This paper describes the design of a liquid-propellant-powered hot-gas actuator appropriate for human-scale power-autonomous robots. A prototype of the actuation system is described, and closed-loop tracking data is shown that demonstrates good motion control. Experiments to characterize the energetic performance of the actuation system indicate that the proposed system with a diluted propellant offers an energetic figure of merit five times greater than a battery-powered by undiluted propellant would offer an energetic figure of merit an order of magnitude greater than battery-powered DC motor actuated systems.