To provide a technology for heating gas by fission reaction, which is suitable for space propulsion.

Gas, for example hydrogen, of relatively low pressure is heated directly by fissile piece emitted from a thin layer of a fissile for example 242mam, deposited on an inner wall of a chamber cooled to a typical temperature of about 1000/1500°K. The gas is discharged as a capillary flow from a wall of a cylindrical tube. A temperature thereof is elevated gradually to reach to an equibrium value of 9500°K. The FF heating and radiation cooling are balanced at the point. Average specific volume power imparted to the hydrogen gas is brought into a high value of 0.66 MWatt/g in a relatively moderate surface power density in a foil of 200 Watt/cm2. Heating power upto a level of MWatt per 1 gram of the gas is possible thereby in an allowable foil surface heating. This heating method is applicable for a rocket engine for deep space propulsion.