface of Mars and show how they pertain to the surface habitat design. A Mars Environment Information Sheet was created and distributed to all other subsystems to ensure consistent parameters were used throughout the design. Table 1 shows relevant parameters for the Martian Environment.

Gravity on Mars is ~1/3 of that on Earth and is basically constant over the planet. The atmospheric pressure varies from 4 mbars to 10 mbars and is also fairly constant over the planet (Tillman, 2003). The surface temperature is dependent on the landing site and was based on the Viking and Pathfinder missions (Tillman, 2003). These temperatures are very site-specific and so once a landing site is chosen, a more refined prediction needs to be obtained in order to prevent over designing the habitat. Radiation is a major concern for a Mars mission. The radiation dosage of 21.2-24.7 cSv came out to be less than the current low earth orbit (LEO) limits of 300 annually for skin and 50 cSv annually for Blood Forming Organ (BFO) dose (Simonsen and Nealy, 1993). BFO dose is the radiation on the organs, which has less tolerance than the skin. This shows that radiation on Mars is less than in LEO, however, the Earth-Mars transit period is of greater concern with respect to radiation exposure. The wind speeds are high compared to Earth, but the actual forces from the wind is less due to the low atmospheric density (Withers, 2002). Wind, therefore, is not a major factor, although dust accumulation from these winds can become a problem on external systems like radiators and solar panels.

Parameters

Maximum

Minimum

Average

Gravity (m/s2)

3.758

3.711

3.735

Atmosphere Pressure (mbars)

10

4

8

Surface Temperature

27

-143

-63

Radiation (BFO) (cSv)

24.7 (22.3)

21.2 (19.7)




Wind Speeds (kph)

36

0




Wind Storm Speeds

127







Table 1: Key Mars Environment Parameters

STRUCTURES - Providing a habitable volume and structural supports for the habitat and subsystem components is a relatively straightforward task, since loads and material properties are generally well known. The primary challenge in this task is finding materials that will fulfill the support requirements with a minimum mass.

The challenge for this design, structurally, was defining an acceptable orientation and overall layout. The baseline design described above is oriented on its side rather than in an upright position, primarily for stability and ease of mobility. This orientation is contrary to the majority of Martian habitat and analog designs that have been published to date. Mars Desert Research Station (The Mars Society, 2003a), Flashline Mars Arctic Research Station (The Mars Society, 2003b), the winning ESA Aurora student design (Fisackerly, et al., 2003), and the Mars Direct design (Zubrin, 1996) are all oriented upright, but have a height half that of the MOB habitat’s length and a larger diameter. One notable example of a habitat design with an orientation similar to the MOB design is the NASA INTEGRITY mission (INTEGRITY, 2003). This program has recently been renamed Advanced Integration Matrix (AIM), and the design may change, but the INTEGRITY habitat design featured several cylinders, similar in orientation to the MOB habitat, connected to each other at their ends by a corridor. Since it is still in initial design phase, published rationale for this orientation was not found. Given the launch vehicle dimensions outlined in the DRM (Hoffman and Kaplan, 1997; Drake, 1998), it was decided that the MOB habitat should be on its side. In addition to improving stability, this limits the stairs necessary to move around within the habitat.

There remain, however, some issues associated with this orientation that were not fully addressed in the MOB design. For instance, the DRM indicates that the habitat will land on its end, in which case the habitat would have to have a single use mechanism to rotate onto its side. The lander was not within the scope of the MOB design, but landing and setup hardware would have to be implemented in the future and will likely have a very large mass. Once the orientation was determined, volume allocations, use of curved wall space, ease of access, equipment noise isolation, systems proximity, center of mass, and radiation shielding were all considered in the layout of the habitat. However, these considerations were not examined in detail and their influence on the orientation of the habitat has yet to be fully determined. These issues must be studied in detail to determine, in a subsequent design stage, whether they could be reasonably met, or if the horizontal orientation selection should be reevaluated.



Some possible alternatives to the MOB habitat design might include designing the habitat to land on its side or a shorter habitat with a larger diameter than prescribed in the DRM (as is the case with most previous designs). These alternative designs would address or eliminate many of the challenges described above.

POWER - The Electrical Power Management and Allocation (EPMA) subsystem stores and distributes power from the nuclear reactor. The power subsystem will manage, distribute and store power throughout the habitat. Both mobile and stationary sources of power will be present within the habitat to provide power for all the functions of the habitat and mission. The primary responsibilities of the EPMA subsystem are to interface with the nuclear power source and other equipment external to the habitat, and to condition power from the onsite nuclear reactor for either distribution throughout the habitat or temporary storage in the mobile and stationary units.

Mission Voltages – High voltage (≥120V AC) is desired to transfer power from the nuclear reactor to the habitat. The MOB habitat voltage is designed to be 120V AC, a decision based primarily on the size of the habitat and the distance that power needed to be transferred within the habitat. However, as a significant amount of COTS technology is designed for 28V DC, a detailed trade study to assess all operating voltages is advisable.

Radiation/Electromagnetic Interference (EMI) Protection – It will be vital to protect the EPMA subsystem from the potentially powerful solar events that may impact the habitat. Recent space storms have disrupted power grids on Earth, and the consequences of such storms on Mars, where the atmospheric protection is much less than that on Earth, may be severe. For this reason it is recommended that the vital equipment be placed along with the communication equipment inside a safe haven where radiation/EMI protection is maximized.

