A compact, unfired, staggered tube, once-through steam generator or boiler of simple construction intended primarily for combined cycle power plants in which the thermal energy utilized to generate steam is obtained from the exhaust gases of a gas turbine engine. The boiler provides steam at two different pressure levels, which maximizes recovery of thermal energy; and all wettable components in the steam/water loop are fabricated of corrosion resistant materials. This eliminates the need for controlling the pH of the feedwater and the need for chemically controlling its dissolved oxygen content, thereby reducing maintenance and operating costs and making automatic, unattended, remotely controlled operation of the boiler practical as well as eliminating the need for blowing down the boiler and minimizing requirements for make-up water. The boiler can be operated dry to remove gas side fouling; and feedwater flow rates can be directly controlled. Orifice-generated pressure drops at the inlets of the boiler tubes which are relatively large compared to the pressure drop in the rest of each tube eliminate the flow instability that is characteristic of once-through boilers.