04ICES-187
Systems Engineering Evaluation of a Mars Habitat Design
Klaus, D., Chluda, H., Ellis, T., Fehring, J., Howard, H., Jairala, J., Lloyd, T., Matthews, D., Morris, K., Rowley, K., Sauers, C. and Stephens, T.
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 tons (mt, or tonnes) to LEO.
Other assumptions are as follows. The crew would 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 80mt launch vehicle, deploying the habitat two years before the first crew and providing capability for a 10-month standby 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 requirements of 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 (10.2 psi) volume of 616 m3, a usable volume of 211 m3, an overall mass of ~68,000 kg, and a maximum power consumption of 43 kWe. The basic 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 infrastructure, including rovers, cargo landers, and nuclear power plants.
The project was divided into 12 subsystems at the beginning of the project termed ‘Mars or Bust’ (MOB), including Program Management and Systems Engineering and Integration. The remaining subsystems and their functional descriptions follow. The 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. The Structures team 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 Accommodations (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 available environmental parameters for the surface of Mars and identify their relevance 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 selected relevant parameters of 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 4-10 millibars (mb) (Tillman, 2003). Surface temperature is site-specific and was based on the Viking and Pathfinder missions for this exercise (Tillman, 2003). Selection of the actual landing site will dictate the ultimate range. Radiation remains a major concern for a Mars mission, although the expected annual dosage of 21.2-24.7 cSv on the surface is less than the current low earth orbit (LEO) limits of 300 cSv for skin and 50 cSv for Blood Forming Organs (BFO) (Simonsen and Nealy, 1993). The Earth-Mars transit period is of greater concern in this regard, and considerable research is underway aboard the ISS to better understand the issues. Surface wind speeds are relatively high compared to Earth, but the actual dynamic pressure is less due to the low atmospheric density (Withers, 2002). Wind, therefore, is not a major factor per se, although abrasion and dust accumulation can become a problem for external elements such as radiators and solar panels.
Parameters / Maximum / Minimum / AverageGravity (m/s2) / 3.758 / 3.711 / 3.735
Atmosphere Pressure (mb) / 10 / 4 / 8
Surface Temperature (˚C) / 27 / -143 / -63
Radiation (cSv) / 24.7 / 21.2
Wind Speeds (kph) / 36 / 0
Wind Storm Speeds / 127
Table 1: Key Mars Environment Parameters
STRUCTURES - The Structures subsystem provides a habitable volume and structural supports for the habitat and subsystem components. The Structures team is also responsible for the overall layout of the habitat, taking into account mass distribution, radiation protection and thermal considerations. Finding materials that will fulfill support requirements with a minimum mass was given a high priority.
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 was the NASA INTEGRITY mission (INTEGRITY, 2003), recently renamed Advanced Integration Matrix (AIM), which featured several horizontally oriented cylinders connected to each other by a corridor. Published rationale for this orientation was not found, however. In addition, the launch vehicle dimensions outlined in the DRM (Hoffman and Kaplan, 1997; Drake, 1998) dictated a longer, narrower pressure shell than that found in most Mars habitat designs. Therefore, it was decided that the MOB habitat would be horizontally oriented. Though habitat stability was the primary driver for this decision, this orientation provided other benefits as well, including ease of exterior maintenance access, emergency ingress/egress from the second floor, and internal mobility (fewer stairs), as well as a more open, psychologically pleasing floor plan.
There remain, however, some issues associated with this orientation that were not fully addressed in this design iteration. For instance, the DRM indicates that the habitat will land on its end, in which case the habitat would require a single-use mechanism to orient the habitat. This mechanism was not included within the scope of the MOB design, and may have a considerable mass penalty.
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 factors 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 in a horizontal orientation or a shorter habitat with a larger diameter than that prescribed in the DRM (as is the case with most previous designs). These alternatives would address many of the challenges described above.
POWER - 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 mobile and stationary power storage 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 solar events have disrupted power grids on Earth, and the consequences of such storms on Mars, where the atmospheric and magnetospheric protection is much less than that on Earth, may be severe. For this reason it is recommended that the vital components be placed with 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 dependence on power should be minimized or eliminated in standby mode, 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 heat (batteries, consumables, etc.). Power storage may alleviate this problem, but careful planning and a high degree of reliability in the Habitat/Nuclear Reactor connection process will likely be required for a robust solution.
Minimizing Heat Production – The size of the radiators was a major concern in this habitat’s design. In future iterations, the radiator design must improve dramatically or the heat load created by the habitat must be reduced. Because a large amount of heat within the habitat is generated through cabling and appliance inefficiency, improving overall system power efficiency, and 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 subsystems: 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 for drinking and to re-hydrate food. Condensate is recovered from the Atmosphere Subsystem that, after undergoing treatment, qualifies as potable water. Urine is also recovered from the Waste Subsystem for subsequent use. Water is provided to the Atmosphere Subsystem for oxygen production using Solid Polymer Water Electrolysis. Finally, unusable waste is passed from the Food Subsystem and Trace Contaminant Control to the Waste Subsystem. This mass includes 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. The required water mass should be determined by a detailed analysis of water use, loss, and recollection in a number of habitat mission processes (i.e. drinking, hydrating food, oxygen production, cooling, showering, vapor leakage, and urine processing). Increasing the efficiency of the water purification system is the best way to minimize the required water mass. However, the regenerative technologies reviewed in the trade study generally had TRLs of less than 6, which was deemed to low for this design. Extracting water from fecal matter would slightly reduce the required water mass. The launch mass of food for subsequent missions can be reduced once it is demonstrated that crops can be successfully grown on the surface of Mars. In addition, further optimization of the ECLSS by increasing consumable recycling and minimizing leakage could result in considerable mass and volume savings.