A conceptual two-stage microthruster employing sublimed molecules as the propellant of the first stage and alpha particles as the propellant of the second stage is analyzed. Transient thermal analysis is developed as a tool to provide design information on material compositions. The numerical result suggests that the radioisotope heating of the nozzle wall could help flatten the subliming surface temperature. The modified Monte Carlo technique for analyzing the vapor flow through the nozzle is shown to be very accurate and flexible. The result clearly points the direction to follow to optimize the performance. Applications of this device are particularly suited in the spin stabilization and the precession damping of small scientific probes.