4 Requirements and Constraints
4.1 General Constraints
The constraints of the system define the boundaries within which the whole system can be operated. The 23 major ones which are needed for this preliminary design are outlined below. A comprehensive list of complementary constraints for the design of a ECLSS can be found in [37]. Every technology to be considered must adhere to these constraints.
a.
Maintain the EnvironmentConstraint: The life support system shall maintain an environment in the crew habitat that is adequate to support and maintain crew health, well-being, and comfort, and that is adequate to support and maintain satisfactory equipment operation. [37]
b. Safety
Constraint: No materials, systems or operations shall be used if they are a threat for the crew.
Rationale: Each concept is considered with respect to fire, contamination, explosion hazards, hot spots, bacteriological problems, and crew hazards to determine if any of these are present. If a problem cannot be eliminated reasonably by careful design, additional control equipment, using different materials, or similar engineering solutions, the concept is eliminated. Hazards are investigated and considered during normal operation, off-design operation, and maintenance downtime.
c. Power Capability
Constraint: The maximum available power is 200 kW at the vicinity of Earth and 86,6 kW at Mars.
Rationale: The maximum available solar irradiance at Mars is 43.3 % that at Earth.
d. Power Consumption
Constraint: The power consumption of the life support system shall be lower than 69.28 kW, with the goal to minimize it.
Rationale: The stated constraint stands for a power use of 80 % of the total available power at mars orbit. The remaining 20 % are assumed to be used by other systems like avionics or lighting. The goal to minimize the power consumption should be used during normal mission operations, but not for emergency phases.
e. Thermal Heat Rejection Capability
Constraint: The maximum heat rejection capability is 100 kW.
f. Thermal Heat Production
Constraint: The thermal heat production of the life support system shall be lower than 80 kW, with the goal to minimize it.
Rationale: The stated constraint stands for a thermal heat rejection of 80 % of the total heat rejection capability. The remaining 20 % are assumed to be used by other systems like avionics. The constraint should be used during normal mission operations, but not for emergency phases.
g. Failure Tolerance
Constraint: All critical systems essential for crew safety shall be designed to be two-fault tolerant, except inter- and intramodule ventilation, heat collection and distribution, and response to hazardous atmosphere, which shall be
single-fault tolerant. When this is not practical, systems shall be designed so that no single failure shall cause loss of the crew. For the purpose of this requirement, maintenance can be considered as the third leg of redundancy so long as mission operations and logistics resupply permit it.
Rationale: For the ISS, the ECLSS is designed that no combination of two failures or operator errors result in a catastrophic hazard, and no single failure or operator error can result in a critical hazard. Maintenance and resupply is a key element of the reliability of the system. Fault tolerance can be seen as the minimum acceptable redundancy, but also has to include cross-link capabilities.
For long duration missions, maintenance and system reconfiguration is a must.
[38]
h. Contingency
Constraint: Life support equipment and commodity stores shall be sized to support a 10 % safety margin on mission duration. [37]
i. Maintenance
Constraint: The life support system shall enable maintenance by a trained crew member. The goal is to enabled this task as simple as possible and reduce it to a minimum of needed time, both for each task and for the entirety of all maintenance work.
j. Level of Repair
Constraint: Components shall be designed and built to be accessible for in-flight maintenance of components inside the box.
Rationale: Levels of repair can range from large assemblies, such as Orbital Replacement Unit (ORU), to small internal components. Repair of failed components at lower levels allows for sparing smaller components and focusing on the items most likely to fail. In many components, the largest mass and volume are in casings and mounting hardware that are very unlikely to fail.
Failures are much more likely to occur in certain internal components such as electronic cards, motors, switches, seals, and numerous other small items.
However, repairs at the box level can be much simpler to execute and thus to train the flight crew to perform. Lower repair levels also require more specialized tools, workbenches, and test equipment to verify successful repairs and require much more time. [39]
k. Commonality
Constraint: Commonality should be used to reduce spare parts and for reducing development and procurement costs.
Rationale: Commonality of components, materials, and tools can save much mass and volume needed for spares, even more if interchangeable components are considered. This includes cannibalism from other non-critical sub-systems.
[39]
l. Redundancy
Constraint: Redundancy should be considered in the design of the different subsystems to increase the reliability of the system.
Rationale: Redundancy, Commonality and Failure Tolerance are heavily interconnected. Whenever rapid failover time is not required, spares are often the better alternative as built-in redundancy. Also, functional redundancy of different sub-system reduces further the need for component redundancy or spare parts. In general 2 types of redundancy can be considered: active or standby. [39]
m. Reliability
Constraint: The assemblies and subsystems of the life support system shall be maintaining a reliability of at least 0.9984. A lower reliability is possible when maintenance and redundancy is considered. The goal is highly reliable system with reliability of at least 0.995.
Rationale: Reliability directly influences the loss of crew (LoC) and the loss of mission. Because multiple failures for a mission of several months must be considered, reliability of components and the whole system plays a major role in the design of the ECLSS. While most components are relatively reliable, it is not likely that the general reliability of proven technologies will improve remarkable. The above stated 0.995 reliability of the ECLSS means, there should be a likelihood of a system failure no greater than 1 in 200 [40, p. 34].
