1. A rotary boiler comprising, a generally cylindrical structure having axially spaced apart side wall portions interconnected by a continuous circumferentially extending peripheral wall defining a boiler chamber, means to admit liquid to the boiler chamber, means to rotationally drive the boiler at a predetermined speed to dispose and maintain a body of the liquid continuously about the inner surface of said peripheral wall with a cylindrical liquid/vapor interface of predetermined radius from the rotation axis providing high boiling heat fluxes in excess of those obtainable at ambient gravity, combustion means adjacent the peripheral wall of the boiler operable to heat and vaporize the liquid therein, and means for withdrawing the vapor from the boiler, the linear axial spacing of the boiler side walls at said liquid/vapor interface and at said peripheral wall being approximately the same and said peripheral wall being contoured transversely and circumferentially of the boiler to provide a combustion heat conduction peripheral wall having inner and outer surfaces respectively confronting the liquid and combustion means having a total length in the axial direction substantially greater than the linear axial spacing of the boiler side wall portions, thereby providing at the peripheral wall a heat flux substantially less than the heat flux at the liquid/vapor interface and a substantially small temperature difference between the wall and the liquid/vapor interface while still maintaining high boiling heat flux at said liquid/vapor interface.