Life support comparisons
The primary life support systems are located in the Service Module, Zvezda, «Звезда» (the FGB, Zarya, provided backup capabilites until Zvezda was in orbit). There are 5 main components: atmospheric control, water supply, food supply, sanitation equipment, fire detection and suppression. All are monitored and controlled by the on-board computer system, the laptops providing a crew interface with these.
The life support system is divided into two components: Atmosphere Revitalization, СОГС, SOGS; and human life support, СОЖ, SOZh – the latter being food, water and sanitation. The information is mainly derived from the LSS ISS; I have linked to more detailed descriptions there in places.
The two sections below, Design Philosophies and RS ECLS Capabilities, are extracted from Living Together in Space: The Design and Operation of the Life Support Systems on the International Space Station, P.O. Wieland, NASA, January 1998.
Design philosophies
The basic philosophies of design that are used by the United States and Russia have some significant differences that must be understood to ensure that the different ECLS systems are compatible. In addition, differences in terminology can lead to confusion. For example, the word “monitor” may be translated into Russian as “control” when the intended meaning is “measure.”
For example, the U.S. approach to ensuring that a capability is provided tends toward using redundant equipment, i.e., having two identical units with one used only in an emergency or operating both at less than their full capability. This leads to having two CO2 removal units, for example, with one in the Hab and one in the Lab, each of which can accommodate the entire normal load. There are exceptions to this approach, e.g., there is only one water processor and one commode. In comparison, the Russian approach is to have an alternative backup rather than a redundant unit, e.g., for oxygen supply if the Elektron O2 generator fails, the backup is the SFOG and stored O2 (gas, liquid, or solid form) for use until a replacement unit is delivered. This method works because of the regular resupply missions. Again, there are exceptions, e.g., there are two fans in the FGB, one of which is for redundancy.
The basic design philosophy used for designing the U.S. ECLS system includes:
- Minimize the use of expendable materials by using regenerable methods where feasible, e.g., for CO2 removal, urine processing, etc.
- Recover as much mass as possible (i.e., close the mass loops) when cost effective, e.g., recovery of the atmospheric moisture during CO2 removal.
- Minimize the amount of redundancy required (i.e., during assembly by adjusting the installation sequence, by appropriate planning of operations, or by relying on the RS to provide redundancy).
- Design for minimum risk of failure of mechanisms, structures, pressure vessels, materials, etc.
The failure tolerance for many of the ECLSS functions is zero (i.e., the function is lost when the equipment fails) at the module level. Exceptions to this are intermodule ventilation and intramodule ventilation, heat collection and distribution, and response to hazardous atmosphere, which must be single-failure tolerant. However, for the complete ISS, there is redundancy for critical functions.
Another example of the effect of different philosophies is the design of the OGA. The United States and Russia both use electrolysis of water as the basic technique, but there are significant design differences. The U.S. approach is to design hardware to be serviceable, so components are designed as orbital replaceable units (ORU) and are accessible for replacement. Safety concerns due to the presence of hydrogen (H2) as an electrolysis byproduct were dealt with by ensuring that the quantities of combustible gases present are negligible. The Russian approach does not require that components be individually replaceable. They also use a different approach to ensuring safety. As a result, for their OGA the electrolyzer was placed inside a pressurized N2 jacket so that any leakage is into the electrolyzer. Also, when the OGA is turned off, the N2 flushes O2 and H2 from the lines. This design precludes the possibility of any leakage of hazardous gases to th e atmosphere, but individual components are not accessible for replacement.
As a result of the differences in design philosophy, integrating the Russian and U.S. ECLS systems must be done carefully. The equipment developed by the different approaches may not be compatible without some modification. The table below lists differences and similarities in the design philosophies of the U.S. and Russian ECLS designers.
