Selection of working fluid is one of the main criterions for designing of heat pipes thermal control systems (TCS) for space application. In this paper we will describe how we solved the task of development of the TCS with working fluid of high thermal physical properties.
In 2004-2006 we developed the Engineering model of Deployable Radiator based on Loop Heat Pipe by CAST purchase order. It was developed for qualification tests.
Ammonia application as LHP working fluid is stipulated by its high thermal physical properties. However Ammonia freezing temperature is of minus 77ºC. Such fact impedes Ammonia application when operation temperatures of LHP Radiator are lower than this value, for example, It takes several tens of hours to orbit a spacecraft and prepare it for work (at that moment the spacecraft is out of power supply) and the working fluid can be frozen in a condenser-radiator when the spacecraft being in the shadow over a long period of time. While working fluid de-freezing in the radiator the condenser tube can be destroyed. To avoid this problem it is better to use the working fluid with lower freezing point. However use of lower freezing point fluids will cause decreasing of thermal control system parameters, in particular, maximum heat transfer power and increasing of LHP thermal resistance, mass, overall dimensions as well as the spacecraft mass.
An important task that shall be solved during ammonia LHP application is recovery of LHP operation ability after working fluid freezing in radiator-condenser. To recover working fluid circulation in LHP it is necessary to defreeze the whole LHP. During multiple freezing/melting cycles of LHP working fluid LHP depressurization may occur. This paper is dedicated to solving aspects of Radiator operation ability recovery after its freezing. Aspects of passive temperature control of LHP evaporator using pressure regulator and aspects of LHP active temperature control using heaters and Peltier elements mounted to compensation chamber are also considered in this paper.
Development of new key components made it possible to design Deployable Radiator with specific mass of less than 10 kg/kW. The Deployable Radiator parameters are described in the paper.