[1] Expansion tunnels use a dual-diaphragm system where the diaphragms act as rupture discs, or a pressure relief.
The driven section is filled with a lower pressure desired test gas, such as carbon dioxide, helium, nitrogen, or oxygen.
When the shock wave hits the second diaphragm, it ruptures causing the two gases to mix with the acceleration and expand down the enclosed test section.
The HET is one of the shock tunnels in the Caltech Hypersonics group at the California Institute of Technology directed by Professor Joanna Austin.
[3][5][6] Large Energy National Shock tunnels (LENS) were constructed over the past 15 years at the Aerothermal/Aero-optic Evaluation Center (AAEC) at CUBRC.
The LENS facilities were developed for the testing of advanced missile seekerheads and scramjet engines.
The drive chamber, filled with helium or hydrogen gas, is compressed to 3,000 psi at 1000 degrees Fahrenheit; this breaks the first diaphragm, causing the driven chamber to experience an influx of hot gas, generating pressures over 20,000 psi before the second diaphragm is ruptured.
[8] It is located at Kakuda Space research centre – JAXA (Japan Aerospace Exploration Agency).
HYFLEX (Hypersonic Flight Experiment) which was a re-entry demonstrator prototype vehicle of JAXA was tested in this facility.
It is a large free piston driven shock tunnel capable of producing sub-orbital flow speeds at a range of Mach numbers.
[10] It is a free piston shock tunnel located at California Institute of Technology, USA.