A gas generator has an elongate combustion housing shaped to fit within a borehole for generating gasses in the borehole for use in the recovery of petroleum products. The gas generator includes a plurality of annular cooling jacket segments that surround the elongate combustion housing, each of the annular cooling jacket segments having an inlet port and an outlet port for allowing water to flow therethrough. A plurality of apertures through the elongate combustion housing allow water to flow from the plurality of annular cooling jacket segments into the elongate combustion chamber.