Mars Or Bust, LLC
6.0.0 ECLSS Subsystem
6.1.0 Overview
6.1.1 Objective
Within the Mars habitat design, it is necessary to develop a new Environmental Control and Life Support System (ECLSS). Some of these issues include the inability to quickly return home, living in a range of gravity conditions, and the psychological factors that will arise from a long duration, high-risk mission. In order to meet the system and mission requirements, available life support technologies will be analyzed and integrated. The Mars Design Reference Mission (DRM) document will be used as the baseline mission and an approach will be developed to provide a comprehensive life support system that will optimally satisfy the needs of the Mars DRM.
6.1.2 Class Specific Scope
The scope of this project is to research relevant technologies and determine an integrated system that will successfully accomplish the above-mentioned objective. This section of the project was completed over the course of the Fall 2002 semester for 3 credit hours. There were two reports from two different teams that were used to find the optimal design for the ECLSS subsystem.
6.1.3 System Design Philosophy
The ECLSS subsystem is separated into four subsystems: Atmosphere, Water, Waste, and Food. The four subsystems are then integrated into one functional system. Figure 6.1.1 shows the subsystem interactions with the human in the loop. All subsystems interact with one another on different levels. All subsystem interactions are taken into account and an iterative system design is implemented to maximize efficiency. The final ECLSS subsystem is based off the optimum individual subsystems that interact best with the other subsystems while maintaining a high overall system performance. Individual subsystem requirements and assumptions will be analyzed while taking into account the overall system requirements and assumptions previously laid out. An iterative design of the overall system and its subsystems is completed to determine the optimal way of meeting all requirements.
Figure 6.1.3.1: Subsystem Interactions
6.2.0 Requirements
As mentioned earlier, The Environmental Control and Life Support subsystem (ECLSS) is responsible for providing a physiologically and psychologically acceptable environment for humans to survive and maintain health in the Mars habitat. This includes providing and managing food, water, waste, and atmospheric conditions, as well as supplying crew accommodations and medical services. To determine how to provide what is necessary requirements must be determined. Top-level requirements, which are also known as level 1 requirements, are requirements that are stated in the DRM. From those, assumptions and level 2 requirements can be derived. Top Level requirements and level 2 requirements for the ECLSS subsystem are given below.
Top Level Requirements
· Provide life support functions for a crew of 6
· 180 worst-case transit time between Earth and Mars
· 600 day worst-case surface stay on Mars
· Perform the entire mission assuming no resupply from Earth
· Take advantage of ISRU when possible
· Operate during launch, transit, descent, and surface g-loads
· Provide 2 levels of backup (life critical)
· Do not rely on biological systems for life support functions
· Provide as much loop closure as possible
· Reliability, maintainability, and safety
Derived Requirements and Assumptions
· Shall provide adequate atmosphere, gas composition, and pressure control.
· Must have necessary Gas Storage for mission duration.
· Must have adequate Ventilation.
· Must provide Trace Contaminant Control.
· Shall provide Temperature and Humidity Control.
· Must have Fire Detection and Suppression.
· Must supply entire crew with adequate sources and amounts of food and potable water for a 4-6 month transit to Mars, and 600-day stay on the surface of Mars.
· Shall be able to collect and store liquid, solid, and concentrated wastes for immediate and/or delayed resource recovery.
· Must provide adequate supply of hygiene water.
· Shall provide psychological support by taking into account crew environment and other human factors.
· Shall monitor and report radiation levels in habitat to other subsystems.
· Mass must not exceed 4661 kg.
· Target life support system power usage of 12.1 kW
· Must allow for crew input to habitat temperature and humidity levels.
6.3.0 Atmosphere Subsystem
The purpose of an atmosphere management subsystem is to maintain an acceptable atmosphere for human life. For a Mission to Mars, analysis of the system level requirements revealed that the subsystem would operate in 1/3g gravity. This exercise will push the current limits in knowledge about long-term reliability and functionality. The following evaluation accounts for these system level concerns in the design approach and in the discussion of subsystem level integration results.
6.3.1 Responsibility and Assumptions
An acceptable atmosphere for human life on a Mars mission consists of providing a safe environment that meets the physiological and psychological needs of the crew. This general requirement translates into subsystem process tasks. These tasks are oxygen provision, pressure regulation, thermal control, trace contaminant control, carbon dioxide control, and fire detection and suppression. For most of these tasks, the selected technologies must provide the basic functionality while maintaining the key parameter(s) within a specific range of values.
The ecosystem of the Earth incorporates most of the tasks in the regulation of the atmosphere as a whole and in specific areas. On a large scale, the atmosphere provides oxygen (~3.1 psia ppO2) to humans, removes metabolic byproducts (trace contaminants and carbon dioxide), and maintains a buffer that regulates temperature, pressure and relative humidity with physical/chemical and biological processes. However, with changes in latitude, longitude, and altitude there are can be distinct changes in temperature, total and partial pressures, and relative humidity.
