d of 0 1 ft and an l d of 20 give the minimum internal volume for a vortex tube producing 550 lb hr of saturated nitrogen vapor product at one atmosphere pressure see table ii in the present invention for the hypersonic aircraft application liquid hydrogen is available as a refrigerant the use of this refrigeration eliminates the need for compression of the feed air above the level needed to operate the vortex tube itself moreover we have determined that vortex tube purity recovery performance should be greatly enhanced by providing as much of the refrigeration as possible by condensing and returning liquid from the product nitrogen stream rather than by partially condensing the feed air as taught in the art this is shown in the present overall flow sheet of fig3 air is cooled to saturation or partially condensed against the onboard hydrogen fuel and the returning nitrogen rich stream the cooled air is fed to the vortex tube through a tangential inlet nozzle located at some distance from the vapor outlet end of the tube nitrogen rich vapor from the cold end of the tube is split into two streams one of these streams is withdrawn as product while the lesser is totally.