The Payload Block is the "cocoon" of the instrument, namely, a structural unit which provides its thermal stability and its mechanical resistance.
The Payload Block is an optimized set from the mechanical and thermal point of view, to guarantee to the accelerometers ("SU") and to their electronic of proximity ("FEEU"), at the same time a good structural behavior during the vibrations of the launch and, a great thermal stability, necessary to scientific measurements.
In order to guarantee the 2 required levels of thermal stability (± 1 mK for the SUs and ± 10 mK for the FEEUs), the BCU must be isolated from the platform and the external environment, which constitute thermal sources of disturbances.
For that, it is fixed on the thermal wall most stable of the satellite: the one that never sees the sun. For that, it is fixed on the thermal wall most stable of the satellite: the one that never sees the sun. Its structure is composed of 2 stages conductively uncoupled by insulating titanium bars, and the unit is radiatively isolated from the remainder of the platform by a cover of super isolating material MLI (Multi-Layer Insulator).
At the base of the BCU, a radiator is installed, necessary to the evacuation of the dissipation of the "FEEU" (12 W). A baffle is installed around this radiator to limit the inputs of variable heat fluxes coming from the Earth, which would penalize the thermal stability of the "FEEU".
The Payload Block weighs approximately 50 kg. Its diameter envelope is 52 cm and its height 55 cm.
A mock-up has been built in the first phase of the project in order to prove the feasibility of a completely passive thermal control (i.e without active regulation which would disturb scientific measurements) enabling to reach the required performances of thermal stability, particularly ambitious.
The mock-up was to be representative in term of thermal inertia (function of the mass and the nature of materials), of geometry, and direct and radiative couplings. Thus in particular, the electric cables connecting the accelerometers to their electronic of proximity were also modelized. Indeed, their particularly heat-conductive copper nuclei, play a considerable part in heat exchange between the various stages of the BCU.
The thermal tests associated with this mock-up consisted in reproducing a certain number of thermal disturbances (representative of those awaited in flight) at the interfaces of this unit and in measuring the effect on the temperature stability of the electronics mock-up. This required to implement a fine methodology enabling to detect temperature variations lower than one mK.