Abstract: The invention provides an engine for obtaining kinetic rotational energy from the explosive decomposition of individual cartridges of an energetic material. The cartridges are individually ignited as needed, producing a bolus of hot, expanding gas, the energy of which is captured by a piston moving in a circular track. Each cartridge ignition produces a single circuit of the piston around the track. A gearing mechanism transfers the angular momentum of the piston to a flywheel. The engine may be coupled to an electrical generator to form a portable, on-demand electric generator system.