Tests were conducted on a straight-tube coaxial heat exchanger to determine heat-transfer rates for the generation of superheated steam from heat supplied by a solid-propellant, hotgas generator. Superheated steam at 1600°F and 1500 psia was generated by hot gas at 2200°F that was introduced into a coaxial counterflow tube-in-tube heat exchanger. Repeated runs were made without burnout or noticeable decrease in heat-transfer efficiency. Stable equilibrium conditions were obtained within 60 sec, and sufficient steam was generated from cold water within 10 sec to provide partial power for a steam-turbine powerplant. Over-all heattransfer rates were determined for superheat, boiling, and preheat regions at high heat flux and large temperature differences. Experimental results were in satisfactory agreement with heat-transfer calculations for forced convection inside tubes. Average heat-transfer rates of over 0.25 X 106 Btu/hr-ft2 were obtained for both steam and hot gas, with local rates as high as 0.85 X 106 Btu/hr-ft2 at a mass velocity of 1.5 X 106 lb/hr-ft2 and a Reynolds number of 1 X 106.