In an enclosed environment, these same tasks operate with a smaller buffer size; however, the same basic requirements still need to be met. For the first task, oxygen needs to meet the base metabolic oxygen demand of 1 kg O2/person/day along with losses due to oxidative technology demands. The ECLSS subsystem is also responsible for supplying the EVA subsystem with all the life support needs. Considering all the leakages and EVA needs, the cabin still has to maintain a 3.1 psia ppO2, with an acceptable range of 2.83 to 3.35 psia ppO2. The total pressure is regulated to 10.2 psia due to the frequency of EVA scheduled to the crew. The combination of oxygen and nitrogen results in variations of the total pressure is possible. The thermal control system maintains relative humidity between 25%-70% and temperature between 18.3oC – 26.7oC. Carbon dioxide removal needs to offset the metabolic production rate of 0.85 kg CO2/person/day in addition to technology products that interact with the crew cabin atmosphere. Humans, material off-gassing, and technologies generate a variety of organic and inorganic compounds (ammonia, nitric oxide, methane, ethylene, and benzene) in volatile state or adsorbed to particulates that need to be controlled below the long term Spacecraft Maximum Allowable Concentrations (SMAC), which are 7 mg/m3, 0.9 mg/m3, 3800 mg/m3, 340 mg/m3, and 0.2 mg/m3, respectively. Finally, on Earth fires are eventually self-limiting, but in an enclosed environment, the final task of fire detection and suppression needs to operate quickly and reliably to avoid both direct (life and limb) and indirect (oxygen consumption) hazards (Eckart 1996).
6.3.2 Design Approach
The evaluation of the atmosphere subsystem entailed a multi-tiered approach. This approach iteratively examined requirements, key mass drivers, and the functionality and integration of different technologies. The air subsystem requirements were driven by the top-level requirements and derived from known and assumed technology specific data. The key mass drivers were identified in the baseline mission scenario, which consisted of only existing (and allowable) non-regenerable technologies. These mass drivers were then initially examined to minimize consumables. After gathering information and ranking current technologies, a functional subsystem was created and then iteratively changed to maximize the reuse or recycle of materials to reduce mass losses while still conforming with the top level and subsystem level requirements.
6.3.3 Technologies and Trade Study
To understand the key mass drivers and interactions, a baseline system with existing non-regenerable technologies used on Space Shuttle, ISS, and MIR was created. The basic system details and interactions are shown in Table 6.3.3.1 and Figure 6.3.3.1.
The key mass drivers for the baseline system were the carbon dioxide removal (46%), oxygen provision (38%), and total pressure regulation (13%) systems. These identified systems are all high in consumable mass. As a first cut for mass savings, the oxygen, nitrogen and carbon dioxide system variables are reviewed to determine options for the minimization of consumables. The consumable mass for the carbon dioxide removal system is due to the LiOH canisters, which translates into mass savings dependent completely upon the selected carbon dioxide removal or reduction technology. The mass of the oxygen system was sized for humans (91.5%), venting (7.9%), leakage (0.6%), and technology (0%) usage; similarly the total pressure regulation system was sized only for leakage and EVA venting. At this level, the potential areas for mass savings are reductions in the leakage and venting of oxygen and nitrogen.
Table 6.3.3.1: Baseline Non-regenerable Physical/Chemical Air Life Support System
Subsystem / Selection / Mass / Power / Volume / Heat Produced / Crew Time / ESMO2 Provision / Chemical oxygen / 7466 / 0.0 / 0.02 / 0.02 / 29.5 / 7496
Total Pressure Regulation / N2 pressure tanks / 2562 / 0.0 / 0 / 0 / 0 / 2562
CO2 removal / LiOH canisters / 8988 / 0.0 / 7 / 0 / 281 / 9333
Temperature & Humidity Control / Rotating Heat Exchanger / 175 / 1.31 / 3.9 / 0.82 / 20.5 / 233
Trace Contaminant Control (TCC) / Space Shuttle TCCS / 178 / 0.15 / 11 / 0.15 / 78 / 361
Fire Detection & Suppression / N2 extinguishers & photoelectric detectors / 65 / 0.003 / 0.047 / 0 / 6 / 71
Total / 19,434 / 1.5 / 21.8 / 1.0 / 415 / 20,056
Given the 1.45 kg/day predicted leakage rate of the Habitat, under normoxic conditions, 1.009 kg of N2 and .441 kg O2 will be lost to the Martian environment each day. For the 600-day mission, therefore, the minimum required buffer tank sizes are 605.6 kg and 264.4 kg for N2 and O2, respectively. However, this system trade has several potential disadvantages: an increase in the percentage of oxygen (and thus flammability), reduced heat rejection capacity of the air, and the unknown long term effects of living at reduced atmospheric pressure with normoxic oxygen levels.