Survival Mode – It may be necessary to perform an EVA during a contingency ‘survival mode’ period in the habitat. Ensuring that power is available for this activity requires that a relatively large amount of power be stored. Hence, this mode may significantly drive the design of the batteries and/or other backup systems.

Habitat Power During Setup – The habitat’s standby mode needs to be virtually power-free, since it may be separated from the nuclear reactor for an indeterminate amount of time after landing on Mars. This will create challenges for all of the other subsystems, particularly those that may require some amount of heat (batteries, consumables, etc.). Power storage may be help with this problem, but careful planning and a high degree of reliability in the Hab/Nuclear Reactor connection process will likely be required for a robust solution.

Minimizing Heat Production – One of the largest concerns in the design of this habitat was the required size of the radiators. Future iterations demand that either the radiator design improve dramatically, or the heat load created by the habitat be reduced. As a large amount of heat within the habitat is generated through cabling and appliance inefficiency, it became apparent that improving overall system power efficiency, thereby minimizing heat generation should be a major driver in the design of this subsystem.

eNVIRONMENTAL CONTROL AND LIFE SUPPORT – ECLSS is comprised of four smaller elements: atmosphere, water, food, and waste management. The integration of these four subsystems is as follows: The Food Subsystem receives potable water from the Water Subsystem. This water is used both to re-hydrate food and for drinking with or without powdered drink mixes. The Water Subsystem also receives water from the Atmosphere Subsystem, which needs to undergo additional treatment to qualify as potable water. The Waste Subsystem is also integrated with the Water Subsystem. When crewmembers eliminate urine, it is then passed to the Water Subsystem for treatment, allowing the ECLSS to reclaim the valuable water for future use. Finally, the ECLSS design integrates collection of waste, by passing waste from the Food Subsystem and Trace Contaminant Control to the Waste Subsystem. The waste from the Food Subsystem will include a combination of packaging plastics and food waste generated during meal preparation and cleanup.

The current ECLSS design is primarily non-regenerative, which results in a large consumable mass demand. It is, therefore, critical to conduct an accurate calculation of consumables required for the mission. For water, this calculation incorporates many interdependent factors because it is an integral part of several habitat and mission processes (i.e. drinking, hydrating food, oxygen production, cooling, showering). This calculation then needs to be factored in with a more detailed analysis of water losses and recollection, such as by vapor leakage from the habitat and by processed urine, respectively. Increasing the efficiency of the water purification system would be the best way to decrease the amount of water needed for the mission. However, the regenerative technologies reviewed in the trade study generally had TRLs of less than 6, which violated the decision that only technologies with TRLs greater than 7 would be incorporated in the design. Extracting water from fecal matter would reduce the water mass even further, but this fraction is relatively small. The launch mass of food for the mission will eventually be able to be reduced once it is demonstrated that crops can be successfully grown on the surface of Mars. In addition, further optimization of ECLSS by increasing consumable recycling and minimizing leakage could result in considerable mass and volume savings.

Other issues remain unresolved. For example, the location of waste storage has yet to be determined. Based on the current design, all waste is stored outside of the habitat, which requires crewmembers to carry it out. If EVAs are not scheduled approximately twice a week, a designated area for waste disposal inside the habitat will be necessary, or an alternate internal location specified. There is also some concern about planet surface contamination if waste is stored outside. Additionally, while advanced life support technologies are continually evolving, the design of this subsystem is limited to technologies with current TRLs of 7 or more. Therefore, although the ECLSS successfully satisfies all the design requirements and assumptions laid out for this project, it is not optimized for future developments.

THERMAL - The thermal subsystem is responsible for maintaining the habitat and all equipment within proper operating temperatures under all mission environmental extremes. This involves collecting heat from or providing heat to all the subsystems and the crew. Any excess heat will be dissipated to the Mars environment. The electronics that are used to run, control and monitor the habitat produce considerable heat.

There are five main components of the thermal subsystem design: cold plates, fluid loops and piping, pumps, heat exchangers, and radiators. The cold plates, collect heat from a local source. This is then transferred into an internal fluid loop and pumped to a heat exchanger. This transfers heat into the external fluid loop, which is then cycled through the radiators where the heat is rejected passively into the environment.



Design Drivers and Challenges - The first major challenge was to determine the heat load of the habitat. This involved determining heat loads from all subsystems in the habitat while they are still in the design phase. As each subsystem is designed, their expected heat loads will change. A first estimate was based on the total power coming into the habitat from the power plant. This estimate, in combination with Martian environment factors, drives the design. However, iterations are required as the subsystems are finalized.

Based on the initial estimated heat load and using the radiation heat transfer equation, the area of required radiative surface can be found from (Cengel, 1998):



where Q is the estimated heat load,  is the Stefan-Boltzman constant,  is the radiator efficiency,  is the emissivity of the radiator, Tr is the radiator operating temperature, and Te is the environment temperature (Larson and Pranke, 2000).

Using the estimated heat load from the other subsystems and the parameters for the Martian environment, a worst-case radiator area was determined. This led to a massive radiator system of nearly 3331 kg. In comparison to the remainder of the thermal system, the panels are a little more than half of the total system mass.

