Required hygiene water mass
6 Life Support Systems
6.1 Atmosphere Control and Supply
6.1.1 High-pressure Storage
Figure 6-3: High pressure gaseous storage concept [50, p. 75]
6.1.1.1 Description
High-pressure storage tanks for O2 and N2 are at ambient temperature with an optimum pressure-to-volume ratio at a pressure of a few million Pa. As can be seen in Figure 6-3, this concept is very simple with only a limited number of parts and is therefore one of the most reliable and safest systems. For small amounts of fluid these systems are the optimum but the disadvantages are the high mass at high pressures and
sometimes a material compatibility concern. For example titanium could not be used for oxygen storage. [2, 13, 50, p. 71]
6.1.1.2 Data and Sizing for Repressurisation and Leakage
Two geometries for high-pressure storage tanks can be considered, spherical and cylindrical tank. Spherical tanks have the advantage that they are very strong structures with no weak points due to the even distribution of stresses on the sphere´s surface. It is also the most weight efficient shape. Therefore, they are selected over cylindrical ones. The disadvantage is that they are more complicated to manufacture and therefore more expensive.
There are different types of pressure vessels. Type I vessels are all-metal construction.
These are by far the cheapest types, but also the heaviest and that with the lowest possible pressure of all pressure vessel types. Type II are mostly steel or aluminum and have composite overwrap to save mass. Type III has only a metal liner with complete composite shell which hold the mechanical loads. The same is true for the Type IV pressure vessel with a polymer liner. Type III and Type IV tanks cost around twice as much as Type II vessels or 3.5 times more than Type I vessels comparing to a mass saving of around 30 % or 75 % respectively. Current development efforts are to get an 82.5 MPa rating for Type IV vessels, which would mean an even further mass saving. The newest development are Type V tanks which consist of composite without a liner. These vessels have even further mass savings and therefore are predestinated for space applications. A new developed tank for the ISS, called NORS for Nitrogen/oxygen recharge system, has a maximum pressure of 41.37 MPa and is a full-composite tank. Additional the propellant tanks of the ITS will be Type V pressure vessels and therefore it is assumed that a possible high pressure storage will be consisted of full-composite tanks. [2, 56, 57]
Oxygen and Nitrogen could potentially be already mixed in the tank. But such a system would be much less flexible and the mass and volume is comparable to a separate system. For that reason separate tanks for O2 and N2 are used. [50, p. 72]
A heater is needed to prevent regulator freeze-up and to warm the fluid for cabin delivery. This heater could use waste heat from the ITCS and would have the benefit to reduce the load on the radiators. But this effect is only appreciable during emergency repressurisation or when the system is constantly used in a storage system. [50, p. 76, 50, p. 72]
For the nitrogen tank, the used ratio of tank-mass to gas-mass (kg kg-1) is 0.556 and for the oxygen tanks it is 0.364 [10, p. 55]. It is not stated for what pressure these values are and if needed equipment like valves are already included. But comparing this with other data from type IV pressure vessels, it is very likely that this values are at least for a type IV or even type V pressure vessel without any additional equipment [58].
The masses for the tanks can be calculated by multiplying the needed N2 and O2
masses given in Table 4-2 and Table 4-4 respectively with the tank ratio mentioned above. For the required valves, flowmeter, tubes etc., a conservative 20 kg per tank is assumed. The last point to consider for the mass is the contingency factor. Storage systems are commonly used in spaceflight and therefore a CoF of 3 is assumed. With
these assumptions, the total mass for the storage tanks can be calculated. A breakdown of the masses is given in Table 6-1.
Eq. ( 6-1 ) can be used for the volume estimation of the tanks. The O2 and N2 masses are stated in Table 4-4 and Table 4-2 and their respective densities at 34.5 MPa of 451.12 kg m-³ and 336.35 kg m-³ [59]. The wall thickness including insulation is assumed to be 10 mm which must be accounted for in the final volume calculation.
The maximum filling degree is assumed to be 95 %. Because the tanks are considered to be outside of the pressurized compartment, no stowage volume factor is applied.
But volume for tank equipment will be inside the pressurized volume and must therefore considered. For simplicity, a cube with a side length of 40 cm is assumed to be sufficient, which would be 0.064 m³ per tank.
