De Laval Nozzle
A de Laval nozzle (or convergent-divergent nozzle) features a length of tube pinched in the middle (the throat). At subsonic velocities a converging tube causes the gas flow to accelerate. At supersonic velocities, however, the tube must be divergent in order for the gas to expand and accelerate. Among many applications of the de Laval nozzle are rocket propulsion and supersonic jet engines.