Phase II Final Report - NASA's Institute for Advanced Concepts

Chapter 3.0 Vehicle Design
3.5 Fuel Storage and Production
3.5.3.1.2.2 Cryocooler for Boil-off Compensation
Based on the results shown in Figure 3-153,
the boil-off rate for the liquid hydrogen is too
high to be useful for a mission that can last for
more then a year from launch to completion. A
way to reduct boil-off therefore must be
devised if liquid hydrogen is to be used. Previous
work focused on eliminating boil-off in
cryogenic systems by incorporating a cryocooler
into the storage tank. [210].
The cryocooler is used to condense the hydrogen
vapor that forms in the ullage (excess area
in the tank not occupied by liquid hydrogen) of
the tank. This vapor is generated by heat leakage
into the tank through the insulation. By
sizing the cryocooler to match the heat leakage,
the system effectively can have no boiloff.
A typical single-stage cryocooler for
reaching liquid hydrogen temperatures is
shown in Figure 3-156.
The energy loss associated with the boil-off
rates shown in Figure 3-153 are given below in
Figure 3-157. This figure shows the amount of
power needed to be supplied by the cryocooler
Figure 3-156: Single-stage 20 o K
Cryocooler
[41]
to compensate for the leakage of heat into the tank through the insulation. This power consumption
is based on the latent heat of vaporization of hydrogen and the rate of heat flow into the
tank, given in Equation 3-55.
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