An oxygen-fuel burner of the rocket burner type includes a graphite burner block for direct exposure to the interior of a furnace, and a cylindrical combustion chamber formed through the hot face of the burner block and extending into the burner block. An oxygen supply conduit delivers oxygen to the combustion chamber along the center line of the combustion chamber and fuel supply ducts deliver fuel to the combustion chamber at the concave surface of the combustion chamber. A plurality of rectilinear cooling bores extend into the burner block and are arranged in a parallel array about the combustion chamber. A liquid coolant header moves cooling liquid through a plurality of parallel supply tubes which are telescopically received within the cooling bores, thereby maintaining the burner block at a reduced temperature. The nozzle that supplies the oxygen to the combustion chamber is movable along the length of the combustion chamber so as to change the shape of the flame emitted from the burner.