4 Requirements and Constraints
4.2 Control Requirements
The requirements to control the ECLSS are the most essential part of the requirements engineering task, because they influence the performance most. They are divided into the different subsystems of the ECLSS (see chapter 6 Life Support Systems for a comprehensive overview).
4.2.1 Atmosphere Control and Supply Control Requirements a. Control Atmosphere Total Pressure
Requirement: The total atmospheric pressure in the crew cabin shall be maintained within the range of 96.5 to 102.7 kPa, with a minimum of 95.8 kPa for 28-days emergencies. [13, 35, 37, 38]
Rationale: In principle, the needed total atmospheric pressure is dependent on the oxygen level. The minimum total pressure consists of pure oxygen at a pressure of 24.13 kPa but this is only feasible for short durations [44, pp. 2-3].
A higher total pressure means lower oxygen levels and reverse. A high pressure requires prolonged times for extravehicular activity (EVA) prebreathing, but since EVA´s not considered, this can be ignored. Since the driving factor on the wall thickness is radiation protection for interplanetary travel (see chapter 9.1), the stress caused by pressure are negligible. Additional the gas leakage is higher, but since the SpaceHab consist of one big module, this is minimal. The benefits of a high-pressure atmosphere are a better cooling efficiency of fans and therefore lower noise. Likewise, the voice communication at a distance is no problem. This is of special interest for a big volume like the SpaceHab.
Requirement: The life support system shall provide enough resources to compensate the atmospheric leakage during nominal operation, which is defined to 0.2 kg day-1. [45, 46]
Rationale: Mean leakage on ISS in 2011 was around 0.227 kg day-1, which includes losses for EVA, docking operations and leakage between modules [46].
In conclusion, a leakage rate of 0.2 kg day-1 for the closed environment of the ITS is a worst-case scenario. Furthermore, for the leakage rate no distinction will be made between the SpaceHab and Evolved-SpaceHab design.
d. Add Metabolically Inert Gas to Atmosphere
Requirement: The metabolically inert gas nitrogen shall be added to the cabin atmosphere as needed during normal operations and to restore the atmosphere following decompression. [13, 37]
Rationale: Two types of inert gases could be considered. Nitrogen is the natural inert gas on earth and commonly used in spaceflight. Helium is often used in deep-sea diving. The benefits of Helium over Nitrogen are the higher resistance to ionizing radiation without forming by-products, the much lower density (⅐) which would lead to reduced power requirements for fans and smaller gas stores since it could be more densified, and the shorter prebreathe time for EVA. On the other hand, the disadvantages are the 6 times higher thermal conductivity which brings a 4 to 6 °C warmer comfort zone, the higher molecular incidence rate and therefore higher loss rates from leakage, and the speech shift of 0.7 octaves to higher frequency which would require technical aids. [13]
e. Control Oxygen Partial Pressure
Requirement: The oxygen partial pressure in the crew cabin shall be maintained within the range of 19.5 to 23.1 kPa for normal operational phases, 16.5 to 23.8 kPa for 90-day degraded phases and 15.9 to 23.8 kPa for 28-day emergencies. The minimum partial oxygen pressure for 1 hour is 15.17 kPa [37, 45]
Rationale: The assumed Loss of Crew (LoC) limit is a decrease of the oxygen partial pressure of under 13.4 kPa for over 3 minutes. [43, p. 35]
f. Add Oxygen to Atmosphere
i. Requirement: Gaseous oxygen shall be added to the cabin atmosphere as needed during normal operations. [37]
ii. Requirement: Gaseous oxygen shall be added to the cabin to restore the atmosphere following decompression. [37]
4.2.2 Temperature and Humidity Control Control Requirements a. Control Atmospheric Temperature
i. Requirement: The atmospheric temperature in the crew cabin shall be maintained in the range of 291.5 to 299.8 K (18.35 – 26.65 °C) during normal operations and 90-day degraded phases. It shall be in the range of 288.7 to 302.6 K (15.6 – 29.5 °C) during 28-day emergencies. [13, 37, 45]
Rationale: The temperature range for the American section on the ISS is 17.8 to 26.7 °C and 18 to 28 °C for the Russian section. [38]
ii. Requirement: The atmospheric temperature in the crew cabin shall be crew selectable within the acceptable operational range. [37]
Rationale: The crew on the ISS generally selects a temperature at or above 22.2 °C. [47]
iii. Requirement: The atmospheric temperature in the crew cabin shall be within ± 1.5 Kelvin of the selected temperature regardless of crew activities. [9, p. 345]
b. Remove or Add Sensible Heat
Requirement: Sensible heat shall be removed from and/or added to the cabin atmosphere as needed during normal operations. [37]
Rationale: Sensible heat is produced by humans and equipment.
c. Control Atmospheric Humidity
i. Requirement: The atmospheric relative humidity (RH) in the crew cabin shall be maintained in the range of 25 to 70 % independent of the current phase. The nominal value is assumed to be 40 %. For a 90-day degraded or 28-day emergency it could be up to 75 %. [13, 37, 45, 48]
Rationale: RH and dew point must be specified independently. The RH is required for crew comfort whereas the dew point is for prevention of condensation. A low RH causes drying of the skin and eyes while a high RH can lead to condensation on surfaces [9, p. 343].
