A Spreadsheet for Preliminary Analysis of Spacecraft Power and Temperatures

Low cost design requires the timely analysis of multiple spacecraft configurations. This paper presents a rapid spreadsheet approach for thermal and electrical power analysis of eleven basic configurations typical of small satellites. The paper justifies using a single thermal node to estimate upper, average and lower temperatures based on linearization of the radiation equation and the orbit average temperature. The spreadsheet Solver routine can be used to adjust temperature limits, by automatic iteration of absorptivity and emissivity subject to user imposed constraints. The program also estimates the maximum power load supportable by body fixed solar panels. The program assumes a circular orbit and fixed vertical or horizontal attitude. Orbit average solar direction cosines, earth view factors and albedo are evaluated for each surface to calculate the total heat load. Program inputs include: altitude, mass, surface dimensions, thermal properties, electrical power system parameters (efficiencies, degradation, battery depth of discharge and solar cell packing factors), solar position (beta angle) and intensity, planet IR, albedo and equipment power dissipation during sunlight and eclipse. Thermal outputs include upper, average and lower temperatures. Power outputs include required battery capacity and maximum supportable power load. The power output feature also allows optimum placement of solar panels. The outputs are shown simultaneously for a sphere, cylinder, and four regular prisms (triangle, square, hexagon and octagon) in both vertical and horizontal attitudes. Comparisons of the spreadsheet approach with detailed simulation show an exact match for maximum supportable electrical power load and orbit average temperature and a 2% match for upper and lower temperatures. The program is intended for use by Systems Engineers and students as an aid in establishing viable baseline configuration options. The program also serves as a reality check on complex thermal analysis programs. The rapid analysis of body fixed solar panel capability in combination with judicious power budgets might avoid (or justify) the expense and risk of deployable solar arrays