Systems Engineering Evaluation of a Mars Habitat Design s1
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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 Key Assumptions - This design effort encompassed the surface habitat only, not transit or external equipment. The overall system architecture of a habitat intended for However, external interfaces were included. The habitat human occupancy on the surface of Mars was analyzed was designed to fully function only on the surface of as part of a graduate aerospace engineering design Mars, and will not be inhabited during transit. This class at the University of Colorado during the 2003 fall design therefore focuses only on the surface operations semester. The process was initiated by summarizing and of the habitat, although relevant aspects of launch, deriving the governing requirements and constraints transit, and Mars landing were considered. Also, it was based on NASA’s “Reference Mission for the Human assumed that the mission architecture delineated by the Exploration of Mars” (Hoffman and Kaplan, 1997; Drake, DRM would be present upon arrival of the habitat, 1998). With emphasis placed on requirement including a nuclear reactor providing 160 kW of power, identification and documentation, a baseline design was with 25 kW allocated to the habitat. The cabling to established that incorporated functional subsystem transfer the power is included with the reactor. The in definition and analysis of integration factors such as situ Resource Utilization (ISRU) plant, along with 2 large structural layout, mass flows, power distribution, data pressurized rovers capable of moving the habitat, will be transmission, etc. In addition, a ‘human-in-the-loop’ located on the surface. Several small rovers will also be focus was stressed by designating a subsystem termed present. The planned launch vehicle will be capable of Crew Accommodations. To further support this function, lifting 80 metric tones (mt) to LEO. Transit, launch, entry, a Mission Operations team was established to ensure descent, and landing modes were considered, but not that relevant crew health and well being factors were included as key drivers for this design. included as integral components of the habitat design and operational planning. Generic human spacecraft Other assumptions include that the crew will have the design requirements, detailed in the Man-Systems capability to perform EVAs. There could be no Integration Standards (MSIS, NASA STD-3000 Rev. B, dependence on the Crew Transfer Vehicle (CTV). 1995), were incorporated as applicable throughout the Communication satellites would be in orbit around Mars. process. Results from the integration analysis were used Up to a 40-minute communication delay would exist with in conjunction with detailed subsystem operational and Earth. Physical profiles of all crewmembers would fall volumetric requirements to assess compatibility of floor between the 5th percentile Japanese female and the 95th plan options proposed in various existing architectural percentile American male human profiles. Environmental habitat concepts. The resultant conceptual design, factors at the habitat site would be within the conditions therefore, represents a unique merger of a traditional found by Viking and Pathfinder (Tillman 2003). systems engineering approach with both architectural interests and human factor considerations. Engineering Requirements - The top-level requirements taken from the DRM are to support a crew of 6 for 600 BACKGROUND days without re-supply while maintaining the health and safety of the crew, as well as minimizing the dependency PROJECT DESCRIPTION on Earth. Other key requirements include utilizing the 80 mt launch vehicle and deploying the habitat two years Design Reference Mission (DRM) - This design exercise before the first crew with a 10-month standby capability used the basic mission outlined in versions 1.0 (Hoffman between crews. and Kaplan, 1997) and 3.0 (Drake, 1998) of the NASA Mars DRM. This document outlines a large-scale Design Philosophy - As with all space missions, it was extended-stay human mission to Mars consisting of essential to minimize mass, power and cost for this three separate crews over 10 years. 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 The design was split into 12 subsystems at the goal of this design was to focus on an overall systems beginning of the project termed ‘Mars or Bust’ (MOB), engineering approach incorporating human factors and including Program Management along with Systems infrastructure interfaces. Hardware choices were limited Engineering and Integration. The remaining subsystems to technologies with TRLs of 7 or better, and subsystems and their functional descriptions follow. The in situ were designed to handle worst-case scenarios to Resource Utilization (ISRU) interface subsystem is establish a baseline design. The design met the full responsible for the interface between the ISRU plant and redundancy required by the DRM without analysis of the habitat’s consumables storage. The Structures reliability. Planetary environmental protection and subsystem provides a habitable volume and structural mission justification factors were not considered, as they supports for the habitat and subsystem components. It is were deemed programmatic rather than engineering also responsible for the overall layout of the habitat, decisions. The design was based heavily on the key taking into account mass distribution, radiation protection DRM requirements, as reevaluation of these top-level and thermal considerations. The Electrical Power requirements was not within the scope of this project. Management and Allocation (EPMA) subsystem stores and distributes power from the nuclear reactor. The Baseline Design Description - The habitat has a total Environmental Control and Life Support system (ECLSS) pressurized volume of 616 m3, an unused volume of 211 is responsible for supplying necessary consumables and m3, an overall mass of ~68000 kg, and a maximum maintaining a ‘shirtsleeve’ environment. The Thermal power consumption of 43 kWe. The overall geometry Control subsystem is responsible for all thermal control and structural layout is shown in Figure 1. 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. Figure 1. Habitat structural layout. KEY DESIGN DRIVERS AND CHALLENGES
