ESA Document - Emits - ESA

s
HMM
Assessment Study
Report: CDF-20(A)
February 2004
page 245 of 422
Automatic assembly is seen as an advantage in particular given the uncertainties on the
availability of LEO infrastructures (ISS, shuttle derivatives). The cost of an automatic capability
(on-board system like docking) would mean a drastic reduction of the payload mass and
proportional increase of launched elements, which could strongly affect for liquid propulsion, in
particular if a heavy LV is not available.
Thermal design depends somewhat on the strategy adopted for the assembly and available
means. Preferred thermal design is nevertheless a hybrid system combining passive insulation
techniques, integrated thermal design on the truss, and an active refrigeration at tank level.
A breadboard model of JT closed loop is presently being tested at the Marshall Space Flight
Centre and a ZBO capability should be available, possibly flight qualified in U.S. within 5 to 10
years. An equivalent capability could be available in Europe within 10 to 15 years if efforts are
oriented to this achievement. A technological basis will be available (compressor in 2004,
turbine possibly in 2006), but are not specifically oriented to ZBO hydrogen storage, although
technically close (no major obstacle). Still, significant efforts have to be conceded (and foreseen)
to reach a ZBO hydrogen tank breadboard.
3.4.3.4.7 Design
Extrapolation of the technology available in a 20-year period is illusive, depending on the efforts
projected. So far, despite an increasing interest, there is no guaranty that a ZBO system or
equivalent will be available in Europe at that present time. A more basic system (and less
efficient) is retained for the moment for this design: integrated thermal design on the truss
(deployable shade) and adequate thermal protection. The BO is accommodated per design and a
tolerance is foreseen.
If required, a lower level of BO could be reached with implementation of a vapour-cooled shield
alimented by centralised system (helium) on the truss. Periodical refilling of this helium tank
could be also an option. Use of such system with three vapour-cooled radiation shields allows a
reduction of the heat loads by a factor of 2.6.
3.4.3.5 MOI and TEI propellant tanks design
Chemical propulsion (UDMH/NTO) is retained as a baseline, with similar tanks for MOI and
TEI stages. Their geometry is respectively:
• a cylinder and half sphere for the UDMH (diameter 4.08 m and length 2.08 m)
• a nested sphere for the NTO (diameter 4.08 m)
The thermal hardware (insulation and heaters) shall preserve the propellant thermal
requirements. In particular, under illumination, thermo-optical properties of the external layer
shall not create an unfavourable imbalance leading to a higher temperature than 40C. the figure
top left in Figure 3-81 indicates a maximum ratio of 1.2 (absorptance over emittance). A
betacloth layer, adequate for anti-reflective purpose (during assembly) fulfils this condition
(between 0.4 and 0.5). Heaters are necessary to maintain temperature above 0C and avoid NTO
freezing (-10C). Temperature is actually a trade-off between MLI performance (quantified by its
equivalent emissivity) and the heater density.