𝑉𝑡𝑎𝑛𝑘 = ( √𝑚𝜌𝑔𝑎𝑠
For the number of tanks, it must be considered that they are assumed to be outside the pressurized compartment. The only bigger unpressurized volume for the Evolved-SpaceHab design is the space between the lowest deck and the propellant tank as can be seen in Figure 6-4. To geometrically determine the maximum usable area between the outer wall, the floor of the lowest deck and the propellant tank wall, a two-dimensional approximation is used. The dome of the propellant tank is assumed to be a half-sphere, or in 2-D a circle, with a radius of 6 m. With the help of a triangle, the maximum tank volume in this space can be calculated. A right, isosceles triangle must be used for this. Therefore, the isosceles sides of the triangle are 3.48 m, subtraction a margin of 10 % gives a side length of 3.13 m (𝑎). With the Pythagorean theorem, the length of the hypotenuse can be calculated to 4.43 m (𝑐). With Eq. ( 6-3 ), the maximum spherical volume inside this space can be calculated to 3.22 m³.
𝑉𝑚𝑎𝑥,𝑠𝑝ℎ𝑒𝑟𝑒 = 4
• 𝑉𝑚𝑎𝑥,𝑠𝑝ℎ𝑒𝑟𝑒 [m³] - maximum spherical volume inside unpressurized space
• 𝑟𝑐𝑖𝑟𝑐𝑙𝑒 𝑖𝑛 𝑡𝑟𝑖𝑎𝑛𝑔𝑙𝑒 [m] - maximum radius of circle in triangle
• 𝐴𝑡𝑟𝑖𝑎𝑛𝑔𝑙𝑒 [m²] - triangle area
• 𝑈𝑡𝑟𝑖𝑎𝑛𝑔𝑙𝑒 [m] - triangle extensive
• 𝑎triangle , 𝑐 [m] - length of triangle sides
Figure 6-4: Unpressurized space between lowest deck and propellant tank [11]
Using Eq. ( 6-1 ) and Eq. ( 6-3 ), the number of required tanks (𝑛𝑡𝑎𝑛𝑘𝑠) can be calculated with Eq. ( 6-7 )10. For oxygen, it is further assumed, that at least 2 tanks are considered for safety reasons.
𝑛𝑡𝑎𝑛𝑘𝑠 = 𝑉𝑡𝑎𝑛𝑘
𝑉𝑚𝑎𝑥,𝑠𝑝ℎ𝑒𝑟𝑒 Eq. ( 6-7 )
This system is mostly passive. The only active component is the high-pressure transducer. A Ultra Precision Pressure Transducer from Honeywell has a power input of under 0.3 W [60], no losses in the transducer and the wiring are stated. To make up for these losses and to remain conservative, 1 W instead of 0.3 W is used for the required power in the further calculations, including an CoF of 3. With 2 O2 tanks and assuming double redundancy per tank, this leads to a total power requirement of 9 W.
Since no active cooling or heating elements shall be integrated the thermal balance is negligible.
High-pressure N2 and O2 storage systems have been used in flight for years, which means the system has a TRL of 9.
For a reliability standpoint, the mean time between failure (MTBF) can be estimated to 328,000 hours when assuming 2 spares for each pressure regulator which is the component with the lowest MTBF in this system [50, pp. 73-74]. The reliability can then be calculated with Eq. ( 4-1 ).
10 Volume for tank equipment is not stored in this space and must technically speaking be subtracted.
Because this volume is only minor compared to the tank volume, this is neglected.
It is further assumed that the system is managed automatically so no crew time is needed for operation or maintenance.
The results from the calculations and considerations above for the O2 system can be found in Table 6-1 and for the N2 system in Table 6-2 below.
Table 6-1: Properties of the O2 high-pressure storage system for repressurisation and leakage
Parameter Value Unit
Number of required O2 tanks (SpaceHab) 2 [-]
Total O2 tank mass (SpaceHab, empty) 156.12 [kg]
Total O2 tank mass (SpaceHab with O2) 462.63 [kg]
Total O2 tank volume (SpaceHab) 0.88 [m³]
Number of required O2 tanks (Evolved-SpaceHab) 2 [-]
Total O2 tank mass (Evolved-SpaceHab, empty) 220.73 [kg]
Total O2 tank mass (Evolved-SpaceHab with O2) 699.57 [kg]
Total O2 tank volume (Evolved-SpaceHab) 1.30 [m³]
Total O2 tank equipment volume 0.13 [m³]
Required power 9 [W]
TRL 9 [-]
Reliability 0.9936 [-]
Table 6-2: Properties of the N2 high-pressure storage system for repressurisation and leakage
Parameter Value Unit
Number of required N2 tanks (SpaceHab) 2 [-]
Total N2 tank mass (SpaceHab empty) 605.65 [kg]
Total N2 tank mass (SpaceHab with N2) 1,612.23 [kg]
Total N2 tank volume (SpaceHab) 3.39 [m³]
Total N2 tank equipment volume (SpaceHab) 0.13 [m³]
Required power (SpaceHab) 9 [W]
Number of required N2 tanks (Evolved-SpaceHab) 3 [-]
Total N2 tank mass (Evolved-SpaceHab empty) 943.09 [kg]
Total N2 tank mass (Evolved-SpaceHab with N2) 2,514.60 [kg]
Total N2 tank volume (Evolved-SpaceHab) 5.25 [m³]
Total N2 tank equipment volume (Evolved-SpaceHab) 0.19 [m³]
Required power (Evolved-SpaceHab) 14 [W]
TRL 9 [-]
Reliability 0.9847 [-]
6.1.1.3 Data and Sizing for Storage System
The needed oxygen mass for consumption by the crew depends on the selected schedule. For a description of the different considered schedules see chapter 3.2.3 and for the required masses see requirement 4.4.1.d. The O2 tank design for a storage system is only depending on the sum of the total consumption during the mission.
