Abstract: A pneumatic/mechanical actuator described herein consists of a variation of a Brayton-cycle machine for generating actuation force. The system avoids the continues heat problem of a Brayton-cycle by using compressed gas to drive a reversible turbine. The turbine only is driven when a change of actuation position. The system includes an internal-combustion engine-air compressor, a pneumatic energy storage and transfer system, a fuel/air mixing means, two electric pilot valves, two lean-ratio high-pressure catalytic-bed burners, a small-diameter and reversible turbine, a ratio speed-reducing transmission, an optical-encoder feedback position and speed sensor and a ball-screw push-rod mechanism. An alternative embodiment uses a free piston engine compressor hybrid to provide the compressed air. Manual or computer control may be used to monitor and direct desired changes in actuator position.