An engine for providing 10.4 kW (14 hp) with efficient throttled performance has been developed for use in a small, remotely piloted airplane. A requirement for operation at very high altitudes resulted in the choice of an oxidizer-independent, monopropellant gas generator to supply energy. One unique feature is a valving concept by which the working piston is used to actuate the gas-inlet-valve poppets. A pneumatic spring is used to provide the proper valve closing force. The exhaust is ported through the lower cylinder. Good life characteristics have been demonstrated by nearly 20 hours of ground and flight testing.