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Common bus spacecraft development and constellations of spacecraft such as ORBCOMM have led to a need for common thermal models. For years, spacecraft design engineers have used common parts stored in a parts library to expedite the process of drafting the spacecraft in a CAD package such as Pro/Engineer. This methodology is implemented into the thermal modeling process for common parts used on one or more spacecraft. Using the previously developed Pro/Engineer parts library as the model template, the component thermal model is developed in FEMAP and translated into a complete thermal model using the TCON thermal software suite. The models are checked for accuracy at the component level to ensure that errors do not propagate to system level models. The result is a component level thermal model ready for integration into a system level model.

Placing the thermal nodes at the corners of the radiation surfaces results in greater thermal-model accuracy for a given number of nodes, as compared to the more common practice of placing the thermal nodes at the center of the radiation surfaces. Placing the thermal nodes at the corners does, however, add complications in computing radiant interchange factors. In this paper the accuracy of subdividing elements into radiation sub-elements is compared to the accuracy of allocating elemental radiation couplings to the corner nodes. The comparison shows that, for a given number of nodes, the two methods have nearly the same accuracy, provided good modeling practice is followed in both cases.

Thermal conductances are computed by finite-element techniques for typical triangular elements.. Heat-transfer rates computed with these conductances are compared to heat-transfer rates computed from exact, closed-form mathematical formulas and from refined models. The comparisons show that significant simulation errors can occur in triangular elements that have an internal angle greater than 90 degrees. An alternative, centroid method is shown to give smaller errors by some measures and in some cases but larger errors in most cases.

Thermal conductances are computed for relatively simple node geometries that are typical of analytical models of spacecraft. Heat-transfer rates computed with these conductances are compared to heat-transfer rates computed from exact, closed-form mathematical formulas. The comparisons show that most accurate results are obtained with rectangular nodal arrangements and with triangular arrangements with interior angles less than ninety degrees. For rectangles, the conductances obtained from finite-difference, finite-element and centroid methods are identical. For triangles, the finite-element conductances are best. For other quadrilateral arrangements and triangles, the centroid method is the most reliable.

How accurate are your thermal models? Few thermal analysts have the time and budget to test the convergence of their thermal models (e.g., SINDA models) as a function of mesh size. This paper presents guidelines for selecting mesh sizes and other simulation parameters to ensure accurate simulations. We considered the following aspects of thermal models: mesh size as determined by combined conduction, convection and radiation; perferred mesh geometries; treatment of boundaries and boundary conditions; and required time steps.

To provide spacecraft thermal design engineers with the data needed to define the earth albedo and emission, we have analyzed data from the Earth Radiation Balance Experiment (ERBE) over a period of 36 months, covering orbital inclinations of 57 and 99 degrees. Design values of the albedo and earth emission can be obtained from this data directly; however, we present an earth-map (zonal) method for computing design values for any orbital parameters. In addition, the zonal method permits less pessimistic assumptions to be made in a proof-of-design simulation.