MARS ENVIRONMENT - The Mars Environment team’s main objective was to collect all environmental SYSTEMS ENGINEERING – The responsibilities of the parameters for the surface of Mars and show how they Systems Engineering team were to ensure cohesiveness pertain to the surface habitat design. A Mars of the habitat design and fulfillment of all mission Environment Information Sheet was created and requirements. Specific tasks include identifying and distributed to all other subsystems to ensure consistent deriving requirements from the DRM, delegating those parameters were used throughout the design. Table 1 requirements to the subsystem teams, reviewing and shows relevant parameters for the Martian Environment. reconciling subsystem designs and coordinating subsystem interfaces. This team also worked closely Gravity on Mars is ~1/3 of that on Earth and is basically with the Mission Operations (MO) group to ensure that constant over the planet. The atmospheric pressure consideration of human factors was addressed from the varies from 4 mbars to 10 mbars and is also fairly beginning of design. They teamed with project constant over the planet (Tillman, 2003). The surface management to oversee, organize and direct the temperature is dependent on the landing site and was subsystem teams, develop report and presentation based on the Viking and Pathfinder missions (Tillman, templates, establish comprehensive project schedules, 2003). These temperatures are very site-specific and so conduct meetings, and provide expertise to individual once a landing site is chosen, a more refined prediction subsystems. Special attention was given to integration of needs to be obtained in order to prevent over designing the habitat with the overall mission elements, including the habitat. Radiation is a major concern for a Mars rovers, cargo landers, and nuclear power plants. 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 design may change, but the INTEGRITY habitat design Blood Forming Organ (BFO) dose (Simonsen and Nealy, featured several cylinders, similar in orientation to the 1993). BFO dose is the radiation on the organs, which MOB habitat, connected to each other at their ends by a has less tolerance than the skin. This shows that corridor. Since it is still in initial design phase, published radiation on Mars is less than in LEO, however, the rationale for this orientation was not found. Given the Earth-Mars transit period is of greater concern with launch vehicle dimensions outlined in the DRM (Hoffman respect to radiation exposure. The wind speeds are high and Kaplan, 1997; Drake, 1998), it was decided that the compared to Earth, but the actual forces from the wind is MOB habitat should be on its side. In addition to less due to the low atmospheric density (Withers, 2002). improving stability, this limits the stairs necessary to Wind, therefore, is not a major factor, although dust move around within the habitat. accumulation from these winds can become a problem on external systems like radiators and solar panels. There remain, however, some issues associated with this orientation that were not fully addressed in the MOB Parameters Maximum Minimum Average design. For instance, the DRM indicates that the habitat will land on its end, in which case the habitat would have Gravity (m/s2) 3.758 3.711 3.735 to have a single use mechanism to rotate onto its side. The lander was not within the scope of the MOB design, Atmosphere 10 4 8 but landing and setup hardware would have to be Pressure (mbars) implemented in the future and will likely have a very large mass. Once the orientation was determined, Surface 27 -143 -63 volume allocations, use of curved wall space, ease of Temperature access, equipment noise isolation, systems proximity, center of mass, and radiation shielding were all Radiation (BFO) 24.7 21.2 considered in the layout of the habitat. However, these (cSv) (22.3) (19.7) 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 Wind Speeds (kph) 36 0 to determine, in a subsequent design stage, whether they could be reasonably met, or if the horizontal Wind Storm 127 orientation selection should be reevaluated. Speeds Some possible alternatives to the MOB habitat design Table 1: Key Mars Environment Parameters 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. STRUCTURES - Providing a habitable volume and structural supports for the habitat and subsystem POWER - The Electrical Power Management and components is a relatively straightforward task, since Allocation (EPMA) subsystem stores and distributes loads and material properties are generally well known. power from the nuclear reactor. The power subsystem The primary challenge in this task is finding materials will manage, distribute and store power throughout the that will fulfill the support requirements with a minimum habitat. Both mobile and stationary sources of power mass. will be present within the habitat to provide power for all the functions of the habitat and mission. The primary The challenge for this design, structurally, was defining responsibilities of the EPMA subsystem are to interface an acceptable orientation and overall layout. The with the nuclear power source and other equipment baseline design described above is oriented on its side external to the habitat, and to condition power from the rather than in an upright position, primarily for stability onsite nuclear reactor for either distribution throughout and ease of mobility. This orientation is contrary to the the habitat or temporary storage in the mobile and majority of Martian habitat and analog designs that have stationary units. been published to date. Mars Desert Research Station (The Mars Society, 2003a), Flashline Mars Arctic Mission Voltages – High voltage (≥120V AC) is desired Research Station (The Mars Society, 2003b), the to transfer power from the nuclear reactor to the habitat. winning ESA Aurora student design (Fisackerly, et al., The MOB habitat voltage is designed to be 120V AC, a 2003), and the Mars Direct design (Zubrin, 1996) are all decision based primarily on the size of the habitat and oriented upright, but have a height half that of the MOB the distance that power needed to be transferred within habitat’s length and a larger diameter. One notable the habitat. However, as a significant amount of COTS example of a habitat design with an orientation similar to technology is designed for 28V DC, a detailed trade the MOB design is the NASA INTEGRITY mission study to assess all operating voltages is advisable. (INTEGRITY, 2003). This program has recently been renamed Advanced Integration Matrix (AIM), and the Radiation/Electromagnetic Interference (EMI) Protection The current ECLSS design is primarily non-regenerative, – It will be vital to protect the EPMA subsystem from the which results in a large consumable mass demand. It is, potentially powerful solar events that may impact the therefore, critical to conduct an accurate calculation of habitat. Recent space storms have disrupted power consumables required for the mission. For water, this grids on Earth, and the consequences of such storms on calculation incorporates many interdependent factors Mars, where the atmospheric protection is much less because it is an integral part of several habitat and than that on Earth, may be severe. For this reason it is mission processes (i.e. drinking, hydrating food, oxygen recommended that the vital equipment be placed along production, cooling, showering). This calculation then with the communication equipment inside a safe haven needs to be factored in with a more detailed analysis of where radiation/EMI protection is maximized. water losses and recollection, such as by vapor leakage from the habitat and by processed urine, respectively. Survival Mode – It may be necessary to perform