Another challenge with using radiators on the surface of Mars is dust accumulation and performance degradation over time. A more accurate analysis of radiator material degradation in this environment is needed. Also, as dust accumulates, an effective method for cleaning the panels must be developed.



Lessons learned - The second design driver was the Martian environment. It is impossible to design a cost effective habitat to function anywhere on Mars, so a landing site must be selected and profiled. The habitat can then be designed to efficiently operate at this site.

Although there are not many options for collecting heat from the subsystems, there are many ways to reject this heat to the environment. Radiators can quickly become massive depending on the environment in which they are designed to operate and lifetime degradation factors. Alternative methods may be more effective on Mars with lower mass. Optional mechanisms for cooling the habitat are convection, conduction and use of phase change materials.

Convective cooling could be incorporated into the outer skin structure of the habitat. Using conduction to the soil of Mars could also reduce mass. The day / night cycle of Mars may allow for the use of a heat sink inside the structure that could store heat produced during the day and then release it at night when the radiator panels would be much more efficient. Finally, phase change material could be incorporated. During the night, the material would freeze through convection or radiation. Throughout the day, heat from the habitat would slowly warm and melt the material, absorbing sensible and latent stored energy.

CREW ACCOMMODATIONS – The success of the mission is clearly dependent on the ability of the crew to execute their assigned tasks, therefore, maintaining crew mental and physical health is a high priority. The Crew Accommodations (CA) subsystem is responsible for everyday necessities beyond those provided by ECLSS, as needed to support the crew physically and psychologically during all stages of the mission. CA support falls into six main categories: hygiene, habitat maintenance and cleanliness, psychological support, routine and emergency medical supplies, exercise equipment, and habitat ergonomics. Most of these categories utilize hardware that will require crew operations; therefore, CA must work closely with Mission Operations to develop the associated operational plans and to select hardware with human factors in mind. In addition to CA hardware operations, CA and Mission Ops integration must occur to develop activities and schedules that promote crew health.

Design Drivers – The level of autonomy designed into the CA equipment is an important design driver, as there is a critical trade off between mass and crew operations. It was determined by a mass balance trade study that a Mars mission of 600 days should utilize such equipment as dishwashers and clothes washing machines, even though these units require mass and power (Larson and Pranke, 2000). The use of such equipment makes it possible to utilize water the most efficiently and allows for reuse of items such as kitchenware and clothes instead of using disposables (Lynn, 2003).

The Mars gravity of approximately 1/3 g is a significant design driver for CA, since the subsystem requires a variety of active hardware that is affected by gravity. Hardware such as medical equipment, dishwasher, washing machine, clothes dryer, and exercise equipment all must be designed to function effectively in the reduced gravity environment.

Crew psychological health is dependent on the layout and use of habitable volume. Human factors studies suggest that Mars crews will benefit greatly from a large gathering area where meals, meetings, and celebrations can be shared. In addition, it is suggested that crews will benefit from personal quarters, ideally including sleep provisions, personal storage and a desk.

The transmission delay between Mars and Earth of approximately 20 minutes will drive the selection of communications equipment, as crewmembers must utilize methods other than real-time voice loops to stay in touch with friends and family on Earth. As a result, ample personal video, audio, and e-mail transfer to and from Earth must be supported.

Crew selection will drive the design of many CA provisions. Size and gender of crewmembers will affect aspects such as food quantity needs, exercise equipment design, clothing requirements, privacy issues, medical equipment, and toilet designs. This project assumed human body size to fall within the MSIS human profile definition of 5th percentile Japanese female to 95th percentile American male (NASA-STD-3000 rev. B, 1995). If crewmembers were selected from the low end of this range, food and clothing mass alone could be considerably reduced. The reduction, however, may not be worth excluding qualified astronauts who fall in the higher end of the range.

Crew well being will be greatly enhanced through workstation designs that consider human reach profiles, encourage comfortable body posture, and provide adequate lighting for the crew members (Larson and Pranke, 2000). Human factors engineers must work with habitat architects early in the design to ensure these needs are met for the Martian gravity level.

Current designs and reliability numbers suggest that EVAs will pose sufficient risks to not permit EVAs solely for recreational purposes. If this changes over the years with technology, the use of EVA as a primary tool for recreation could reduce the exercise equipment need, consequently reducing mass, design and fabrication time, and space for equipment use and stowage.

Challenges – Human factors and CA will be more crucial in a Mars mission than for any space mission to date, due to the duration and distance from Earth. Therefore, CA must work with the other Habitat subsystems early in the design process to ensure that the integrated system will be capable of supporting the crew physically and psychologically. The simple distance-driven fact that the crew cannot readily abort to Earth in an emergency situation or talk with people on Earth in real-time poses daunting challenges.

Although our project focused on the increment of the mission on the surface of Mars, the transfer to and from Earth brings many psychological concerns for humans. With current propulsion technologies, the crew will endure long-duration exposure to weightlessness and harmful radiation, along with dramatic acceleration load changes before and after each transfer orbit

Careful consideration needs to be given towards the selection of a variety of mission equipment. An area that still needs much attention is the selection of medical equipment and consumables. Due to the mass constraints, medical provisions will be limited. Not all medical emergencies can be foreseen, so developing a limited suite of equipment that can handle almost any condition that could arise is difficult. Another area that presents a challenge is the selection of exercise equipment. Thorough analyses need to be performed to determine what exercise equipment and routines will be most effective in the Mars gravity environment. Attention also needs to be given to the selection of recreational equipment to address needs associated with topics such as news, entertainment, religion, sports, and family.

