As has been mentioned above, the schematic diagram and the design of LHPs make it possible to use capillary materials for making wicks with quite a small effective pore radius. Such materials were specially created on the basis of fine-grained nickel, titanium and copper powders. Despite their high porosity, which is usually equal to 60-70%, they possess a sufficient strength, required for complex mechanical processing in making wicks.

Since such wicks are used in hermetic systems, which LHPs are, it is necessary to ensure their chemical compatibility with the working fluids during a long operation. Appropriate

"body — wick — working fluid” combinations have been determined by experiment [2]. They may also be recommended for use in LHPs. Some of them are presented in Table 1.

Wicks with the indicated working fluids are capable of creating a high capillary pressure, which is necessary for pumping a working fluid for a sufficiently great distance. Table 2 gives values of the capillary pressure created by a wick with an effective pore radius of 1 micron.

As can be seen from this table, the highest pressure may be achieved when the working fluid is water, which is a consequence of the high coefficient of surface tension this liquid possesses. Besides, water retains its advantages in the widest temperature range. At the same time at temperatures below 70oC the efficiency of using water in LHPs decreases owing to the decreasing value of 3P/3T. In this case ammonia becomes preferable.