an EVA Increasing the efficiency of the water purification system during a contingency ‘survival mode’ period in the would be the best way to decrease the amount of water habitat. Ensuring that power is available for this activity needed for the mission. However, the regenerative requires that a relatively large amount of power be technologies reviewed in the trade study generally had stored. Hence, this mode may significantly drive the TRLs of less than 6, which violated the decision that only design of the batteries and/or other backup systems. technologies with TRLs greater than 7 would be incorporated in the design. Extracting water from fecal Habitat Power During Setup – The habitat’s standby matter would reduce the water mass even further, but mode needs to be virtually power-free, since it may be this fraction is relatively small. The launch mass of food separated from the nuclear reactor for an indeterminate for the mission will eventually be able to be reduced amount of time after landing on Mars. This will create once it is demonstrated that crops can be successfully challenges for all of the other subsystems, particularly grown on the surface of Mars. In addition, further those that may require some amount of heat (batteries, optimization of ECLSS by increasing consumable consumables, etc.). Power storage may be help with recycling and minimizing leakage could result in this problem, but careful planning and a high degree of considerable mass and volume savings. reliability in the Hab/Nuclear Reactor connection process will likely be required for a robust solution. Other issues remain unresolved. For example, the location of waste storage has yet to be determined. Minimizing Heat Production – One of the largest Based on the current design, all waste is stored outside concerns in the design of this habitat was the required of the habitat, which requires crewmembers to carry it size of the radiators. Future iterations demand that either out. If EVAs are not scheduled approximately twice a the radiator design improve dramatically, or the heat week, a designated area for waste disposal inside the load created by the habitat be reduced. As a large habitat will be necessary, or an alternate internal location amount of heat within the habitat is generated through specified. There is also some concern about planet cabling and appliance inefficiency, it became apparent surface contamination if waste is stored outside. that improving overall system power efficiency, thereby Additionally, while advanced life support technologies minimizing heat generation should be a major driver in are continually evolving, the design of this subsystem is the design of this subsystem. limited to technologies with current TRLs of 7 or more. Therefore, although the ECLSS successfully satisfies all ENVIRONMENTAL CONTROL AND LIFE SUPPORT – the design requirements and assumptions laid out for ECLSS is comprised of four smaller elements: this project, it is not optimized for future developments. atmosphere, water, food, and waste management. The integration of these four subsystems is as follows: The THERMAL - The thermal subsystem is responsible for Food Subsystem receives potable water from the Water maintaining the habitat and all equipment within proper Subsystem. This water is used both to re-hydrate food operating temperatures under all mission environmental and for drinking with or without powdered drink mixes. extremes. This involves collecting heat from or providing The Water Subsystem also receives water from the heat to all the subsystems and the crew. Any excess Atmosphere Subsystem, which needs to undergo heat will be dissipated to the Mars environment. The additional treatment to qualify as potable water. The electronics that are used to run, control and monitor the Waste Subsystem is also integrated with the Water habitat produce considerable heat. Subsystem. When crewmembers eliminate urine, it is then passed to the Water Subsystem for treatment, There are five main components of the thermal allowing the ECLSS to reclaim the valuable water for subsystem design: cold plates, fluid loops and piping, future use. Finally, the ECLSS design integrates pumps, heat exchangers, and radiators. The cold plates, collection of waste, by passing waste from the Food collect heat from a local source. This is then transferred Subsystem and Trace Contaminant Control to the Waste into an internal fluid loop and pumped to a heat Subsystem. The waste from the Food Subsystem will exchanger. This transfers heat into the external fluid include a combination of packaging plastics and food loop, which is then cycled through the radiators where waste generated during meal preparation and cleanup. the heat is rejected passively into the environment. Design Drivers and Challenges - The first major and then release it at night when the radiator panels challenge was to determine the heat load of the habitat. would be much more efficient. Finally, phase change This involved determining heat loads from all material could be incorporated. During the night, the subsystems in the habitat while they are still in the material would freeze through convection or radiation. design phase. As each subsystem is designed, their Throughout the day, heat from the habitat would slowly expected heat loads will change. A first estimate was warm and melt the material, absorbing sensible and based on the total power coming into the habitat from latent stored energy. the power plant. This estimate, in combination with Martian environment factors, drives the design. CREW ACCOMMODATIONS – The success of the However, iterations are required as the subsystems are mission is clearly dependent on the ability of the crew to finalized. execute their assigned tasks, therefore, maintaining crew mental and physical health is a high priority. The Based on the initial estimated heat load and using the Crew Accommodations (CA) subsystem is responsible radiation heat transfer equation, the area of required for everyday necessities beyond those provided by radiative surface can be found from (Cengel, 1998): ECLSS, as needed to support the crew physically and psychologically during all stages of the mission. CA Q support falls into six main categories: hygiene, habitat A 4 4 maintenance and cleanliness, psychological support, ( T r T r ) routine and emergency medical supplies, exercise equipment, and habitat ergonomics. Most of these where Q is the estimated heat load, is the Stefan- categories utilize hardware that will require crew Boltzman constant, is the radiator efficiency, is the operations; therefore, CA must work closely with Mission emissivity of the radiator, Tr is the radiator operating Operations to develop the associated operational plans temperature, and Te is the environment temperature and to select hardware with human factors in mind. In (Larson and Pranke, 2000). addition to CA hardware operations, CA and Mission Ops integration must occur to develop activities and Using the estimated heat load from the other schedules that promote crew health. subsystems