Although considerable work remains to be done to ensure a physically and psychologically safe mission for the crew, the CA subsystem conceptual design to date did not identify any ‘show-stoppers’.



COMMAND, CONTROL AND COMMUNICATIONS - The C3 subsystem supports and manages the habitat’s data flows. It provides the data processing and communications equipment required to monitor and control the habitat’s environment and subsystems, monitor crew health and safety, communicate with Earth, rovers and EVA crewmembers, and support data processing related to the mission objectives. The C3 subsystem is comprised of two major functional components. The first component is the habitat’s internal computing network (ICN). The ICN manages the data flows required to monitor and control the habitat’s automated systems and provides a computing interface for the crew. The external communications subsystem (EC) handles data flows outside the habitat, including communication with Earth, rovers and EVA crewmembers.

Design Drivers – Two key drivers had the greatest impact on the C3 subsystem design. DRM requirements for real-time communication with rovers and Earth drove the EC system architecture. Human factors-related communication requirements drove C3 subsystem transmission and data processing capacity.

The DRM requires the habitat to permit (near) real-time communication with Earth. This was assumed to refer to continuous link availability because the light delay for radio propagation is unavoidable. An additional requirement for direct communication between the habitat and robotic rovers working beyond the habitat’s direct line of sight was derived from the DRM. To meet these requirements, the EC design assumed an aero-stationary satellite to be in place above the habitat site providing direct communications coverage over approximately 50% of the Martian surface and roughly 50% Earth-Mars link availability. EC architecture, power requirements and throughput estimates are tied to the Mars orbiting communication satellite capabilities.

The C3 subsystem throughput capacity resulted almost exclusively from human factors related communication needs. Communication needs were determined by polling each of the other subsystems. Based on subsystem input, a time-averaged data rate of roughly 11.6 kbps is necessary to meet the habitat’s communication needs. Human factors communications requirements account for 11.1 kbps of the time averaged data rate. Despite the uncertainty in these estimates it is obvious that human factors will account for the majority of the communication load. These communications include personal and mission related transmission directed toward or generated by the crew. These messages consist of large, high data rate voice, video and TCP/IP packets. This communication driver represents a major difference between human and robotic missions, and must therefore be considered early in the C3 design process.

Challenges – A human mission to Mars creates C3 related challenges that must be resolved to help ensure mission success. Three key challenges were identified during the design process.

Flight ready C3 technological options are currently very limited. The C3 subsystem design utilized only flight-qualified technologies currently in use on the International Space Station and Shuttle. The primary benefit of these technologies is their ability to support proven operational needs outside Earth’s environment. Flight-qualified technologies are more mass intensive than their contemporary counterparts and have limited processing capabilities by today’s standards. The result of using flight ready technologies was a complex, dated C3 subsystem that exceeded our mass budget. Newer technologies would likely resolve these issues, but research and testing will be necessary to establish TRL’s greater than 7 and ensure they can meet the demands of a human mission to Mars.

The Earth-based communications infrastructure is currently inadequate to support a human mission to Mars based on needs identified in our analysis. The Deep Space Network (DSN) is the primary means of communicating with spacecraft outside Earth’s immediate vicinity. The DSN is currently overtaxed supporting unmanned missions. It is unlikely that the existing DSN will be able to handle the high communication volume required to support a human mission to Mars, so the existing infrastructure will likely need to be upgraded prior to sending humans to Mars.

Key operational issues also need to be addressed in order to design an appropriate C3 architecture. To date, crewed space missions have relied on near-continuous contact with Earth. This level of communication will be impossible for a manned Mars mission. A Mars based crew will need to function autonomously due to the roughly 20 minute round-trip communications delay and limited Earth-Mars link availability. The operational strategy selected will affect the C3 requirements, so extensive coordination with the mission operations team is required during the design process.

ROBOTICS AND AUTOMATION – Due to the harsh conditions on the Martian surface and the many time consuming and monotonous tasks required, robotics and automation will play a vital role in habitat operations. The robotics and automation subsystem is responsible for designing the automated systems and interfaces that are outside of the scope of other subsystems (Hoffman and Kaplan, 1997). This subsystem is also responsible for designing the interfaces between all robotics systems and the habitat. The habitat’s robotic systems have a major impact on the development of infrastructure for the habitat and operations, including site analysis, habitat assembly, instrument deployment and scientific investigation (Hoffman and Kaplan, 1997). The main robotic systems that will be required for habitat operations are three types of rovers: the small scientific rover, the local unpressurized rover, and the large pressurized rover (Hoffman and Kaplan, 1997).

Two small scientific rovers will be mainly used for exploration. These rovers will be autonomous a majority of the time, but will have the capability to perform telerobotic operations, with the controller stationed in the habitat. Also, the rover will be capable of recharging through solar panels. This type of rover will be required to: deploy scientific instruments, collect and return samples from the Martian surface, determine safe routes for crew travel, and act as a communications relay in contingency situations. The interface between these rovers and the habitat will be minimal. Only data will be transferred to and from the habitat, consisting of telemetry, audio, video, and any necessary data from the scientific instruments. The small rover’s power requirements were estimated at 0.3 kW using the Mars Exploration Rover as a reference point and sizing up.


