The flow in a low-powered arc gas heater combined with a supersonic nozzle of throat diameter less than1 mm is quite complicated and difficul to describe in quantitative detail. Experiments on arc-heated supersonic jet thrusters of monatomic gases argon and helium have been carriedoutandtheirperformancemeasured.Theflowcharacteristics are analyzed with the help of numerical simulation.Results show that the viscous effect is the most important factor causing the large difference between ideal and real performance. A large outer section of the exit flow is slowmoving. This is especially pronounced in helium, where 70 %of the exit area of the nozzle might be in subsonic flow. Friction forces can be much larger than the net thrust, reaching several times higher in helium, resulting in very low efficien cies. Other factors causing the differences between ideal and real flow include: complex flow in the throat region, electric arc extending to the nozzle expansion section, heat transfer to the inlet gas and from the hot plasma, and environmental pressure in the vacuum chamber. It is recognized that the ordinary concepts of supersonic nozzle flow must be greatly modifie when dealing with such complicated situations. The general concepts presented in this paper could be helpful in guiding the design and operation of this equipment.