The hotshot tunnel, with its short-duration arc and its run times on the order of 100 milliseconds, has a companion facility known as the arc-jet, which can operate continuously. Here the test gas is preheated in the stilling chamber upstream of the nozzle by means of a continuous electric arc. The heated gas reaching temperatures of 10 000 to 20 000 ° F, is injected under pressure into the nozzle. Flow can be sustained for several minutes but only at relatively low-density levels and supersonic Mach numbers. Power requirements for major facilities are tremendous-some requiring over 100 000 kilowatts. The arc-jet is an extremely valuable facility for testing spacecraft heatshield materials under the high heating rates associated with planetary reentry. The test stream, however, is subject to contamination from the arc.