The cargo carrier will also be bringing one Local Unpressurized Rover (LUR). This rover will be required to provide local transport, ~100 km from the habitat, for EVA crews and their tools. The LUR will be required to operate for 10 hours, with the charge/discharge cycle being under one day. This type of rover will have two interfaces with the habitat for exchanging power and data. Power will be transferred via a direct connection, with the outlet positioned on the outside of the habitat, and the inlet connection placed on the outside of the rover. The rover will also be sending and receiving audio and relaying telemetry information to and from the habitat. The medium rover’s power requirements were estimated to be 2.5 kW using the large pressurized rover as a reference, taking into account that the LUR does not have a life support system, and sizing down.
The third type of rover required for habitat operation is the Large Pressurized Rover (LPR). Two LPRs will be brought to the surface on the first cargo carrier. The LPRs will have critical responsibilities that must be carried out before the first crew arrives, including site preparation, moving, deploying, and inspecting the habitat’s infrastructure, and connection and inspection of the ISRU and power plant. This will be done using 2 mechanical arms and a locomotive system. With all of these tasks being performed without the assistance of EVA crews, the LPR must be able to be fully automated, but will also have telerobotic capabilities. The LPR will interface with the habitat directly, though this is mostly for EVAS concerns. Other interfaces will involve the transfer of data, including telemetry, audio, and video. Weight and power constraints of the large pressurized rover were specified in the DRM at 15.5 metric tons and a 10 kW, respectively.
For the tasks of initially leveling the habitat and radiator deployment, research was conducted on existing commercial off the shelf technology for mass, power and volume estimates. The initial leveling of the habitat will utilize 12 linear actuators with two on each of the six legs for redundancy purposes. The actuators have 720 mm of travel and can produce a total force of 50,000 N. Their mass is 60 kg each for a total of 720 kg and they use 35 W each. Any increase in the amount of travel needed will result in a slight increase in mass. For deployment of the radiator panels, 8 total actuators will be used with 2 on each panel. The actuators have 1 m of travel and produce 7,500 N of force. Their mass is 9 kg each for a total of 72 kg and each uses 5 W.
The following are examples of items also suggested for automation on the habitat. Similar actuators, motors, and servos will be incorporated and sized based on the specific requirements of the task.

  • Automated doors in case of depressurization

  • Deployment of communications hardware

  • External monitoring equipment

  • Deployment of radiator panels

  • Leveling of habitat

  • Compaction of waste

  • Deploy airlock

  • Connection of power plant to habitat and ISRU

  • Connect ISRU to habitat

  • Inspection and maintenance of habitat and ISRU

evaS and planetary surface vehicle interfaces - EVAS is primarily responsible for providing the ability for individual crew members to move around and conduct useful tasks outside the pressurized habitat. It includes any activity performed by a crewmember wearing the EVA suit or operating the large pressurized rover in the Martian environment.

The following components are integrated into the design of the habitat to provide the primary elements that make EVA possible: an EVA suit designed to be used on the surface of Mars and be compatible with other EVA equipment, tools, and transport aids; an umbilical system to provide connections from the habitat to the airlock and rover systems; a large pressurized rover that allows the crew to safely explore distant sites; and an airlock providing the suited crewmembers with the ability to safely transition from the habitat pressure to the Martian surface.

Going through the design process identified several key driving parameters. During the 600 days on Mars, the crew will be performing up to 2 EVAs per week (Hoffman and Kaplan, 1997). With this in mind, an end-to-end timeline is proposed as shown in Figure 2. A main consideration was pre-breathe time for the astronauts. If the habitat was at an internal total pressure of 14.7 psi (1 atm, 21% O2), and the EVA suit has a internal pressure of 4.3 psi (100% O2) as does the current shuttle / ISS space suit, an oxygen pre-breathe time of several hours is needed. This was deemed unacceptable, so keeping in mind that there is a minimum of 50 minutes before depress and egress, the EVAS team calculated that if the internal pressure of the habitat was set at 10.2 psi (30% O2), with the same suit pressure, only 40 minutes of pre-breathe time would be required. So as soon as the astronauts start to don and check their suits, they start to pre-breathe, and by the time the donning procedure is done, they are finished with pre-breathe and ready to work (Larson and Pranke, 2000). This represented a primary design driver for the overall habitat.

Figure 2: Timeline for Airlock Don/Doff Cycle



Spacesuit – The suit was assumed to have an internal pressure of 4.3 psi (based on the current NASA design) in order to safely meet these pre-breathe protocols. If the current pre-breathe time was increased, the internal pressure of the suit could be lowered even further (3.7 psi) to increase the mobility and dexterity of the suit. One of the most important features will be a regenerable, non-venting heat sink. If sublimated water was used to cool the suit, the required mass of water transported from Earth would more than double. A robust, durable suit is also needed to minimize the mass of spare parts needed. This mass will further decrease if parts are designed to be modular and easily interchangeable (especially those that experience the most wear and tear of use).

This design accommodates astronauts with a total of thirteen suits over the 600 days, providing seven backup suits. Based on 2 EVA’s per week, each primary suit would, therefore, have to be able to withstand over 1200 hours of use.



