A cylindrical rocket engine exhaust duct has a liner that fits inside an outer jacket. The outer periphery of the liner has circumferentially spaced ribs defining a sinuous path for a fluid to be heated. The liner has a least one, preferably several, transverse fins with internal serpentine passages for carrying the fluid to be heated through the fins. The fluid to be heated is directed around the periphery of the liner (between the liner and the jacket), and consecutively through the separate fins, along sinuous and serpentine paths for heating of the fluid by heat transfer from the exhaust gas of the rocket engine.