There is a third type of tunnel not really discussed here: hypersonic wind tunnels. They are closed but not looped. At one end are giant tanks of compressed and chilled air, almost liquid. At the other, even bigger expansion tanks filled with vacuum. Tests last only seconds and are more like explosions than wind.

https://asiatimes.com/2023/06/chinas-jf-22-hypersonic-wind-t...

In the western world, hypersonic testing has been traditionally done live. For groups like NASA, it was always a tossup between biulding a monster tunnel, or just strapping the test articles to the sharp end of a rocket and letting the upper atmosphere be the tunnel.

I worked next door to one in UQ. It was cobbled together from structurally strong steel parts, said to include former gun barrels, and major mining equipment prop shafts. I still have a pressure plate diaphragm, explosively burst like a bubble when the air is forced through it to make the hypersonic shock wave. The chamber is big enough for a wee 1/72nd scale model thing: typically a ramjet engine.

I used to have a teletype 33 from the same lab. At the end, it was only being used to submit orders to university Central stores for cleaning products.