Required hygiene water mass
6 Life Support Systems
6.1 Atmosphere Control and Supply
6.1.2 Cryogenic Storage
Figure 6-5: Cryogenic storage system schematic [50, p. 97]
6.1.2.1 Description
Two alternatives are considered for cryogenic O2 storage. Dedicated storage tanks are of similar setup as the high-pressure tanks in section 6.1.1, and a size increase of the already existing O2 propellant tank. Cryogenic N2 storage is of comparable structure as the high-pressure tanks.
Subcritical cryogenic storage tanks, like the one in Figure 6-5, operate at relatively low pressures (around 340 kPa) but need to be isolated to hold the low temperatures and maybe require active thermal cooling. The envisioned cryogenic storage system will operate at a pressure of 34.5 MPa. Such a high pressure for cryogenic storage are common in industry [61, 62]. This approach has several advantages over the high-pressure tanks like a high storage density due the fact that the O2 or N2 are liquid. That means that the tanks have reduced volume. Eventually it could be used as a refrigeration source when designed properly. But there are also some disadvantages
like the sensitive to heat leaks or that the fluid delivery is more complex. Additional, due the fact that some venting is occurring and that the specific tank mass is higher, the tank often weigh more than a high-pressure tank. [2, 11, 50, pp. 93-95, 61]
6.1.2.2 Data and Sizing for Repressurisation and Leakage
The same shape consideration as that of the high-pressure vessel under 6.1.1.2 are used and therefore a spherical shape is considered for both the O2 and N2 tanks.
For flexibility and safety reasons, the species are in separate tanks. SpaceX uses a cryogenic temperature of 66.15 K (-207°C) for their Falcon 9 LOX system which is slightly lower than the boiling point of O2 of 90.19 K (−182.96 °C). For calculations, a temperature of 73.15 K for the liquid O2 is assumed. The boiling point of N2 is 77.36 K (−195.8 °C). For simplification, the same temperature for liquid N2 as for O2 is assumed.
The density of O2 and N2 at the cryogenic temperature of 73.15 K is around 1268.76 kg m-3 and 883.42 kg m-3 respectively. When further assuming that all tanks should be filled at 95 % at beginning of the mission, the required volumes can be calculated by dividing the needed masses stated in Table 4-2 through the densities to get to the required fluid volumes stated in Table 6-4. For the wall, only a single wall is assumed.
Normally double walls are needed, because a vacuum is generated between the two walls to reduce the boil-off rate. Due to the fact that the tanks are assumed outside the pressurized section of the SpaceHab, a vacuum is already given and therefore only one wall is needed. The same type V for the wall material is assumed as for the high-pressure vessel. This is again in compliance with the proposed propellant tank design by SpaceX. The wall thickness is thus assumed to be 5 mm.
Table 6-4: Minimum required fluid volumes for cryogenic repressurization storage
Volume of fluid (m³) O2 N2
SpaceHab 0.24 1.14
Evolved-SpaceHab 0.38 1.78
The used tank-to-gas mass-ratio is 0.429 for the O2 storage [63] and 0.524 for the N2
tank [10, p. 55]. The same assumptions as stated for the high-pressure storage tanks are used.
Insulation is needed to prevent a pressure rise due to warming of the fluid. Normally a vacuum is used to reduce or prevent heat loading due to gas conduction. Because it is assumed that the tanks for the analyzed system are outside the crew compartments and not in the pressurized section of the SpaceHab, a near perfect vacuum can be assumed. But even then, the tanks can be heated up by radiation. Because of this, a multi-layer insulation shield around the tanks is considered to minimize the radiation effects. The heat transfer though MLI can be defined by Eq. ( 6-10 ). When assuming a 40 layer MLI consisting of aluminum foils and the tank wall has the same temperature as the liquid (73.15 K) and a conservative 200 K environment, the heat leak is around 9.0545*10-2 W m-2. When considering the volumes of the tanks stated in Table 6-5 and Table 6-6, the total heat transfer through the MLI is between 0.2820 W and 0.8063 W.
Dividing the total heat transfer through the vaporization enthalpy of 214 kJ kg-1 for O2
and 199 kJ kg-1 for N2, gives the needed venting consumption or boil-off rate. The
lowest is for one O2 tank and assuming the shortest mission duration of 88 days with SpaceHab design which would be 12.02 kg of O2, or 7.71 % of the oxygen in the tank.
The highest is for one N2 tank with the Evolved-SpaceHab design which is 36.93 kg or 4,64 % of the nitrogen in the tank. Because of this low boil-off rates, no active cooling is considered. The thickness of such a MLI shield would be around 42.7 mm. [64]
𝑞̇ = ( 𝜀𝑀𝐿𝐼
For power calculations, the same transducers as described under 6.1.1.2 are used with the same total needed power. Because the system is otherwise passive, no additional power is required.
Cryogenic N2 and O2 storage systems have already been used in flight, which means the system has a TRL of 9.
The MTBF for the cryogenic system with minimal spares is 81,400 hours. [50, p. 96]
Like for the high-pressure system it is assumed that the system is managed automatic and consequently no crew time is needed for operation or maintenance.
The alternative concept for a cryogenic storage system is the use of the O2 propellant tank of the SpaceHab. Because it is not clear if this approach is feasible, since the propellant tank is autogenous pressurized, it is not further considered in this thesis.
