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Actual measured temperatures within the interior of LDEF ranged from a low of 39 degrees Farenheit to a maximum of 134 degrees Farenheit, and were well within design specifications. External thermal profiles varied greatly, depending on orientation, absorptance/emittance, and material mounting and shielding. The thermal stability of LDEF adds to the accuracy of existing thermal models and enhances our ability to model LDEF's thermal history, as well as that of other spacecraft.

Initial functional tests were performed for each of the three heat pipe experiments flown on LDEF, and the heat-pipe systems were found to be intact and fully operational. No heat-pipe penetration occurred due to micrometeoroid or debris impact.

The LDEF experiment, Low Temperature Heat Pipe Experiment Package (HEPP), located on tray F12, was designed to evaluate the performance of two different heat pipes and a low-temperature (182 K) phase change material. The HEPP did not cool below 188.6 K during the LDEF mission. As a result, the preprogrammed transport test sequence which initiates when the phase change material (n-heptane) temperature drops below 180 K was never activated. Therefore, in-flight transport tests with both heat pipes, and the diode reverse test could not be run. Because n-heptane has a melt temperature of 182 K, its freeze/thaw behaviour could not be evaluated on-orbit. The failure of the HEPP to obtain the desired on-orbit temperatures was attributed to an apparent error in the original thermal model that overestimated the HEPP radiators's cooling capacity.

Both flight and ambient post-flight testing showed all electrical systems were functioning properly after retrieval and that the mechanical and thermal integrity of the HEPP were intact. Following the functional test at KSC during deintegration, HEPP was removed from its LDEF tray and underwent thermal vacuum testing at NASA Goodard Space Flight Center (GSFC). This test was performed to simulate actual flight conditions, enabling the acquisition of temperature data for comparison with orbital data and correlation with thermal model predictions. Results show that the n-heptane canister, constant conductance heat pipe, and the diode heat pipe, performed as they had 14 years ago. The phase change material froze at a temperature only 1 K higher than was measured 15 years ago, well within instrumentation accuracy. The HEPP experiment was the only known LDEF hardware to undergo post-flight thermal vacuum testing.

The change in performance of a wide variety of thermal-control coatings and surfaces was moderate, with a few exceptions. A significant amount of these changes has been attributed to contamination effects. Certain metals (especially chromic acid anodize aluminum), ceramics, coatings (YB-71, Z-93, PCB-Z), aluminum coated stainless-steel reflectors, composites with inorganic coatings (Ni/SiO₂), and siloxane-containing polymers exhibited spaceflight environment resistance that is promising for longer missions. Other thermal-control and silicone-based conformal coatings, uncoated polymers and polymer matrix composites, metals (Ag, Cu) and silver Teflon^® thermal-control blankets and second-surface mirrors displayed significant environmental degradation. In addition, post-flight measurements may be optimistic because of bleaching effects from the ambient environment.

The results of thermal measurements on different samples of the same materials made at different laboratories have proven to be remarkably consistent and in agreement, lending additional credibility to the results. Confidence in designers' thermal margins for longer flight missions has been increased.

One of the most notable observations made during the on-orbit photo survey was the loose aluminized Mylar multi-layer insultion (MLI) blankets located on a space-end experiment. Tape was used to hold the edges of the MLI blankets to the experiment-tray frame. The blankets apparently shrunk in flight, causing the blankets to detach from the frame. Portions of the tape were attached to both the blanket and frame, indicating that the tape had failed in tension. Post-flight adhesion testing showed that the tape retained adequate adhesive properties.

Another notable observation was made in a study of these same aluminized Mylar blankets more than three years after LDEF's retrieval that illustrates the interrelation of the different aspects of the LEO environment. Seven of the eight rectangular MLI sections had torn at or near the edges, as previously described. The eighth blanket failed across a diagonal. Investigation revealed that two debris particles had impacted this blanket along the tear line. One was pure silver, presumed to be from an electrical connection on another spacecraft's solar array. The other particle was aluminum 6061-T6, presumed to be from an LDEF drilled hole. Both were on the order of a centimeter in diameter.

The loss of specularity of silver Teflon thermal blankets, one of the earliest observations noted at the time of retrieval, was determined to have had no significant effect on the thermal performance of those materials. The loss of specularity was the result of surface erosion and roughening by atomic oxygen.

The thermal performance (absorptance/emittance) of many surfaces was degraded by both line-of-sight and secondary contamination. The specific contamination morphology in various locations was affected by ultraviolet radiation and atomic oxygen impingement. Overall, the macroscopic changes in thermal performance from contamination appear to be moderate at worst. Limited measurements on surfaces from which the contamination was removed post-flight suggest that the surfaces beneath the contamination layers have undergone minimal thermal degradation.

Betacloth which was exposed to the atomic oxygen flux was seen to have been cleansed of the many minute fibers that normally adorn its surface. This has been observed to have no measurable effect on the thermal preformance of the betacloth, although some associated contamination issues are raised.

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