| System | Russian | U.S. |
|---|---|---|
| Trace contaminant detection/control before entry | No capability to verify clean air prior to entering a module. For the FGB and SM, a special filter is activated 2 days prior to first entry. Other modules are purged prior to launch and attached before offgassing contaminates the atmosphere. | Samples may be collected through the MPEV and analyzed before opening the hatch by CHeCS instrumentation. Node 1 has filters to remove contaminants prior to ingress. |
| Trace contaminant removal | Trace contaminant removal equipment sizing considers that atmospheric contaminants are removed by the humidity control assembly and due to atmospheric leakage to space. | Trace contaminant removal equipment sizing does not consider other ways in which atmospheric contaminants are removed. Therefore the design is conservative. |
| Trace contaminant generation | Generation rate prediction is based on the surface area of materials. | Generation rate prediction is based on the mass of non-metallic materials. |
| SMAC level selection 1 | SMAC levels are based on the capabilities of the available TCCS technologies, as well as health reasons. | SMAC levels are based on the best information available concerning possible health impacts of contaminants. |
| (Note: A result of this different approach is that Russian SMAC values tend to be smaller than U.S. SMAC values. The U.S. TCCS equipment is capable, however, of maintaining concentrations well below the SMAC values for most compounds.) | ||
| Failure tolerance | For repairable systems there could be many failures with no long-term loss of function. Loss of one leg of redundancy does not mean that a system has failed. | Specified for each function and system. ECLS functions are zero- or one-failure tolerant. One-failure-tolerant hardware requires a redundant functional path. |
| Response to rapid decompression | Protect from rapid depressurization rather than design for depressurization. | Design for depressurization, as well as protect from depressurization. |
| Internal hatches | Operable from the inside only (EVA hatch and Progress cargo hatch). | All hatches operable from both sides (except for the Airlock hatch). |
| Intermodule ventilation | Drag-through ducts that must be disconnected before the hatches can be closed. | Hard ducts that allow IMV with the hatches closed. |
| Fire protection | Non-flammable or slow-burning materials are used where possible. Smoke detectors and PFEs are provided. | Non-flammable or slow-burning materials are used where possible. Smoke detectors and PFEs are provided. |
| Emergency equipment – breathing masks | Emergency mask generates O2 by chemical reaction of CO2 and water vapor with the material in the mask. | Emergency mask has a supply of gaseous O2. |
| Overall water recovery architecture | Separate recovery of condensate, waste hygiene, and urine water; recovered condensate reserved for potable use; recovered urine reserved for electrolysis. | Recovered urine water is combined with all other waste waters and processed to potable specification for reuse in all applications. |
| Water quality measurement | On-line measurement of conductivity only. Off-line measurement of samples returned to Earth. | On-line measurement of conductivity, pH, iodine, and TOC. Off-line measurement of micro-organisms, TOC, and specific ions. |
| Biocide in water 2 | Ag (silver) | l2 (Iodine) |
| (Note: There is an integration concern that if the waters are mixed, Agl2 would precipitate out, removing biocide activity and potentially clogging lines.) | ||
| Metabolic design requirements | O2 consumption: 0.86 kg/day/person (1.89 lb/day/person); CO2 production: 1.00 kg/day/person (2.20 lb/day/person). |
O2 consumption: 0.84 kg/day/person (1.84 lb/day/person); CO2 production: 1.00 kg/day/person (2.20 lb/day/person). |
| Oxygen concentration | Materials must be compatible with 40% ppO2. | Materials must be compatible with 24.1% ppO2 (except for the Airlock, where the maximum is 30% ppO2). |
| Oxygen supply | During normal operation: 100% generated by electrolysis. During crew exchange or other off-nominal condition: 75% generated by electrolysis and 25% from perchlorate or other source. |
Initial operation: Supplied by the Russian Segment or Shuttle. After the Hab OGA is operating: 100% is generated by electrolysis. (O2 for EVAs is resupplied from the Space Shuttle in tanks.) |
| CO2 partial pressure 3 | 5.3 mmHg (0.10 psia) with a maximum 7.6 mmHg (0.147 psia). | See Figure 93. [1.0 HEU = 1.0 kg/person/day (2.2 lb/person/day) CO2 removed. The equation of the line is defined by: HEU³ 1.723 ppCO2 (mmHg)−0.37975 for 2.0² ppCO2² 3.9] |
| (Note: During crew exchange, the specifications allow 7.6 mmHg with peaks to 9.9 mmHg.) | ||
| Humidity removal | Moisture is removed from the atmosphere as necessary. Temperature control and humidity removal are separate functions. | Moisture is removed from the atmosphere continuously. Temperature control and humidity removal are performed by the same device. |
