The development of high-temperature heat exchangers, such as the pebble-bed storage heater, made possible the high-performance hypersonic wind tunnel operating on an intermittent basis. In a typical cycle, air is compressed to high pressure, dried, then stored in large tanks. At the same time, a large bed of ceramic pebbles is heated for several hours to incandescence by a gas-fired burner. A typical run begins by opening a throttling valve, allowing highpressure air to surge through the heater, picking up heat as it goes, and then expanding the heated gas to hypersonic speeds in the nozzle. High temperatures in the settling chamber are required to prevent the liquefaction of the air as it expands to very low temperatures in the nozzle. Stagnation temperatures of 3500° F at pressures of several hundred atmospheres provide test Mach numbers from 6 to 15 for run durations on the order of 1 minute. Even during such short exposures, the nozzle throat must be water cooled, for the heat transfer to the wall is highest in this region. Although "real-gas effects" are not simulated in such a tunnel, valid force and pressure data, as well as selective heat transfer data, can be obtained on relatively large models.
For hypersonic runs at more moderate temperatures (2000° F), continuous operation can be obtained by providing an array of continuously operating compressors with air heating provided by electrical resistance heaters. The resulting compressor and drive motor installation, as well as the tunnel cooling requirements, is substantial.