Airlock – The airlock designed by the EVAS team was required to be an independent element capable of being relocated or ‘plugged.’ The airlock will be a solid shell as opposed to the inflatable airlocks that have proposed used in other designs. This was decided upon for durability issues and for ease of relocation during the mission. One of the major drivers for the EVAS team was to ensure that in case of fire or rapid depress, it must always be possible to get all astronauts out of the habitat. With this in mind, it was decided that there would be three airlocks, two operational on the bottom floor and one emergency/back up airlock on the second floor. The third airlock ensures that astronauts on the second floor (living quarters) could easily escape the habitat. In addition to the emergency airlock, each airlock is equipped with three EVA suits, two operational and one emergency/backup suit. The emergency suit will be sized so that any of the six crewmembers can fit into it. The dimensions of the airlock are 4 m long, 3.5 m wide, and 2.5 m high, with a total volume of 35 m3. The choice of this design was to ensure enough room for donning, doffing, storing, and servicing the EVA suits.

The main door to the surface of Mars is the airlock. It will provide the astronauts with the ability to access the surface at a high cost: the possible introduction of toxic particulate contaminants through the airlock and into the astronaut living area (Shidemantle et. al., 2003). If not controlled, this permeation may cause health concerns (such as cancer which may be induced by breathing the hexavelant chromium within the dust) as well as equipment failure. For this reason, the airlock must be designed to incorporate a particulate cleaning system.



In Situ Resource Utilization - The responsibility of the ISRU team was to design the interfaces between the in situ propellant production (ISPP) plant and the habitat. The ISPP will provide additional reserve of 4.5 mt of oxygen, 3.9 mt of nitrogen and 23.2 mt of water, as specified by the DRM (Hoffman and Kaplan, 1997). For the surface habitat designed in this document, all consumable mass is being launched with the habitat. The ISPP plant will demonstrate that these consumables can be produced in a timely, efficient manner. This demonstration will be monitored and will be used primarily to save mass for future missions to Mars. The first crew may use the consumable reserve if it is needed.

Key Design Drivers and Challenges - The ISRU subsystem major design tasks were to transfer the consumables, oxygen, nitrogen and by-products such as water, safely into to the habitat. One key design driver for the ISRU was transferring the consumables from the un-pressurized Martian environment through the pressurized shell of the habitat cabin. Another driver was keeping the pipes insulated from the varying diurnal temperatures. Redundant safety valves must also be implemented in the design to keep the habitat pressurized in case of failure in the pipes.

The challenges involved with designing the ISRU are to choose the most efficient ISPP, understand the inputs and outputs of the plant, and then design the interfaces. Another design challenge is location of the habitat relative to the ISPP. This distance drives the method used to transfer the consumables to the habitat. Possible methods considered were pumping of the consumables directly to the habitat or transferring via the rover’s storage tanks.

Using Martian soil can be a very efficient way to build radiation shielding or provide other forms of insulation (Larson and Pranke, 2000). Soil could either be poured into bags or combined with water and heated in a kiln to form bricks. Subsequent Mars missions might take advantage of the abundant iron, silicon, and other metals in the soil as well.

Using the ISPP to provide consumables to the crew will save on overall mass of the mission and thus bring the cost of the mission down. This mass and cost savings is proportional to which ISPP and which method of transfer is chosen.

MISSION OPERATIONS AND HUMAN FACTORS - Mission Operations is an element often left out of academic design classes, where the focus tends to be placed on designing “hardware” subsystems. Because of the length of this mission, however, it was deemed imperative to include human factors considerations as encompassed by the MO and Crew Accommodations subsystems. By including these “subsystems” at the beginning of the design process, the habitat design can be further optimized with respect to crew time, safety, and comfort.

The purpose of the MO team is to oversee all activities during the mission. These activities may be manual, automated, or Earth-controlled, and will include a wide range of functions, such as maintenance of crew psychological and physiological health, science investigations, habitat maintenance and operations, public relations, and communications and data transfer with Earth.

The primary task of the MO support team is to identify and coordinate operations within each of these functions, and schedule activities so that stated mission goals are achieved. The MO team works closely with the design engineers to establish clear hardware operational requirements incorporating human factors and scheduling considerations and to revise these requirements as needed. Before and during the mission, the MO team is responsible for creating and modifying the operations schedule (in concert with the engineering and science teams), developing procedures for all operations and failure scenarios, identifying and delivering relevant system status data to the crew, and working with crew during the mission to identify and respond to any off-nominal situations that may arise. Ultimately, the MO team will help to engineer a habitat that meets all physiological and psychological needs of the crew, in addition to developing and implementing a comprehensive mission activity schedule that leads to the successful achievement of the mission goals.

For this four-month class exercise, the scope of the MO team was limited to developing operations within the habitat (as it was being designed), but excluded crew operations during transit or training. Consideration was given, however, to automated operations that may occur within the habitat during transit and to functions of the ground operations crew.

There were two key drivers for the MO team. The first was to integrate human factors considerations from the beginning of the design, rather than “forcing” them in at the end. This philosophy allows the crew’s time, comfort, and safety, and therefore the overall mission’s success, to be optimized by design. The second design driver was the time delay for communication between Earth and Mars and resupply opportunities. Because real-time communication and control are not possible, and flights bringing cargo or crew from Earth are separated by years, the crew and habitat must attain an unprecedented level of independence.

As a result of this project, some important points were brought to light which should be included as key considerations in future design efforts.



