Thermal cycling effects
A spaceflight vehicle in low Earth orbit will receive radiant thermal energy from three primary sources; the incoming solar radiation (solar constant), reflected solar energy (Earth albedo), and outgoing longwave radiation emitted by the Earth and atmosphere itself with a blackbody radiation spectrum of 288K. Certain amounts of this energy will be reflected by the vehicle, and the vehicle itself radiates energy into the cold sink of space at 3K. The thermal environment of any spacecraft surfaces will tend towards a temperature which balances these energy fluxes with any energy produced internally within the vehicle. A similar thermal balance process applies to the Earth itself. The Earth/atmosphere is in radiative equilibrium with the Sun. However, it is not in balance everywhere on the globe, as there are variations with respect to geography, and atmospheric conditions. A space vehicle's motion with respect to the Earth results in its viewing only a continuously changing portion across the full global thermal profile, so it sees these variations as functions of time depending on the thermal time constants of the vehicle. The thermal environment is therefore primarily dependant upon the orbital parameters.
The LDEF thermal design was completely passive, relying on surface coatings and internal heat paths for temperature control and equalisation. To maximise the internal radiation coupling between the spacecraft components, high-emittance coatings were used. All interior surfaces were coated with Chemglaze Z-306 black paint. This unexposed coating did not suffer any degradation during the LDEF mission. However, because the LDEF structure was not baked out after being painted, the Chemglaze Z-306 became one the leading sources of contamination. Actual internal flight temperatures were recorded at intervals of approximately 112 minutes for the first 390 days of the LDEF mission. Temperature measurements were taken using five copper-constantan thermocouples, a radiometer and two thermistors for reference measurements. The recorded temperature range for all seven locations was from 3.8°C to a maximum of 56.7°C.
The effects of this temperature cycling on a deposited multilayer causes thickness contraction and expansion of each of the layers, the size of which depend on the thermal expansion coefficient properties of the materials. As dielectric coatings deposited on optical components are usually brittle, and may be under constant tensile or compressive stress, delamination of the coating can result if the induced thermal mismatch between adjacent layers becomes too large. The resultant failure of the coating significantly degrading the spectral performance of the optical system. In addition to the unwanted thickness variations, which can displace the spectral profile from its desired position, temperature cycling effects can also cause the spectral performance to deteriorate, as the optical dispersion properties of the materials change with temperature, these variations can become significant and adversely affect the spectral profile of the filter. The coatings must therefore be constructed of materials with sufficient adhesion between the interfaces to withstand the thermally induced expansions and contractions and possess a low susceptibility to thermally induced dispersion.
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