and the parameters for the Martian environment, a worst-case radiator area was Design Drivers – The level of autonomy designed into determined. This led to a massive radiator system of the CA equipment is an important design driver, as there nearly 3331 kg. In comparison to the remainder of the is a critical trade off between mass and crew operations. thermal system, the panels are a little more than half of It was determined by a mass balance trade study that a the total system mass. Mars mission of 600 days should utilize such equipment as dishwashers and clothes washing machines, even Another challenge with using radiators on the surface of though these units require mass and power (Larson and Mars is dust accumulation and performance degradation Pranke, 2000). The use of such equipment makes it over time. A more accurate analysis of radiator material possible to utilize water the most efficiently and allows degradation in this environment is needed. Also, as dust for reuse of items such as kitchenware and clothes accumulates, an effective method for cleaning the instead of using disposables (Lynn, 2003). panels must be developed. The Mars gravity of approximately 1/3 g is a significant Lessons learned - The second design driver was the design driver for CA, since the subsystem requires a Martian environment. It is impossible to design a cost variety of active hardware that is affected by gravity. effective habitat to function anywhere on Mars, so a Hardware such as medical equipment, dishwasher, landing site must be selected and profiled. The habitat washing machine, clothes dryer, and exercise equipment can then be designed to efficiently operate at this site. all must be designed to function effectively in the reduced gravity environment. Although there are not many options for collecting heat from the subsystems, there are many ways to reject this Crew psychological health is dependent on the layout heat to the environment. Radiators can quickly become and use of habitable volume. Human factors studies massive depending on the environment in which they suggest that Mars crews will benefit greatly from a large are designed to operate and lifetime degradation factors. gathering area where meals, meetings, and celebrations Alternative methods may be more effective on Mars with can be shared. In addition, it is suggested that crews lower mass. Optional mechanisms for cooling the habitat will benefit from personal quarters, ideally including are convection, conduction and use of phase change sleep provisions, personal storage and a desk. materials. The transmission delay between Mars and Earth of Convective cooling could be incorporated into the outer approximately 20 minutes will drive the selection of skin structure of the habitat. Using conduction to the soil communications equipment, as crewmembers must of Mars could also reduce mass. The day / night cycle of utilize methods other than real-time voice loops to stay in Mars may allow for the use of a heat sink inside the touch with friends and family on Earth. As a result, structure that could store heat produced during the day ample personal video, audio, and e-mail transfer to and determine what exercise equipment and routines will be from Earth must be supported. most effective in the Mars gravity environment. Attention also needs to be given to the selection of recreational Crew selection will drive the design of many CA equipment to address needs associated with topics such provisions. Size and gender of crewmembers will affect as news, entertainment, religion, sports, and family. aspects such as food quantity needs, exercise equipment design, clothing requirements, privacy issues, Although considerable work remains to be done to medical equipment, and toilet designs. This project ensure a physically and psychologically safe mission for assumed human body size to fall within the MSIS human the crew, the CA subsystem conceptual design to date profile definition of 5th percentile Japanese female to 95th did not identify any ‘show-stoppers’. percentile American male (NASA-STD-3000 rev. B, 1995). If crewmembers were selected from the low end COMMAND, CONTROL AND COMMUNICATIONS - of this range, food and clothing mass alone could be The C3 subsystem supports and manages the habitat’s considerably reduced. The reduction, however, may not data flows. It provides the data processing and be worth excluding qualified astronauts who fall in the communications equipment required to monitor and higher end of the range. control the habitat’s environment and subsystems, monitor crew health and safety, communicate with Earth, Crew well being will be greatly enhanced through rovers and EVA crewmembers, and support data workstation designs that consider human reach profiles, processing related to the mission objectives. The C3 encourage comfortable body posture, and provide subsystem is comprised of two major functional adequate lighting for the crew members (Larson and components. The first component is the habitat’s Pranke, 2000). Human factors engineers must work with internal computing network (ICN). The ICN manages habitat architects early in the design to ensure these the data flows required to monitor and control the needs are met for the Martian gravity level. habitat’s automated systems and provides a computing interface for the crew. The external communications Current designs and reliability numbers suggest that subsystem (EC) handles data flows outside the habitat, EVAs will pose sufficient risks to not permit EVAs solely including communication with Earth, rovers and EVA for recreational purposes. If this changes over the years crewmembers. with technology, the use of EVA as a primary tool for recreation could reduce the exercise equipment need, Design Drivers – Two key drivers had the greatest consequently reducing mass, design and fabrication impact on the C3 subsystem design. DRM requirements time, and space for equipment use and stowage. for real-time communication with rovers and Earth drove the EC system architecture. Human factors-related Challenges – Human factors and CA will be more crucial communication requirements drove C3 subsystem in a Mars mission than for any space mission to date, transmission and data processing capacity. due to the duration and distance from Earth. Therefore, CA must work with the other Habitat subsystems early in The DRM requires the habitat to permit (near) real-time the design process to ensure that the integrated system communication with Earth. This was assumed to refer to will be capable of supporting the crew physically and continuous link availability because the light delay for psychologically. The simple distance-driven fact that the radio propagation is unavoidable. An additional crew cannot readily abort to Earth in an emergency requirement for direct communication between the situation or talk with people on Earth in real-time poses habitat and robotic rovers working beyond the habitat’s daunting challenges. direct line of sight was derived from the DRM. To meet these requirements, the EC