This paper presents the results of the design and development of a reciprocating free piston Auxiliary Power Supply. The Auxiliary Power Supply is a gas-driven reciprocating hydraulic pump powered by a hot-gas source such as a solid-propellant gas generator. The simplicity, ruggedness, response, design flexibility and low fabrication cost of the Pneumatic-hydraulic Power Converter should allow it to be competitive for future Auxiliary Power Supply requirements on missiles, boosters and re-entry vehicles. The incorporation of this power converter into the control systems of future missiles will simplify the total power system, reduce volume requirements and result in overall system reliability gains at a reduced system cost. The operation, performance, system advantages, growth potential, and some of the problems encountered during the development program are discussed.