Operations Scheduling – A vital component enabling MO’s integration with all subsystems was the operations (ops) list. This list defined scheduling of activities, which in turn allowed the MO team to ensure that all activities could be accomplished within a specified period (day, week, month, mission). The ops list helped to determine which operations should be automated, Earth-controlled, or performed by the crew, by elucidating factors such as ease, frequency and duration of the task, as well as any related safety concerns.

Each of the subsystems provided MO with a list of operational activities that they expected to require during the mission. Each operation was then designated as either automated, Earth-controlled, requiring crew, or some combination of the three. In addition to the tasks required by each subsystem, there were a number of activities required by the overall mission that were not included under any specific subsystem. These activities, which include obvious elements such as sleeping, eating, and cleaning, as well as less obvious tasks such as collecting medical data from crewmembers and writing e-mails to family, were gathered into a separate ops list. MOB compiled a list of 153 maintenance and day-to-day type tasks that need to be scheduled throughout the mission.

Once all of these mission operations were catalogued, they were consolidated into representative mission timelines. These timelines were a useful tool that provided a reality check of the expectations put upon the crew to perform maintenance, science, and all other mission tasks. In a habitat that required too much crew time for maintenance, the creation of a representative timeline could expose design shortcomings that may otherwise be missed. It became apparent that mission scheduling in this manner needed to be concurrently iterated as the design evolved.

Crew Day/Night Schedules – Additional trade studies need to be conducted to optimize the crew’s day/night schedules. MOB chose a crew schedule that is synchronized with the Mars day/night cycle. In each daily schedule, the six crew members are divided into three groups, and each group follows separate schedules, which are similar but shifted in starting and ending times (wake and sleep times) by a period of 15 minutes. This division of schedules allows time for the Mission Commander (MC) and Second-in-Command (SIC) to engage in mission planning and other activities special to the crew commanders, while the slight shifts in the schedules helps to avoid potential “traffic jams” in areas such as the bathroom and laundry room. It was thought that the “all-awake, all-asleep” (or four-A) schedule would be easiest on the crew psychologically, in addition to minimizing sleep interference from “awake” crew members showering, doing laundry, etc. This type of schedule should be carefully weighed against a schedule that rotates the crew through 8-hour sleep shifts so that someone is always awake in the habitat, which does provide an element of safety that may outweigh the benefits of a “four-A” schedule. Thus, a careful trade study should be done before opting for the specific type of schedule. Circadian rhythm desynchronization may pose long-term adaptation issues, since the Martian day differs from Earth’s.

Ground Support / Crew Command Architecture – At this early stage of development, it is difficult to clearly plan the separation of duties between the ground (Earth-based) segment and Mars-based segment of the operations team. However, it is important to begin to delineate tasks between these two segments.

It is expected that the ground operations shifts will be set up in a similar hierarchical structure to the ISS operations team, with the notable exception that the chain of command from the operations team to the Mars-based astronauts will be hampered by the up to 25-minute one-way time delay. Because of this delay, real-time conversations between the ground segment and the astronauts will not be possible. Any ground control/crew architecture will need to take into account the up to 25-minute one-way time delay, as well as the usage of the Deep Space Network (DSN) for communication with Mars. The different duration of days also poses a unique problem in keeping both crews in synch.



Due to the time delay, as well as the bandwidth limitations, the ground team will not be monitoring all data at all times. Rather, they will monitor key telemetry items from each subsystem in near real time, and analyze long-term trends using more complete data downlinked at regular intervals. Telemetry, including sensors and data types, should be designed with this in mind.

Automated vs. Crewed Processes – Consideration needs to be given for determining an appropriate balance between automated and crewed processes throughout the entire mission. Ideally, a maximum number of processes would be automated so that the crew’s time could be utilized for value-added activities, yet these automated processes should still be able to be overridden and controlled by the crew to ensure safety and reliability. Optimization of this balance is key to mission success, and may differ from previous space missions in terms of the unique demands associated with the transit period versus surface operations.

Specific Design Influences – In addition to the general considerations noted above, integration of MO from the beginning of the project influenced a number of specific elements of the habitat design in ways that may have been overlooked and are relevant to future endeavors.

  • Consideration of space suit design parameters and expected number of EVAs drove the decision to keep the habitat internal pressure at 10.2 psi, because it significantly reduces pre-breathe time.

  • The habitat will be deployed horizontally. This orientation was chosen for a number of structural and MO reasons. As a result of the horizontal orientation, the crew will have fewer stairs to deal with, and in the event of an emergency they can potentially egress from a second story airlock, which may not be an option in a vertical configuration. This layout was also considered to allow a more open floor plan than would a vertical configuration. It is important to remember that in a mission of this magnitude, psychological health of the crew will be a significant factor for success.

  • ECLSS waste disposal will not recycle feces. Though recycling feces would create a more closed life support system, the crew’s psychological discomfort from eating or drinking recycled waste is a very real concern. Conversely, waste storage or disposal is equally an issue yet to be fully resolved.

  • C3’s data flows were driven by MO’s need for high-bandwidth communication, such as audio and video; if MO were not integrated into the design from the beginning this driver might have been missed.

  • Numerous hardware choices will be affected by MO considerations. For example, radiator performance is expected to degrade when covered in dust after a Martian storm, and they will therefore need cleaning. Cleaning can be automated or manual, which will be a factor in the radiator trade study.