design assumed an aero- Although our project focused on the increment of the stationary satellite to be in place above the habitat site mission on the surface of Mars, the transfer to and from providing direct communications coverage over Earth brings many psychological concerns for humans. approximately 50% of the Martian surface and roughly With current propulsion technologies, the crew will 50% Earth-Mars link availability. EC architecture, power endure long-duration exposure to weightlessness and requirements and throughput estimates are tied to the harmful radiation, along with dramatic acceleration load Mars orbiting communication satellite capabilities. changes before and after each transfer orbit The C3 subsystem throughput capacity resulted almost Careful consideration needs to be given towards the exclusively from human factors related communication selection of a variety of mission equipment. An area that needs. Communication needs were determined by still needs much attention is the selection of medical polling each of the other subsystems. Based on equipment and consumables. Due to the mass subsystem input, a time-averaged data rate of roughly constraints, medical provisions will be limited. Not all 11.6 kbps is necessary to meet the habitat’s medical emergencies can be foreseen, so developing a communication needs. Human factors communications limited suite of equipment that can handle almost any requirements account for 11.1 kbps of the time averaged condition that could arise is difficult. Another area that data rate. Despite the uncertainty in these estimates it is presents a challenge is the selection of exercise obvious that human factors will account for the majority equipment. Thorough analyses need to be performed to of the communication load. These communications include personal and mission related transmission and the habitat. The habitat’s robotic systems have a directed toward or generated by the crew. These major impact on the development of infrastructure for the messages consist of large, high data rate voice, video habitat and operations, including site analysis, habitat and TCP/IP packets. This communication driver assembly, instrument deployment and scientific represents a major difference between human and investigation (Hoffman and Kaplan, 1997). The main robotic missions, and must therefore be considered early robotic systems that will be required for habitat in the C3 design process. operations are three types of rovers: the small scientific rover, the local unpressurized rover, and the large Challenges – A human mission to Mars creates C3 pressurized rover (Hoffman and Kaplan, 1997). related challenges that must be resolved to help ensure mission success. Three key challenges were identified Two small scientific rovers will be mainly used for during the design process. exploration. These rovers will be autonomous a majority of the time, but will have the capability to perform Flight ready C3 technological options are currently very telerobotic operations, with the controller stationed in the limited. The C3 subsystem design utilized only flight- habitat. Also, the rover will be capable of recharging qualified technologies currently in use on the through solar panels. This type of rover will be required International Space Station and Shuttle. The primary to: deploy scientific instruments, collect and return benefit of these technologies is their ability to support samples from the Martian surface, determine safe routes proven operational needs outside Earth’s environment. for crew travel, and act as a communications relay in Flight-qualified technologies are more mass intensive contingency situations. The interface between these than their contemporary counterparts and have limited rovers and the habitat will be minimal. Only data will be processing capabilities by today’s standards. The result transferred to and from the habitat, consisting of of using flight ready technologies was a complex, dated telemetry, audio, video, and any necessary data from the C3 subsystem that exceeded our mass budget. Newer scientific instruments. The small rover’s power technologies would likely resolve these issues, but requirements were estimated at 0.3 kW using the Mars research and testing will be necessary to establish TRL’s Exploration Rover as a reference point and sizing up. greater than 7 and ensure they can meet the demands of a human mission to Mars. The cargo carrier will also be bringing one Local Unpressurized Rover (LUR). This rover will be required The Earth-based communications infrastructure is to provide local transport, ~100 km from the habitat, for currently inadequate to support a human mission to EVA crews and their tools. The LUR will be required to Mars based on needs identified in our analysis. The operate for 10 hours, with the charge/discharge cycle Deep Space Network (DSN) is the primary means of being under one day. This type of rover will have two communicating with spacecraft outside Earth’s interfaces with the habitat for exchanging power and immediate vicinity. The DSN is currently overtaxed data. Power will be transferred via a direct connection, supporting unmanned missions. It is unlikely that the with the outlet positioned on the outside of the habitat, existing DSN will be able to handle the high and the inlet connection placed on the outside of the communication volume required to support a human rover. The rover will also be sending and receiving audio mission to Mars, so the existing infrastructure will likely and relaying telemetry information to and from the need to be upgraded prior to sending humans to Mars. habitat. The medium rover’s power requirements were estimated to be 2.5 kW using the large pressurized rover Key operational issues also need to be addressed in as a reference, taking into account that the LUR does order to design an appropriate C3 architecture. To date, not have a life support system, and sizing down. crewed space missions have relied on near-continuous contact with Earth. This level of communication will be The third type of rover required for habitat operation is impossible for a manned Mars mission. A Mars based the Large Pressurized Rover (LPR). Two LPRs will be crew will need to function autonomously due to the brought to the surface on the first cargo carrier. The roughly 20 minute round-trip communications delay and LPRs will have critical responsibilities that must be limited Earth-Mars link availability. The operational carried out before the first crew arrives, including site strategy selected will affect the C3 requirements, so preparation, moving, deploying, and inspecting the extensive coordination with the mission operations team habitat’s infrastructure, and connection and inspection of is required during the design process. the ISRU and power plant. This will be done using 2 mechanical arms and a locomotive system. With all of ROBOTICS AND AUTOMATION – Due to the harsh these tasks being performed without the assistance of conditions on the Martian surface and the many time EVA crews, the LPR must be