The calculated values are showing, what considerable mass and volume could potentially be saved when such an approach is used. The N2 storage system stays the same. To accommodate the O2, the propellant tank must be slightly increased. The measured diameter in Figure 2-2 is around 5.27 m which leads to a total volume of 76.70 m³ for the spherical O2 propellant tank. It is further assumed that the tank is filled at 95 % at the beginning of the mission. It is unlikely that the pressure of such a big tank will be the same as the previously calculated O2 storage tank. Hence, a pressure of 1 MPa is assumed with the same temperature of 73.15 K. This leads to a density of 1,224 kg m-³ for the oxygen. With this assumption, it can be calculated that there would be 89,182.89 kg of O2 in the tank. When the required oxygen for repressurisation and leakage are added to this tank (see Table 4-2), then there must be up to 89,661.74 kg O2 into the tank, a O2 mass increase of only 0.54 %. Using Eq. ( 6-2 ), the diameter of the tank must be 9.4 mm larger. This leads to a tank volume increase of 0.41 m³.
For the mass calculations, it is assumed that T1000G from TORAYCA® is used by SpaceX, as this is the most advanced carbon fiber created to date. T1000G has a density of 1,800 kg m-³ [65]. The Volume of fibers in a composite is normally 60%, where the other 40 % are an epoxy resin. Epoxy resins have a density of around 1,100 kg m-³ which means the composite has a density of around 1,520 kg m-³. With an expected wall thickness of 10 mm, the mass increase of the O2 propellant tank is maximal 4.75 kg.
The results from the calculations and considerations above for the O2 system can be found in Table 6-5 and that for the N2 system in
Table 6-6 below.
Table 6-5: Properties of the cryogenic O2 storage system for repressurisation and leakage
Parameter Value Unit
Number of required O2 tanks 2 [-]
Total O2 tank mass (SpaceHab, empty) 189.54 [kg]
Total O2 tank mass (SpaceHab with O2) 525.24 [kg]
Total O2 tank volume (SpaceHab) 0.52 [m³]
Growth O2 propellant tank mass (SpaceHab, empty) 3.04 [kg]
Growth O2 propellant tank mass (SpaceHab with O2) 309.55 [kg]
Growth O2 propellant tank volume (SpaceHab) 0.26 [m³]
Total O2 tank mass (Evolved-SpaceHab, empty) 268.04 [kg]
Total O2 tank mass (Evolved-SpaceHab with O2) 781.41 [kg]
Total O2 tank volume (Evolved-SpaceHab) 0.70 [m³]
Growth O2 propellant tank mass (Evolved-SpaceHab, empty) 4.75 [kg]
Growth O2 propellant tank mass (Evolved-SpaceHab with O2) 483.59 [kg]
Growth O2 propellant tank volume (Evolved-SpaceHab) 0.41 [m³]
Total O2 tank equipment volume 0.13 [m³]
Required power 9 [W]
TRL 9 [-]
Reliability (best-case) 0.9744 [-]
Reliability (worst-case) 0.9397 [-]
Table 6-6: Properties of the cryogenic N2 storage system for repressurisation and leakage
Parameter Value Unit
Number of required N2 tanks 2 [-]
Total N2 tank mass (SpaceHab empty) 613.79 [kg]
Total N2 tank mass (SpaceHab with N2) 1,697.21 [kg]
Total N2 tank volume (SpaceHab) 1.71 [m³]
Total N2 tank mass (Evolved-SpaceHab empty) 921.48 [kg]
Total N2 tank mass (Evolved-SpaceHab with N2) 2,586.28 [kg]
Total N2 tank volume (Evolved-SpaceHab) 2.50 [m³]
Total N2 tank equipment volume 0.13 [m³]
Required power 9 [W]
TRL 9 [-]
Reliability (best-case) 0.9744 [-]
Reliability (worst-case) 0.9397 [-]
The storage system for consumption consist of cryogenic oxygen tanks, where most parameters are identical to the O2 repressurisation system. Notable are the fact, that the mass of the cryogenic tanks is around 10 % heavier than a high-pressurization system despite that 14 tanks less are required. This means that for the worst-case scenario of 100 people and 211 days, nearly 3 tons of mass could be saved when no cryogenic storage system is used. The properties of the cryogenic storage system for a best-case and worst-case scenario are listed below, including the propellant tank growth.
Table 6-7: Properties of the cryogenic O2 storage system for consumption
Parameter Value Unit
Number of required O2 tanks (best-case) 2 [-]
Total O2 tank mass (best-case, empty) 562.81 [kg]
Total O2 tank mass (best-case with O2) 1,534.75 [kg]
Total O2 tank volume (best-case) 1.17 [m³]
Total O2 tank equipment volume (best-case) 0.13 [m³]
Growth O2 propellant tank mass (best-case, empty) 9.62 [kg]
Growth O2 propellant tank mass (best-case with O2) 981.57 [kg]
Growth O2 propellant tank volume (best-case) 0.84 [m³]
Required power (best-case) 9 [W]
Number of required O2 tanks (worst-case) 7 [-]
Total O2 tank mass (worst-case, empty) 10,535.69 [kg]
Total O2 tank mass (worst-case with O2) 29,956.13 [kg]
Total O2 tank volume (worst-case) 18.31 [m³]
Total O2 tank equipment volume (worst -case) 0.45 [m³]
Growth O2 propellant tank mass (worst-case, empty) 186.24 [kg]
Growth O2 propellant tank mass (worst-case with O2) 19,606.68 [kg]
Growth O2 propellant tank volume (worst-case) 16.70 [m³]
Required power (worst-case) 31 [W]
TRL 9 [-]
Reliability (best-case) 0.9744 [-]
Reliability (worst-case) 0.9397 [-]