applications of our invention within the scope of the appended claims we wish to specifically state that our invention is not limited to use in the propulsion system of a torpedo or other types of vehicle as illustrated in the drawing the first and second tubes 32 54 are of different diameters specifically the first tube 32 is of a smaller diameter then the second tube 54 although such an arrangement is normally preferred because the volume of the working fluid increases significantly as it vaporizes while traveling from the inlet 76 to the outlet 78 of the boiler reactor 10 there is no necessity that tubes of differing sizes be utilized furthermore those having skill in the art will recognize that our invention may be practiced in boiler reactors having a cylindrical rather than an annular cavity for receipt of the reactant and that in such a boiler it may be desirable to use only a single boiling tube structure rather than a first and second boiling tube structure as illustrated in the drawing the drawing generally illustrates a casing constructed by integrally joining the first and second sleeves and first and second end closures by a process such as welding but other means of joining the constitutant parts such as bolting may also be utilized within the scope of our invention although the convoluted shape of the first and second helically shaped channels 48 70 will generally inhibit circulation of the reaction bath through the helically shaped channels to such a degree that film boiling will be precluded it may be.