The self-consistent mathematical model in a Variable Specific Impulse Magnetoplasma Rocket (VASIMR)1 is examined. Of particular importance is the effect of a magnetic nozzle in enhancing the axial momentum of the exhaust Also, different geometries and rocket asymmetries are considered. The magnetic configuration is modeled with an adaptable mesh, which increases accuracy without compromising the speed of the simulation. The single particle trajectories are integrated with an adaptive time-scheme, which can quickly solve extensive Monte-Carlo simulations for systems of hundred thousands of particles in a reasonable time (1–2 hours) and without the need for a powerful supercomputer.