A thermodynamic engine which converts heat generated by a radioisotope into mechanical energy pulses is described. The mechanical energy pulses are produced by first heating a curved bimetallic disk to a temperature at which it becomes unstable and reverses curvature and then by cooling the disk to a temperature where it again becomes unstable and assumes its original curvature. The initial disk curvature is determined by the operating temperature limits desired and physical properties of the disk components. An approximate theoretical analysis of engine performance has been carried out. For a mean disk temperature of 434° F (223° C), a maximum engine temperature of 750° F (399° C), a minimum engine temperature of 68° F (20° C), and a disk temperature change of 50° F (28° C), an ideal output of 10 W-s/cycle appears attainable from an engine with the following characteristics: disk thickness 0.075 in. (1.91 mm), disk diameter 3.5 in. (8.9 cm), radioisotope thermal power 150 W, and cycle time 11 s.