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3.3.3.5 Cryogenic storage for oxygen tanks option
HMM
Assessment Study
Report: CDF-20(A)
February 2004
page 180 of 422
Although it was decided to store the oxygen for the life support system at high pressure, it has
been also investigated the cryogenic technology. This section describes the design for this
option.
Life support subsystem requires 394 kg of liquid oxygen to be stored and maintained for the
duration of the mission (31.6 months). Considering the mass and related volume, it is assumed
that these tanks can be accommodated in the same conditions of storage than for the fuel cells
(no environmental loads from the Sun and planets but radiative environment from surrounding
structures).
The required heat lift is shown in Figure 3-40. The dotted line indicates an ideal solution (heat
lift exactly compensates heat losses through MLI) and the others lines where heat losses exceed
heat lift, solution possible with a tolerance on the BO.
The number of tanks and their diameter are traded off so that despite possible boil-off (BO
dependent on the diameter), the required capacity is reached at the end of the 31.6 months.
Within that hypothesis, Figure 3-40 shows the relationship between tank diameter, number of
tanks and BO. A sensitivity with two different temperatures of saturation (90 and 120K) is
shown, taking into account the variation of density with temperature (higher temperature
improves the performance of cryocooler).
cryocooler heat lift [W]
2.0
1.5
1.0
0.5
Oxygen (90K, 0.1MPa, sink 300K)
0.0
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
tank diameter [m]
1 tank 2 tanks 3 tanks 4 tanks 6 tanks
8 tanks 11 tanks 16 tanks BO=0
cryocooler heat lift [W]
2.0
1.5
1.0
0.5
Oxygen (120K, 1MPa, sink 300K)
0.0
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
tank diameter [m]
1 tank 2 tanks 3 tanks 4 tanks 6 tanks
8 tanks 11 tanks 16 tanks BO=0
Figure 3-40: Oxygen tank dimensions
Given a sufficient tolerance on the boil-off, it is therefore not possible to cool down the tank and
reach the required usable mass. The cost in terms of additional mass and volume is however
acceptable, and an active cooling capability is preferred as a mass saving solution.
A hybrid thermal design is retained with maximization of insulation and use of a cooler: the
number of MLI layers is set to 40 layers (DAM) and the pulse tube cooler is chosen as the most
suitable cooling technology for this range of temperature (40-120K).
Significantly funded in the past years, the pulse tube offers now good performance and an
increasing range of application. Very similar to Stirling coolers in its concept, the pulse tube has
a simpler cold finger (no displacer mechanism) allowing a lower vibration level, an increased
robustness and a wider temperature range. The compressor remains identical with the same
configuration as for the Stirling (split configuration where two compressors are mounted back to
back).