Various trends in the spacecraft industry are driving the development of low-thrust propulsion systems.
These may be needed for fine attitude control, or to reduce the mass of the propulsion system through the
use of small lightweight components. The nozzle converts the stored energy in a pressurized gas into kinetic
energy through an expansion. The nozzle efficiency is characterized by the amount of kinetic energy leaving
the nozzle, and is governed by the exit velocity. Because of the increase in viscous losses as scale is reduced,
it was feared that high Mach number supersonic flows could not be generated in microdevices. However, a
scaling analysis indicates that the reduction in throat area can be offset by an increase in operating pressure
to maintain a constant Reynolds number. Therefore, thrust can be decreased by reducing the nozzle scale,
and viscous losses mitigated by running at higher chamber pressures.