ESA Document - Emits - ESA

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HMM
Assessment Study
Report: CDF-20(A)
February 2004
page 242 of 422
Constrained by the high solar absorptance of the betacloth, using exclusively an MLI solution
would be a costly solution, either in term of mass (320 layers of DAK would weight 580kg to
cover the tank surface), or financial cost (190 layers of DGK). Installing these different blankets
on the shields (five blankets of 38 layers distributed on two or three external aluminium shells)
might prove delicate along with probable degradation of the efficiency.
Sunshade allows a drastic reduction of heat losses along with the minimum required insulation
(decrease of a factor 3.6) but its implementation raises certain constraints to be dealt at system
level:
• reflective coatings (OSR, SSM) are incompatible with nearby operations requirements
(docking, EVA), unless this reflective surface can be pointed away during these periods
• a non-axisymetric shield requires a pointing capability with its associated penalties
(propellant, energy supply, electronics)
• a decrease of the allowable tank envelope (constraint from the launcher fairing diameter)
3.4.3.4.2 Passive vapour cooling
Using of hydrogen to cool down the shields requires an acquisition system. Extracting the vapour
is a particular challenge for Newtonian fluids in zero-g (like hydrogen and oxygen) since there is
no equivalence of helium superfluid properties (fountain effect) to exploit. In the absence of
buoyancy, the gas stands at its evaporation point and forces have to be created (per capillarity or
acceleration) to displace and separate the gas from the fluid. Related technical solutions appear
globally inappropriate for the tank dimensions and complex to validate in 1-g.
Phase separation system Advantages Disadvantages
Artificial gravity Distinct separation of the phases Requires a spin of the tank or a rotation of the
fluid (motorization)
Friction of the fluid (possible heating,
although viscosity is low)
Sensitive to adverse acceleration (vehicle for
example)
Capillary acquisition Appropriate to hydrogen low surface tension High weight penalty with important surface
Simple, no moving parts, well known system Sensitive to adverse acceleration
Table 3-70: Phase separation systems
Since direct venting of the gas appears challenging, BO minimisation is to be sought. Extraction
of the liquid hydrogen (or a two-phase mixture) for cooling external shields can be considered.
Several schemes are possible depending if an open or closed loop is considered.
In the first case, liquid hydrogen can be throttled (Joule-Thomson effect) from the saturated state
into a two-phase regime and then vapourised in a heat exchanger controlling the shell and liquid
heat losses. A venting valve controlled by a tank pressure sensor regulates the system below a
safety value.
The use of another cryogen (helium) to maintain the cryostat temperature could be considered
provided it meets mass and volume constraints. With a rate of 5 mg/s, 271 kg (2170 l) helium
would be needed for a 24-month period. Volume however is another penalty that could restrain
the use of such system on the ground before launch.