able to be fully automated, consuming and monotonous tasks required, robotics and but will also have telerobotic capabilities. The LPR will automation will play a vital role in habitat operations. The interface with the habitat directly, though this is mostly robotics and automation subsystem is responsible for for EVAS concerns. Other interfaces will involve the designing the automated systems and interfaces that are transfer of data, including telemetry, audio, and video. outside of the scope of other subsystems (Hoffman and Weight and power constraints of the large pressurized Kaplan, 1997). This subsystem is also responsible for rover were specified in the DRM at 15.5 metric tons and designing the interfaces between all robotics systems a 10 kW, respectively. space suit, an oxygen pre-breathe time of several hours For the tasks of initially leveling the habitat and radiator is needed. This was deemed unacceptable, so keeping deployment, research was conducted on existing in mind that there is a minimum of 50 minutes before commercial off the shelf technology for mass, power and depress and egress, the EVAS team calculated that if volume estimates. The initial leveling of the habitat will the internal pressure of the habitat was set at 10.2 psi utilize 12 linear actuators with two on each of the six legs (30% O2), with the same suit pressure, only 40 minutes for redundancy purposes. The actuators have 720 mm of pre-breathe time would be required. So as soon as of travel and can produce a total force of 50,000 N. the astronauts start to don and check their suits, they Their mass is 60 kg each for a total of 720 kg and they start to pre-breathe, and by the time the donning use 35 W each. Any increase in the amount of travel procedure is done, they are finished with pre-breathe needed will result in a slight increase in mass. For and ready to work (Larson and Pranke, 2000). This deployment of the radiator panels, 8 total actuators will represented a primary design driver for the overall be used with 2 on each panel. The actuators have 1 m habitat. 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 Figure 2: Timeline for Airlock Don/Doff Cycle Inspection and maintenance of habitat and ISRU
EVAS AND PLANETARY SURFACE VEHICLE INTERFACES - EVAS is primarily responsible for Spacesuit – The suit was assumed to have an internal providing the ability for individual crew members to move pressure of 4.3 psi (based on the current NASA design) around and conduct useful tasks outside the pressurized in order to safely meet these pre-breathe protocols. If habitat. It includes any activity performed by a the current pre-breathe time was increased, the internal crewmember wearing the EVA suit or operating the large pressure of the suit could be lowered even further (3.7 pressurized rover in the Martian environment. psi) to increase the mobility and dexterity of the suit. One of the most important features will be a The following components are integrated into the design regenerable, non-venting heat sink. If sublimated water of the habitat to provide the primary elements that make was used to cool the suit, the required mass of water EVA possible: an EVA suit designed to be used on the transported from Earth would more than double. A surface of Mars and be compatible with other EVA robust, durable suit is also needed to minimize the mass equipment, tools, and transport aids; an umbilical system of spare parts needed. This mass will further decrease if to provide connections from the habitat to the airlock and parts are designed to be modular and easily rover systems; a large pressurized rover that allows the interchangeable (especially those that experience the crew to safely explore distant sites; and an airlock most wear and tear of use). providing the suited crewmembers with the ability to safely transition from the habitat pressure to the Martian This design accommodates astronauts with a total of surface. thirteen suits over the 600 days, providing seven backup suits. Based on 2 EVA’s per week, each primary suit Going through the design process identified several key would, therefore, have to be able to withstand over 1200 driving parameters. During the 600 days on Mars, the hours of use. crew will be performing up to 2 EVAs per week (Hoffman and Kaplan, 1997). With this in mind, an end-to-end Airlock – The airlock designed by the EVAS team was timeline is proposed as shown in Figure 2. A main required to be an independent element capable of being consideration was pre-breathe time for the astronauts. If relocated or ‘plugged.’ The airlock will be a solid shell as the habitat was at an internal total pressure of 14.7 psi (1 opposed to the inflatable airlocks that have proposed atm, 21% O ), and the EVA suit has a internal pressure 2 used in other designs. This was decided upon for of 4.3 psi (100% O2) as does the current shuttle / ISS durability issues and for ease of relocation during the Possible methods considered were pumping of the mission. One of the major drivers for the EVAS team consumables directly to the habitat or transferring via the was to ensure that in case of fire or rapid depress, it rover’s storage tanks. must always be possible to get all astronauts out of the habitat. With this in mind, it was decided that there would Using Martian soil can be a very efficient way to build be three airlocks, two operational on the bottom floor radiation shielding or provide other forms of insulation and one emergency/back up airlock on the second floor. (Larson and Pranke, 2000). Soil could either be poured The third airlock ensures that astronauts on the second into bags or combined with water and heated in a kiln to floor (living quarters) could easily escape the habitat. In form bricks. Subsequent Mars missions might take addition to the emergency airlock, each airlock is advantage of the abundant iron, silicon, and other metals equipped with three EVA suits, two operational and one in the soil as well. emergency/backup suit. The emergency suit will be sized so that any of the six crewmembers can fit into it. Using the ISPP to provide consumables to the crew will The dimensions of the airlock are 4 m long, 3.5 m wide, save on overall mass of the mission and thus bring the and 2.5 m high, with a total volume of 35 m3. The choice cost of the mission down. This mass and cost savings is of this design was to ensure enough room for donning, proportional to which ISPP and which method of transfer doffing, storing, and servicing the EVA suits. is chosen.