Figure 6.3.3.1: Baseline non-regenerable Physical/Chemical Air ECLSS Schematic
This schematic illustrates the basic subsystem level intra-and interactions for the air management system.
The remaining mass savings in this system are based on the selection of individual candidate technologies and maximization of recyclables for compatible subsystem and system level interactions. Initially, information on a variety of technologies for meeting the different tasks was compiled into spec sheets. These technologies were then sized in a similar fashion to the baseline system calculations to meet the associated task. In the case of missing information for a specific technology, some assumptions were made. After the technologies were reasonably detailed, the technologies were ranked based upon their equivalent system mass in Table 6.3.3.2.
Table 6.3.3.2: Air Management Life Support Technology Rankings
Technologies / TRL / ESM / RANKOxygen
SPWE (w/ EDC) / 9 / 2119 / 1
SPWE (w/ Sabatier) / 9 / 4394 / 2
SPWE (w/o Sabatier) / 9 / 5515 / 3
Tank- cryogenic / 9 / 6573 / 4
O2 chemical (TRK) / 9 / 7496 / 5
Tank- pres vessel / 9 / 7681 / 6
Total Press Reg (Nitrogen)
Tank- cryogenic / 9 / 2224 / 1
Tank- pres vessel / 9 / 2562 / 2
Carbon Dioxide Removal or Reduction
EDC / 6 / 162 / 1
SAWD / 6 / 190 / 2
4BMS (CDRA) / 8 / 418 / 3
Sabatier (CDReA w/ H2 tanks) / 9 / 450 / 4
Bosch / 6 / 885 / 5
LiOH / 9 / 9333 / 6
Thermal and Humidity Control
CHX-rotating / 6 / 267 / 1
TCC
Detection
GC/MS / 9 / 150 / 1
Treatment
TCCA / 9 / 135 / 1
TCCS / 9 / 201 / 2
Activated Charcoal / 9 / In TCCS & TCCA
Catalytic Oxidation / 9 / In TCCS & TCCA
Particulate Filters / 9 / In TCCS & TCCA
FDS
Detection
ISS Photoelectric / 9 / 21 / 1
STS Ionization / 9 / 21 / 2
Suppression
Nitrogen Agent / 9 / 68 / 1
Halon 1301 Agent / 9 / 68 / 2
CO2 / 9 / 85 / 3
Depressurization / 9 / 694 / 4
Note: ESM = mass + 0.0115 kg/kW (power) + 9 kg/m3 (volume) + 0.0069 kg/kW (heat rejection) + 1 kg/crew-hour (crew time) + 5% kg total surcharge/TRL less than 9.
The highest ranked systems were subsequently analyzed and deemed compatible. However, further analysis was required to determine the potential recyclables between the selected oxygen generation system (solid polymer water electrolysis [SPWE]) and the carbon dioxide reduction/removal systems (Sabatier, Bosch, and electrochemical depolarized concentrator). For the primary option A (Figure 6.3.3.2), there is a high degree of water return from the EDC to the SPWE, which reduces the additional water supply required to produce oxygen from 5250 kg H2O to 1,854 kg H2O, see Table 6.3.3.3. Option B (Figure 6.3.3.3) has a lower degree of water return from the Sabatier reactor which only reduces the additional water supply required to produce oxygen from 5,250 kg H2O to 4,129 kg H2O, see Table 6.3.3.4. Based on these tradeoffs along with the fixed mass differences, the option A saves 2,500 kg of mass over option B and 14,000 kg of mass over the baseline scenario.
Table 6.3.3.3: Physical/Chemical Air Life Support System for Mars Mission–Option A
Subsystem / Selection / Mass / Power / Volume / Heat Produced / Crew Time / TRL / ESMO2 Provision / SPWE / 2096 / 1.84 / 2.24 / 1.84 / 2 / 9 / 2119
Total Pressure Regulation / N2 pressure tanks / 2562 / 0 / 0 / 0.00 / 0 / 9 / 2562
CO2 removal / EDC / 133 / 0.30 / 0.2 / 0.67 / 5.4 / 6 / 162
Temperature & Humidity Control / Rotating Heat Exchanger / 175 / 1.31 / 3.9 / 0.82 / 20.5 / 6 / 267
Trace Contaminant Control (TCC) / GC/MS,TCCA / 273 / 0.08 / 0.9 / 0.18 / 4 / 9 / 286
Fire Detection & Suppression / N2 extinguishers & photoelectric detectors / 80 / 0.003 / 0.3 / 0.00 / 6 / 9 / 88
Total / 5319 / 3.53 / 7.5 / 3.51 / 37.9 / 5485
Notes: