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Technical Discipline Area: Solar & Thermal

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Solar

Solar Exposure

The solar UV exposure rate is relatively stable in comparison with the variation of atomic oxygen flux. However, there are variations in the flux rates for specific vacuum ultraviolet wavelengths of 50 to 100% over the range of solar minimum to solar maximum conditions. Short term activity from solar flares can also temporarily increase the intensity of vacuum ultraviolet energy. This variability is not reflected in the calculation of hours of solar exposure.

Most of the Earth albedo data available were derived from satellite-borne infrared radiometer data. We have assumed, lacking better knowledge, that the spectral dependence is approximately constant from the infrared to the ultraviolet. Based on this assumption, we have used the monthly average albedo data derived from Nimbus 7 short wave radiometer measurements of Smith et al. as updated by Rutan to calculate an annual average Earth albedo under the LDEF orbit of 0.246. Bourassa and Gillis used this albedo value to calculate direct solar and Earth reflected solar exposure to LDEF. The best measurement of the total solar irradiance at the top of the atmosphere is 1368 +/- 7 W/m².

Solar Wind

Solar electrons and protons were not a major factor due to the low altitude of the spacecraft. The majority of the experiments were passive in nature. Those that were active did not require substantial amounts of electronic hardware, or operated for only the first 11/2 years, and/or were well shielded. LDEF was a large structure with the necessary electronics mounted on the back of the experiment trays (i.e., on the interior of the spacecraft). These factors minimized the potential for single-event upsets which could scramble information, induce unplanned signals in electronics hardware, and therefore send spurious commands to active systems.

Thermal

LDEF Thermal Design

The thermal control of LDEF was completely passive, and it used radiation coupling inside the structure and surface coatings. The LDEF structure was essentially a hollow polygon with high emissivity values on the internal surfaces. The internal surfaces were coated with Chemglaze Z-306 flat-black paint, which had a preflight emissivity of 0.90. While this coating appeared not to deteoriate due to the space exposure, the fact that it had not been baked out prior to flight led to its being one of LDEF's larger sources of contamination. Internal radiation blockage was decreased by minimizing the number of structural components inside the facility. Solar flux was kept from the interior by closing all possible entrances through the tray locations and at both ends of the cylinder. Vent holes were placed uniformly around the facility and occupied approximately 0.15% of the total external surface area.

The tray-mounting scheme minimized the contact area through which the heat could be transferred between the facility structure and experiment trays. Different tray types were distributed on the surface of LDEF. Over 50% of the thermal-control surface area was provided by chromic anodized coatings of the facility's aluminum structure, trays, and the meteoroid&debris witness panels of Experiment S0001.

LDEF Thermal Environment

LDEF flight temperature data was recorded by the Thermal Measurement System (THERM), Experiment P0003 at intervals of approximately every 112 minutes for the first 390 days of the 2,105 day mission. These were compared, postflight, with the predictions made with the Thermal Mathematical Model (TMM). This model was unverified prior to flight. The postflight analysis reduced the uncertainty of the model at the temperature sensor locations from +/- 40 degrees Farenheit to +/- 18 degrees Farenheit.

Actual measured temperatures within the interior of LDEF ranged from a low of 35 degrees Farenheit to a maximum of 134 degrees Farenheit, and were well within design specifications. More information from analyses by the LDEF Systems Special Investigation Group can be found in the Thermal Section of the LDEF Systems pages.

References

1. Berrios, W.M., Use of LDEF's Thermal Measurement System for the Verification of Thermal Models, 69 Months in Space: First LDEF Post-Retrieval Symposium, NASA CP-3134, June 1991.

2. Dursch, H.W., W.S. Spear, E.A. Miller, G.L. Bohnhoff-Hlavacek, and J. Edelman, Analysis of Sytems Hardware Flown on LDEF - Results of the Systems Special Investigation Group, NASA CR-189628, April 1992.

3. Berrios, W.M., Long Duration Exposure Facility: Post-Flight Thermal Analysis, Orbital / Thermal Environment Data Package.

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