Because the ECLSS subsystem are critical for crew survival and a failure of one subsystem results in loss of crew, the total system can be assumed to be in series. Therefore, they must all have the same reliability. The reliability of a system with elements in series can be calculated with Eq. ( 4-1 ) when the MTBF of the components is known. With the above assumption of equal reliability of every element, Eq. ( 4-1 ) can be simplified to Eq. ( 4-2 ). To get the required reliability of the elements from a series system with equal importance, Eq. ( 4-2 ) could be converted to Eq. ( 4-3 ) to get the stated reliability of 0.9984 for the 3 critical subsystems5. For the reliability of a system with parallel elements, Eq. ( 4-4 ) could be used. [41–43]
For all equations below, a constant failure rate was assumed, meaning components are in the middle region of their lifetime and therefore increasing time does not increase the failure rate. While this is an assumption, it is reasonable given the fact that LSS technologies are mature and the failure rates for components and systems are well known. Therefore, the systems are not susceptible to the higher failure rates at the early stage. Further it is assumed that components and systems sent into space are not reaching the end of their lifetime, as this would not be practical. Therefore, systems are also not subject to increased failure due to components reaching the end of their life span.
𝑅𝑠𝑒𝑟𝑖𝑒𝑠(𝑡) = ∏𝑛𝑖=1𝑒𝑙𝑒𝑚𝑒𝑛𝑡𝑠𝑅𝑖(𝑡) = 𝑒−𝑡 𝜆𝑎𝑠𝑠𝑒𝑚𝑏𝑙𝑦 Eq. ( 4-1 )
5 The 3 considered critical subsystems are temperature and humidity control, atmosphere revitalization, and water recovery and management. Other subsystems like atmosphere control and supply are considered as independent.
𝑅𝑠𝑒𝑟𝑖𝑒𝑠,𝑠𝑎𝑚𝑒(𝑡) = 𝑅(𝑡)𝑛 Eq. ( 4-2 ) modular and/or scalable system for different crew sizes.
o. Consideration of Schedule
Constraint: All subsystems shall consider the predefined schedules in chapter 3.2.3. From the nominal schedules6, the one with the lowest impact on ECLSS mass, volume, and power has to be chosen. The goal is a ECLSS that can handle all defined schedules.
Rationale: It is important to know the daily variation and range of metabolic loads and demands to plan a robust LSS design. [17, p. 5]
p. TRL
Constraint: The TRL of components and assemblies shall be greater than 4.
Rationale: A TRL of 5 is described as a "Component and/or breadboard validation in relevant environment" [19]. For the scope of this thesis, a TRL of at least 5 is considered, because an even lower TRL would mean the technology is far from mature and therefore has a lower reliability, higher costs for development and increasing risk for under-performance.
q. Internal Interfaces
Constraint: Interfaces between different subsystems of the life support system shall be provided. The goal is to minimize the number and mass flows between different subsystems or assemblies.
Rationale: A low number of interfaces means a simpler and safer system. It would also be more cost-effective.
6 All schedules with exercise are considered as nominal schedules.
r. Use Synergetic Effects
Constraint: Synergetic effects between subsystems or components should be observed and used.
Rationale: Synergy effects defined here, provide secondary functions for components or subsystems in addition to the main function, e.g. water can be used for drinking and oxygen generation.
s. Pressurized Volume
Constraint: The pressurized volume of the system will be 1,090.28 m³ for a crew up to 40 people and 1,710.76 m³ for crew above 40 persons.
Rationale: The storage area in the Evolved-SpaceHab design has to be accounted as habitable area too, because only the upper part would be too small for 100 people (see section 3.2.2.1 for further details). The pressurized volume of the storage area is around 620.47 m³.
t. Volume Usage
Constraint: The maximum volume of the life support system shall be less than one third of the pressurized volume. This includes all supply masses of gases, liquids, food and spare parts. The goal is to minimize the occupied space.
Rationale: The maximum allowable volume for the SpaceHab is 363.33 m³ and 570.33 m³ for the Evolved-SpaceHab design.
u. Payload Mass
Constraint: The useable total payload mass will be in the range of 200,000 to 450,000 kg, depending on the mission scenario (see chapter 3.2 Operational Scenarios).
Rationale: A payload boundary of 200,000 kg and 450,000 kg is given at a ∆v of 6 km s-1 or 4 km s-1 respectively. The payload of 450,000 kg is only possible with a transfer in LEO, because the booster has a maximum capability of only 300,000 kg to LEO. (see section 2.4 Payload Capacity for further details) v. Mass Usage
Constraint: The maximum mass of the life support system shall be less than 75 % of the total payload mass. This includes all supply masses of gases, liquids, food and spare parts. The goal is to minimize the needed total mass.
Rationale: Depending on the considered mission time, the maximum allowable ECLSS mass is 150,000 kg for 88 days and 337,500 kg for 211 days.
w. Mission Duration
Constraint: The mission duration will be between 88 and 211 days, depending on mission scenario (see section 3.2 Operational Scenarios). The worst-case scenario of 211 days, which includes a margin of 10 % on the longest trip time of 192 days, must be feasible.
Rationale: The best-case of 80 days is given in the year 2035 with a ∆v of 6 km s-1. Added to this are a margin of 10 %, or 8 days. The worst-case is 192 days in the years 2024 and 2029 at ∆v of 4 km s-1. See also section 2.4 Payload Capacity for further details.