On the assumption that a Lockhart-Martinelli type of correlation is applicable, an analytical approach to the interpretation of pressure drop data on condensing flow in tapered tubes was developed with particular reference to the condensation of potassium in a radiant condenser of a type that may be attractive for space power plants. It appears that instability can be encountered in such condensers only through the introduction of excess liquid inventory at reduced power or by design of the condenser for insufficient pressure drop at design power. (auth)