Regulative Characteristics of Methanol – Copper Heat Pipes for Asteroid Lander “MASCOT”

Abstract

Variable conductance heat pipes (VCHPs) are the main part of MASCOT (Mobile Asteroid surface SCOuT) lander passive thermal control system (TCS). They provide variable conductivity by utilizing the physical heat transfer limitations. This allows the heat pipes to act as thermal switches without additional constructive elements. The advantage is the simplicity and compactness of a conventional heat pipe design. Two cylindrical copper-methanol heat pipes with shell length 0.482 m and 0.438 m and external diameter of 0.006 m, having copper discrete metal fiber wick and copper shell were constructed and verified in the temperature range −75…+60 °C. The purpose is to apply this design into the MASCOT project and to investigate their regulative characteristics and longitudinal physical heat transfer limitations. VCHPs show a change of thermal resistance from 70 K/W, at a sink temperature of −60 °C, to 0.8 K/W at a sink temperature of +60 °C; with an obtained maximal heat transfer rates of 5 W and 16 W, respectively. It is found, that the switching effect of the heat pipes is governed by the sonic velocity limitation, the saturation vapor pressure of the working fluid and the maximal capillary pressure of the fibrous capillary structure.