A question was asked how to get Mach five air flow through a wind tunnel. Getting supersonic airflow through a tunnel is conceptually straightforward. If you have a high pressure (typically 1.4 times the outside pressure, or higher) gas source, such as a gas cylinder, and you simply poke a hole in it, the air flowing from the high pressure to the low pressure will flow right at the speed of sound. If you mount a nozzle downstream of the hole (the “throat”), the gas will accelerate, trading thermal energy for kinetic (the gas can get cryogenically cold and even begin to liquify if the difference between upstream and downstream pressures is high enough).
By adding upstream heaters, you can make the gas *very* hot at the throat. The hotter the gas, the higher the speed of sound, and thus the higher the velocity. Additionally, the hotter the gas, the longer it takes to cool down to cryogenic temperatures, thus the faster you can accelerate the gas. This is the basic process used at the NACA-Langley “Hypersonic Continuous Flow Facility.” This facility also adds a whole lot of compressor power, which allows the system to run essentially non-stop. Smaller facilities would use a compressor to pump up tanks; when the tanks blow down, the test is over.