| Operating pressure | 79.9 to 114.4 kPa (11.6 to 16.6 psia); 93.0 kPa (13.5 psia) normal minimum. (In operation, the RS total pressure matches that of the USOS.) | 97.9 to 102.7 kPa (14.2 to 14.9 psia). 95.8 kPa (13.9 psia) normal minimum. |
| Crew accommodation | With the SM, three people normally with up to five during crew exchange. After activation of the Russian Life Support Module, six people normally with TBD during crew exchange. | After the Hab is activated, six people normally, and TBD during crew exchange (includes Space Shuttle and JEM/APM). |
| EVA atmosphere | Prior to activation of the Docking Module, venting of atmosphere in the Pirs airlock for EVA. After activation of the DM, recovery of atmosphere in the AL prior to EVA. | Recovery of atmosphere in the AL (Quest) prior to EVA. |
| EVA suits | 36.5 kPa (5.3 psia) (Orlan-M) | 29.66 kPa (4.3 psia) (EMU) |
| Shower water usage | One 10 L (0.35 ft³, 22 lb) shower per person each week. | 5.5 L (0.19 ft³, 12 lb) shower every 2 days per person. |
| Food supply | Almost all food is dehydrated and requires potable water to rehydrate. | Diet includes moist food, which provides a source of water to the system. |
| Potable water | Minerals are added to the processed condensate water, which add flavor and provide a pH-balanced water. | No additives to the potable water. |
| Hardware location | When possible, hardware items performing related or connected functions are located in the same module to avoid the need to plumb fluids between modules. | When possible, hardware items performing related or connected functions are located in the same module; however, fluids are plumbed between modules. |
| Hardware maintenance | Components are replaced after failure, or based on statistical expectation of failure. | Components are replaced after failure or, for limited-life items, on a scheduled basis. |
RS ECLS capabilities
The specified RS ECLSS capabilities are listed below:
Control Total Atmospheric Pressure
The atmospheric total pressure is manually monitored over the range of 0 to 960 mmHg (0.0 to 18.5 psia) with an accuracy of ±2 mmHg (0.04 psia). The atmospheric total pressure is automatically monitored over the range of 1 to 1,000 mmHg (0.02 to 19.4 psia) with an accuracy of ±30 mmHg (0.58 psia). The total pressure is maintained between 734 and 770 mmHg (14.2 and 14.9 psia) with a minimum pressure of 700 mmHg (13.5 psia). N2 is added to replenish losses, but the ppN2 is maintained below 600 mmHg (11.6 psia). The cargo vehicle has the capability to introduce atmospheric gases (nitrogen, oxygen, or air) into the habitat to maintain the atmospheric pressure.
Control Oxygen Partial Pressure
The ppO2 is monitored over a range of 0 to 300 mmHg (0 to 5.8 psia) with an accuracy of ±12 mmHg (0.23 psia). The ppO2 is maintained between 146 an 173 mmHg (2.83 and 3.35 psia) with a maximum concentration of 24.8 percent by volume. Oxygen is added at a rate of 0.86 kg/person/day (1.89 lb/person/day) for three people during normal operations and six people during crew transfer operations.
Relieve Overpressure
The total pressure is maintained below the maximum allowable design pressure for the ISS, the maximum allowable design pressure is 104.7 kPa (15.2 psia, 786 mmHg). The RS modules are designed to accommodate pressures as high as 128.8 kPa (18.7 psia, 970 mmHg).
Equalize Pressure
The pressure differential between adjacent, isolated volumes at 775 mmHg (15.0 psia) and 740 mmHg (14.3 psia) can be equalized to less than 0.5 mmHg (0.01 psia) within 3 min.
Control Atmospheric Temperature
The atmospheric temperature is monitored over the range of 15.5 to 32.2 °C (60 to 90 °F) with an accuracy of ±1 °C (2 °F). The atmospheric temperature in the cabin aisleway is maintained within the range of 18 to 28 °C (64 to 82 °F) and within ±1.5 °C (3 °F) of the selected temperature.
Control Atmospheric Moisture
The atmospheric relative humidity in the cabin aisleway is maintained within the range of 30 to 70 percent, the dewpoint within the range of 4.4 to 15.6 °C (40 to 60 °F), and the water vapor pressure is monitored over a range of 1 to 35 mmHg (0.02 to 0.68 psia) with an accuracy of ±1.5 mmHg (0.029 psia). For the Soyuz, while attached to the ISS, the dewpoint is maintained in the range of 4.4 to 14.0 °C (40 to 57 °F). Moisture removed as humidity condensate is delivered at an average rate of 1.5 kg/person/day (3.3 lb/ person/day) to the SM water processor.
Circulate Atmosphere Intramodule
The effective atmospheric velocity in the FGB cabin aisleway is maintained within the range of 0.05 to 0.2 m/sec (10 to 40 fpm). The effective atmospheric velocity pertains to the time-averaged velocity in the cabin, using averages over time periods sufficient to achieve stability. Two-thirds of the local velocity measurements are within the design range, with a minimum velocity of 0.036 m/sec (7.1 fpm) and a maximum velocity of 1.02 m/sec (200 fpm). Atmospheric velocities within 15 cm (6 in) of the cabin interior surfaces are not considered.