DESIGN APPROACH SUMMARY

The design process undertaken in this semester long effort highlighted several significant aspects of how closely the engineering requirements for a planetary base depend upon exact mission parameters (number of crew, duration, mission and scientific objectives, etc.). Optimization of the life support system, trades between recycling and single use elements (as diverse as from oxygen, food and water to clothing and packaging material), selection of the habitat and space suit operating pressures, enabling communication needs and thermal control, are among the many variables that cannot begin to be adequately addressed without specific mission objectives having been identified. Going to the moon or Mars will likely drive ultimate designs in very different directions, although the fundamental goals are virtually identical – to keep humans alive and healthy, both physiologically and psychologically, in a hostile, alien environment far from Earth. Giving due consideration to systems engineering analyses, architectural guidelines and human factor concerns concurrently from the initial mission planning stages through actual operations will allow the future human exploration of space to proceed as safely and efficiently as possible.



Acknowledgments

The authors would like to acknowledge the efforts of their classmates S. Baker, E. Dekruif, B. Duenas, K. Hill, N. Kungsakawin, E. Schleicher, M. Silbaugh, J. Uchida and T. White; and also thank the following individuals for their assistance in technical matters and design reviews: D. Anderson (Mars Society), M. Benoit (CU), B. Clark (Lockheed Martin), J. Clawson (CU), C. Craig (Lockheed Martin), T. Gasparrini (Lockheed Martin), A. Hoehn (CU), K. Mankoff (Honeybee Robotics), R. McCall (CU and NexTerra), T. Muscatello (Mars Society and Pioneer Astronautics), S. Price (Lockheed Martin), J. Russell, (CU), K. Stroud (CU) and R. Zubrin (Mars Society and Pioneer Astronautics).



References

  1. Drake, BG, ed. Reference Mission Version 3.0: Addendum to the Human Exploration of Mars: The Reference Mission of the NASA Mars Exploration Study Team. Lyndon B. Johnson Space Center, Houston, TX: June 1998.

  2. Fisackerly, et al. Cranfield Aurora: Mars Excursion Module. Cranfield University; 1st Aurora Student Design Contest, Barcelona: 2003.

  3. Hoffman, SJ and Kaplan, DL. Human Exploration of Mars: The Reference Mission of the NASA Mars Exploration Study Team. Lyndon B. Johnson Space Center, Houston, TX: July 1997.

  4. “INTEGRITY Team Kickoff Meeting.” PowerPoint Presentation. NASA JSC INTEGRITY: 26 Feb. 2003.

  5. Larson, WJ. and Pranke, LK. Human Spaceflight Mission Analysis and Design. The Mc-Graw-Hill Companies, Inc., New York: 2000.

  6. Lynn, V. Peace in Space. theGuardians.com, 6 Dec. 2003

  7. Man-Systems Integration Standards. NASA-STD-3000. NASA Johnson Space Center, Houston, TX: 1995 < http://msis.jsc.nasa.gov/>

  8. Simonsen, L. C., and Nealy, J. E. Mars Surface Radiation Exposure for Solar Maximum Conditions and 1989 Solar Proton Events. NASA Technical Paper 3300. NASA Langley Research Center, Hampton, Virginia: 1993.

  9. The Mars Society: Mars Desert Research Station. The Mars Society: 13 Dec. 2003a

  10. The Mars Society: The Flashline Mars Arctic Research Station (FMARS) 2003 Field Season. The Mars Society: 13 Dec. 2003b

  11. Tillman, J. E. “Mars Atmospheric Pressure Overview.” 1 Oct. 2003

  12. Withers, P. “Winds in the Martian upper atmosphere from MGS aerobraking density profiles.” Eos Trans. AGU, 83(47), Fall Meet. Suppl., 2002.

  13. Zubrin, R. The Case For Mars: The Plan to Settle The Red Planet and Why We Must. Touchstone, New York: 1996.



CONTACT

Dr. Klaus is an Assistant Professor in the Aerospace Engineering Sciences Department at the University of Colorado, Boulder. (email: klaus@colorado.edu)



Additional Sources

Course: http://www.colorado.edu/ASEN/asen5158



Project: http://www.colorado.edu/ASEN/project/mob

Definitions, Acronyms, Abbreviations

AIM: Advanced Integration Matrix

BFO: Blood-Forming Organ

C3: Command, Control, and Communication subsystem

CA: Crew Accommodations

COTS: Commercial Off-The-Shelf

CTV: Crew Transfer Vehicle

CU: University of Colorado (Boulder)

DRM: Design Reference Mission

DSN: Deep Space Network

EC: External Communications

ECLSS: Environmental Control and Life Support Subsystem

ESA: European Space Agency

EVA: Extravehicular Activity

EVAS: Extravehicular Activity Subsystem

ICN: Internal Computing Network

INTEGRITY: Integrated Human Exploration Mission Simulation Facility (now AIM)

ISPP: in situ Propellant Production

ISRU: in situ Resource Utilization

LEO: Low Earth Orbit

LPR: Large Pressurized Rover

LUR: Local Unpressurized Rover

MO: Mission Operations

MOB: ‘Mars Or Bust’

MSIS: Man-Systems Integration Standards

PMDU: Power Management and Distribution Unit

TRL: Technology Readiness Level

URL: Uniform Resource Locator (internet)

VAC: Volts, Alternating Current

VDC: Volts, Direct Current


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