The main door to the surface of Mars is the airlock. It will MISSION OPERATIONS AND HUMAN FACTORS - provide the astronauts with the ability to access the Mission Operations is an element often left out of surface at a high cost: the possible introduction of toxic academic design classes, where the focus tends to be particulate contaminants through the airlock and into the placed on designing “hardware” subsystems. Because astronaut living area (Shidemantle et. al., 2003). If not of the length of this mission, however, it was deemed controlled, this permeation may cause health concerns imperative to include human factors considerations as (such as cancer which may be induced by breathing the encompassed by the MO and Crew Accommodations hexavelant chromium within the dust) as well as subsystems. By including these “subsystems” at the equipment failure. For this reason, the airlock must be beginning of the design process, the habitat design can designed to incorporate a particulate cleaning system. be further optimized with respect to crew time, safety, and comfort. IN SITU RESOURCE UTILIZATION - The responsibility of the ISRU team was to design the interfaces between The purpose of the MO team is to oversee all activities the in situ propellant production (ISPP) plant and the during the mission. These activities may be manual, habitat. The ISPP will provide additional reserve of 4.5 automated, or Earth-controlled, and will include a wide mt of oxygen, 3.9 mt of nitrogen and 23.2 mt of water, as range of functions, such as maintenance of crew specified by the DRM (Hoffman and Kaplan, 1997). For psychological and physiological health, science the surface habitat designed in this document, all investigations, habitat maintenance and operations, consumable mass is being launched with the habitat. public relations, and communications and data transfer The ISPP plant will demonstrate that these consumables with Earth. can be produced in a timely, efficient manner. This demonstration will be monitored and will be used The primary task of the MO support team is to identify primarily to save mass for future missions to Mars. The and coordinate operations within each of these first crew may use the consumable reserve if it is functions, and schedule activities so that stated mission needed. goals are achieved. The MO team works closely with the design engineers to establish clear hardware operational Key Design Drivers and Challenges - The ISRU requirements incorporating human factors and subsystem major design tasks were to transfer the scheduling considerations and to revise these consumables, oxygen, nitrogen and by-products such as requirements as needed. Before and during the mission, water, safely into to the habitat. One key design driver the MO team is responsible for creating and modifying for the ISRU was transferring the consumables from the the operations schedule (in concert with the engineering un-pressurized Martian environment through the and science teams), developing procedures for all pressurized shell of the habitat cabin. Another driver was operations and failure scenarios, identifying and keeping the pipes insulated from the varying diurnal delivering relevant system status data to the crew, and temperatures. Redundant safety valves must also be working with crew during the mission to identify and implemented in the design to keep the habitat respond to any off-nominal situations that may arise. pressurized in case of failure in the pipes. Ultimately, the MO team will help to engineer a habitat that meets all physiological and psychological needs of The challenges involved with designing the ISRU are to the crew, in addition to developing and implementing a choose the most efficient ISPP, understand the inputs comprehensive mission activity schedule that leads to and outputs of the plant, and then design the interfaces. the successful achievement of the mission goals. Another design challenge is location of the habitat relative to the ISPP. This distance drives the method For this four-month class exercise, the scope of the MO used to transfer the consumables to the habitat. team was limited to developing operations within the habitat (as it was being designed), but excluded crew Crew Day/Night Schedules – Additional trade studies operations during transit or training. Consideration was need to be conducted to optimize the crew’s day/night given, however, to automated operations that may occur schedules. MOB chose a crew schedule that is within the habitat during transit and to functions of the synchronized with the Mars day/night cycle. In each ground operations crew. daily schedule, the six crew members are divided into three groups, and each group follows separate There were two key drivers for the MO team. The first schedules, which are similar but shifted in starting and was to integrate human factors considerations from the ending times (wake and sleep times) by a period of 15 beginning of the design, rather than “forcing” them in at minutes. This division of schedules allows time for the the end. This philosophy allows the crew’s time, comfort, Mission Commander (MC) and Second-in-Command and safety, and therefore the overall mission’s success, (SIC) to engage in mission planning and other activities to be optimized by design. The second design driver was special to the crew commanders, while the slight shifts in the time delay for communication between Earth and the schedules helps to avoid potential “traffic jams” in Mars and resupply opportunities. Because real-time areas such as the bathroom and laundry room. It was communication and control are not possible, and flights thought that the “all-awake, all-asleep” (or four-A) bringing cargo or crew from Earth are separated by schedule would be easiest on the crew psychologically, years, the crew and habitat must attain an in addition to minimizing sleep interference from “awake” unprecedented level of independence. crew members showering, doing laundry, etc. This type of schedule should be carefully weighed against a As a result of this project, some important points were schedule that rotates the crew through 8-hour sleep brought to light which should be included as key shifts so that someone is always awake in the habitat, considerations in future design efforts. which does provide an element of safety that may outweigh the benefits of a “four-A” schedule. Thus, a Operations Scheduling – A vital component enabling careful trade study should be done before opting for the MO’s integration with all subsystems was the operations specific type of schedule. Circadian rhythm (ops) list. This list defined scheduling of activities, which desynchronization may pose long-term adaptation in turn allowed the MO team to ensure that all activities issues, since the Martian day differs from Earth’s. could be accomplished within a specified period (day, week, month, mission). The ops list helped to determine Ground Support / Crew Command Architecture – At this which operations should be automated, Earth-controlled, early stage of development, it is difficult to clearly plan or performed by the crew, by elucidating factors such as the separation of duties between the ground (Earth- ease, frequency and duration of the task, as well as any based) segment and Mars-based segment of the related safety concerns. operations team. However, it is important to begin to delineate tasks between these two segments. Each of the subsystems provided MO with a list of operational activities that they expected to require during It is expected that the ground operations shifts will be set the mission. Each operation