Circulate Atmosphere Intermodule
The SM exchanges atmosphere with the USOS at a rate of 60 to 70 L/sec (127 to 148 cfm).
Respond to Fire
Fire safety criteria are shown in figure 13. Isolation of the fire (by removal of power and forced ventilation in the affected location) will occur within 30 sec of detection. Detection of a fire will initiate a Class I alarm and a visual indication of the fire event will be activated. Forced ventilation between modules will stop within 30 sec of annunciation of a Class I fire alarm. PBA’s and PFE’s are provided.
Fires will be suppressed using PFE’s within 1 min of suppressant discharge. The capability to restore the habitable environment after a fire event is present.
Respond to Rapid Decompression
A decompression of more than 90 mmHg per hr (1.74 psi per hr) will be detected and a Class I alarm will be activated when such a decompression rate is detected.
Respond to Hazardous Atmosphere
PBA’s (breathing masks) with a 15-min supply of O2 (generated by chemical reaction from CO2 and water vapor) are provided for each crew member. The FGB provides such capability for three people.
Accommodate Crew Hygiene and Wastes
Facilities are provided for personal hygiene and collection, processing, and disposal of crew metabolic waste. The wastes include menstrual discharge and associated absorbent material; emesis; fecal solids, liquids, gases, and particulates; urine and associated consumable material; soap, expectorants, hair, nail trimmings, and hygiene water; and crew wastes collected during EVA’s. Facilities are provided for personal grooming, including skin care, shaving, hair grooming, and nail trimming. Simultaneous whole body skin and hair cleaning are accommodated.
Control CO2
The atmospheric ppCO2 is maintained at a maximum daily average of 4.50 mmHg (0.08 psia), with peak levels no greater than 7.60 mmHg (0.147 psia). CO2 is removed and disposed of at an average rate of 0.96 kg/person/day (2.12 lb/person/ day) for three people during normal operations and six people during crew exchanges. The ppCO2 level is monitored over a range of 0.00 to 25.00 mmHg (0.00 to 0.48 psia) with an accuracy of ±2.00 mmHg (0.038 psia).
Control Gaseous Contaminants
Atmospheric trace gas contaminants that are generated during normal operations are maintained at levels below the Maximum Allowable Concentration (MAC) levels. The removed gases are discarded. The MAC levels are listed in table 6. Provisions are made to accom-modate the U.S. air monitoring equipment according to SSP 50065, the CHeCS to RS ICD.
Control Airborne Particulate Contaminants
The daily average concentration of airborne particulates is limited to less than 0.15 mg/m3 for particles from 0.5 to 300 microns in size.
Control Airborne Microbial Growth
The daily average concentration of airborne microorganisms is limited to less than 1,000 CFU/m3. (Present Russian capabilities can limit airborne microbes to 500 CFU/m3 for bacteria and to less than 100 CFU/m3 for fungi.) Microbial monitoring is performed using U.S. and Russian equipment.
Provide Water for Crew Use
An average of 2.5 kg/person/day (5.5 lb/person/day) of potable water is provided for six people for food rehydration, consumption, and oral hygiene. The SM provides an average of 1.1 kg/person/day (2.42 lb/person/ day) of hygiene water for three people. After activation of the LSM, the LSM and SM combined provide an average of 4.53 kg/person/day (9.96 lb/person/day) of hygiene water. The qualities of the waters meet the specifications defined in the “System Specification for the International Space Station,” SSP41000E, 3 July 1996. Humidity condensate is processed to potable water quality. Urine is collected and disposed of at an average rate of 1.2 kg/person/day (2.64 lb/person/day). This function is performed in the SM until the LSM is activated. After activation of the LSM, urine is processed and provided to the Elektron to produce breathing oxygen.
To monitor the water quality, the SM accommodates U.S. provided water monitoring equipment, according to SSP 50065, the CHeCS to RS ICD. Sample ports for manual collection of water samples are provided to facilitate off-line monitoring and analysis of processed water, and for archiving of water samples.
Support Station Ingress
The DC supports the controlled, tethered entry into the RS by a person in a pressurized spacesuit. The DC supports repressurization from vacuum to the RS atmospheric pressure at a nominal repressurization rate of 5 mmHg per sec. The maximum emergency repressurization rate is 10 mmHg per sec. In the event of an emergency during an EVA, an unimpaired crew member can reenter the AL within 30 min.
Distribute Gases to User Payloads
This capability is not presently required on the RS.
Diagrams
Environmental Control and Life Support System (ECLSS) (external link, 402 KB.) Page illustration from the Reference Guide to the International Space Station PDF.
