Pre-Phase A Report - Lisa - Nasa

126 Chapter 5 Payload Design
5.5 Payload thermal requirements
The major science requirement on the payload thermal control subsystem is one of temperature
stability, with the optical bench fluctuations due to solar intensity variations and
other sources of disturbances kept below 10−6 K/ √ Hz at 10−3 Hz. The telescope thermal
stability should be below 10−5 K/ √ Hz at 10−3 Hz to achieve the desired performance.
The optical bench part of the payload shall be maintained at 20 ◦C±10 ◦C, but should be
known with an accuracy of TBD ◦C at the design stage. Temperature gradients within
the optical bench should be less than TBD ◦C. Electronics boxes need to be maintained
within their operational temperature limits of TBD ◦CtoTBD ◦C. The laser diodes are to be kept at their operational temperature, and this will reduce
over the mission life from 295±TBD KatBOL to 280±TBD KatEOL (beginning, end of
lifetime), to accommodate changes in the diode operating wavelength.
In addition to the specific thermal requirements defined above the thermal design must,
together with the thermoelastic design, prevent deformations of the structure that compromise
the scientific performance of the payload.
5.6 Payload thermal design
It is clear that many thermal and system level trade-offs need to be performed before an
optimised thermal design may be established. However some thermal analysis has been
performed and has enabled the definition of certain design parameters.
The first stage of isolation from the sun should be provided by the spacecraft as either a
solar shield with optimised αs/ɛ or, preferably, multilayer insulation (MLI). This would
probably be at the level of the top of the spacecraft structural ring. A second stage
of solar isolation is provided by the Y-shaped thermal shield. This will be goldized as
extensively as possible on external and internal surfaces, although if the electronics boxes
are radiatively cooled then certain parts of this thermal shield will have to be blackened,
and also the internal surfaces of the baffles forward of the primary mirrors will probably
be blackened for control of scattered light.
The external and internal surfaces of the optical bench support cylinders are goldized to
radiatively isolate them from the Y-shaped tube, thus providing a third stage of radiative
isolation from the sun. The optical bench and sensor assemblies have been assumed to
have their natural surface properties, but further modelling may show that these too need
to have controlled low emissivity coatings.
Conductive isolation is used throughout the payload and at the interfaces with the spacecraft
as defined in Section 5.2 . For this purpose, Pyroceram cylinders are used to support
the optical benches, the electronics plates and the telescope thermal shields off the internal
support cylinders. Glass fibre reinforced bands are assumed for mounting the internal
support cylinders off the Y-shaped tube and carbon fibre brackets for mounting the laser
radiator off the Y-shaped tube and the primary mirror off the support cylinder.
Current modelling has indicated that the electronics boxes operate somewhat warm at
about 30 ◦C but the current design study considered radiative losses only from the box to
3-3-1999 9:33 Corrected version 2.08