was then designated as up in a similar hierarchical structure to the ISS either automated, Earth-controlled, requiring crew, or operations team, with the notable exception that the some combination of the three. In addition to the tasks chain of command from the operations team to the Mars- required by each subsystem, there were a number of based astronauts will be hampered by the up to 25- activities required by the overall mission that were not minute one-way time delay. Because of this delay, real- included under any specific subsystem. These activities, time conversations between the ground segment and the which include obvious elements such as sleeping, astronauts will not be possible. Any ground control/crew eating, and cleaning, as well as less obvious tasks such architecture will need to take into account the up to 25- as collecting medical data from crewmembers and minute one-way time delay, as well as the usage of the writing e-mails to family, were gathered into a separate Deep Space Network (DSN) for communication with ops list. MOB compiled a list of 153 maintenance and Mars. The different duration of days also poses a unique day-to-day type tasks that need to be scheduled problem in keeping both crews in synch. throughout the mission. Due to the time delay, as well as the bandwidth Once all of these mission operations were catalogued, limitations, the ground team will not be monitoring all they were consolidated into representative mission data at all times. Rather, they will monitor key telemetry timelines. These timelines were a useful tool that items from each subsystem in near real time, and provided a reality check of the expectations put upon the analyze long-term trends using more complete data crew to perform maintenance, science, and all other downlinked at regular intervals. Telemetry, including mission tasks. In a habitat that required too much crew sensors and data types, should be designed with this in time for maintenance, the creation of a representative mind. timeline could expose design shortcomings that may otherwise be missed. It became apparent that mission Automated vs. Crewed Processes – Consideration scheduling in this manner needed to be concurrently needs to be given for determining an appropriate iterated as the design evolved. balance between automated and crewed processes throughout the entire mission. Ideally, a maximum number of processes would be automated so that the recycling and single use elements (as diverse as from crew’s time could be utilized for value-added activities, oxygen, food and water to clothing and packaging yet these automated processes should still be able to be material), selection of the habitat and space suit overridden and controlled by the crew to ensure safety operating pressures, enabling communication needs and and reliability. Optimization of this balance is key to thermal control, are among the many variables that mission success, and may differ from previous space cannot begin to be adequately addressed without missions in terms of the unique demands associated specific mission objectives having been identified. with the transit period versus surface operations. Going to the moon or Mars will likely drive ultimate designs in very different directions, although the Specific Design Influences – In addition to the general fundamental goals are virtually identical – to keep considerations noted above, integration of MO from the humans alive and healthy, both physiologically and beginning of the project influenced a number of specific psychologically, in a hostile, alien environment far from elements of the habitat design in ways that may have Earth. Giving due consideration to systems engineering been overlooked and are relevant to future endeavors. analyses, architectural guidelines and human factor concerns concurrently from the initial mission planning Consideration of space suit design parameters and stages through actual operations will allow the future expected number of EVAs drove the decision to human exploration of space to proceed as safely and keep the habitat internal pressure at 10.2 psi, efficiently as possible. because it significantly reduces pre-breathe time. ACKNOWLEDGMENTS The habitat will be deployed horizontally. This orientation was chosen for a number of structural and MO reasons. As a result of the horizontal The authors would like to acknowledge the efforts of orientation, the crew will have fewer stairs to deal their classmates S. Baker, E. Dekruif, B. Duenas, K. Hill, with, and in the event of an emergency they can N. Kungsakawin, E. Schleicher, M. Silbaugh, J. Uchida potentially egress from a second story airlock, which and T. White; and also thank the following individuals for may not be an option in a vertical configuration. This their assistance in technical matters and design reviews: layout was also considered to allow a more open D. Anderson (Mars Society), M. Benoit (CU), B. Clark floor plan than would a vertical configuration. It is (Lockheed Martin), J. Clawson (CU), C. Craig (Lockheed important to remember that in a mission of this Martin), T. Gasparrini (Lockheed Martin), A. Hoehn (CU), magnitude, psychological health of the crew will be a K. Mankoff (Honeybee Robotics), R. McCall (CU and significant factor for success. NexTerra), T. Muscatello (Mars Society and Pioneer Astronautics), S. Price (Lockheed Martin), J. Russell, ECLSS waste disposal will not recycle feces. (CU), K. Stroud (CU) and R. Zubrin (Mars Society and Though recycling feces would create a more closed Pioneer Astronautics). life support system, the crew’s psychological discomfort from eating or drinking recycled waste is REFERENCES a very real concern. Conversely, waste storage or disposal is equally an issue yet to be fully resolved. 1. Drake, BG, ed. Reference Mission Version 3.0: C3’s data flows were driven by MO’s need for high- Addendum to the Human Exploration of Mars: The bandwidth communication, such as audio and video; Reference Mission of the NASA Mars Exploration if MO were not integrated into the design from the Study Team. Lyndon B. Johnson Space Center, beginning this driver might have been missed. Houston, TX: June 1998. 2. Fisackerly, et al. Cranfield Aurora: Mars Excursion Numerous hardware choices will be affected by MO Module. Cranfield University; 1st Aurora Student considerations. For example, radiator performance is Design Contest, Barcelona: 2003. expected to degrade when covered in dust after a 3. Hoffman, SJ and Kaplan, DL. Human Exploration of Martian storm, and they will therefore need cleaning. Cleaning can be automated or manual, which will be Mars: The Reference Mission of the NASA Mars a factor in the radiator trade study. 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 DESIGN APPROACH SUMMARY Mission Analysis and Design. The Mc-Graw-Hill Companies, Inc., New York: 2000. The design process undertaken in this semester long 6. Lynn, V. Peace in Space. theGuardians.com, 6 effort highlighted several significant aspects of how Dec. 2003 closely the engineering requirements for a planetary