A rocket engine chamber has a back structure and a heat exchanger structure located radially inwardly of the back structure. The heat exchanger structure has a plurality of integrally formed cooling passageways with adjacent ones of the passageways having a common sidewall structure. Each passageway has a top section which forms the wall that contacts the hot gases. The top section is formed by a wall structure having an inner and outer surface formed by an arc of a circle and a substantially constant wall thickness.