It is further assumed, that for safety reasons, the repressurization tanks and O2 tanks for consumption are separated. This enhances redundancy and results in a more reliable system.
For the tank masses, the same ratio as for the O2 repressurization tanks are used.
The required O2 tank volumes for a storage system can be calculated by Eq. ( 6-1 ), where the gas mass is the required mass from 4.4.1.d. and the density of O2 at 34.5 MPa is 451.12 kg m-³.
The same transducer as for the repressurization system is assumed, with the same parameters.
The parameters TRL and MTBF are the same as for the oxygen tanks for repressurization.
Table 6-3 represent a breakdown of the O2 tanks for the storage system. Only the best-case with 88 days and 12 crew members and the worst-best-case scenario for 211 days and 100 crew members are shown for clarity. For both scenarios, a nominal schedule is assumed.
For evaluation that the calculated number of tanks can fit inside the unpressurized space (shown in Figure 6-4), the circumference is used. By applying Eq. ( 6-8 ) and using 0.82 m as usable radius, the circumference of the unpressurized space can be calculated to 35.13 m. For the worst-case scenario with 100 people, 211 days and Evolved-SpaceHab design, 8.01 m of this circumference is used by tanks, or 27.12 m are still left. Actually 42 tanks with maximum radius could be installed inside the unpressurized space.
𝑈𝑢𝑛𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑖𝑧𝑒𝑑 𝑠𝑝𝑎𝑐𝑒 = 𝑟𝑢𝑛𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑖𝑧𝑒𝑑 𝑠𝑝𝑎𝑐𝑒 𝜋 Eq. ( 6-8 ) 𝑟𝑢𝑛𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑖𝑧𝑒𝑑 𝑠𝑝𝑎𝑐𝑒 = 𝑟𝑆𝑝𝑎𝑐𝑒𝐻𝑎𝑏− 𝑟𝑐𝑖𝑟𝑐𝑙𝑒 𝑖𝑛 𝑡𝑟𝑖𝑎𝑛𝑔𝑙𝑒 Eq. ( 6-9 ) with:
• 𝑈𝑢𝑛𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑖𝑧𝑒𝑑 𝑠𝑝𝑎𝑐𝑒 [m] - circumference of unpressurized space
• 𝑟𝑢𝑛𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑖𝑧𝑒𝑑 𝑠𝑝𝑎𝑐𝑒 [m] - usable radius of unpressurized space
• 𝑟𝑆𝑝𝑎𝑐𝑒𝐻𝑎𝑏 [m] - radius of SpaceHab (6 m)
• 𝑟𝑐𝑖𝑟𝑐𝑙𝑒 𝑖𝑛 𝑡𝑟𝑖𝑎𝑛𝑔𝑙𝑒 [m] - maximum radius of circle in triangle
Table 6-3: Properties of O2 high-pressure storage system for consumption
Parameter Value Unit
Number of required O2 tanks (best-case) 2 [-]
Total O2 tank mass (best-case, empty) 405.60 [kg]
Total O2 tank mass (best-case with O2) 1,377.54 [kg]
Total O2 tank volume (best-case) 2.48 [m³]
Total O2 tank equipment volume (best-case) 0.13 [m³]
Required power (best-case) 9 [W]
Number of required O2 tanks (worst-case) 21 [-]
Total O2 tank mass (worst-case, empty) 7,713.71 [kg]
Total O2 tank mass (worst-case with O2) 27,134.15 [kg]
Total O2 tank volume (worst-case) 47.29 [m³]
Total O2 tank equipment volume (worst -case) 1.34 [m³]
Required power (worst-case) 94 [W]
TRL 9 [-]
Reliability (best-case) 0.9936 [-]
Reliability (worst-case) 0.9847 [-]