Impulse tunnels depend on the explosive release of energy to create extremely high temperatures and pressures in the test gas (which need not be air). This energy-rich burst of gas expands through a nozzle to hypervelocity speeds, and in a fleeting 10 to 100 milliseconds sweeps past the model mounted in the test section. The two basic types of impulse tunnels that provide hypervelocity flows are termed " hotshot " and "shock." They differ mainly in the way in which energy is added to the test gas. In the hotshot tunnel, an initial charge of nitrogen test gas is heated by an electric-arc discharge, which generates pressures up to 2000 atmospheres and temperatures to 10 000° F. The high-pressure gas ruptures a diaphragm and then expands through a nozzle to the test section with a useful run time of approximately 100 milliseconds. Note that the highly energized gas serves as the test medium. Typical Mach numbers from 8 to 25 are produced.
The shock tunnel on the other hand, uses an initial primary shock wave-created from the rupture of the primary diaphragm by the high-pressure expansion of driver gas-to accelerate and compress the driventube gas. When the shock wave hits the relatively small nozzle throat, it is fully reflected, acting to decelerate the test gas briefly and further compress it into an arrested, hot, high-pressure condition. It is this "driven gas" that bursts through a restraining diaphragm into the expanding nozzle. For a few milliseconds the model in the test section is enveloped by the driven gas at stagnation temperatures up to 20 000 ° F and velocities to 15 000 feet per second. Happily, the response of modern wind tunnel instrumentation is so fast that meaningful results can be gathered even in these short periods of time.