Strength Analysis - Finite Element Analysis for Satellite Structures - page 185

through the fluidic boundary layer by conduction, and then fluid action distributes
the heat. Thermal radiation is the transfer of energy by electromagnetic waves
through a gas or the vacuum of space. Radiation is the driving mode of heat
rejection in space; heat generated internally or absorbed from the environment
(also by radiation) must eventually radiate to deep space.
Actual thermal-design problems for a satellite are complicated. The design
problem typically must combine multiple modes of heat transfer with time-varying
boundary conditions that require transient instead of steady-state solutions. To
predict satellite's temperatures, the thermal analysis problem combines two types
of heat transfer models. The first one is a thermal-radiation model to calculate
external heating rates by simulating the external geometry of the satellite including
surface properties. By subjecting this model to a simulated orbit, the output from
this model consisting of; the environmental heating rates due to direct solar,
albedo, and planetary emissions, and external radiation between satellite surfaces;
becomes input for the second model.
The second thermal model uses a thermal analyzer. The satellite is modeled
much the same as the structural analysis model with internal details, but the
analysis is based on the finite difference method. Then the heating sources are
defined. Finally, the model is solved to simulate the heat transfer paths of con-
duction, convection, and radiation within the satellite body. The thermal analyzer
calculates temperatures at all nodes for steady state or transient conditions by
solving energy equations. The thermal analysis process for on-orbit satellite
structure is performed by the thermal analysis engineer.

4.14.2 Performing an On-Orbit Thermoelastic Analysis

The procedure of a thermal deformation (thermoelastic) analysis for Small Sat
structure due to on-orbit cyclic temperature changes consists of the following steps:

1. Build the model: The most critical structural module in Small Sat regarding on-
orbit thermal deformation is the basis unit block module. This module is very
sensitive to thermal deformation, because it carries the precise equipments of the
satellite. The rest of structural modules do not have severe restrictions on their
equipment mounting accuracy. Therefore, only the basis unit block is studied. To
speed up calculations, the substructure technique is not applied, and new sim-
plified finite-element model of the entire satellite is built where fine structural
details are ignored for all structural modules except the basis unit block module.
The following points are taken into consideration during building this model:

• The basis unit block module consists of the basis plate, four basis walls, two
diagonal struts, and star sensor bracket.
• The material is Aluminum alloy AMg6. Its properties are mentioned in
Sect. 4.4 . For thermal analysis, it has the following properties:
• Thermal conductivity (K) = 117 W/m

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