“Note: Cabin atmospheric relative humidity and dew point are not independent quantities, but rather different assessments of the moisture in the cabin atmosphere. This understanding, however, does not waive either (relative humidity) or (dew point). Rather, both must be satisfied.”
[48]
ii. Requirement: The atmospheric dew point in the crew cabin shall be maintained in the range of 277.6 to 288.7 K for normal operational phases with the goal to no higher than 287.2 K, and 274.8 to 294.3 K during all other phases. [37, 38]
Rationale: It should be noted that a lower dew point means the reliability and redundancy gets better. [37]
d. Remove or Add Moisture
Requirement: Moisture shall be removed from and/or added to the cabin atmosphere as needed during normal operations. [37]
Rationale: Humidity condensate is delivered to the wastewater bus at a rate up to 1.45 kg h-1 and a pressure up to 55 kPa on ISS [38]
e. Ventilation Velocities in the Crew Habitable Volume
Requirement: Atmospheric velocities in the crew habitable volume shall been between 0.051 to 0.203 m s-1 for nominal operational situations with a lower and upper limit of 0.036 and 1.02 m s-1 respectively. [37, 38]
f. Exchange Atmosphere between Modules
Requirement: Atmosphere exchange between adjacent, non-isolated pressurized volumes shall be provided to maintain sufficiently uniform conditions for atmosphere composition control when atmospheric constituents are controlled by centralized systems. [37]
Rationale: The exchange rate between different modules on ISS is 63.7 to 68.4 L s-1 (229.32 to 246.24 m³ hr-1) [38]
4.2.3 Atmosphere Revitalization Control Requirements
a. Control Partial Pressures of Atmospheric Contaminants
Requirement: The partial pressures of contaminants, such as carbon dioxide and other trace contaminants, in the cabin atmosphere shall be maintained at or below current applicable Spacecraft Maximum Allowable Concentration limits for various exposure periods defined in [49]. The maximum allowable
concentration of CO2 is 0.68 to 0.72 kPa for up to 180 days. It also depends on total pressure. Therefore, it is allowed to rise up to 2.03 kPa for 1 hour, and 0.9066 kPa within 24 hours. The goal is a mean CO2 partial pressure below 267 Pa (2 mmHg). [17, p. 5, 35, 37, 45]
Rationale: Several different sources of trace contaminants must be considered.
These are humans, structural and aesthetic materials, payload chemicals, propellants (e.g. hydrazine), coolants, and thermodegradation of materials heated. [45]
The LoC limit is assumed to be CO2 partial pressure of over 3 kPa for longer than 15 minutes [43, p. 35].
b. Remove Gaseous Atmospheric Contaminants
Requirement: Atmospheric contaminants, such as carbon dioxide and other trace contaminants, shall be removed from the cabin atmosphere as needed.
[37]
c. Control Airborne Particulates
Requirement: The concentration of airborne particulates greater than 0.5 µm in the cabin atmosphere shall be maintained to be lower or equal to 3,500,000 particles per m³ (0.05 mg m-³ with periodic peaks to 1 mg m-3 are allowed) during normal operational phases. [37, 38]
Rationale: Typical sources of airborne particulates are hair, food debris, fabric lint, skin fragments, and paper and plastic debris. The typical generation load to control is between 0.6 and 1.6 mg CM-min-1. [17, p. 6]
d. Remove Airborne Particulates
Requirement: Airborne particulates shall be removed from the cabin atmosphere as needed. [37]
e. Control Microbes
Requirement: The concentration of microbes within the cabin, whether airborne or on a surface, shall be controlled within less than 500 CFU7 per m³ during normal operational phases, less than 750 CFU per m³ during 90-day degraded phases and less than 1000 CFU per m³ during 28-day emergencies. [37]
f. Remove Airborne Microbes
Requirement: Airborne microbes shall be removed from the cabin atmosphere as needed. [37]
4.2.4 Water Recovery and Management Control Requirements a. Control Water Quality
Requirement: Water provided for crew use and consumption shall meet current established water quality requirements as defined in [45]. [37]
7 CFU stands for colony forming units
Rationale: The parameters to monitor includes total organic carbon, total inorganic carbon, total carbon, conductivity, pH, turbidity, color, iodine, iodide, and iodine compounds.
b. Water System Decontamination
Requirement: Capabilities shall be provided for in-flight decontamination of water processing and storage systems. [37]